International Journal of Coal Geology 90–91 (2012) 135–148

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International Journal of Coal Geology

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Failure modes of the surface venthole casing during longwall coal extraction:A case study

Jinhua Chen a, Tao Wang b,c,⁎, Yong Zhou b, Yuanle Zhu b, Xiangxiang Wang b

a National Key Laboratory of Gas Disaster Detecting, Preventing and Emergency Controlling, Chongqing, Chinab State Key Laboratory of Water Resource and Hydropower Engineering Science, Wu Han University, Chinac Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering, Ministry of Education, China

⁎ Corresponding author at: Key Laboratory of Rock Meal Engineering, Ministry of Education, China. Tel.: +71511155(mobile).

E-mail address: htwang@whu.edu.cn (T. Wang).

0166-5162/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.coal.2011.11.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 July 2011Received in revised form 11 November 2011Accepted 12 November 2011Available online 4 December 2011

Keywords:Surface venthole casingCoal extractionNumerical simulationFailure modesDeformation monitoring

The effective control of gas emissions from longwall faces and gob areas in the coal mining industry in Chinahas become increasingly important. Drilling underground gas drainage holes from a surface is an effectiveway to release gas. Once a coal seam is extracted, the rock strata above a gob subside and move, afterwhich the gas drainage borehole and casing become damaged due to state changes in the rock. In thispaper, we perform a deformation and failure analysis of a surface venthole casing using the numerical simu-lation method. The numerical investigation is carried out on the casing under compression, tension, and shearconditions. The general factors that affect casing failure are discussed, and basic failure modes are summa-rized. The experimental casing of the surface venthole at Cheng Zhuang Colliery (CZC) is also analyzed by nu-merical simulation. The actual deformation and failure of the casing are examined using on-site monitoringdata, to which the numerical simulation results are then compared. We deduce that the tensile and shear fail-ures occur at the lower part of the casing or at the bedding plane separation. Monitoring must be focused onthese areas, and protective measures should be used. The numerical simulation model is simplified given var-ious restrictions. The integration of numerical simulation and monitoring technology can serve as an efficientmethod for the quantitative analysis of failure on surface venthole casings. The research concepts presentedin this paper and the rules deduced are useful references.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

China is the world's largest producer and consumer of coal (Chenget al., 2011). However, China has experienced more serious coal minedisasters than has any other country. In particular, accidents causedby gas explosions result in numerous casualties and substantial prop-erty losses; therefore, these accidents have elicited significant publicconcern (Wang and Xue, 2008). Coal mine methane has alwaysbeen considered as a hazard of underground coal mining because itcan create a serious threat to mining safety and productivity due toits explosion risk (Karacan et al., 2011). In mining a longwall coalpanel, the rock strata above and below the panel are disturbed.These disturbances result in methane gas release from adjacent coalseams, which flow through the fractured rock strata into the road-ways and face line of the panel (Jozefowicz, 1997). The uncontrolledemission of gas is hazardous to both mines and miners, as well asleads to lost time and production (Whittles et al., 2007). One of themost important duties of the ventilation system in underground

chanics in Hydraulic Structur-86 27 68773941, +86 138

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coal mines is to keep methane levels well below the explosive limitby diluting the methane emissions that occur during mining(Karacan et al., 2011).

Gob vent boreholes and horizontal and vertical drainage boreholesare drilled and used as supplementary methane control measures inmanymines because ventilation alone may be insufficient for control-ling the methane levels on longwall operations (Karacan, 2009). Asurface vertical drainage borehole is a simple and high-performancemethod for drawing gas from a coalbed and gob. Vertical drilling isconducted from the earth's surface to an overlying rock fissure zoneor coal seam. In-seam horizontal methane drainage boreholes areusually completed open-hole and are logistically difficult to drill be-cause of limited workspace. Vertical methane drainage boreholes,on the other hand, have the distinct advantage of not being conductedin a restrictive underground environment. They are mostly suited fordeep, low-permeability coal seams with a lot of gas where adequatelydegasifying the coalbed requires a long length of time before miningcan proceed (Karacan, 2009; Thakur, 2006).

The purpose of a surface venthole for gas drainage is to reduce gasemission, alleviate the pressure of gas overrunning, and develop coalseam gas. The surface venthole method can drain gas to the surfacedirectly from the fissure network and borehole. Permeability is en-hanced by the pre-crack or mining disturbance effect. Surface drilling

Fig. 1. Schematic representation of the gas drainage drilling.

Fig. 2. Simplified mechanical model of casing, surrounding rock, and cement. P1 and P2are different boundary conditions of pressure.

136 J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

is generally divided into three types: drainage before mining, drain-age in the mining, and drainage in the gob. The usual practice is todrill gob gas ventholes from the surface before mining. As mining pro-ceeds under the venthole, the gas-bearing strata that surround thewell will fracture and establish preferential pathways for the releasedgas to flow toward the ventholes (Diamond et al., 1994; Karacan et al.,2007). With the high cost of surface drilling projects, numerous min-ing companies in China have recently attempted the “one well withthree” mode of operation to use the three types of surface drillingprocesses through one well: drainage before mining, drainage in themining, and drainage in the gob, successively. Casings of different di-ameters and thicknesses are installed using the strata conditions asbases. Consolidation materials, such as cement and mortar, are usedto fill in between the casings and the borehole wall to improve the ef-fect of pore consolidation.

However, certain practical engineering experiences have indicatedthat coal extraction considerably affects surface ventholes. When aworking face passes near a drainage borehole, the borehole well isoften damaged and cannot adequately perform.

Although gas drainage can alleviate the adverse effect of massivegas accumulation and enable the effective collection of gas in a coalseam, the price of a surface venthole is expensive. At present, the sta-bility is also poor. The normal operation of a borehole is closely relat-ed to the safety of gas drainage. Enhancing the utilization ratio of aborehole is an urgent problem. Little information is available in theliterature about the effect of rock disturbance on borehole deforma-tions within coal mines. In contrast, substantial information on thedeformation of well bores is available because of the reservoir com-paction that occurs in the petroleum industry (Bull, 2003). The cas-ings of surface ventholes and oil extraction differ. Under miningdisturbance effects, rock stress adjusts sharply (especially at the over-lying rock of the stope). The strata exhibit cutting slippage, tension orcompression deformation, and separation layer displacement, leavingcasings and boreholes in danger of being crushed, cut, or snapped.The compaction of reservoir rocks occurs because of pore collapse ofweak reservoir rocks during oil and gas extraction, which induceshorizontal rock shear along distinct horizons within overlying strata.Underground coal mining affects the surface venthole and rockmass around casings and causes extensive disturbances. These distur-bances cause a particularly remarkable pattern of destruction of thecasing in the upper part of the mined-out area. Rock mass fails andplastically “flows” as it deforms around the casing; thus, the casingis deformed to a lesser extent than that of rock deformation. Finite el-ement models of well bore/rock interactions predict that the maxi-mum lateral displacement of a casing is less than the applied sheardisplacement because of this plastic deformation (Whittles et al.,2006). In the current paper, an analysis of the numerical simulationresults serves as a method for quantifying the effects of geologicalconditions and the failure conditions that occur. The effect of coalmining is taken into consideration in the calculation. The unloading,moving, and separation effects of the rock strata are also taken intoaccount.

2. Typical models of casing failure in surface ventholes

Fig. 1 shows the layout of the gas drainage drilling. The zone mostlikely to sustain damage is highlighted. The initial stress is larger atthe bottom of the gas drainage pipe, and the perturbation effect ofthe lower rock mass is more pronounced during the mining process.These regions are often considered to be key because the gas is con-centrated primarily in these areas. Closer attention should be paidto the lower part of the casings.

The surface subsidence andmovement of overlying rock caused byunderground mining are highly complex mechanical change process-es; the entire process includes the redistribution of stress and dis-placement fields. Mining causes the direct roof of a gob to produce a

series of phenomena with downward movement, bending, breakage,and even collapse. At the same time, the old roof moves along thenormal direction of a bedding plane as the bend of a beam or cantile-ver generates a local fracture and abscission layer. The stress redistri-bution of the rock mass surrounding a gob caused by these stratamovements is likely to affect the stress field near the casing, andthis influence may diminish casing stability. Generally, the causes ofdamage of gas casings based on a mechanical mechanism can be di-vided into compression, tension, shear, and other factors. Given thecomplex characteristics induced by geological factors, we studiedthese features using the numerical simulation method of rock me-chanics. The calculation will be carried out using FLAC3D(Fast La-grangian Analysis of Continua in 3 Dimensions) (Itasca, 2006),which is used for stress and deformation analyses of surrounding sur-faces and underground structures in soil and rock (Badr et al., 2003;Likar and Čadež, 2006; Wang, 2006; Xie et al., 2009).

2.1. Casing failure under compression conditions

The rock at depth is subjected to stresses resulting from theweight of overlying strata and locked-in stresses of tectonic origin.When an opening is excavated in the rock, the stress field is locallydisrupted, and a new set of stresses is induced in the rock surround-ing the opening (Hoek, 2006). As the free face appears in the drillhole of a gas drainage, stress is redistributed around the casing. Theinteraction of wall rock and casing leads to casing deformation ordamage. In casing deformation and failure caused by compression

Fig. 3. Mesh of the numerical simulation for the compression test (the casing is simulated by the shell structure element).

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conditions, compression pressure is produced in the release of initialgeostress and due to strata movement because of coal seam mining.

The formulas for the collapse pressure of the American PetroleumInstitution (API) (Lyons, 1996) have serious limitations when used inthe design of borehole casings in a coal seam. For instance, thesesemi-rational formulas can analyze only the deformation and ruptureproblems of casings under even pressure without considering the ef-fects of the surrounding rock and cement. Because of the redistribu-tion of the stress field in the drainage gas project of a coal seam,obvious stress concentration phenomena occur at two ends of thegob, and varying degrees of stress release occur on the roof and thefloor. Casings receive non-uniform compression forces when theyare placed in these zones; thus, the casings may finally be destroyed.Studying the destruction rules under non-uniform compression ac-tions is necessary. Current studies are largely based on elasticity the-ory and relative test statistical data. The critical compression pressureformula is then derived under uniform and non-uniform compressionconditions. In mining engineering, clearly observable plasticity yieldis generated in rock material; the wrapped concrete ring shows ex-tensive plasticity failure before the large deformation failure of thecasing occurs. Thus, the calculation results based on elasticity theoryare erroneous and cannot be used in a practical project. In the currentwork, elasto-plastic constitution models are used to simulate the me-chanical behavior of rock and concrete to obtain more accurateresults.

Generally, the exterior of a gas casing has an established layer ofconcrete protection. Considering that the material behaviors of thesurrounding rock and concrete have an obvious influence on thefinal stress state of casings, it is possible to effectively decrease thedegree of casing deformation and failure using suitable materials.The casing as well as the surrounding rock and concrete are unifiedin the model, which can be simplified as a plane strain problem.Fig. 2 shows the mechanical model with three parts under compres-sion conditions. Under these conditions, the casing exhibits differentdeformation degrees. Plastic deformation and casing failure occurwhen the pressure reaches or exceeds the yield strength. Considering

Table 1Mechanical parameters of cement and surrounding rock mass.

Parameters Cement Rock mass

Density (kg/m3) 2500 2689Elasticity modulus (GPa) 7 8.12Poisson's ratio 0.167 0.32Cohesion (MPa) 4 4.55Friction angle (°) 45 36.62Tensile strength (MPa) 2.5 0.75Compressive strength (MPa) 30 4.21

that the steel casing is relatively thin, it can be simulated using thestructural element (shell element) in FLAC3D. We can compare theequivalent stress of the “Von Mises” with the minimum yield stressto estimate the yield degree of the casing (Itasca, 2006). When thetwo stress values are equal, the outside compression pressure is de-fined as the collapsing strength. In FLAC3D, positive stresses indicatetension, whereas negative stresses indicate compression. Principalstresses (i.e., σ1, σ2, and σ3) are assigned such that compressive stres-ses are negative, and σ1≤σ2≤σ3. The maximum principal stress isthe least negative stress (σ3), whereas the minimum principal stressis the most negative stress (σ1).

The numerical simulation based on the model shown in Fig. 2 isdescribed as follows. In the simulation process, changing stressboundary conditions are used to analyze the deformation and failurefeatures of the casing. A set of material parameters used in the prac-tical projects is selected. The model is 2 m long (x coordinate), 2 mwide (y coordinate), and 0.4 m high (z coordinate). Cement thicknessis 38 mm. The calculation grids are shown in Fig. 3. The parameters ofthe casing are as follows: external diameter, 139.7 mm; thickness,6.2 mm; and steel grade, N80. Based on the Plastic Collapse PressureFormula of the API (Lyons, 1996), we derive a casing yield strengthof 552 MPa, a tensile strength of 689 MPa, an elastic modulus of206 GPa, a Poisson's ratio of 0.3, and a density of 78.5 kg/m3. The

Fig. 4. SZ of the casings in the compression process (the zone surrounded by the redline is based on the Mohr–Coulomb model; the zone surrounded by the blue line isbased on the elastic model).

Fig. 5. Minimum principal stress distribution of the casing under different non-uniform initial stresses. P1/P2=1 (left) and P1/P2=3 (right).

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other parameters used in the model are shown in Table 1. The casingand concrete each use a single set of mechanical parameters.

The non-uniform stress distribution around an opening and casinggenerally exists in many underground excavations, particularly whendrilling within a long section of salt formations in deep wells (Peng etal., 2007; Zhang and Roegiers, 2005). Fig. 4 shows the numerical cal-culation analysis results with which non-uniform stress is considered.The zone surrounded by the red line or blue line is defined as theSafety Zone (SZ). In this calculation, the materials of rocks and con-crete are simulated by the Mohr–Coulomb or elasticity constitution.The collapsing strength of the casing decreases with the increasingnon-uniformity degree of the two-directional compression load.Overall, the SZ of the casing is calculated using the two kinds of con-stitutive models with certain overlaps. The collapsing strengths arealmost the same when the uniform compressive stress is loaded.However, some “independent zones” remain. Through the numericalcalculation, the characteristics of the casing damage can be obtainedunder different compression pressures. Fig. 5 (left) shows that thestress distribution on the casing is even and consistent when uniformcompression pressure (P1=P2) is loaded. The casing cannot be easilydamaged under this condition. Fig. 5 (right) shows that the stress dis-tribution on the casing is uneven when non-uniform compressionpressure (P1≠P2) is loaded, and part of the direction towardswhich the smaller compression pressure is loaded first reaches theminimum yield stress.

Although gob stress redistribution under mining disturbance ishighly complex, we can use the numerical simulation method to ob-tain stress distribution and analyze casing deformation and failurein accordance with the above-mentioned law. The results can serve

Fig. 6. Mesh of the numerical simulation for the shear test (left)

as excellent guidance for engineering practices. This specific imple-mentation is reflected in a case study discussed later in this paper.

2.2. Casing failure under shear conditions

Casing stability under shear conditions is related primarily to theslippage of rock strata along bedding planes or fault planes(Dusseault et al., 2001). In a coal mine project, slippage is caused bythe rock mass near the casing that is disturbed by coal mining.These effects produce significant shear stress near the interface andenable the casing to cause shear deformation and failure. In general,as the regions of coal mining advance, roof strata bend to a gob orform a local cave, providing a sliding space for the displacement ofdiscontinuities in rock mass. Shear slippage at the strata interfacenear the casing can be correlated with the degree of casing deforma-tion and damage. When shear slippage is small, the deformation of acasing does not affect normal operations. Only when the slippagereaches a critical value does the casing produce larger shear deforma-tion and even failure. Eventually, the casing stops working normallybecause it has been cut off by the strata interface. For a casing stabilityissue directly caused by interfacial shear slippage, the specific slip-page level at which the casing is just cut off can be defined as the crit-ical slippage point for the corresponding casing.

The interface logic of FLAC3D is used to simulate the physical re-sponses of the bedding plane. The shear slip between the rock strataoccurs when part of a rock is subjected to the initial velocity. The sim-ulation results indicate that the primary effect factors are the initialstress conditions, the angle between the axial casing and interface,and the physical mechanical parameters of the interface, among

. The image on the right shows the location of the interface.

Fig. 8. Simplified mechanical model with the strata of the non-horizontal angle undershear conditions.

Table 2Mechanical parameters of the interface.

Normalstiffness

Shearstiffness

Internalfriction angle

Cohesion

/(GPa·m−1) /(GPa·m−1) /(°) /MPa

Interface betweenthe rock strata

50 50 25 1

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others. Fig. 6 demonstrates a simplified three-dimensional numericalsimulation model that consists of two horizontal strata including abedding plane. The mechanical parameters are the same as those de-scribed in the previous section. The parameters of the interface arelisted in Table 2. The horizontal slip velocity is incorporated into theupper block of the rock to simulate the rock slip process.

According to the calculation results, the stability of the casing isgradually affected by the increasing slippage along the beddingplane until the yield is finally obtained. Throughout this process, thedestruction area of the casing is distributed mainly on the loadedside. The first yield region is located near the interface (Fig. 7). Asexpected, the casing breaks in the location of the interface when itsdeformation reaches a certain level. Concrete can play a protectiverole in casing stability. The cement around the loaded side of the cas-ing is compressed; the main cement is a shearing failure. The stresson the other side gradually decreases, and tensile stress develops.The cement and rock in this region are destroyed primarily by thetension. This kind of damage pattern may also cause the squeezed de-formation and failure of the casing around the strata interface.

In the actual project, the bedding plane may not be a completelylevel surface. Therefore, it is necessary to study the influence of thestrata dip angle. Fig. 8 shows the simplified mechanical model ofthe non-horizontal angle strata under shear conditions. Further re-search on the relationship between the dip angle of rock strata andcritical slippage is required.

2.3. Casing failure under tensile conditions

Research and field monitoring data show that casing deformationand failure phenomenon under tensile conditions is caused mainly bybedding plane separations during mining. In China, when a coal bed isburied deeply, the full caving method is usually adopted. A thick frac-tured zone and caving zone form above a gob. These zones may forma series of bedding plane separations because of the different bend

Fig. 7. Minimum principal stress distribution of the casing after the shear test.

degrees of rock strata, especially in the bedding plane where the dif-ference in lithology is considerable.

When a coalface is close or located within the region of an embed-ded casing, rock layers may exhibit the tendency to move towards anexposed surface. In the process, the over-burdened rock strata maycollapse or bend gradually from the bottom to the top (Fig. 9). Atthis time, separations between rock strata may occur, and relativeslippage or fixed conditions may appear among gas casings, cement,and rock mass. If the casing is locked and cannot slide along the sur-rounding wall, it may be pulled off near the separation location.

If rock mass can slide along a casing wall in the process of separa-tion, sliding can play a protective role to avoid the effect of the gravityof the rock mass below. If the disturbance caused by this exploitationis severe, several groups of rock strata may move down together. Thecasing can be pulled off instantaneously because the friction betweenthe casing and rock is sufficiently large and the rock cannot slidealong the casing wall. Therefore, the mining intensity should be con-trolled to ensure that the over-burdened rock moves slowly towardsa gob when the coalface is near the region of an embedded casing. Ifthe rock mass can slide along a casing under the action of the over-burdened bedding plane separation, the casing may not be pulled off.

Fig. 9 shows the conceptual model of the deformation and failureof the surface venthole and casing under tensile conditions. If thelength of the casing segment is L m, the sliding resistance force canbe derived from the Coulomb–Slip formula (Cundall and Board,1988; Itasca, 2006) as follows:

Fsmax ¼ c⋅2πRLþ tanϕFn ð1Þ

where c is the cohesion between the casing and rock mass, A is thelateral area, and Fn denotes the normal force applied to the casing.

The casing can bear the maximum axial tension as follows:

T max ¼ σS ð2Þ

where σ is the tensile stress along the cross-section and S is the cross-sectional area.

In the calculation model, the casing size is the same as that men-tioned previously. The horizontal principal stress is approximately5 MPa near the casing. The interface parameters between the casing

Fig. 9. Conceptual model of bedding plane separations near the venthole (left). The right figure shows a simplified mechanical model of the casing under tensile conditions (ν rep-resents the casing's downward slip trend).

140 J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

and external medium are as follows: the cohesion is 0.15 MPa and theinternal friction angle is 15°. If L is equal to 8 m, then the sliding resis-tance force is 1750 KN. The maximum axial tension is also 1750 KN. Ifthe length of the casing segment is more than 8 m, the casing seg-ment is locked, and the rock strata cannot slide along the casingwall. Therefore, if larger thick rock strata exist near a casing, increas-ing the strength of a casing or decreasing the friction coefficientamong casings, cement, and rock mass becomes necessary. A deeplyburied casing should have a certain monitoring measure and appro-priate engineering treatment.

2.4. Other failure modes

The quality of casing materials may cause damage to the casing it-self (e.g., when the casing itself has micropores and microfractures),such as when the thread of a connector does not satisfy requirementsor the shear and tensile strengths of the casing are low. At the sametime, the accuracy of casing size, such as the circular degree, andthe irregularity of wall thickness may influence casing stability.

Fig. 10. Plane layout of the 4308 face and entry points. The

Some domestic casings currently suffer from quality problems thatshould be considered in the selection process of casings.

The cementing quality of a casing also has certain effects on casingstability. The borehole cementation problem imposes a variety of ef-fects, such as channeling, weak bonding between cement and casing,and weak bonding between cement and rock, among other concerns.In many cases, casing damage is the result of poor cementing quality.However, some researchers believe that strengthening the casing–cement system seldom eliminates shear, although in somecircumstances, it may retard it (Dusseault et al., 2001).

2.5. Discussion

In this section, the investigation of casing stability under compres-sion, tension, and shear conditions is carried out, and the importantrules of practical engineering are employed. However, the engineer-ing disturbance effect of coal mining is highly complex, combiningnon-uniform compression stress, bedding plane separation, andshear-slipping phenomena. Considering that the combined effects ofthese various forms are most common, we can infer that the above-

picture on the right was taken at the construction site.

Table 3Material mechanical parameters of the CZC. Boldfaced entries show the thicker rock strata.

Rock strata Altitude Density Tensile strength Elasticity modulus Poisson's Cohesion Friction angle

m kg/m3 MPa GPa ratio MPa °

Mudstone 946.4–915.1 2533 0.042 2.50 0.30 1.58 22.40Kern stone+mudstone ~893.9 2642 0.035 3.17 0.26 2.91 27.65Fine sandstone+siltstone ~884.6 2652 0.024 2.78 0.28 2.09 28.52Mudstone+siltstone ~869.3 2639 0.048 3.94 0.31 4.19 28.58Mudstone+siltstone ~850.0 2576 0.040 3.92 0.31 2.83 29.97Mudstone+siltstone ~828.7 2602 0.025 2.83 0.35 1.77 28.75Sand shale ~818.4 2650 0.029 3.43 0.33 2.19 30.38Sand shale+siltstone ~804.1 2652 0.032 3.81 0.29 2.51 31.23Mudstone ~799.8 2564 0.011 1.43 0.30 0.41 24.30Fine sandstone+mudstone ~780.5 2652 0.041 4.68 0.31 2.86 32.66Fine sandstone+mudstone ~754.2 2637 0.032 4.32 0.28 2.24 33.46Siltstone+kern stone ~717.8 2623 0.055 5.98 0.28 3.96 34.23Mudstone ~710.5 2627 0.022 2.20 0.20 0.82 25.37Mudstone ~689.8 2649 0.040 4.80 0.32 3.44 33.33Siltstone+mudstone ~653.5 2675 0.035 4.70 0.27 2.73 34.16Siltstone+mudstone ~647.2 2587 0.055 6.76 0.27 3.11 36.20Siltstone+kern stone ~627.7 2689 0.075 8.12 0.32 4.55 36.62Fine sandstone ~611.2 2709 0.040 5.50 0.29 2.24 35.45Mudstone+kern stone ~601.1 2725 0.075 8.82 0.31 4.50 38.02Mudstone ~588.7 2751 0.111 5.83 0.36 2.37 27.79Mudstone ~575.7 2700 0.104 5.30 0.30 2.11 26.78Coal ~569.1 1436 0.040 2.84 0.32 0.96 24.69Basement rock ~369.1 2744 0.140 1.41 0.31 7.97 41.24

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mentioned conclusions are in an ideal state. Therefore, when the re-sults above are used to design the casing based on practical engineer-ing, these influencing factors should be considered. Subsequently, theform, location, and degree of casing deformation and failure can bedetermined more accurately. Through optimum design, if the miningdisturbance can be controlled within a tolerable limit or if the casingcan be placed in the SZ, the casing damage can thus be reduced.

3. Cheng Zhuang Colliery: s case study

For several years, coal exploitation at Cheng Zhuang Colliery(CZC), Shanxi Province, has been carried out using fully mechanizedtop-coal caving. Many casing breakages have occurred in the courseof the operation of the gas drainage project in CZC. Heavy economiclosses have been incurred, and many hidden dangers remain becauseof hindrances to gas extraction. Working face 4308 was selected asthe experimental area for the investigation of the deformation andstability of the surface venthole casing. To extract gas from the work-ing face, the drillings are bored using the ADR 250 high-efficiency

Fig. 11. Scatter plot of the maximum (minimum) horizontal principal stress

drilling rig, as shown in Fig. 10. The influence on the borehole causedby the mining of coal seams is simulated as accurately as possiblethrough the numerical method.

3.1. Geological conditions and geostress distribution

3.1.1. Geological conditionsAn investigation of the geological conditions in CZC showed that

the lithology consists mainly of fine-grained sandstone, siltstone,sandy mudstone, and mudstone. The rock strata distribution is prox-imately horizontal. Using the CZC geological survey data as bases, wedivide the model into 23 types of materials. The top part of the coalseams is divided into 21 types, and the bottom part is considered tobe composed of only one type.

The mechanical parameters of rock mass are the basis of the defor-mation and stability analysis. In 1997, Hoek and Brown proposed thewidely used theory of rock mass mechanical parameter estimation(Hoek and Brown, 1997). Laboratory testing derives the initial prop-erties. We combine the Hoek–Brown rock mass parameter method

and vertical stress at depth in the coring borehole (Wang et al., 2011).

Fig. 12. Entire large model mesh (left); B–B cross-section profile (right).

142 J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

with the characteristics of the FLAC3D results and select the final pa-rameters in Table 3 for the numerical simulation.

3.1.2. Geostress distributionIn-situ stresses are measured using the hydraulic fracturing tech-

nique. Fig. 11 shows the calculation of the nearly linear relationshipat depth for the measurements in CZC. Vertical stresses are assumedto increase linearly with depth caused by the weight of the overlyingrock strata. At the depth of the panel, the vertical stress is approxi-mately 10.1 MPa. This horizontal maximum principal stress measure-ment is close to the vertical stress. Within the coalfields, thehorizontal stresses are affected by the tectonic strain and thereforecannot be directly determined from the lateral restraint provided bythe vertical loading.

According to the in-situ stress measurements, the initial stress re-gressive formula can be expressed as follows:

σx ¼ 0:0201z−18:6218σy ¼ 0:0249z−23:1782τxy ¼ 0:6533−0:00069z

ð3Þ

where x, y, and z are the coordinate systems of the calculations, z isthe altitude, MPa is the stress unit, and m is the scale unit.

Fig. 13. Principal stress-vector field of the surrounding rocks in the center of t

3.2. Simulation method

The casing deformation and failure caused by coal extraction canbe simulated by two methods based on various theories and practicalresults regarding casing deformation and damage.

(1) Direct simulation: Casing geometry is included in the entiremodel. The coal mining process and the process of casing em-bedding are considered; thus, the deformation or failure ofthe surface venthole and casing can be analyzed directly duringcoal mining.

(2) Indirect simulation: Casing geometry is not included in the en-tire model. Indirect simulation is a simulation of the entiremining process without considering the drill hole. Subsequent-ly, the distribution and variation of the stress and displacementnear the drill hole are analyzed. The most dangerous regionand corresponding stress and displacement boundary condi-tions are also identified. Finally, based on the analytical results,the numerical simulation is carried out for a small model, in-cluding the casing geometry.

Considering that the influence area is only 3–5 times the size ofthe outer diameter of the cement ring during casing embedment,this area cannot affect the entire model. The calculation results shouldbe similar under certain conditions in the two simulation methods to

he face (section A–A), the coalface is close to the location of the borehole.

Fig. 14. Principal stress vector field of the surrounding rocks in the center of the face (section A–A), the panel is mined completely.

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the overall appearance. Using the direct method, building the modelis difficult when the appropriate mesh density is used. Simulating asmall casing model is more feasible when the indirect method isemployed.

3.3. Model implementation and analysis

The stress, displacement, and plastic zone from numerical simula-tion can serve as bases for casing stability analysis. The deformationand failure of a surface venthole and casing under the compressionconditions are caused by a non-uniform stress field. The tension con-ditions caused by separation layers and shear conditions caused bythe strata cutting-slippage during the mining process are simulated.The relative parameters are consistent with those in Table 3.

3.3.1. Large model calculation and analysisThe designed exploitation round is approximately 10 m, and the

single excavation step in the numerical analysis consists of elementsthat are removed inside the 200 m×10 m×6.5 m zone. Therefore,75 excavation steps are calculated, with each excavation step consist-ing of a one-day exploitation round. Cubic elements are used for themodel construction. The model is composed of 520,000 elements

Fig. 15. Plastic zone distribution(section A

and 540,918 grid points, as shown in Fig. 12. There are core holesand drainage holes in the site conditions; however, these details arenot simulated in the large model, as explained previously. The indi-rect method is used in the analysis.

After the numerical calculation, the distribution of the stress, dis-placement, plastic zone, and other results can be obtained for the en-tire calculation domain. When the mining steps gradually increase,the stress on the gob roof and floor is dramatically released. However,both sides of the gob increase sharply and result in a concentration ofstress. According to the investigation of the entire mining process, theinitial state of the maximum compressive stress is approximately10 MPa on both sides of the gob, slowly increasing to approximately20 MPa before finally stabilizing (Figs. 13 and 14).

Predictably, the casing may experience various levels of stressrelease or concentration phenomena during the mining process.Comparing the numerical results with the SZ (Fig. 4) of a casing,the casing in CZC is safe under the compression conditions if thestress state occurs in the SZ. The surface venthole and casing aresafe and reliable under the pure compression conditions. Consider-ing the complex mining process, including aspects such as the dis-continued nature of rock mass, the redistribution of initialgeostress, sudden fracture of rock mass, and other geological

–A), the panel is mined completely.

Fig. 16. Finite difference mesh of Model A (left). The right figure shows the location of the interface.

Fig. 17. Vertical displacement (left) and maximum principal stress (right) distribution near the casing at cross-section A–A (interface 2, non-slip).

Table 4Mechanical parameters of the interfaces.

Normalstiffness

Shearstiffness

Internalfriction angle

Cohesion

/(GPa·m−1) /(GPa·m−1) /(°) /MPa

Interface 1 (betweenthe casing and cement)

100,000 100,000 30 0.1

Interface 2 (betweenthe rock strata)

500 500 25 1

144 J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

phenomena, it is impossible for most of the casings to be in a purecompression state. Casing failures may be more common undershear or tensile conditions.

The numerical calculation of the CZC shows that the thicknessof the plastic zone gradually increases with the mining process.The thickness of the plastic zone stabilizes when the coalfacepushes forward at 300 m and finally at approximately 95 m (theelevation is 575.7–670 m). Fig. 15 shows the distribution of thestope plastic zone after mining is completed. Given that the nu-merical simulation reflects the mining process under the full cav-ing method, we can assume that caving, cracking, bedding planeseparation, and strata bending may occur at the coal seam roof.These seriously damaging phenomena in the rock mass are partic-ularly observable near the gob. We speculate that a certain rela-tionship exists between the deformation and damage of the rockmass and the plastic zone from the numerical simulation. Interest-ingly, the area of the plastic zone from the numerical simulation isclose to that obtained from the empirical formula using in China.Explanations of terms given in the legend of Fig. 15 are as follows(Yasitli and Unver, 2005):

– None: no-failure zone.– Shear-n: the region failed under shear loading and failure process

is still in progress.– Tension-n: the region failed under tensile loading and failure pro-

cess is still in progress.

3.3.2. Numerical investigation of the surface venthole and casing undertension or shear conditions

As for the casing stability of the gas drainage under the shear andtension conditions, two sets of small simulation models are explored,

and several pieces of beneficial data are obtained for practical engi-neering. The direct method is used in the analysis (Fig. 16).

(A) CZC casing under tension conditionsAs indicated by the results of previous studies, tensile deforma-tion and failure most likely occur at separation layers. The sim-ulation model under the shear conditions is also composed ofupper and lower parts. The model is designated as Model A,which has a length of 8 m (x coordinate), width of 8 m (y coor-dinate), and altitude of 600–604 m (z coordinate). It consists oftwo layers of rock, as shown in Fig. 17. The casing is simulatedby a solid element (not a shell element) to obtain more accu-rate results. Cubic elements are used for model construction.The model is composed of 155,520 elements and 160,766grid points. Roller boundaries are located along the sides. Thetop and bottom of the model are free surfaces. Two sets of in-terfaces are built between the upper and lower blocks of therock, i.e., the casing and the cement ring, respectively. The me-chanical parameters of the interface elements are shown inTable 4. The real initial stress field is simulated in Model A.The process of the bedding plane separation is simulated by

Fig. 18. Vertical displacement (left) and maximum principal stress (right) distribution near the casing at cross-section A–A (interface 2, slip).

145J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

applying the field velocity of the z direction to the lower rockmass. The tensile behavior of the casing caused by the beddingabscission can be divided into two forms: slipping and non-slipping. Therefore, interface 1 of Model A is set to slip ornon-slip in the FLAC3D.When interface 1 is set to non-slip, the casing is easily pulledoff. Figs. 17 and 19 show that when the crack of the beddingplane separation is still very small, a large area of the casingwith 500–1200 MPa appears where the bedding abscission oc-curs. Considering that the tensile strength of the N80 steel cas-ing is 682 MPa, distortion and destruction take place at thismoment. When the tensile stress is larger than the strength,the casing can be pulled off first, and the high stress zone canspread towards the inner wall of the casing; finally, the casingcan be pulled off completely. The casing can be pulled off at theinterface between the upper and lower blocks. When interface1 is set to slip, the stress on the casing is in the same order ofmagnitude as that at the initial stress of the region. The rockstrata slip along the wall of the casing as the opening of thebedding abscission increases. Generally, the casing cannot bepulled off. Figs. 18 and 19 show that the maximum tensilestress in the casing is relatively small and does not reach thelevel of the tensile strength.The three strata of boldfaced entries located in the majorexcavation-disturbed region are thicker than others, asshown in Table 3. The strata are deeply buried, with the adja-cent strata having large differences in lithology. Based on theconclusion regarding the failure of the casings under tension

Fig. 19. Maximum principal stress distribution of the ca

conditions, we infer that casings can be seriously damagednear the interface of strata; hence, considerable attentionshould be given to the casing. The expansion joint (the pipesection that has the freedom to expand and exhibit micro-angle deflection as well as exhibit good seal performance)can be used to connect the interfaces between the differentsegments of the casing. In general, the scale of progressiverock caving near the casing must be controlled in the miningprocess. If the trend of rock strata fixed along a casing wallcan be avoided, the tension fracture damage caused by a bed-ding plane separation can be reduced effectively. The law ofcasings damage is in agreement with the results in the sectionon failure modes.

(B) CZC casing under shear conditionsAccording to the results of previous studies, the area most vul-nerable to shear deformation and failure is the part at whichthe strata slippage occurs. The simulation model under shearconditions is also composed of upper and lower parts. Themodel is defined as Model B. The geometrical measurementsof Model B are a length of 8 m (x coordinate), width of 8 m (ycoordinate), and altitude of 599–601 m (z coordinate), asshown in Fig. 20. Cubic elements are used inmodel construction.The model is composed of 25,344 elements and 26,425 gridpoints. The boundary conditions are roller boundaries locatedalong all of the sides except the two x direction surfaces. Thereal initial stress field is simulated in Model B. The slippage pro-cess of the rock strata is simulated by the exerting field velocityon the upper rock formation in the x direction. One interface is

sing at cross-section B–B (left, non-slip; right, slip).

Fig. 20. Finite difference mesh of Model B (left). The right figure shows the location of the interface.

146 J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

built between the rock and the casing. The mechanical parame-ters of the interface elements are similar to those at interface 1in Table 4. The casing is simulated by the shell element.

At the initial stage of sliding in the upper rock strata, the compres-sive stress of the cement ring and surrounding rock abruptly in-creases, but the casing deformation is only minimally observable. Asthe strata slippage increases, the destruction area of the cement andthe surrounding rock rapidly expands. The casing deformation is vis-ible, and the yield is finally generated (Fig. 21).

Fig. 22 (left) shows that the casing deformation in the upper partis more serious and that the maximum deformation occurs on theloaded side. Fig. 22 (right) shows that the stress in some areas ofthe casing is higher than 552 MPa when the slippage is 0.035 m. Con-sidering that the minimum yield strength of the N80 steel casing is552 MPa, distortion and destruction can take place at this moment,and they may be initially cut off at the interface of the rock strata.The cement ring and round rock mass bear varying degrees ofdamage.

3.4. Analysis of deformation monitoring

Deformation monitoring is a well-known practical method fordetecting material damage. In this project, monitoring work is verydifficult to accomplish because the depth of the casing is greaterthan 300 m. Deep displacement monitoring exploration is carriedout, and the test points are arranged in the core hole. The distance be-tween the drainage hole and core hole is 50 m. A borehole inclinom-eter is used to monitor the inner displacement of the rock mass. Theinclination angle sensors are laid aside at the lithology interfaces.There are two kinds of borehole inclinometers: the mobile boreholeinclinometer and the fixed borehole inclinometer. In the fixed bore-hole inclinometer, a connecting rod and fixed inclinometer probeare placed in a series of different bedding plane positions. The endof the link rod hangs over the orifice. In the process of strata move-ment, each measuring point of the inclinometer probe can obtainthe dip angle value of the segment. The deformation of the segment

Fig. 21. Displacement distribution at cross-section A–A. Strata slippage

can be obtained according to the distance between the two measur-ing points and the inclination angle value. Considering that coal min-ing can cause large deformations in overlying strata, a fixed inclinedinstrument is more suitable for determining the deformation of theentire process. Fig. 23 shows the brief foundations and placement ofthe fixed borehole inclinometer.

Fig. 24 shows the horizontal displacement monitoring situation atdifferent time points. The 10 different color curves represent the cor-responding monitoring data separately when a certain distance ap-pears between the monitoring regions and the starting coalface. Thedisplacement increases substantially when the coalface moves closerto the core hole. The value of the rock mass displacement is relativelyhigh below 700 m, and a clearly observable change occurs at this ele-vation. These monitoring data indicate that the stability of the rockmass around the monitoring region worsens when the mining posi-tion is near the monitoring hole. The disturbance of the rock massnear the gob becomes more obvious, and a large degree of slippageoccurs on the bedding plane. By unifying the failure modes discussedin this paper, the shear failure of the casing occurs when the miningposition is close to the monitoring hole; the region below the eleva-tion of 700 m is primarily considered a failure area. When the miningface is 300 m away from the monitoring well casing, the maximumstrata slippage is more than 5 cm, as shown in Fig. 24. Shear damageoccurs as indicated by the conclusion of the shear condition simula-tion. Thus far, we remain unable to obtain the deformation monitor-ing data on the casing in the vertical direction due of technicallimitations. However, the casing can be predicted to break easily atthe bedding plane separation position near the three highlighted stra-ta. Considering that the load conditions of the gas drainage from acoal seam are very complex, the stress, displacement, and other con-ventional physical quantity monitoring techniques in the regionshould be used in the course of the operation. The result can serveas an excellent guideline for analysis of the failure of casings whenthe field monitoring data and numerical simulations are systematical-ly integrated.

Current developments in microseismic monitoring technology,which has been widely used in the field of deep coal mining, make

along the interface is approximately 3.5×10−2 m at this moment.

Fig. 22. Displacement (left) and maximum principal stress (right) distribution of thecasing. Strata slippage along the interface is approximately 3.5×10−2 m at thismoment.

Fig. 24. Horizontal displacement versus altitude of the monitoring points. The 10curves represent monitoring data taken from different distances to the starting coal-face, and the red dashed line indicates 700 m in altitude.

147J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

it suitable for use as an auxiliary method to monitor the damage ex-tent and tendency of changes of the rock mass. With the help of an in-terpretation program, microseismic technology can be used to deducethe location and size of fractures, separations, and slips. The conditionof casings can also be forecasted and evaluated ahead of time.

4. Conclusion

In this study, the surface venthole and casing deformation and fail-ure caused by coal extraction are numerically simulated. Severalmeaningful conclusions are drawn. The general factors that affectthe casings failure are discussed, and the basic failure modes are sum-marized. The basic rules are deduced from the compression, tension,and shear conditions.

The experimental casing of the surface venthole in CZC is also an-alyzed via numerical simulation. The changes in the rock stress andstrain conditions near the borehole in the process of mining are calcu-lated. The calculation and monitoring results derived using the bore-hole inclinometers are compared and analyzed. Casing damage underpure compression is less likely to occur, but the cement ring is easilydestroyed. As indicated by the field monitoring data and simulation ofthe entire mining process, the formation of separation layers and stra-ta slippage phenomena are likely to take place at the overlying rockstrata of the gob, especially on the bedding plane where the lithology

Fig. 23. Displacement monitoring instrument principle of the fixed borehole incl

varies considerably. We deduce that the tension or shear failures arethe most common failure at the lower part of the surface ventholecasing. Particular attention must be paid to monitoring, and protec-tive measures must be used in these areas.

The numerical simulation model is simplified given various re-strictions. The disturbance caused by mining cannot be accuratelysimulated because there is no comprehensive consideration of thevarious factors involved. For these reasons, the calculated and actualresults deviate from each other to a certain extent. The integrationof numerical simulation and monitoring technology can serve as anefficient method for the quantitative analysis of failure on surfaceventholes and casings. Therefore, the optimal design can be used tocontrol mining disturbance in accordance with previous laws to effec-tively decrease the loss caused by coal extraction-induced surfaceventhole casing.

Acknowledgments

This study was funded by the National Science and TechnologyMajor Project under Contract No. 2011ZX05040-004 and by the Na-tional Natural Science Foundation of China (NSFC) under ContractNos. 50879063, 51079111, and 90715042.

inometer (left). Field surface suspension devices of the inclinometer (right).

148 J. Chen et al. / International Journal of Coal Geology 90–91 (2012) 135–148

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