Received 16 December 2009; accepted 21 February 2010 *Corresponding author. Tel: 86 13952117141 E-mail address: liangw1982@126.comdoi: 10.1016/S1674-5264(09)60234-9

Controlling the effect of a distant extremely thick igneous rock in overlying strata on coal mine disasters

WANG Liang1,2,*, CHENG Yuanping1,2, YANG Yuekui1,2, CHEN Haidong1,2, LI Peiqing1,2, LIU Jie1,2, WANG Feng1,2

1School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China 2National Engineering Research Center for Coal & Gas Control, China University of Mining & Technology, Xuzhou 221008, China

Abstract: Based on theoretical analysis, similarity simulation tests, numerical simulation analysis and field observations, we ana-lyzed rock collapse and rules of fraction evolution of overlying rocks and studied the rules in controlling the effect of an extremely thick igneous rock, found above a main mining coal seam in an area prone to coal mine disasters in the Haizi Coal Mine. The re-sults show that this igneous rock, called a “main key stratum”, will not subside nor break for a long time, causing lower fractures and bed separations not to close. The presence of igneous rock plays an important role in rock bursts, mine floods, gas outburst and surface subsidence in coal mines. By analyzing the rules in controlling the effect of this igneous rock, we provide useful references for safety and high efficiency mining in coal mines under special geological conditions. Keywords: extremely thick igneous rock; separation; rock burst; mine flood; gas outburst; surface subsidence

1 Introduction

Intrusive igneous rock, as part of sedimentary rock, is usually found in the overlying strata above stopes and its occurrence, lithology and distribution play important roles in the safety of coal mining. Espe-cially when an extremely thick igneous rock bed (oc-casionally more than 100 m thick with 100 MPa in compressive strength) is present in overlying strata, rules on collapse and failure take on new meaning and make their effect directly felt in disasters, such as rock bursts, mine floods, gas outburst and surface subsidence. Already for a long time, some experts have carried out research on field production and ac-cident investigation in order to analyze the effect of extremely thick igneous rock, see Tan et al., Zhang et al., and Li[1-3]. Other studies have dealt with failures of overlying strata and surface subsidence in coal mining under extremely thick but weak layers, mainly composed of mud and sandstone[4-6]. Rules on frac-ture evolution and pressure relief gas drainage from distant, protected coal seams under extremely thick igneous rock were discussed by Wang et al[7]. None of these studies dealt in detail with coal mine disas-ters brought on by extremely thick igneous rock in-

trusion. Targeting the extremely thick igneous rock intrusion in the Haizi coal mine, we discuss the effect of control exerted by igneous rock during great dis-asters in the No.10 coal seam when mining, which should provide useful references for safety and high efficiency mining in coal mines under special geo-logical conditions.

2 Igneous rock intrusion situation in the Haizi Coal Mine

The Haizi Coal Mine is located in Suixi county, Huaibei city, Anhui province. Its site belongs to the Huaibei coal field in the Linhaitong mining area. The Nos.7, 8, 9 and 10 coal seams in the Haizi Mine are primary mineable coal seams and they are all outburst seams. The outburst risk of the No.10 coal seam is relatively small and the occurrence of the coal seam is stable, so we choose the No.10 coal seam as the protective layer for the Nos.7, 8 and 9 coal seams. The distance in height between the Nos.10 and 9 coal seams is 84 m by average, and the No.7 coal seam is 115 m above the No.10 coal seam, which in turn, is 55 m above a 120 m thick igneous rock. The coal and rock profile is shown in Fig. 1.

The magmatic rock (i.e., the extremely thick igne-ous rock) is distributed as a rock bed which intrudes along the No.5 coal seam in the middle and western part of the mine. The strike is 6.5 km long and the

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length of the trend about 140 km. This extremely thick igneous rock is found in stable condition in the

102 mining area above the No.7 coal roof and is usually more than 120 m thick. The rock contains mainly diorite and diorite porphyrite. By core testing of mechanical index parameters (core photos are shown in Fig. 2), we determined that the average compression strength of the rock is 144.21 MPa, with an average tensile strength of 10.91 MPa and an av-erage RQD (Rock Quality Designation) of more than 90%. There are few primary structural planes in the rock, the lithology is single and the effect of ground-water is not obvious. Given these conditions, we con-clude this rock is an extremely massive structure and the main key stratum in this coal mine.

Fig. 1 Coal and rock profile

Fig. 2 Photo of igneous rock cores

3 Rules of movement in overlying strata under extremely thick igneous rock

Based on similar simulation tests and numerical simulation experiments, we studied rules of move-ment and deformation in overlying strata under ex-tremely thick igneous rock. Our simulation test used a plane strain model of 2.0 m×2.0 m×0.3 m, laid along the strike and we selected 1/150 as the geometric similarity constant. The model was excavated along a 1.4 m stretch with 0.3 m coal pillars left on both sides. The numerical simulation experiment used UDEC numerical simulation software, which is based on the principle of a discrete element method. The model was 600 m long, where we mined 200 m in 20 steps and left 200 m coal pillars on both sides.

When the working face of the No.10 coal seam was pushed forward, the strata in the roof of this coal seam generated fractures and separations which gradually spread upward and the scope of the de-struction expanded. At first, some very small bedding parallel fractures appeared below the igneous rock and expanded gradually into obvious separations with the occasional collapse, which then developed in an upwards direction. As the separations under this thick igneous rock gradually expanded, separations in the pressure relief coal-rock mass began to close slowly, compared with the initial stage. With mining of theNo.10 coal seam completed (excavation by 140 cm in the model, as shown in Fig. 3), the collapse on both sides was approximately symmetric, although the collapse on the left side had further advanced. The pressure relief angle was larger and fractures devel-oped. From a macroscopic point of view, the igneous rock did not bend, but the separations below it were well developed. In the middle group coal seams, bending and subsidence was most evident. A few vertical fractures and separations were not completely closed at the roof and floor of the protected layer. The details are shown in Fig. 4.

Fig. 3 140 cm of model excavation

When the stress reached an equilibrium, after fin-ishing the mining in our numerical simulation ex-periment (200 m of excavation), subsidence of the overlying strata appeared from the effect of rock gravity, which caused the lower strata to become compacted and the separations below the igneous

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rock were largely developed. The igneous rock stopped the upward movement of its overlying strata and the height of pressure relief terminated at the ig-neous rock. At this time, the movement and deforma-tion of the strata had developed symmetrically, with a “cap” like shape as shown in Fig. 5.

Fault fissures

Bed separated fissures

Longitudinal fissures

Fig. 4 Distribution of fracture and separation in the model

We used displacement sensors and close-range photography to observe the movement and deforma-tion of the igneous rock, as shown in Fig. 6. At the end of the excavation, the #W5 and #W6 measuring points had not experienced any subsidence within the

igneous rock, but a 16.85 mm deep separation was formed with the #W4 measuring point (as shown in Fig. 6a). By comparison with a coordinate calculation of the two images before and after the exploitation, the depth of the separation under the igneous rock was 15.39 mm (see Fig. 6b). The measurements were almost the same, which verified the igneous rock as the main key stratum in the mine. Given its enormous thickness and bending rigidity, it would not break and prevented subsidence along the entire overlying strata. Bed separations below the igneous rock were largelydeveloped.

Fig. 5 Cloud picture of stress equilibrium

(a) Measured by displacement sensors (b) Measured by close-range photography

Fig. 6 Separation development below igneous rock

4 Controlling the effect of igneous rock on mine rock bursts

Extremely thick igneous rock has a whole structure.The calculations for its initial caving interval are as follows[8-11]:

2 2 10.91120 2057.5

tL hqσ ×= = × = m

(bending and tensile failure model)

1

2 2 144.21120 3325 5 7.5

cL hqσ ×= = × =

× m

(clamped-clamped beam model) 2 210.91 120 269

6 6 0.04829 7.5b

mhbkq

σ ×= = =× ×

m

(square rock plates model)

We can see that the initial caving interval of the igneous rock, of more than 200 m, is clearly larger

than that of general rocks. Combined with core ob-servations, mechanical characteristics and other re-lated indices, the igneous rock is judged to be the key stratum[12]. When a key stratum is found in overlying strata, its failure will cause a wide range of rockmovements. In particular, failure of a main key stra-tum usually brings strong weighting on the roof in the stope[12]. Table 1 shows the core testing results of the

Table 1 Bump proneness parameters testing of the igneous rock

Bump proneness Boreholes

Dt WET KEEvaluation of bump proneness

R455 Medium Strong Strong Strong

R456 Medium Medium to strong Strong Medium to strong

R457 Medium Medium to strong Strong Medium to strong

R459 Medium Strong Strong Strong

Average Medium Strong Strong Strong

Note: Dt, dynamic failure time; WET, elastic energy index; KE, impact en-ergy coefficient.

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tendency to form bumps, suggesting that this ten-dency is strong. Along with the working face of the No.10 coal seam being pushed forward, the hanging arch area increased in size, and when this area reached the limit of the caving interval of the rock bed, the igneous rock became unstable and a large amount of elastic strain energy was instantly released, causing a large rock burst.

5 Controlling the effect of igneous rock on mine floods

The main hydro-geological consequences of igne-ous rock intrusion are the following. In first instance, igneous rock forms a regional impermeable boundary for groundwater which, under different hydro-geo- logical conditions, clearly constitutes a hydro-geo- logical block. Secondly, the igneous rock forms wa-ter-bearing bodies or a hydraulic connection to adja-cent aquifers. As well, the existing igneous rock af-fects groundwater quality[13]. When an extremely thick hard igneous rock is present in the roof of coal mine seams, the rock constitutes an impermeable bar-rier for regional groundwater movement, called “a water-impermeable main key stratum”, which is the main factor for controlling or affecting regional groundwater movement[14]. By water permeability testing of cores, the average permeability coefficient is found to be 1.15×10–5 cm/s, the rock is considered an impermeable rock bed.

Table 2 Water permeability testing of igneous rock

Boreholes R455 R456 R458 R459 Average

Permeability coefficient K20 (cm/s)

1.2×10–5 3.2×10–4 4.4×10–4 3.3×10–4 1.15×10–5

After mining of the distant No.10 coal seam, this thick igneous rock must lead to displacement, given the principle of energy conservation. Due to its large thickness, high strength and structural integrity, its displacement was very small and the internal stress not high. In the short run, it would not break until its carrying capacity were to reach its limit. Between the igneous rock and its underlying series of coal strata, a large separation space would appear, which will often fill with water. When the strength of the igneous rock becomes suddenly unleashed, a large elastic strain energy would instantly be released, leading to a strong impact on this stored water, forming and ex-erting considerable pressure on the water. This high pressure water would cause instant ruptures along the weak zones of the series of coal strata at the top of the working face, form water inrush channels, quickly resulting in water inrush. A large sandstone roof wa-ter inrush accident occurred during the mining of the #745 working face in the Haizi Coal Mine on May 21, 2005, recorded and described by Li[3]. The investiga-tion concluded that the water inrush was a sudden,

extremely large water inrush, without any warning. The water originated mainly from the separated bed.The investigation also concluded that the water from the separation was the most important direct reason and responsible for mine rock burst.

6 Controlling the effect of igneous rock on gas disasters

Present research, both at home and abroad, shows that coal and gas outburst is caused by the combined effect of gas, ground stress and coal structure. Gener-ally, the mechanical strength of coal seams, prone to outburst, is low and varies considerably. This type of coal is usually called a “tectonic coal”, its seam is subject to strong geological and structural forces, has poor permeability, a high initial methane release rate, low humidity and inordinate bedding and the location where outburst occur is often the soft coal seam[15]. The presence of the 120 m thick igneous rock in the Haizi Coal Mine causes the occurrence of gas, ground stress and coal structure to vary greatly and provides favorable conditions for coal and gas outburst. At 1:47 on April 25th, 2009, a coal and gas outburst ac-cident occurred in the II1026 heading face at the third stage of the west II102 mining area in the No.10 coal seam. The amount of coal involved was 656 t and the amount of gas released 13210 m3. One person died in the accident and the direct economic loss amounted to 764800 RMB.

6.1 Distribution of the effect of thick igneous rock on coal structure

Under the condition of a high baking temperature in the No.10 coal seam, the coal consistent coefficient was reduced and pores and fractures developed. The coal in the soft seam was crushed to powder, which took on the typical characteristics of “tectonic coal”. The coal is identified as lean coal in the II102 mining area and is covered by this extremely thick igneous rock, while the coking coal in the II101 mining area is not covered by igneous rock. The high baking tem-perature increases the metamorphic grade of the coal seam.

Below the igneous rock, P (initial gas releasing rate) increased, f (the coal sturdiness coefficient) re-duced. Table 3 shows that the outburst index P of the No.10 coal seam was 26 mmHg in the II102 min-ing area and f was 0.17. The test results showed that the outburst risk in the II102 mining area was larger than that in the II101 mining area.

According to requirements of the “regulation for the prevention and control of coal and gas outburst”, we must carry out coal and gas outburst predictionsfor outburst coal seams, where the main indices are gas pressure and gas content. In Table 3, the values of gas pressure and content in the II102 mining area, covered by the thick igneous rock, are much higher

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than that in the II101 mining area at the same level. Table 3 Outburst (prediction) indices of tests in

different mining areas Regina prediction indices for outburst Outburst indices

Serial number

Sampling spots Gas

pressure P (MPa)

Gas content X(m3/t)

P (mmHg) f

1 II101 mining area 0.63 5.36 9 0.41

2 II102 mining area 1.75 15.36 26 0.17

6.2 Effect of change in thick igneous rock on distribution of ground stress

The effect of magmatic intrusion produces addi-tional tectonic stress and thermal stress on a nearby coal-rock mass which destroys the surrounding rocks and changes the distribution of ground stress. The presence of the igneous rock, the main key stratum in this coal mine, prevented the surface from sinking after the No.10 coal seam was mined. The pressure relief effect was good which provided excellent con-ditions for gas drainage[7,16-18]. At the same time, the stress distribution at both ends of the working face changed, the maximum stress concentration de-creased gradually to less than 2~4 H, while the range of stress concentration increased. Fig. 7 shows the situation of stress distribution. It is well known that the geological structure and mining stress concentra-tion zones are the key zones for outburst. Given the increase in stress concentration around the stope, covered by igneous rock, the outburst risk also in-creased.

Hγ

Fig. 7 Pressure distribution around the stope of No.10 coal seam working face

6.3 Effect of igneous rock on gas storage

The action of magmatic intrusion caused the coal seam to form twice a gas generation under high tem-perature and high pressure, where the amount of gas produced and the output ratio greatly improved. The high baking temperature increased the metamorphic grade of the coal seam, the number of micro-pores increased and gas absorption of the coal seam was enhanced which improved the capacity for gas stor-age. Given the extreme thickness and low permeabil-ity of the igneous rock in the Haizi Coal Mine, thepreexisting fissures did not develop, stopped gas dis-sipation and trapped the gas. The igneous rock be-

came the natural barrier for gas migration and pro-vided excellent conditions for gas storage.

Adsorption constants of tests carried out and gas emissions from the working face of the No.10 coal seam in the II101 and II102 mining areas are shown in Table 4. The results show that limiting the amounts of adsorbed (index a) and emitted gas in the II102 mining area are much larger than that in the II102 mining area. Table 4 Adsorption constants of tests and gas emission from

working face of No.10 coal seam in two mining areasResults of gas emission Absorption results

Serial number

Sampling spots

Total amount of absolute gas emission (m3/min)

Total amount of relative gas emission(m3/t)

a (m3/t)

b(MPa–1)

1 II102mining area

27.71 39.90 31.6753 1.0432

2 II101mining area

19.02 27.39 19.3298 0.8713

Note: a, b, adsorption constants.

7 Controlling the effect of igneous rock on surface subsidence

When there was no thick igneous rock above the roof of the protected seam, its supporting effect was greatly reduced. After mining the coal seams, the original stress equilibrium of the overlying strata was destroyed and formed movement and deformation in the overlying strata. When the hanging arch area of the goaf expanded within a certain range, rock movements would develop on the surface and cause itto move and deform. When the surface stopped mov-ing after the mining was completed, the correspond-ing surface of the goaf formed a basin-shaped subsi-dence region[9]. Under full mining of the working faces and compaction of the goaf, the surface sub-sided to some extent and the subsidence coefficientremained stable at about 0.8 without considering other factors[9].

According to information provided by the Haizi Coal Mine, the largest surface subsidence was only 212 mm after mining the #1022 working face for 13 months (result from the #W6 measuring point), with a subsidence coefficient of only 0.11. After mining for 17 months, the largest surface subsidence was only 310 mm (result from the #I37 measuring point), witha subsidence coefficient of 0.12. After mining the II1024 working face for 14 months, the subsidence coefficient was only 0.16. Based on measured surface subsidence data from the Huaibei and Huainan min-ing areas, the subsidence coefficient is generally 0.8 to 1.0[15]. Given that the subsidence coefficient in the Haizi Coal Mine is far smaller, we can see that this igneous rock will not break after mining the No.10 coal seam for a certain area, which verifies that this extremely thick igneous rock controls the effect on

WANG Liang et al Controlling the effect of an distant extremely thick igneous rock in … 515

surface subsidence.

8 Conclusions

1) By core testing the mechanical index parameters of an extremely thick igneous rock (average thickness 120 m), we calculated that the average compression strength of the rock is 144.21 MPa, its average tensile strength 10.91 MPa and the average RQD more than 90%. The rock is an ‘extremely massive structure’, with an initial caving interval larger than 200 m, which proved to be the main key stratum in the coal mine.

2) Based on similar simulation tests and numerical simulation experiments, our results show that this igneous rock would not break and prevented subsi-dence of the entire overlying strata. The bed separa-tions below the igneous rock were largely developed. The igneous rock stopped the upward movement of overlying strata. The developed height of pressure relief terminated at the igneous rock. The movementand deformation of strata was distributed symmetri-cally and was shaped liked a “cap”.

3) When an extremely thick and hard igneous rock is present in the roof of a coal mine seam, the rock constitutes an impermeable barrier for regional groundwater movement, referred to as the main “wa-ter-impermeable key stratum” and becomes the main factor for controlling or affecting regional groundwa-ter movements. By testing bump proneness in the cores, we found that this proneness is strong and when the area reached the limit of the caving interval of the rock bed, the igneous rock lost its strength, became unstable, released its elastic strain energy instantly, which resulted in a large rock burst and flooding of the mine.

4) The existence of the 120 m thick igneous rock in the Haizi Coal Mine made considerably changed for the occurrence of gas, ground stress and coal struc-ture, providing favorable conditions for coal and gas outburst.

Acknowledgements

The authors are grateful to the National Basic Re-search Program of China (No.2005CB221503), the National Natural Science Foundation of China (Nos.70533050 and 50674089), the National Founda-tion for the Youth of China (No.50904068) and the Research Fund for the Youth of China University of Mining & Technology (No.OY091223).

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