Research ArticleStudy on Fracture and Stress Evolution Characteristics ofUltra-Thick Hard Sandstone Roof in the Fully MechanizedMining Face with Large Mining Height A Case Study ofXiaojihan Coal Mine in Western China

Feng Ju Meng Xiao Zequan He Pai Ning and Peng Huang

State Key Laboratory for Geomechanics and Deep Underground Engineering China University of Mining amp TechnologyXuzhou 221116 China

Correspondence should be addressed to Feng Ju juf1983163com and Meng Xiao xiao_meng1993126com

Received 30 August 2018 Accepted 10 October 2018 Published 13 November 2018

Guest Editor Dengke Wang

Copyright copy 2018 Feng Ju et al1is is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Ultra-thick hard sandstone roofs present high thickness poor delamination and wide caving range 1e strata pressure of theworking face during actual mining increases having a significant influence on the safe mining of the working face Especially inthe mining areas of western China the fully mechanizedmining faces with highmining height and high-strength mining are moreprominent Understanding the fractures and stress evolution characteristics of the ultra-thick hard sandstone roof during actualmining is of high significance to control the dynamic pressure on the working face In this paper the typical ultra-thick hardsandstone roof of the Xiaojihan coal mine was taken as an example 1e structural and chemical composition characteristics wereanalyzed Besides the fracture characteristics of ultra-thick hard roof during the working face mining were analyzed Moreoverthe fracture structure consistency was verified through physical simulation and a field measurement method Finally the stressevolution laws in the ultra-thick hard sandstone roof fracture were studied through numerical simulation 1e findingsdemonstrated that (1) the ultra-thick hard sandstone roof was composed of inlaid coarse minerals which had compact structurewhile the Protodyakonov hardness reached up to 307 (2) under the high-strength mining condition of fully mechanized miningface with large mining height the ultra-thick hard sandstone roof had the characteristics of brittle fracture with a caving span of12m (3) under the high-strength mining condition of fully mechanizedmining face with large mining height the ultra-thick hardsandstone roof followed the stress evolution laws that were more sensitive to the neighboring goaf 1erefore it was necessary toreduce the fracture span or layering of ultra-thick hard sandstone roof through the manual intervention method adoption orincrease either the strength of coal pillar or supporting body to resist the impact generated during ultra-thick hard sandstoneroof fracture

1 Introduction

Coal still constitutes the main fossil fuel for energy pro-duction throughout the world To reduce the index of deathrate per million-ton coal (DRPMT) in coal mines the goalachievement of a highly intensive production of modern coalmines is an important approach At present coal minesboth domestically and abroad have been developed as large-scale modern mines with a capacity of tens of millions oftons in the mode of ldquoone mine one facerdquo In China up to 36

pairs of mines exist of tens of millions tons in capacity while34 pairs are under construction or being expanded Most ofthese mines are distributed in the mining areas withinwestern China demonstrating the characteristics of high-intensive mining such as having working faces with largemining height high advance speed and long advance dis-tance [1] Ultra-thick hard sandstone roof is a certaingeological condition recently discovered but commonlyfound during the high-intensive coal mining in westernChina 1is type of roof has several characteristics such as

HindawiAdvances in Civil EngineeringVolume 2018 Article ID 5474165 12 pageshttpsdoiorg10115520185474165

high strength high elastic modulus undeveloped joint fis-sures high thickness strong wholeness and strong self-bearing capacity Following coal mining the roofs move andsustain fracture easily leading to stress concentrationconsequently causing strata behaviors such as rib spallingsevere deformation of tunnels and abrupt increase ofsupport loading Instantaneous caving of large areas of roofsand roof weights severely threatens the production safety inmines [2ndash5] Large-scale coal bases in China are thenortheastern China the north of Shanxi Province the east ofShanxi Province the central part of Shanxi the central partof Hebei Province the southwest of Shandong ProvinceHuainan and Huaibei Shendong the north of ShaanxiNingdong Yunnan and Guizhou as well as the west ofHenan Province In the mining areas of Yulin located at thenorth of Shaanxi Datong and Yangfangkou in the north ofShanxi Zaozhuang of Shandong province TonghuaHegang and Qitaihe in northeast of China and Jingyuan ofHuanglong [6ndash11] the problems regarding hard roofs areubiquitous among which the accidents of roof falls accountfor approximately 70 of the accidents on the working faces1e thick hard roof constitutes the root cause of strong minepressure 1e mechanical properties of this type of roof aswell as the movement deformation damage and fractureduring the working face mining determine the rock pressureand sphere of influence for the entire stope 1ereforestudying the fracture characteristics and stress evolutionlaws of these roof conditions during the high-strengthmining of fully mechanized mining face with large min-ing height is of high significance to control the minepressure on the working face and guarantee the productionsafety on the working face

Many studies on ultra-thick hard sandstone roof miningexist both domestically and abroad With focus on thefracture mechanism of ultra-thick hard sandstone roofWang [12] analyzed the instability mechanism of thick hardroof in Tashan coal mine through the ldquokey stratardquo theorywhile proposing the relationship between roof fracture andabnormal gas emission at the working face Besides theReissner thick plate theory the Vlasov plate theory [13] andthe long-beam theory [14] are often used to analyze theinitial and periodic fracture laws of the ultra-thick hard roofRelevant research techniques mainly include similar simu-lation numerical simulation [15] microseismic monitoringand other methods Wu [16] successfully analyzed thevariation of strata behavior and fissure zone developmentheight in the stope prior to and following ultra-thick hardroof fracture through the physical similar simulationmethod Lu [17 18] analyzed the change law of microseismicsignals on the working face coal body prior to and followingultra-thick hard roof fracture which constituted importantreference data for the fracturemonitoring of ultra-thick hardroof Yu [19] and Xu [20] obtained similar conclusionsthrough numerical simulations 1e simulation resultsdemonstrated that when approximately 350m of workingface was mined full mining was achieved while severe stratabehaviors often appeared In the aspect of processingmethod for ultra-thick hard roof the most commonly usedmethods are the blasting method and the fracturing method

Guo [8] and Ning [10] successfully solved the problem ofsevere strata behaviors caused by ultra-thick hard roofsthrough the decompression blasting and deep-hole pre-splitting blasting methods respectively He [21] and Zheng[22] relieved the early-warning of high strata pressure on theworking face guaranteeing safe mining at the working facethrough the hydrofracturing and sleeve fracturing methodsrespectively Scholars both domestically and abroad con-ducted research on hard roofs mostly from the perspectivesof mine pressure model energy aggregation and rock stratamovement through the methods of numerical simulationand theoretical analysis However insignificant attentionwas paid to the fracture and stress evolution characteristicsof ultra-thick roofs under high-intensive mining In thispaper through the 11215 fully-mechanized mining face inthe Xiaojihan coal mine consideration as an example theroof fracture and stress evolution characteristics duringmining in the fully-mechanized mining face under thecondition that the ultra-thick sandstone roof was near goafwere revealed through the methods of theoretical analysissimilar simulation numerical simulation and field mea-surement 1is provided the important theoretical basis forthe overlying strata control and roadway support at theworking face under this condition

2 General Situation of Engineering

21 Layout of Working Face 1e Xiaojihan coal mine waslocated approximately 20 km northwest of Yulin 1e minefield was bounded by the north boundary of the Yuhengmine area in the north adjacent to the Xihongdun andHongshixia mine fields in the south as well as connected toKekegai exploration area in the west and bounded by theYuxi River in east 1e 11215 working face was located inpanel 11 of 2 coal bed in the well field laid out along thecoal bed in an inclined manner with a strike length of4888m an inclined length of 280m and a mining area of585600m2 At the present stage the 11213 working facemining ended and in the 11215 working face the minereached the open-off cut position and goaf for the 11213working face 1e mining of the 11215 working face and itslayout are presented in Figure 1

1e 11215 working face mining had entered Area II A20m-section coal pillar was laid out between the 11213working face and the 11215 working area when the sur-rounding rock on the air return way of 11215 working facesustained severe deformation Rib spalling and deformationof hydraulic single prop occurred which severely affectedthe normal production 1e cross section of the air returnway for 11215 working face was rectangular with a sectionalarea of 38 times 55m 1e roadway excavation was conductedalong the bottom plate with solid coal on one side and coalpillar on the other side

22 Occurrence of Coal-Series Strata At the present stagethe Xiaojihan coal mining was mainly focused on 2 coal forwhich the depth of the coal bed in the 11215 working facewas 17398ndash46036m 1e coal bed pitch was 1degsim3deg the coal

2 Advances in Civil Engineering

bed thickness was 325sim504m and the average miningheight for the coal bed was 456m 1e working face miningwas conducted through the large-mining-height long wallbackward caving method In the advancing direction of the11215 working face the drilling-hole detection was con-ducted and the geology columnar section of the working facewas obtained through analysis as presented in Figure 2

It could be observed from the geology columnar sectionthat the immediate roof was medium sandstone of 65m inaverage thickness with a color of gray white and a layereddistribution 1e internal minerals presented angular shapeof dark color displaying poorer sorting characteristics 1emain roof was divided into three parts the lower part wasmedium sandstone the upper part was stratified siltstoneand the middle part was stratified sandstone grit 1e mainroof presented approximately horizontal layered distribu-tion with relatively high thickness high strength and goodstability 1is led to difficult fracture occurrence Mostdistricts of the coal seam floor were mudstone with relativelylow strength which was easy to generate heaving floor1rough the entire borehole columnar section analysis itcould be observed that the roof of 11215 working facebelonged to interbedding of sandstone and mudstone inwhich the sandstone accounted for a high proportion 1ecumulative proportion of sandstone over 5m of thicknessfor a single layer exceeded 582 and the cumulativeproportion of sandstone over 10m of thickness exceeded389 It could be considered that the entire roof of theworking face was dominated by relatively intact sandstonewhile the main roof was felspar sandstone stratum of1979m (ultra-thick sandstone of nearly 20m)

23 Structure and Mechanical Characteristics of Ultra-ick Sandstone Roof 1e density of the rock sample for

the thick sandstone roof was analyzed through SEM(scanning electron microscope) 1e SEM pictures of themain roof under conditions of different resolutions arepresented in Figure 3

It could be observed that the coarse-grained mineralsinside the sandstone roof mostly followed an inlaid distri-bution mode 1e grains were high-sized with high crys-tallization 1e structure was significantly dense and thepores were not developed

1rough physical mechanics testing on the rock samplefor the sandstone roof its physical mechanics parameterswere obtained as presented in Table 1

3 Assumption of Ultra-Thick Sandstone HardRoof Fracture

Figure 4 shows the difference of pressure distribution be-tween complete and layered roof caving Comparing rooflayered caving the stress above roadway that has ultra-thickroof is much higher which causes the surrounding rock havelarge deformation When the 11215 working face miningprogressed to the position of open-off cut of 11213 workingface (entering the influence sphere of the goaf for 11213working face) the goafs of 11215 and 11213 working facescontact interconnected At this time the structures of thestope roofs for the two working faces formed the ldquoL-shapedrdquospace structure of the overlying rock with the bearing pointbeing in the goaf area and fracturing lines being in the side ofsolid coal as presented in Figure 5 It could be observed thatblue lines (inside the solid coal) denoted the fracturing linesof roofs while the red lines (in the goaf) denoted the linesformed through the bearing point connections of rooffracturing in the goaf Figure 6 presents the caving state ofthe suspended roof in the goaf of the 11213 working face

11203 working face

11205 working face

11207 working face

112011 working face

11213 working face

11215 working faceI area II area

875870865860

855850

840

845

880

885

840

835

830

820

815

Figure 1 Mining engineering plane diagram

Advances in Civil Engineering 3

ickness(m)

Columnar Keybed Rock name Description of the rock

Gray-white block medium feldspar sandstone Medium sorting staggered bedding roundness is sub-angularobvious pore contact

Black semi-bright coal Dark brown streaks asphalt luster stepped fractures brittleness medium hardnessstrip-like structure layered structure e coal and rock components are mainly bright coal followed by dark coaland a small amount is mirror coal and silk coal e coal seam is in obvious contact with the top and bottom plates

Light gray thick layered siltstone horizontal bedding obvious contact with the lower layer

Gray-white block medium feldspar sandstone Medium sorting staggered bedding roundness is sub-angularporous mud cementation obvious contact with the lower layer

Gray-white block fine feldspar sandstone Medium sorting staggered bedding roundness is sub-circular porousmud cementation obvious contact with the lower layer

Dark gray medium thick layered silty mudstone horizontal bedding obvious contact with the lower layer

Black semi-bright coal Dark brown streaks asphalt luster stepped fractures brittleness medium hardnessstrip-like structure layered structure e coal and rock components are mainly bright coal followed by dark coaland a small amount is mirror coal and silk coal e coal seam is in obvious contact with the top and bottom plates

Dark gray medium thick layered silty mudstone horizontal bedding obvious contact with the lower layer

Gray-white block fine feldspar sandstone Medium sorting staggered bedding roundness is sub-circular porousmud cementation obvious contact with the lower layer

1087

122

607

1979

653

053

510

304

1126

2 coal

Siltymudstone

Siltstone

1 coal

Feldsparsandstoner

Feldsparsandstoner

Feldsparsandstoner

Feldsparsandstoner

Siltymudstone

Figure 2 Composite borehole columnar section for 11215 working face

(a) (b)

(c) (d)

Figure 3 Continued

4 Advances in Civil Engineering

(e) (f )

Figure 3 SEM images at different resolutions in the main roof (a) 100X (b) 250X (c) 350X (d) 500X (e) 800X (f ) 1000X

Table 1 Physical and mechanical parameters of extra-thick sandstone roof

Item Density (kgmminus3) Broken expand coefficientStrength (MPa)

Frictional angle (deg)Compression Tensile Cohesive

Parameters 2566 1440 3067 493 790 3104

Roof completely caving

(a)

Roof layered caving

(b)

Figure 4 Lateral support pressure distribution of coal seam under different roof caving forms (a) roof completely caving (b) roof layeredcaving

Advances in Civil Engineering 5

subsequently to being affected by the 11215 working facemining

It could be observed from the morphological charac-teristics of the ldquoL-shapedrdquo space structure of the overlying-rock that the stress above the structure transferred to solidcoal and the gangue at the goaf through supporting point1ereby two areas of stress concentration formed in boththe goaf and solid coal sides 1e ldquoL-shapedrdquo space structureof the overlying rock could be interpreted as overlying-rock

structures in two working faces cooperatively moving incrosswise dimension forming high pressure arch with theinclined arch front and back feet being at a certain areaof this working face and the upper working face In verti-cal dimension the balanced structure developed upwardsbelow which was fracturing articulated structure Periodicfracture of the rock strata generated characteristics of pe-riodic pressure appearance 1e high rock stratum on thebalanced structure transferred gravity stress to the gangue in

11213 air return

11213 working face(mined)

11213 haulage gate

20 m coal pillar

11215 air return

11215 working face

11215 haulage gate11215 auxiliary

transport gateway

Figure 5 ldquoL-shapedrdquo overlying rock space structure

65

207

12

Medium-granular arkose

Light gray siltstone

Medium-granular arkose

Miliary arkose

Silty mudstone

Overburdencracking

Overburdenfracture line

11215 working face

11215 air return

20 m coal pillar 11213 gob

Figure 6 Suspended roof structure of the No 11213 goaf affected by mining of No 11215 face

6 Advances in Civil Engineering

the goaf and the solid coal in working face through layer-by-layer transmission 1e instability of the balanced structurewould cause dynamic pressure

When the 11215 working face mining was conductednear the open-off cut position of the 11213 working face thesynergic movement of overlying rocks in the two workingfaces led to the ldquoL-shapedrdquo space structure formationcausing stress concentration to form above the 20m isolatedcoal pillar set in-between the working faces 1erefore se-vere surrounding rock deformation and dynamic pressurephenomenon occurred at the end of the 11215 working faceand return air roadway1e roof above the coal pillar rotatedto the goaf of the 11213 working face causing deformation ofnonproduction side (coal pillar side) on the air returnroadway of 11215 working face to far exceed the productionside (solid coal side)

1e position calculation of the fracturing line on the roofin the goaf of the 11213 working face was of high significanceto determine the rational coal pillar width isolate synergicmovement of overlying rocks on the neighboring goafs andcontrol the roof weighting1e position of fracturing line forthe main roof in the working face could be obtained throughthe elastic foundation beam model 1rough the stressconditions on the fractured roof strata the model could besimplified into the ldquocantilever-simply supportrdquo beamstructure in the direction along working face as presented inFigure 7

1e fractured rock length Lx can be calculated throughthe following equation on the key strata theory basis

Lx tanminus1 β 2αM0s + rQ0( 1113857( 1113857 r2M0 + αrQ0( 11138571113858 1113859

β (1)

where

Q0 qcL + QminusF

M0 qc

2L2

+ QL + Nh

2minusFZ

N LQ

2(hminusΔs)qcL

Q qcL

Δs h

6

s N

EI

r2

k

EI

(2)

where α approximately equals to β the corresponding valuecan be taken as 009mminus1 Qc presents the uniformly dis-tributed load imposed by the weight of main roof and itsupper bearing strata (8 times the mining height) h denotesthickness of the main roof which was 20m k denotes elasticfoundation coefficient of 500MPa in value E denotes the

elastic modulus of the main roof of 55GPa in value Idenotes inertia moment for section of the main roof Fdenotes supporting force of the coal pillar Z denotes theapex distance from the action point of supporting force ofthe coal pillar to the fixed end and L denotes the originalcantilever length in the main roof According to the ultimatespan equation for the mechanical model of the simplysupported beam for the main roof

L 2h

RT

3q

1113971

(3)

where h denotes the main roof thickness which is 20m RTdenotes the rock tensile strength of 596MPa q denotes theuniformly distributed load which is 8 times the miningheight q 09MPa

1e hanging arch length of the main roof for the 11213working face under the condition of ultra-thick sandstonecan be calculated as 594m

It was discovered through data statistics that as thesupporting force of coal pillar F gradually increased Lxgradually decreased signifying that the higher the sup-porting force of coal pillar to the main roof was the shorterthe fracturing length of the main top was Supposing theultimate limit state the supporting force of coal pillar to themain roof was 0 consequently the fracturing length of themain top as maximum 1rough calculations it could beobtained that Lx(max) 435m

From Figure 8 the fracturing length of the main top is

Lx L1 + L2 + L3 (4)

where L1 denotes the suspension length of the main roof inthe goaf which is determined to be 15m through fieldmeasurement L2 denotes the coal pillar width of 20m inthis case and L3 denotes the extended length of 85mthrough calculation 1rough the width consideration of theair return way of 5m the position of the main roof frac-turing was approximately 35m inside the coal

4 Similar Simulation Testing and Detection ofUltra-Thick Hard Sandstone Roof Fracture

In order to obtain the fracturing position and fracturingcharacteristics of the roof for 11215 working face under theneighboring goaf influence similar simulation testing wasadopted to conduct research as well as the fracturing position

0 x

y

N

Q

M0

Q0

F

Z

L

Figure 7 Mechanical model of main roof suspension above coalpillar after mining face

Advances in Civil Engineering 7

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

bed thickness was 325sim504m and the average miningheight for the coal bed was 456m 1e working face miningwas conducted through the large-mining-height long wallbackward caving method In the advancing direction of the11215 working face the drilling-hole detection was con-ducted and the geology columnar section of the working facewas obtained through analysis as presented in Figure 2

It could be observed from the geology columnar sectionthat the immediate roof was medium sandstone of 65m inaverage thickness with a color of gray white and a layereddistribution 1e internal minerals presented angular shapeof dark color displaying poorer sorting characteristics 1emain roof was divided into three parts the lower part wasmedium sandstone the upper part was stratified siltstoneand the middle part was stratified sandstone grit 1e mainroof presented approximately horizontal layered distribu-tion with relatively high thickness high strength and goodstability 1is led to difficult fracture occurrence Mostdistricts of the coal seam floor were mudstone with relativelylow strength which was easy to generate heaving floor1rough the entire borehole columnar section analysis itcould be observed that the roof of 11215 working facebelonged to interbedding of sandstone and mudstone inwhich the sandstone accounted for a high proportion 1ecumulative proportion of sandstone over 5m of thicknessfor a single layer exceeded 582 and the cumulativeproportion of sandstone over 10m of thickness exceeded389 It could be considered that the entire roof of theworking face was dominated by relatively intact sandstonewhile the main roof was felspar sandstone stratum of1979m (ultra-thick sandstone of nearly 20m)

23 Structure and Mechanical Characteristics of Ultra-ick Sandstone Roof 1e density of the rock sample for

the thick sandstone roof was analyzed through SEM(scanning electron microscope) 1e SEM pictures of themain roof under conditions of different resolutions arepresented in Figure 3

It could be observed that the coarse-grained mineralsinside the sandstone roof mostly followed an inlaid distri-bution mode 1e grains were high-sized with high crys-tallization 1e structure was significantly dense and thepores were not developed

1rough physical mechanics testing on the rock samplefor the sandstone roof its physical mechanics parameterswere obtained as presented in Table 1

3 Assumption of Ultra-Thick Sandstone HardRoof Fracture

Figure 4 shows the difference of pressure distribution be-tween complete and layered roof caving Comparing rooflayered caving the stress above roadway that has ultra-thickroof is much higher which causes the surrounding rock havelarge deformation When the 11215 working face miningprogressed to the position of open-off cut of 11213 workingface (entering the influence sphere of the goaf for 11213working face) the goafs of 11215 and 11213 working facescontact interconnected At this time the structures of thestope roofs for the two working faces formed the ldquoL-shapedrdquospace structure of the overlying rock with the bearing pointbeing in the goaf area and fracturing lines being in the side ofsolid coal as presented in Figure 5 It could be observed thatblue lines (inside the solid coal) denoted the fracturing linesof roofs while the red lines (in the goaf) denoted the linesformed through the bearing point connections of rooffracturing in the goaf Figure 6 presents the caving state ofthe suspended roof in the goaf of the 11213 working face

11203 working face

11205 working face

11207 working face

112011 working face

11213 working face

11215 working faceI area II area

875870865860

855850

840

845

880

885

840

835

830

820

815

Figure 1 Mining engineering plane diagram

Advances in Civil Engineering 3

ickness(m)

Columnar Keybed Rock name Description of the rock

Gray-white block medium feldspar sandstone Medium sorting staggered bedding roundness is sub-angularobvious pore contact

Black semi-bright coal Dark brown streaks asphalt luster stepped fractures brittleness medium hardnessstrip-like structure layered structure e coal and rock components are mainly bright coal followed by dark coaland a small amount is mirror coal and silk coal e coal seam is in obvious contact with the top and bottom plates

Light gray thick layered siltstone horizontal bedding obvious contact with the lower layer

Gray-white block medium feldspar sandstone Medium sorting staggered bedding roundness is sub-angularporous mud cementation obvious contact with the lower layer

Gray-white block fine feldspar sandstone Medium sorting staggered bedding roundness is sub-circular porousmud cementation obvious contact with the lower layer

Dark gray medium thick layered silty mudstone horizontal bedding obvious contact with the lower layer

Black semi-bright coal Dark brown streaks asphalt luster stepped fractures brittleness medium hardnessstrip-like structure layered structure e coal and rock components are mainly bright coal followed by dark coaland a small amount is mirror coal and silk coal e coal seam is in obvious contact with the top and bottom plates

Dark gray medium thick layered silty mudstone horizontal bedding obvious contact with the lower layer

Gray-white block fine feldspar sandstone Medium sorting staggered bedding roundness is sub-circular porousmud cementation obvious contact with the lower layer

1087

122

607

1979

653

053

510

304

1126

2 coal

Siltymudstone

Siltstone

1 coal

Feldsparsandstoner

Feldsparsandstoner

Feldsparsandstoner

Feldsparsandstoner

Siltymudstone

Figure 2 Composite borehole columnar section for 11215 working face

(a) (b)

(c) (d)

Figure 3 Continued

4 Advances in Civil Engineering

(e) (f )

Figure 3 SEM images at different resolutions in the main roof (a) 100X (b) 250X (c) 350X (d) 500X (e) 800X (f ) 1000X

Table 1 Physical and mechanical parameters of extra-thick sandstone roof

Item Density (kgmminus3) Broken expand coefficientStrength (MPa)

Frictional angle (deg)Compression Tensile Cohesive

Parameters 2566 1440 3067 493 790 3104

Roof completely caving

(a)

Roof layered caving

(b)

Figure 4 Lateral support pressure distribution of coal seam under different roof caving forms (a) roof completely caving (b) roof layeredcaving

Advances in Civil Engineering 5

subsequently to being affected by the 11215 working facemining

It could be observed from the morphological charac-teristics of the ldquoL-shapedrdquo space structure of the overlying-rock that the stress above the structure transferred to solidcoal and the gangue at the goaf through supporting point1ereby two areas of stress concentration formed in boththe goaf and solid coal sides 1e ldquoL-shapedrdquo space structureof the overlying rock could be interpreted as overlying-rock

structures in two working faces cooperatively moving incrosswise dimension forming high pressure arch with theinclined arch front and back feet being at a certain areaof this working face and the upper working face In verti-cal dimension the balanced structure developed upwardsbelow which was fracturing articulated structure Periodicfracture of the rock strata generated characteristics of pe-riodic pressure appearance 1e high rock stratum on thebalanced structure transferred gravity stress to the gangue in

11213 air return

11213 working face(mined)

11213 haulage gate

20 m coal pillar

11215 air return

11215 working face

11215 haulage gate11215 auxiliary

transport gateway

Figure 5 ldquoL-shapedrdquo overlying rock space structure

65

207

12

Medium-granular arkose

Light gray siltstone

Medium-granular arkose

Miliary arkose

Silty mudstone

Overburdencracking

Overburdenfracture line

11215 working face

11215 air return

20 m coal pillar 11213 gob

Figure 6 Suspended roof structure of the No 11213 goaf affected by mining of No 11215 face

6 Advances in Civil Engineering

the goaf and the solid coal in working face through layer-by-layer transmission 1e instability of the balanced structurewould cause dynamic pressure

When the 11215 working face mining was conductednear the open-off cut position of the 11213 working face thesynergic movement of overlying rocks in the two workingfaces led to the ldquoL-shapedrdquo space structure formationcausing stress concentration to form above the 20m isolatedcoal pillar set in-between the working faces 1erefore se-vere surrounding rock deformation and dynamic pressurephenomenon occurred at the end of the 11215 working faceand return air roadway1e roof above the coal pillar rotatedto the goaf of the 11213 working face causing deformation ofnonproduction side (coal pillar side) on the air returnroadway of 11215 working face to far exceed the productionside (solid coal side)

1e position calculation of the fracturing line on the roofin the goaf of the 11213 working face was of high significanceto determine the rational coal pillar width isolate synergicmovement of overlying rocks on the neighboring goafs andcontrol the roof weighting1e position of fracturing line forthe main roof in the working face could be obtained throughthe elastic foundation beam model 1rough the stressconditions on the fractured roof strata the model could besimplified into the ldquocantilever-simply supportrdquo beamstructure in the direction along working face as presented inFigure 7

1e fractured rock length Lx can be calculated throughthe following equation on the key strata theory basis

Lx tanminus1 β 2αM0s + rQ0( 1113857( 1113857 r2M0 + αrQ0( 11138571113858 1113859

β (1)

where

Q0 qcL + QminusF

M0 qc

2L2

+ QL + Nh

2minusFZ

N LQ

2(hminusΔs)qcL

Q qcL

Δs h

6

s N

EI

r2

k

EI

(2)

where α approximately equals to β the corresponding valuecan be taken as 009mminus1 Qc presents the uniformly dis-tributed load imposed by the weight of main roof and itsupper bearing strata (8 times the mining height) h denotesthickness of the main roof which was 20m k denotes elasticfoundation coefficient of 500MPa in value E denotes the

elastic modulus of the main roof of 55GPa in value Idenotes inertia moment for section of the main roof Fdenotes supporting force of the coal pillar Z denotes theapex distance from the action point of supporting force ofthe coal pillar to the fixed end and L denotes the originalcantilever length in the main roof According to the ultimatespan equation for the mechanical model of the simplysupported beam for the main roof

L 2h

RT

3q

1113971

(3)

where h denotes the main roof thickness which is 20m RTdenotes the rock tensile strength of 596MPa q denotes theuniformly distributed load which is 8 times the miningheight q 09MPa

1e hanging arch length of the main roof for the 11213working face under the condition of ultra-thick sandstonecan be calculated as 594m

It was discovered through data statistics that as thesupporting force of coal pillar F gradually increased Lxgradually decreased signifying that the higher the sup-porting force of coal pillar to the main roof was the shorterthe fracturing length of the main top was Supposing theultimate limit state the supporting force of coal pillar to themain roof was 0 consequently the fracturing length of themain top as maximum 1rough calculations it could beobtained that Lx(max) 435m

From Figure 8 the fracturing length of the main top is

Lx L1 + L2 + L3 (4)

where L1 denotes the suspension length of the main roof inthe goaf which is determined to be 15m through fieldmeasurement L2 denotes the coal pillar width of 20m inthis case and L3 denotes the extended length of 85mthrough calculation 1rough the width consideration of theair return way of 5m the position of the main roof frac-turing was approximately 35m inside the coal

4 Similar Simulation Testing and Detection ofUltra-Thick Hard Sandstone Roof Fracture

In order to obtain the fracturing position and fracturingcharacteristics of the roof for 11215 working face under theneighboring goaf influence similar simulation testing wasadopted to conduct research as well as the fracturing position

0 x

y

N

Q

M0

Q0

F

Z

L

Figure 7 Mechanical model of main roof suspension above coalpillar after mining face

Advances in Civil Engineering 7

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

ickness(m)

Columnar Keybed Rock name Description of the rock

Gray-white block medium feldspar sandstone Medium sorting staggered bedding roundness is sub-angularobvious pore contact

Black semi-bright coal Dark brown streaks asphalt luster stepped fractures brittleness medium hardnessstrip-like structure layered structure e coal and rock components are mainly bright coal followed by dark coaland a small amount is mirror coal and silk coal e coal seam is in obvious contact with the top and bottom plates

Light gray thick layered siltstone horizontal bedding obvious contact with the lower layer

Gray-white block medium feldspar sandstone Medium sorting staggered bedding roundness is sub-angularporous mud cementation obvious contact with the lower layer

Gray-white block fine feldspar sandstone Medium sorting staggered bedding roundness is sub-circular porousmud cementation obvious contact with the lower layer

Dark gray medium thick layered silty mudstone horizontal bedding obvious contact with the lower layer

Black semi-bright coal Dark brown streaks asphalt luster stepped fractures brittleness medium hardnessstrip-like structure layered structure e coal and rock components are mainly bright coal followed by dark coaland a small amount is mirror coal and silk coal e coal seam is in obvious contact with the top and bottom plates

Dark gray medium thick layered silty mudstone horizontal bedding obvious contact with the lower layer

Gray-white block fine feldspar sandstone Medium sorting staggered bedding roundness is sub-circular porousmud cementation obvious contact with the lower layer

1087

122

607

1979

653

053

510

304

1126

2 coal

Siltymudstone

Siltstone

1 coal

Feldsparsandstoner

Feldsparsandstoner

Feldsparsandstoner

Feldsparsandstoner

Siltymudstone

Figure 2 Composite borehole columnar section for 11215 working face

(a) (b)

(c) (d)

Figure 3 Continued

4 Advances in Civil Engineering

(e) (f )

Figure 3 SEM images at different resolutions in the main roof (a) 100X (b) 250X (c) 350X (d) 500X (e) 800X (f ) 1000X

Table 1 Physical and mechanical parameters of extra-thick sandstone roof

Item Density (kgmminus3) Broken expand coefficientStrength (MPa)

Frictional angle (deg)Compression Tensile Cohesive

Parameters 2566 1440 3067 493 790 3104

Roof completely caving

(a)

Roof layered caving

(b)

Figure 4 Lateral support pressure distribution of coal seam under different roof caving forms (a) roof completely caving (b) roof layeredcaving

Advances in Civil Engineering 5

subsequently to being affected by the 11215 working facemining

It could be observed from the morphological charac-teristics of the ldquoL-shapedrdquo space structure of the overlying-rock that the stress above the structure transferred to solidcoal and the gangue at the goaf through supporting point1ereby two areas of stress concentration formed in boththe goaf and solid coal sides 1e ldquoL-shapedrdquo space structureof the overlying rock could be interpreted as overlying-rock

structures in two working faces cooperatively moving incrosswise dimension forming high pressure arch with theinclined arch front and back feet being at a certain areaof this working face and the upper working face In verti-cal dimension the balanced structure developed upwardsbelow which was fracturing articulated structure Periodicfracture of the rock strata generated characteristics of pe-riodic pressure appearance 1e high rock stratum on thebalanced structure transferred gravity stress to the gangue in

11213 air return

11213 working face(mined)

11213 haulage gate

20 m coal pillar

11215 air return

11215 working face

11215 haulage gate11215 auxiliary

transport gateway

Figure 5 ldquoL-shapedrdquo overlying rock space structure

65

207

12

Medium-granular arkose

Light gray siltstone

Medium-granular arkose

Miliary arkose

Silty mudstone

Overburdencracking

Overburdenfracture line

11215 working face

11215 air return

20 m coal pillar 11213 gob

Figure 6 Suspended roof structure of the No 11213 goaf affected by mining of No 11215 face

6 Advances in Civil Engineering

the goaf and the solid coal in working face through layer-by-layer transmission 1e instability of the balanced structurewould cause dynamic pressure

When the 11215 working face mining was conductednear the open-off cut position of the 11213 working face thesynergic movement of overlying rocks in the two workingfaces led to the ldquoL-shapedrdquo space structure formationcausing stress concentration to form above the 20m isolatedcoal pillar set in-between the working faces 1erefore se-vere surrounding rock deformation and dynamic pressurephenomenon occurred at the end of the 11215 working faceand return air roadway1e roof above the coal pillar rotatedto the goaf of the 11213 working face causing deformation ofnonproduction side (coal pillar side) on the air returnroadway of 11215 working face to far exceed the productionside (solid coal side)

1e position calculation of the fracturing line on the roofin the goaf of the 11213 working face was of high significanceto determine the rational coal pillar width isolate synergicmovement of overlying rocks on the neighboring goafs andcontrol the roof weighting1e position of fracturing line forthe main roof in the working face could be obtained throughthe elastic foundation beam model 1rough the stressconditions on the fractured roof strata the model could besimplified into the ldquocantilever-simply supportrdquo beamstructure in the direction along working face as presented inFigure 7

1e fractured rock length Lx can be calculated throughthe following equation on the key strata theory basis

Lx tanminus1 β 2αM0s + rQ0( 1113857( 1113857 r2M0 + αrQ0( 11138571113858 1113859

β (1)

where

Q0 qcL + QminusF

M0 qc

2L2

+ QL + Nh

2minusFZ

N LQ

2(hminusΔs)qcL

Q qcL

Δs h

6

s N

EI

r2

k

EI

(2)

where α approximately equals to β the corresponding valuecan be taken as 009mminus1 Qc presents the uniformly dis-tributed load imposed by the weight of main roof and itsupper bearing strata (8 times the mining height) h denotesthickness of the main roof which was 20m k denotes elasticfoundation coefficient of 500MPa in value E denotes the

elastic modulus of the main roof of 55GPa in value Idenotes inertia moment for section of the main roof Fdenotes supporting force of the coal pillar Z denotes theapex distance from the action point of supporting force ofthe coal pillar to the fixed end and L denotes the originalcantilever length in the main roof According to the ultimatespan equation for the mechanical model of the simplysupported beam for the main roof

L 2h

RT

3q

1113971

(3)

where h denotes the main roof thickness which is 20m RTdenotes the rock tensile strength of 596MPa q denotes theuniformly distributed load which is 8 times the miningheight q 09MPa

1e hanging arch length of the main roof for the 11213working face under the condition of ultra-thick sandstonecan be calculated as 594m

It was discovered through data statistics that as thesupporting force of coal pillar F gradually increased Lxgradually decreased signifying that the higher the sup-porting force of coal pillar to the main roof was the shorterthe fracturing length of the main top was Supposing theultimate limit state the supporting force of coal pillar to themain roof was 0 consequently the fracturing length of themain top as maximum 1rough calculations it could beobtained that Lx(max) 435m

From Figure 8 the fracturing length of the main top is

Lx L1 + L2 + L3 (4)

where L1 denotes the suspension length of the main roof inthe goaf which is determined to be 15m through fieldmeasurement L2 denotes the coal pillar width of 20m inthis case and L3 denotes the extended length of 85mthrough calculation 1rough the width consideration of theair return way of 5m the position of the main roof frac-turing was approximately 35m inside the coal

4 Similar Simulation Testing and Detection ofUltra-Thick Hard Sandstone Roof Fracture

In order to obtain the fracturing position and fracturingcharacteristics of the roof for 11215 working face under theneighboring goaf influence similar simulation testing wasadopted to conduct research as well as the fracturing position

0 x

y

N

Q

M0

Q0

F

Z

L

Figure 7 Mechanical model of main roof suspension above coalpillar after mining face

Advances in Civil Engineering 7

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

(e) (f )

Figure 3 SEM images at different resolutions in the main roof (a) 100X (b) 250X (c) 350X (d) 500X (e) 800X (f ) 1000X

Table 1 Physical and mechanical parameters of extra-thick sandstone roof

Item Density (kgmminus3) Broken expand coefficientStrength (MPa)

Frictional angle (deg)Compression Tensile Cohesive

Parameters 2566 1440 3067 493 790 3104

Roof completely caving

(a)

Roof layered caving

(b)

Figure 4 Lateral support pressure distribution of coal seam under different roof caving forms (a) roof completely caving (b) roof layeredcaving

Advances in Civil Engineering 5

subsequently to being affected by the 11215 working facemining

It could be observed from the morphological charac-teristics of the ldquoL-shapedrdquo space structure of the overlying-rock that the stress above the structure transferred to solidcoal and the gangue at the goaf through supporting point1ereby two areas of stress concentration formed in boththe goaf and solid coal sides 1e ldquoL-shapedrdquo space structureof the overlying rock could be interpreted as overlying-rock

structures in two working faces cooperatively moving incrosswise dimension forming high pressure arch with theinclined arch front and back feet being at a certain areaof this working face and the upper working face In verti-cal dimension the balanced structure developed upwardsbelow which was fracturing articulated structure Periodicfracture of the rock strata generated characteristics of pe-riodic pressure appearance 1e high rock stratum on thebalanced structure transferred gravity stress to the gangue in

11213 air return

11213 working face(mined)

11213 haulage gate

20 m coal pillar

11215 air return

11215 working face

11215 haulage gate11215 auxiliary

transport gateway

Figure 5 ldquoL-shapedrdquo overlying rock space structure

65

207

12

Medium-granular arkose

Light gray siltstone

Medium-granular arkose

Miliary arkose

Silty mudstone

Overburdencracking

Overburdenfracture line

11215 working face

11215 air return

20 m coal pillar 11213 gob

Figure 6 Suspended roof structure of the No 11213 goaf affected by mining of No 11215 face

6 Advances in Civil Engineering

the goaf and the solid coal in working face through layer-by-layer transmission 1e instability of the balanced structurewould cause dynamic pressure

When the 11215 working face mining was conductednear the open-off cut position of the 11213 working face thesynergic movement of overlying rocks in the two workingfaces led to the ldquoL-shapedrdquo space structure formationcausing stress concentration to form above the 20m isolatedcoal pillar set in-between the working faces 1erefore se-vere surrounding rock deformation and dynamic pressurephenomenon occurred at the end of the 11215 working faceand return air roadway1e roof above the coal pillar rotatedto the goaf of the 11213 working face causing deformation ofnonproduction side (coal pillar side) on the air returnroadway of 11215 working face to far exceed the productionside (solid coal side)

1e position calculation of the fracturing line on the roofin the goaf of the 11213 working face was of high significanceto determine the rational coal pillar width isolate synergicmovement of overlying rocks on the neighboring goafs andcontrol the roof weighting1e position of fracturing line forthe main roof in the working face could be obtained throughthe elastic foundation beam model 1rough the stressconditions on the fractured roof strata the model could besimplified into the ldquocantilever-simply supportrdquo beamstructure in the direction along working face as presented inFigure 7

1e fractured rock length Lx can be calculated throughthe following equation on the key strata theory basis

Lx tanminus1 β 2αM0s + rQ0( 1113857( 1113857 r2M0 + αrQ0( 11138571113858 1113859

β (1)

where

Q0 qcL + QminusF

M0 qc

2L2

+ QL + Nh

2minusFZ

N LQ

2(hminusΔs)qcL

Q qcL

Δs h

6

s N

EI

r2

k

EI

(2)

where α approximately equals to β the corresponding valuecan be taken as 009mminus1 Qc presents the uniformly dis-tributed load imposed by the weight of main roof and itsupper bearing strata (8 times the mining height) h denotesthickness of the main roof which was 20m k denotes elasticfoundation coefficient of 500MPa in value E denotes the

elastic modulus of the main roof of 55GPa in value Idenotes inertia moment for section of the main roof Fdenotes supporting force of the coal pillar Z denotes theapex distance from the action point of supporting force ofthe coal pillar to the fixed end and L denotes the originalcantilever length in the main roof According to the ultimatespan equation for the mechanical model of the simplysupported beam for the main roof

L 2h

RT

3q

1113971

(3)

where h denotes the main roof thickness which is 20m RTdenotes the rock tensile strength of 596MPa q denotes theuniformly distributed load which is 8 times the miningheight q 09MPa

1e hanging arch length of the main roof for the 11213working face under the condition of ultra-thick sandstonecan be calculated as 594m

It was discovered through data statistics that as thesupporting force of coal pillar F gradually increased Lxgradually decreased signifying that the higher the sup-porting force of coal pillar to the main roof was the shorterthe fracturing length of the main top was Supposing theultimate limit state the supporting force of coal pillar to themain roof was 0 consequently the fracturing length of themain top as maximum 1rough calculations it could beobtained that Lx(max) 435m

From Figure 8 the fracturing length of the main top is

Lx L1 + L2 + L3 (4)

where L1 denotes the suspension length of the main roof inthe goaf which is determined to be 15m through fieldmeasurement L2 denotes the coal pillar width of 20m inthis case and L3 denotes the extended length of 85mthrough calculation 1rough the width consideration of theair return way of 5m the position of the main roof frac-turing was approximately 35m inside the coal

4 Similar Simulation Testing and Detection ofUltra-Thick Hard Sandstone Roof Fracture

In order to obtain the fracturing position and fracturingcharacteristics of the roof for 11215 working face under theneighboring goaf influence similar simulation testing wasadopted to conduct research as well as the fracturing position

0 x

y

N

Q

M0

Q0

F

Z

L

Figure 7 Mechanical model of main roof suspension above coalpillar after mining face

Advances in Civil Engineering 7

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

subsequently to being affected by the 11215 working facemining

It could be observed from the morphological charac-teristics of the ldquoL-shapedrdquo space structure of the overlying-rock that the stress above the structure transferred to solidcoal and the gangue at the goaf through supporting point1ereby two areas of stress concentration formed in boththe goaf and solid coal sides 1e ldquoL-shapedrdquo space structureof the overlying rock could be interpreted as overlying-rock

structures in two working faces cooperatively moving incrosswise dimension forming high pressure arch with theinclined arch front and back feet being at a certain areaof this working face and the upper working face In verti-cal dimension the balanced structure developed upwardsbelow which was fracturing articulated structure Periodicfracture of the rock strata generated characteristics of pe-riodic pressure appearance 1e high rock stratum on thebalanced structure transferred gravity stress to the gangue in

11213 air return

11213 working face(mined)

11213 haulage gate

20 m coal pillar

11215 air return

11215 working face

11215 haulage gate11215 auxiliary

transport gateway

Figure 5 ldquoL-shapedrdquo overlying rock space structure

65

207

12

Medium-granular arkose

Light gray siltstone

Medium-granular arkose

Miliary arkose

Silty mudstone

Overburdencracking

Overburdenfracture line

11215 working face

11215 air return

20 m coal pillar 11213 gob

Figure 6 Suspended roof structure of the No 11213 goaf affected by mining of No 11215 face

6 Advances in Civil Engineering

the goaf and the solid coal in working face through layer-by-layer transmission 1e instability of the balanced structurewould cause dynamic pressure

When the 11215 working face mining was conductednear the open-off cut position of the 11213 working face thesynergic movement of overlying rocks in the two workingfaces led to the ldquoL-shapedrdquo space structure formationcausing stress concentration to form above the 20m isolatedcoal pillar set in-between the working faces 1erefore se-vere surrounding rock deformation and dynamic pressurephenomenon occurred at the end of the 11215 working faceand return air roadway1e roof above the coal pillar rotatedto the goaf of the 11213 working face causing deformation ofnonproduction side (coal pillar side) on the air returnroadway of 11215 working face to far exceed the productionside (solid coal side)

1e position calculation of the fracturing line on the roofin the goaf of the 11213 working face was of high significanceto determine the rational coal pillar width isolate synergicmovement of overlying rocks on the neighboring goafs andcontrol the roof weighting1e position of fracturing line forthe main roof in the working face could be obtained throughthe elastic foundation beam model 1rough the stressconditions on the fractured roof strata the model could besimplified into the ldquocantilever-simply supportrdquo beamstructure in the direction along working face as presented inFigure 7

1e fractured rock length Lx can be calculated throughthe following equation on the key strata theory basis

Lx tanminus1 β 2αM0s + rQ0( 1113857( 1113857 r2M0 + αrQ0( 11138571113858 1113859

β (1)

where

Q0 qcL + QminusF

M0 qc

2L2

+ QL + Nh

2minusFZ

N LQ

2(hminusΔs)qcL

Q qcL

Δs h

6

s N

EI

r2

k

EI

(2)

where α approximately equals to β the corresponding valuecan be taken as 009mminus1 Qc presents the uniformly dis-tributed load imposed by the weight of main roof and itsupper bearing strata (8 times the mining height) h denotesthickness of the main roof which was 20m k denotes elasticfoundation coefficient of 500MPa in value E denotes the

elastic modulus of the main roof of 55GPa in value Idenotes inertia moment for section of the main roof Fdenotes supporting force of the coal pillar Z denotes theapex distance from the action point of supporting force ofthe coal pillar to the fixed end and L denotes the originalcantilever length in the main roof According to the ultimatespan equation for the mechanical model of the simplysupported beam for the main roof

L 2h

RT

3q

1113971

(3)

where h denotes the main roof thickness which is 20m RTdenotes the rock tensile strength of 596MPa q denotes theuniformly distributed load which is 8 times the miningheight q 09MPa

1e hanging arch length of the main roof for the 11213working face under the condition of ultra-thick sandstonecan be calculated as 594m

It was discovered through data statistics that as thesupporting force of coal pillar F gradually increased Lxgradually decreased signifying that the higher the sup-porting force of coal pillar to the main roof was the shorterthe fracturing length of the main top was Supposing theultimate limit state the supporting force of coal pillar to themain roof was 0 consequently the fracturing length of themain top as maximum 1rough calculations it could beobtained that Lx(max) 435m

From Figure 8 the fracturing length of the main top is

Lx L1 + L2 + L3 (4)

where L1 denotes the suspension length of the main roof inthe goaf which is determined to be 15m through fieldmeasurement L2 denotes the coal pillar width of 20m inthis case and L3 denotes the extended length of 85mthrough calculation 1rough the width consideration of theair return way of 5m the position of the main roof frac-turing was approximately 35m inside the coal

4 Similar Simulation Testing and Detection ofUltra-Thick Hard Sandstone Roof Fracture

In order to obtain the fracturing position and fracturingcharacteristics of the roof for 11215 working face under theneighboring goaf influence similar simulation testing wasadopted to conduct research as well as the fracturing position

0 x

y

N

Q

M0

Q0

F

Z

L

Figure 7 Mechanical model of main roof suspension above coalpillar after mining face

Advances in Civil Engineering 7

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

the goaf and the solid coal in working face through layer-by-layer transmission 1e instability of the balanced structurewould cause dynamic pressure

When the 11215 working face mining was conductednear the open-off cut position of the 11213 working face thesynergic movement of overlying rocks in the two workingfaces led to the ldquoL-shapedrdquo space structure formationcausing stress concentration to form above the 20m isolatedcoal pillar set in-between the working faces 1erefore se-vere surrounding rock deformation and dynamic pressurephenomenon occurred at the end of the 11215 working faceand return air roadway1e roof above the coal pillar rotatedto the goaf of the 11213 working face causing deformation ofnonproduction side (coal pillar side) on the air returnroadway of 11215 working face to far exceed the productionside (solid coal side)

1e position calculation of the fracturing line on the roofin the goaf of the 11213 working face was of high significanceto determine the rational coal pillar width isolate synergicmovement of overlying rocks on the neighboring goafs andcontrol the roof weighting1e position of fracturing line forthe main roof in the working face could be obtained throughthe elastic foundation beam model 1rough the stressconditions on the fractured roof strata the model could besimplified into the ldquocantilever-simply supportrdquo beamstructure in the direction along working face as presented inFigure 7

1e fractured rock length Lx can be calculated throughthe following equation on the key strata theory basis

Lx tanminus1 β 2αM0s + rQ0( 1113857( 1113857 r2M0 + αrQ0( 11138571113858 1113859

β (1)

where

Q0 qcL + QminusF

M0 qc

2L2

+ QL + Nh

2minusFZ

N LQ

2(hminusΔs)qcL

Q qcL

Δs h

6

s N

EI

r2

k

EI

(2)

where α approximately equals to β the corresponding valuecan be taken as 009mminus1 Qc presents the uniformly dis-tributed load imposed by the weight of main roof and itsupper bearing strata (8 times the mining height) h denotesthickness of the main roof which was 20m k denotes elasticfoundation coefficient of 500MPa in value E denotes the

elastic modulus of the main roof of 55GPa in value Idenotes inertia moment for section of the main roof Fdenotes supporting force of the coal pillar Z denotes theapex distance from the action point of supporting force ofthe coal pillar to the fixed end and L denotes the originalcantilever length in the main roof According to the ultimatespan equation for the mechanical model of the simplysupported beam for the main roof

L 2h

RT

3q

1113971

(3)

where h denotes the main roof thickness which is 20m RTdenotes the rock tensile strength of 596MPa q denotes theuniformly distributed load which is 8 times the miningheight q 09MPa

1e hanging arch length of the main roof for the 11213working face under the condition of ultra-thick sandstonecan be calculated as 594m

It was discovered through data statistics that as thesupporting force of coal pillar F gradually increased Lxgradually decreased signifying that the higher the sup-porting force of coal pillar to the main roof was the shorterthe fracturing length of the main top was Supposing theultimate limit state the supporting force of coal pillar to themain roof was 0 consequently the fracturing length of themain top as maximum 1rough calculations it could beobtained that Lx(max) 435m

From Figure 8 the fracturing length of the main top is

Lx L1 + L2 + L3 (4)

where L1 denotes the suspension length of the main roof inthe goaf which is determined to be 15m through fieldmeasurement L2 denotes the coal pillar width of 20m inthis case and L3 denotes the extended length of 85mthrough calculation 1rough the width consideration of theair return way of 5m the position of the main roof frac-turing was approximately 35m inside the coal

4 Similar Simulation Testing and Detection ofUltra-Thick Hard Sandstone Roof Fracture

In order to obtain the fracturing position and fracturingcharacteristics of the roof for 11215 working face under theneighboring goaf influence similar simulation testing wasadopted to conduct research as well as the fracturing position

0 x

y

N

Q

M0

Q0

F

Z

L

Figure 7 Mechanical model of main roof suspension above coalpillar after mining face

Advances in Civil Engineering 7

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

detection of the roof at the side of goaf in combination withthe hole-drilling method for mutual verification

A two-dimensional areal model was utilized as thephysical model 1e scheme was the process simulation ofthe first mining influence produced by 11213 working facemining and the second mining produced by the 11215working face mining on 11215 air return way Based on thegeological conditions and similarity parameters the modelbasic parameters were obtained and are presented inTable 2

From Figure 9 it could be observed that subsequent tothe 11215 working face mining due to the influence of 11213goaf the immediate roof sustained caving and contacted thebottom plate At 34m above the coal bed apparent roofbedding separation occurred with significant fracture de-velopment and the maximum height of fracture develop-ment reached up to approximately 256m At this time framethe fracturing angle at the left side above the coal pillar was74deg and the hanging arch distance of immediate roof was19m and for 20m overlying rock it was 75m 1e frac-turing angle at the right side was 58deg the hanging archdistance of immediate roof was 44m and for the 20moverlying rock it was 17m Since the 11215 working faceexcavated the displacement of overlying strata is very largewhich reached up to 5900mm (Figure 10)

In order to further prove the suspension state of the roofin the neighboring goaf the tunnel detection and the drillingmethod was used1e boreholes were drilled from the 11215air return way to the side of 11213 goaf When the 11215working face advanced to the influence sphere of 11213 goafa group of fan-shaped boreholes was drilled into the 11215air return way as aforementioned 1e drill site position ispresented in Figure 11 in which the drill site was 518mfrom the 11213 open-off cut and 89m ahead of the 11215working face

1e specific construction location of three detectionholes is as follows the roadway sides of coal pillar in the airreturn way of 11215 working face were 072m 01m and01m from the roof 1e field construction parameters arepresented in Table 3 and the site construction scheme ispresented in Figure 12

Table 3 and Figure 12 present the hanging arch length Lof the initial roof and the boundary line for the roof fractureand the fracturing angle the following could be observed

(1) 1e overlying rock detected at 32m of altitude rangeabove the coal pillar in the 11215 air return way wasfine and the medium-grained sandstone And thestratification was not apparent and the integrity wasrelatively good

(2) 1e overlying rock detected at the 32m altituderange above the coal pillar in the 11215 air returnway had suspended roof structure in the 11213 goaf1e length of the hanging arch detected throughthree boreholes was 95m 116m and 141m

(3) 1e detected fracturing lines of roof are presented inFigure 9 1e line formed by linking points A B andC was initially regarded as boundary line of the rooffracturing in which the fracturing line was 71deg

5 Fracture Stress Evolution of Ultra-ThickHard Sandstone Roof

According to geological conditions in the 11215 workingface the UDEC was utilized to conduct research on thefracture stress evolution of roof in the inclined direction ofthe 11215 working face 1e mechanical parameters ofcoalrock mass and their contact surfaces adopted in nu-merical simulation are given in Tables 4 and 5 Followingthe 11213 working face mining the overlying rock fractureis presented in Figure 13(a) 1e detection lines werearranged on the main roof of 20m in thickness while itsstress variation curves were obtained as presented in

11215 air return roadway Coal pillar

LxL1L2L3

11213 goaf

Figure 8 Main roof suspension state above the coal pillar after No 11213 working face

Table 2 Basic parameter table for physical similarity model

Item Parameters Item ParametersModel type 2D pane Excavation height 15 cm

Model length 25m Excavationdistance 20m

Model thickness 03m Model boundary 25 cmModel height 1273 cm Excavation steps 50Height of coalseam 15 cm Single excavation

distance 4 cm

Geometricsimilarity ratio 300 1 Excavation

interval 05 h

Volume-weightratio 1667 1 Excavation time 25 h

Stress ratio 501 1 Upper load 0

8 Advances in Civil Engineering

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

Figure 13(c) Regarding the stress of the 11215 workingface the closer the distance to the 11213 working face wasthe higher the stress of the main roof was for which the

maximum occurred at 10m on the left side of the air returnroadway Under the influence of air return roadway thestress variation displayed hump shape In the middle part of

ndash35

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

5

Ver

tical

disp

lace

men

t of i

mm

edia

te ro

of (m

m) Mode length (mm)

500 1000 1500 2000 2500

(a)

ndash16

ndash12

ndash8

ndash4

0500 1000 1500 2000 2500

Ver

tical

disp

lace

men

t of m

ain

roof

(mm

)

Mode length (mm)

(b)

Figure 10 Rock movement characteristics during excavation of working face (a) the sinking curve of immediate roof (b) the sinking curveof main roof

11213 working face

11215 working faceDrill site

Figure 11 Location map of No 11215 working face

(a) (b)

Figure 9 Development figures of roof cracks in No 11215 working face the Chinese characters in (a) mean ldquophysical similarity simulationtest of Xiaojihan coal minerdquo

Table 3 Construction parameters of borehole detection

Number Elevation angle Horizontal angle Hole depth (m) Aperture (mm) Distance (m) Distance from the roof (m)1 29 0 45 93

1072

2 35 0 405 93 013 40 0 50 93 01

Advances in Civil Engineering 9

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

coal pillar the stress reached up to the maximum of1455MPa

Subsequent to the 11213 working face excavationcompletion the 11215 working face excavation was carriedout Similarly the detection lines were arranged on the mainroof while its stress variation curves were obtained as

presented in Figure 13(d) Following the 11215 working faceexcavation the overlying rock stress in the main roof wasbasically zero Also the stress in the middle part of theworking face was higher was 135MPa Due to the goafinfluence in the 11213 working face the stress above the coalpillar first increased and consequently decreased Near the

65

20

39

183

513

8

07

Silty mudstone

Miliary arkose

Medium-granulararkose

HardSoft

Hard

Soft

11215 air return roadway20

1 rock separatrix

2 rock separatrix

3 rock separatrix

29deg

41deg35deg 71deg

Position of the crack startingCrack section

Position where the crack ends

Flushing fluid completely lost(starting point)

Crack development

Entering the gob

A

B

C

95

116

14129deg hole 45 m final hole 35deg hole 405 m final hole 41deg hole 50 m final hole

Figure 12 Lateral suspension roof shape detection hole above the coal pillar of the 11215 return airway

Table 4 Mechanical parameters of coal and rock mass in numerical model

Rock type 1ickness (m) Density (KNm3) Elastic module (GPa) Poissonrsquos ratio Cohesion (MPa) Fractional angle (deg)Mudstone 6 18 98 032 14 20Sandstone 14 22 15 028 114 23Mudstone 12 19 102 032 14 20Fine sandstone 12 20 16 029 59 30Mudstone 3 19 102 032 14 20Siltite 7 21 144 029 102 31Mudstone 5 19 96 032 14 19Medium-grained stone 23 26 152 030 79 30Siltite 8 23 144 029 102 29Medium-grained stone 198 25 152 033 79 31Fine sandstone 7 23 16 033 59 31Coal 45 14 22 030 48 13Mudstone 3 18 98 032 14 20Sandstone 11 24 142 028 110 23

Table 5 Mechanical parameters of contact surface of strata in the model

Rock type Normal stiffness (GPa) Shear stiffness (GPa) Cohesion (MPa) Frictional angle (deg) Tensile strength (MPa)Mudstone 10 22 0 10 0Sandstone 26 58 0 22 0Mudstone 08 20 0 10 0Fine sandstone 35 74 0 23 0Mudstone 08 22 0 10 0Siltite 34 70 0 22 0Mudstone 10 22 0 12 0Medium-grained stone 36 74 0 22 0Siltite 34 70 0 20 0Medium-grained stone 36 74 0 22 0Fine sandstone 35 70 0 23 0Coal 05 20 0 16 0Mudstone 10 22 0 12 0Sandstone 32 68 0 22 0

10 Advances in Civil Engineering

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

position of 5m from the goaf of the 11213 working face thestress reached the maximum of 491MPa

6 Conclusions

(1) 1rough field measurement and laboratory testanalysis it was discovered that the roof of 11215working face in Xiaojihan coal mine belonged to theinterbedding of sandstone and mudstone as a wholein which the sandstone played a main role 1e roofstrata had good integrity lower porosity and highdensity 1e main roof was 1979 m feldspar sand-stone stratum (nearly 20m ultra-thick sandstone)

(2) 1e working face side was simplified to ldquocantilever-simply supportedrdquo beam structure1e force situationof fractured stratum on the roof was analyzed throughthe elastic foundation beam model According tocalculation results it could be obtained that thefracturing line of the main roof in 11215 working facewas approximately 35m within the coal

(3) 1rough the methods combination of similar simu-lations and field measurements of drilled holes thehanging arch lengths in the goaf of 64m 221m and

296m high above the coal bed were determined to be95m 116m and 141m respectively Furthermorethe fracturing angle of the main roof was determinedto be 71deg through the same method combination

(4) 1e overlying rock fracture and stress evolution lawsabove the coal pillar prior to and following mining of11215 working face under the influence of 11213 goafwere simulated through the UDEC numerical sim-ulation method 1e simulation results demon-strated that the 11215 working face was highlyinfluenced by the 11213 goaf whereas its lateralsupporting stress reached the maximum at the po-sition of 5m from 11213 goaf consequently havinga significant influence on the roadway deformation

Data Availability

1e data used to support the findings of this study are in-cluded and shown within the article

Conflicts of Interest

1e authors declare that there are no conflicts of interestregarding the publication of this paper

(a) (b)

16

14

12

10

8

6

4

2

040 60 80

1121

3 w

orki

ng fa

ce st

ress

(MPa

)

100 120 140 160 180 200 220Distance from the model boundary (m)

(c)

40

50

40

30

20

10

0

60 80 100 120 140 160 180 200 220Distance from the model boundary (m)

1121

5 w

orki

ng fa

ce st

ress

(MPa

)

(d)

Figure 13 Roof breaking and stress evolution (a) 11213 working face excavation (b) 11215 working face excavation (c) stress distributionof the 11213 working face excavation (d) stress distribution of the 11215 working face excavation

Advances in Civil Engineering 11

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

Acknowledgments

1e work presented in this paper was funded by the generalprogram of the National Natural Science Fund project inChina (No 51674241)

References

[1] China Coal Industry Association Annual report on the de-velopment of Chinese coal industry in 2017 China Coal In-dustry Association Beijing China 2017

[2] C L Han N Zhang B Y Li G Y Si and X G ZhengldquoPressure relief and structure stability mechanism of hard rooffor gob-side entry retainingrdquo Journal of Central South Uni-versity vol 22 no 11 pp 4445ndash4455 2015

[3] J He L M Dou A Y Cao S Y Gong and J W Lu ldquoRockburst induced by roof breakage and its preventionrdquo Journal ofCentral South University of Technology vol 19 no 4pp 1086ndash1091 2012

[4] W L Shen J B Bai X Y Wang and Y Yu ldquoResponse andcontrol technology for entry loaded by mining abutmentstress of a thick hard roofrdquo International Journal of RockMechanics and Mining Sciences vol 90 pp 26ndash34 2016

[5] T B Zhao Y C Yin and Y L Tan ldquoSafe mining andnew prediction model in coal seam with rock burst in-duced by roofrdquo Disaster Advances vol 5 no 4 pp 961ndash965 2012

[6] Q S Bai S H Tu F T Wang and C Zhang ldquoField andnumerical investigations of gateroad system failure inducedby hard roofs in a longwall top coal caving facerdquo InternationalJournal of Coal Geology vol 173 pp 176ndash199 2017

[7] J Guo G R Feng P F Wang T Y Qi X R Zhang andY G Yan ldquoRoof strata behavior and support resistance de-termination for ultra-thick longwall top coal caving panela case study of the Tashan coal minerdquo Energies vol 11 no 52018

[8] W B Guo H S Wang G W Dong L Li and Y G HuangldquoA case study of effective support working resistance and roofsupport technology in thick seam fully-mechanized facemining with hard roof conditionsrdquo Sustainability vol 9 no 62017

[9] J L Jia L W Cao D J Zhang et al ldquoStudy on the fracturecharacteristics of thick-hard limestone roof and its controllingtechniquerdquo Environmental Earth Sciences vol 76 no 172017

[10] J G Ning J Wang L S Jiang N Jiang X S Liu andJ Q Jiang ldquoFracture analysis of double-layer hard and thickroof and the controlling effect on strata behavior a case studyrdquoEngineering Failure Analysis vol 81 pp 117ndash134 2017

[11] Y M Shu and Y L Liu ldquo1e mechanism analysis of rockburst in Xinxing minerdquo in Proceedings of 4th InternationalConference on Energy and Environmental Protectionpp 528ndash531 Shenzhen China December 2015

[12] W Wang Y P Cheng H F Wang et al ldquoFracture failureanalysis of hard-thick sandstone roof and its controlling effecton gas emission in underground ultra-thick coal extractionrdquoEngineering Failure Analysis vol 54 pp 150ndash162 2015

[13] X L Li C W Liu Y Liu and H Xie ldquo1e breaking span ofthick and hard roof based on the thick plate theory and strainenergy distribution characteristics of coal seam and its ap-plicationrdquo Mathematical Problems in Engineering vol 2018pp 1ndash14 2017

[14] T Zhao C Y Liu K Yetilmezsoy B S Zhang and S ZhangldquoFractural structure of thick hard roof stratum using long

beam theory and numerical modelingrdquo Environmental EarthSciences vol 76 no 21 2017

[15] W Wang Y P Cheng H F Wang W Li and L WangldquoCoupled disaster-causing mechanisms of strata pressurebehavior and abnormal gas emissions in underground coalextractionrdquo Environmental Earth Sciences vol 74 no 9pp 6717ndash6735 2015

[16] Q S Wu L S Jiang Q L Wu Y C Xue and B Gong ldquoAstudy on the law of overlying strata migration and separationspace evolution under hard and thick strata in undergroundcoal mining by similar simulationrdquo Dyna Ingenieria eIndustria vol 94 no 1 pp 175ndash181 2018

[17] C P Lu L M Dou N Zhang et al ldquoMicroseismic frequency-spectrum evolutionary rule of rockburst triggered by rooffallrdquo International Journal of Rock Mechanics and MiningSciences vol 64 pp 6ndash16 2013

[18] C P Lu Y Liu H Y Wang and P F Liu ldquoMicroseismicsignals of double-layer hard and thick igneous strata sepa-ration and fracturingrdquo International Journal of Coal Geologyvol 160-161 pp 28ndash41 2016

[19] Z X Yu F X Jiang Q J Zhu and S T Zhu Study onStability of ick-Hard Key Stratum Based on NumericalSimulation in Longwall Mining 2014

[20] C Xu L Yuan Y P Cheng K Wang A T Zhou andL Y Shu ldquoSquare-form structure failure model of mining-affected hard rock strata theoretical derivation applicationand verificationrdquo Environmental Earth Sciences vol 75no 16 2016

[21] H He L M Dou J Fan T T Du and X L Sun ldquoDeep-holedirectional fracturing of thick hard roof for rockburst pre-ventionrdquo Tunnelling and Underground Space Technologyvol 32 pp 34ndash43 2012

[22] Z T Zheng Y Xu D S Li and J H Dong ldquoNumericalanalysis and experimental study of hard roofs in fullymechanized mining faces under sleeve fracturingrdquo Mineralsvol 5 no 4 pp 758ndash777 2015

12 Advances in Civil Engineering

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom

International Journal of

AerospaceEngineeringHindawiwwwhindawicom Volume 2018

RoboticsJournal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Active and Passive Electronic Components

VLSI Design

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Shock and Vibration

Hindawiwwwhindawicom Volume 2018

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawiwwwhindawicom

Volume 2018

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

Control Scienceand Engineering

Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom

Journal ofEngineeringVolume 2018

SensorsJournal of

Hindawiwwwhindawicom Volume 2018

International Journal of

RotatingMachinery

Hindawiwwwhindawicom Volume 2018

Modelling ampSimulationin EngineeringHindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawiwwwhindawicom Volume 2018

Hindawiwwwhindawicom Volume 2018

Navigation and Observation

International Journal of

Hindawi

wwwhindawicom Volume 2018

Advances in

Multimedia

Submit your manuscripts atwwwhindawicom