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Failure behavior of a rock-coal-rock combined body with a weak coal interlayer
International Journal of Mining Science and Technology 23 (2013) 907–912
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Failure behavior of a rock-coal-rock combined body with a weak coal interlayer Zuo Jianping a,b,⇑, Wang Zhaofeng b, Zhou Hongwei a,b, Pei Jianliang c, Liu Jianfeng c a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining & Technology, Beijing 100083, China c College of Hydraulic and Hydroelectric Engineering, Sichuan University, Chengdu 610065, China b
a r t i c l e
i n f o
Article history: Received 14 March 2013 Received in revised form 25 April 2013 Accepted 12 May 2013 Available online 22 November 2013 Keywords: Rock-coal-rock combined body Weak coal interlayer Failure mechanism Axial failure
a b s t r a c t Using an MTS 815 testing machine, the deformation and failure behavior of a rock-coal-rock combined body containing a weak coal interlayer has been investigated and described in this paper. Uniaxial loading leads to the appearance of mixed cracks in the coal body which induce instability and lead to bursts in coal. If the mixed crack propagates at a sufficiently high speed to carry enough energy to damage the roof rock, then coal and rock bursts may occur – this is the main mechanism whereby coal bumps or coal and rock bursts occur after excavation unloading. With increasing confining pressure, the failure strength of a rock-coal-rock combined body gradually increases, and the failure mechanism of the coal interlayer also changes, from mixed crack damage under low confining pressures, to parallel crack damage under medium confining pressures, and finally to single shear crack damage or integral mixed section damage under high confining pressures. In general, it is shown that a weak coal interlayer changes the form of overall coal damage in a rock-coal-rock combined body and reduces the overall stability of a coal body. Therefore, the whole failure behavior of a rock-coal-rock combined body in large cutting height working faces is controlled by these mechanisms. Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction In order to ensure continuous, rapid and healthy development of the economy in our country, the demand for energy must also increase significantly. Since the quantity of shallow natural resources is limited, energy development in China has increasingly focused on the resources available at greater depth in the earth. However, in deep coal mining operations, a number of technical problems need to be solved, including roof falls, floor heave, large deformation of gateways and coal or gas outburst etc. Most of the rock strata are comprised of sandstone, mudstone and shale in alternating layers; the strengths of these rocks are normally low, so that under the combined effects of gravity and tectonic stress, deformation or failure of surrounding rocks is inevitable, especially in rock masses with weak intercalated layers [1]. While investigating the stability of underground tunnels in a layered rock mass using the finite element method (FEM), Wang et al. considered that a layered rock mass or a weak layer can be regarded as a transverse isotropic material and the interlayer contact might be selected as Goodman contact elements with rotational degrees of freedom [2]. The influence of a weak interlayer (such as a fault, spandrel
⇑ Corresponding author. Tel.: +86 10 62331358.
and the sidewall) on the stability of the rock around an underground cavern and the behavior of a spray anchor bracing structure were theoretically analyzed by Zhang et al. [3]. Zhao et al. studied the initiation, propagation, connection, and closure of sub-cracks in rock materials. Research on cement mortars including weak fillers under uniaxial compression was carried out by Zhao et al. [4]. Combining the results from indoor mechanical tests on weak structural planes in soft rocks with damage criteria, a damage evolution model for weak intercalated layers was established and applied to stability analysis of the excavation of a deep soft rock roadway in Huainan coal mine by Yang et al. [5]. Utilizing knowledge of rock mechanics, material mechanics, a theoretical approach, and an analysis of the hinge rock beam stability of layered composite rock beam, Ma et al. studied the effect of the location and thickness of a soft interlayer on roadway roof stability [6]. Analyzing the characteristics of ground stress in two models, with and without a weak interlayer, Wang et al. concluded that both jointing and the weak interlayer contributed to the deformation and damage of the rock mass [7,8]. With large-scale high intensity coal mining now being carried out with a continuing improvement in coal exploitation technology, the design of roadways is gradually being transferred from rock roadways to coal roadways, from rectangular shapes to arch shapes, and from small cross-section to large-cross section. These large cross-section, rectangular-shaped coal roadways which
E-mail address: [email protected] (J. Zuo). 2095-2686/$ - see front matter Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.11.005
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J. Zuo et al. / International Journal of Mining Science and Technology 23 (2013) 907–912
are under high ground stress and having low intensity between the coal seam roof and floor strata, are prone to undergo large deformation or instability, which greatly affects the stability of deep coal roadways. As shown in Fig. 1, the stability of roadways in coal seams at great mining depth depends greatly on the overall failure properties of the coal-rock combined body under high stress conditions. In addition, stability is controlled by the distribution and mechanical characteristics of the weak interlayer coal.
2. Preparation and testing of coal-rock specimens In order to investigate the mechanisms of coal mine disasters such as roof falls and coal-rock bursts, a series of experiments have been carried out to study the overall failure characteristics of a rock-coal-rock combined body in a typical coal layer containing a weak intercalation. From the results, three different failure mechanisms and strength characteristics have been analyzed. Samples were taken from 2071 working face in Hebei Kailuan coal mine, at a depth of approximately 850 m, the thickness of the coal seam being 3.5 m. Generally speaking, the coal seam is soft and friable, resulting in large deformations in the rock around the roadway. Under the coupled action of high gravity and tectonic stress, large contractions occur between the two sides of the coal floor resulting in serious floor heave and a high risk of dangerous coal bursts. Fig. 2 shows typical coal burst and floor heave in Kailuan Qianjiaying coal mine caused by the softness of the coal seam and the coupled effects of gravity and high ground stress. The figure illustrates the deformation and failure characteristics of both coal and rock. Since coal is a comparatively weak and broken material, coal samples should be cut into small pieces. Two rock samples are then used to clamp the coal sample forming a rock-coal-rock combined body which approximately meets the recommended 2:1 ratio of height to diameter in accordance with international standards for rock mechanics testing. To further comply with international rock testing standards, both ends of the rock and coal samples must be
Roof Coal
Roadway
Floor
(a) Geological engineering model
(b) Indoor model
Fig. 1. Rock-coal-rock combined body concept model.
Lateral wall
Coal burst and floor heave
Fig. 2. Coal burst and floor heave in Kailuan Qianjiaying coal mine.
polished to ensure parallelism of the specimen faces such that the non-parallelism should be less than 0.01 mm and the end diameter deviation is less than 0.02 mm. 3. Test system and loading mode All tests were carried out using the advanced MTS 815 testing machine at Sichuan University. The machine has a maximum axial load capacity of 4600 kN, a uniaxial lateral extensometer range of 4–4 mm, and an axial stroke of 0–100 mm [9–15]. The precision of each sensor is 0.5% of the range for the current geometric calibration point. Due to the relative weakness of a coal or coal-rock combined body, the displacement loading mode was used in these tests. 4. Uniaxial failure experiments on a rock-coal-rock combined body containing a weak coal layer For a rock-coal-rock combined body, a specimen test identifier RMR-A-B is applied. The first R refers to the roof rock, M refers to the middle weak intercalation coal, and the second R represents the floor rock. A defines the confining pressure in MPa and B represents the test number under the same confining pressure. For example, RMR-5-1 indicates the first test of a rock-coal-rock combined body using a confining pressure of 5 MPa and a loading rate of 3 MPa/min. The displacement loading mode was applied in all RMR experiments. First, a loading rate of 0.06 mm/min is applied until failure of the specimen occurs. Then, after failure of the specimen, displacement control mode is applied using a loading rate of 0.1 mm/min. The stress value used is the overall stress of the rockcoal-rock combined body and the axial strain is the deformation of the whole coal, half of roof rock and half of the floor rock. A lateral strain measuring device was installed on the coal body, as the strain is mainly the lateral strain in the coal. Uniaxial compression experiments for three groups of RMR specimen were carried out. For specimen RMR-0-1, the initial load was 0.15 kN, and the maximum peak load of the combination was 33.69 kN. As the rock was being crushed, noise was emitted from the specimen accompanied by the formation of rock fragments. Finally, the rock failed by crushing of the bottom end of the sample, with the formation of two connected axial cracks. Though the coal was soft and broken, the energy of the high speed propagating crack was large enough to destroy the hard rock. After all the coal was crushed, rock failure occurred from the bottom section, as shown in Fig. 3a. Two cracks are found in the upper section of rock, which were caused by the two end faces not being flat. Vertical cracks were observed in the failed coal, together with several horizontal cracks in which crack bifurcation phenomena were observed. Additionally, four larger, approximately parallel, cracks appeared at an angle of 60° to the horizontal. For specimen RMR-0-2, the initial load was 0.3 kN. With increasing load, micro cracks first appeared in the middle of the weak intercalation. When the load reached 41.26 kN, the rockcoal-rock combined body suddenly failed with many micro cracks evident in the coal body, as shown in Fig. 3b. Similarly, the highspeed propagation of micro cracks in the coal led to roof rock fracture. The brittle nature of this specimen is apparent when compared with specimen RMR-0-1. After peak load had occurred, the load rapidly dropped accompanied by noise. Exfoliation occurred in both rock and coal. For specimen RMR-0-3, the initial load was 0.16 kN, and the maximum peak load of the combination was about 38.53 kN. The central coal section failed massively and dilatancy in the coal was highly evident. There were also many small coal fragments produced, as shown in Fig. 3c.
J. Zuo et al. / International Journal of Mining Science and Technology 23 (2013) 907–912
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Fig. 3. Stress–strain curves and failure images of coal-rock combined body under uniaxial load.
As shown in Fig. 4, the stress-strain curves for specimen RMR-0 under uniaxial load, show clearly that a compressive process is taking place during initial loading resulting in deformation of the central weak coal. With increasing load, the stress-strain curve transforms into a linear elastic phase. When the load reaches 70– 90% of the peak load, nonlinear behavior is seen on the stressstrain curve because of micro fractures or macro fractures inside the coal. After reaching peak load, the coal-rock combined body suffered complete failure. Lateral deformation was far greater than axial deformation, and coal dilatancy was also evident. Mixed cracks occurred mainly in the coal. Under uniaxial load, hard rock failure due to high speed crack propagation of the coal and coal burst due to mixed crack development, are likely to be the main mechanisms of coal-rock burst after excavation, which coincides with experience from mining engineering.
5. Triaxial failure experiments on a rock-coal-rock combined body containing a weak coal layer In order to study the failure behavior of RMR specimens under different depth conditions, confining pressures of 5, 10, 15 and 20 MPa were respectively applied to the combination of coal and rock specimens.
5.1. Experiments under 5 MPa confining pressure
5.2. Experiments under 10 MPa confining pressure For the RMR-10-3 specimen, peak load was 56.69 kN. As shown in Fig. 7, lateral deformation occurred in the coal which fractured at a roughly 20° angle to the vertical axis. In addition, several micro cracks appeared in the coal besides the obvious large crack. Basically, rock failure was not observed. The group of experiments indicated that the peak strengths were greater than uniaxial strengths, and showed some evidence of ductility damage. Due to the restraining effect of confining pressure, the crack changed from a mixed crack to a parallel crack at an angle to the vertical direction. Experiments indicated that the failure mechanism would turn from brittle failure to shear failure with an increase in confining pressure, as shown in Fig. 8. 5.3. Experiments under 15 MPa confining pressure For the RMR-15-1 specimen, peak load was 101.19 kN, and the residual load was 68.21 kN. Damage occurred mainly in the coal body which showed a small amount of dilatancy. Compared with the experiments carried out at low confining pressure, the dilatancy was clearly reduced. Local damage occurred on the interface between the coal body and the rock body, as shown in Fig. 9. In the case of specimen RMR-15-2, since there were original microcracks distributed randomly in the coal body, the peak load at failure was low-approximately 49.2 kN. Damage also occurred
Stress
(MPa)
For the RMR-5-1 specimen, the peak load was 62.09 kN, and the residual load was 31.4 kN. Two vertical cracks were observed in the floor rock, and were interconnected at the base. Many micro cracks appeared in the coal at the same time. This indicates that rock failure is affected by micro crack development and propagation. In this case, high speed crack propagation in coal also caused failure of the hard rock, which is similar in behavior to combined body failure under uniaxial load. Due to the effects of confining pressure, both
peak values of stress and corresponding strain values increased (Fig. 5). From the stress-strain curves under 5 MPa, as shown in Fig. 6, the failure of the specimen was still predominantly brittle failure, as. However, compared with the uniaxial experiments, micro fracturing decreased, the distribution of micro cracks having a similar distribution to the approximately parallel vertical cracks orientated at an angle of 0–10° with the vertical axis. Few mixed cracks appeared.
Fig. 4. Stress–strain curves of RMR-0 under uniaxial load.
Fig. 5. Stress–strain curve and failure image of RMR-5-1.
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J. Zuo et al. / International Journal of Mining Science and Technology 23 (2013) 907–912
Fig. 6. Stress–strain curves under confining pressure 5 MPa.
Fig. 10. Stress–strain curve and failure image of RMR-15-2.
From the stress-strain curves for the specimen, the failure mode displayed typical ductility characteristics. Due to the restraining effects of a high confining pressure, the number of cracks in the coal is significantly reduced, with few parallel cracks, highlighting the overall instability of coal and rock, as shown in Fig. 11. 5.4. Experiments under 20 MPa confining pressure
Fig. 7. Stress–strain curve and failure image of RMR-10-3.
Fig. 8. Stress–strain curves under confining pressure 10 MPa.
For specimen RMR-20-1, the lower section of the specimen was very non-homogeneous. The peak load was 70.39 kN, and the residual load was 60 kN. Damage occurred mainly in the coal, which divided into two parts. In the coal, a vertical crack could be observed accompanied by several small inclined cracks, as shown in Fig. 12. For the RMR-20-3 specimen, the peak load was 72.86 kN, and the residual load was 46.6 kN. The specimen emitted loud noises on two occasions, corresponding with the two sharp declines seen in the stress-strain curve. A 45° shear crack developed in the coal, which led to the formation of an approximately parallel crack in the rock, inclined at 45°. The lower section of the rock also displayed a triangular destruction belt, as shown in Fig. 13. From the stress-strain curves produced in the whole failure process of the specimens, a high degree of ductility was also evident. One failure mechanism in coal and rock was, in part, overall buckling failure resulting from the use of high confining pressures; another mechanism was shear failure, also resulting from the high confining pressures used, as shown in Fig. 14. 6. Failure analysis of coal-rock combination with a weak coal interlayer Based on all the RMR uniaxial and triaxial failure experiments, the physical and mechanical parameters of the specimens were derived, as shown in Table 1. Wave velocity tests on coal and rock are used mainly for the evaluation of the integrity of the coal-rock combined body. Generally, the lower the wave velocity, the greater
Fig. 9. Stress–strain curve and failure image of RMR-15-1.
mainly in the coal body. There were also some parallel inclined cracks and a large crack through the entire coal body. However, the specimen showed high ductility under the high confining pressure, as shown in Fig. 10.
Fig. 11. Stress–strain curves under 15 MPa confining pressure.
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J. Zuo et al. / International Journal of Mining Science and Technology 23 (2013) 907–912
Fig. 12. Stress–strain curves and failure image of RMR-20-1.
Fig. 13. Stress–strain curve and failure image of RMR-20-3.
Fig. 14. Stress–strain curves under 20 MPa confining pressure.
the number of internal fractures in the coal or rock body. Conversely, the higher the wave velocity, the fewer the number of internal cracks. With increasing confining pressure, the number of internal cracks in the combined body reduces. This is mainly because the confining pressure restrains crack production. With
Fig. 15. Peak strengths of coal, rock, and coal-rock combined body.
increasing confining pressure, the overall elastic modulus of the coal-rock combined body also has a tendency to increase. As the confining pressure increases from 0 to 10 MPa, the elastic modulus increases rapidly. However, as the confining pressure increases from 10 to 20 MPa, the elastic modulus increases more slowly. Poisson’s ratio for the RMR combined body does not change significantly with increasing confining pressure, and is generally about 0.45. The failure strength of the RMR combined body increases slowly with increasing confining pressure. When the confining pressure was increased from 0 to 10 MPa, the failure strength rapidly increased, whilst from 10 to 20 MPa, the failure strength increased slowly. The main reason for this behavior is the low failure strength of the coal body; at high confining pressure, the overall failure of the RMR combined body is caused by failure of the coal body. However, at low confining pressure, the failure mechanism of the RMR combined body is structural failure. We also analyzed the peak failure strength of the coal and rock samples described in reference, for comparison with the tests on the rock-coal-rock combined body with a weak coal interlayer, with damage behavior in coal and rock, and with their strength characteristics [9]. These comparisons are illustrated in Fig. 15. It can be seen that the failure strength of the combined body is greater than that of coal, but less than that of rock. Experimental results from uniaxial and triaxial testing of the RMR combined body indicate that brittle failure occurred frequently in coal due to its low strength properties, which thereby affects the stability of roadways. The failure mode was also influenced by the confining pressure.
7. Conclusions Using the MTS 815 testing system, the deformation and failure behavior of a rock-coal-rock combined body containing a weak
Table 1 Physical and mechanical parameters of RMR combined body with a weak coal interlayer under different confining pressures. Specimen code
Wave velocity V (m/s)
Elasticity modulus E (GPa)
Poisson’s ratio l
Peak strength rc (MPa)
RMR-0-1 RMR-0-2 RMR-0-3 RMR-5-1 RMR-5-2 RMR-5-3 RMR-10-1 RMR-10-2 RMR-10-3 RMR-15-1 RMR-15-2 RMR-20-1 RMR-20-2 RMR-20-3
1812.9687 901.7530 1373.7589 1327.8677 1298.0021 1321.9590 1409.5094 1375.0605 598.9874 715.1626 884.2235
7.724866 5.795695 9.962877 9.330667 8.206638 9.873470 10.22807 11.17452 10.67823 11.86291 8.013938 10.63099 12.39705 11.52835
0.415305 0.486088 0.458710 0.407667 0.427589 0.476940 0.427756 0.409306 0.499431 0.439825 0.474579 0.477872 0.457807 0.457150
35.90809 43.67907 40.54892 72.43493 69.48120 50.74946 86.70815 93.64627 60.21350 106.1981 51.42236 74.47479 105.2938 77.56996
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coal interlayer was experimentally studied and described in this paper. The following conclusions can be drawn: (1) The results of tests on RMR combined body with a weak coal interlayer subjected to uniaxial load indicate that uniaxial loading leads to mixed cracking in coal, which leads to instability of the coal. This is the main mechanism controlling coal bursts. If the energy of high speed crack propagation is sufficient to destroy roof rock, then further rock and coal bursts may occur. (2) With increasing confining pressure, the failure strength of an RMR combined body has a tendency to increase. The failure mechanism of the coal interlayer changes from a mixed crack condition under low confining pressures, to a parallel crack condition under medium confining pressures and to a single shear crack condition or integral cross section damage under high confining pressures. (3) In general, the existence of the weak coal interlayer changes the overall failure mechanism and strength in an RMR combined body, thereby reducing the overall stability of a coal roadway. Furthermore, it should be noted that the deformation and failure of an RMR combined body has an important influence on the design and excavation of coal roadways in large cutting height coalfaces. Acknowledgments The work was supported by the Special Funds for Major State Basic Research Project (Nos. 2011CB201201 and 2010CB732002), the National Natural Science Foundation of China (Nos. 11102225 and 51374215) and the National Excellent Doctoral Dissertation of China (No. 201030).
References [1] Han DX. Coal petrology of China. Beijing: China University of Mining and Technology Press; 1996. [2] Wang XQ, Yang LD, Gao WH. 2D nonlinear FEA for underground openings located in the layered rock mass with softening joints. Chin J Geotech Eng 2002;24(6):729–32. [3] Zhang ZQ, Li N, Swoboda G. Influence of weak interbed distribution on stability of underground openings. Chin J Rock Mech Eng 2005;24(18):3252–7. [4] Zhao YH, Yang ZT. Research on fracturing around cemented slot in rock specimen. Chin J Rock Mech Eng 2005;24(13):2350–6. [5] Yang JP, Chen WZ, Zheng XH. Stability study of deep soft rock roadways with weak intercalated layers. Rock Soil Mech 2008;29(10):2864–70. [6] Ma NJ, Zhan P, He G, Xiang QA. On soft interlayer affecting roadway stabilization in roof. Mineral Eng Res 2009;24(4):1–4. [7] Wang YZ, Li XZ, Zhang YS, Zhao XB, Yuan L, Cheng JY. Deformation and damage characteristics of hard rock mass with thick weak interlayer. J Disaster Prev Mitigation Eng 2011;31(4):441–9. [8] Kang HP, Wang JH. Rock bolting theory and complete technology for coal roadways. China Coal Industry Publishing House; 2007. [9] Zuo JP, Xie HP, Wu AM, Liu JF. Investigation on failure mechanisms and mechanical behaviors of deep coal-rock single body and combined body. Chin J Rock Mech Eng 2011;30(1):84–92. [10] Qiao W, Li WP. Effect of geo stress on permeability of groundwater in karst fractured rock mass. J China Univ Mining Technol 2011;40(1):73–9. [11] Yue ZW, Yang RS, Sun ZH, Dou BY, Pan ChC. Experimental study of the failure mechanism in surrounding rock of an inclined shaft excavated through a thick gravel stratum. J China Univ Mining Technol 2011;40(3):368–72. [12] He MC, Nie W, Zhao ZY, Cheng Ch. Micro- and macro-fractures of coarse granite under true-triaxial unloading conditions. Mining Sci Technol 2011;21(3):389–94. [13] Zhang HQ, He YN, Liu HG, Han LJ, Shao P. Ringlike failure experiment of thickwalled limestone cylinder specimens in triaxial unloading tests. Mining Sci Technol 2011;21(3):445–50. [14] Fang HF, Ge SR, Cai LH, Hu EY. Buckling analysis of the shell of a refuge chamber in a coal mine under uniform axial compression. Int J Mining Sci Technol 2012;22(1):85–9. [15] Chen X, Liao ZH, Peng X. Deformability characteristics of jointed rock masses under uniaxial compression. Int J Mining Sci Technol 2012;22(2):213–21.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Failure behavior of a rock-coal-rock combined body with a weak coal interlayer Zuo Jianping a,b,⇑, Wang Zhaofeng b, Zhou Hongwei a,b, Pei Jianliang c, Liu Jianfeng c a
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining & Technology, Beijing 100083, China c College of Hydraulic and Hydroelectric Engineering, Sichuan University, Chengdu 610065, China b
a r t i c l e
i n f o
Article history: Received 14 March 2013 Received in revised form 25 April 2013 Accepted 12 May 2013 Available online 22 November 2013 Keywords: Rock-coal-rock combined body Weak coal interlayer Failure mechanism Axial failure
a b s t r a c t Using an MTS 815 testing machine, the deformation and failure behavior of a rock-coal-rock combined body containing a weak coal interlayer has been investigated and described in this paper. Uniaxial loading leads to the appearance of mixed cracks in the coal body which induce instability and lead to bursts in coal. If the mixed crack propagates at a sufficiently high speed to carry enough energy to damage the roof rock, then coal and rock bursts may occur – this is the main mechanism whereby coal bumps or coal and rock bursts occur after excavation unloading. With increasing confining pressure, the failure strength of a rock-coal-rock combined body gradually increases, and the failure mechanism of the coal interlayer also changes, from mixed crack damage under low confining pressures, to parallel crack damage under medium confining pressures, and finally to single shear crack damage or integral mixed section damage under high confining pressures. In general, it is shown that a weak coal interlayer changes the form of overall coal damage in a rock-coal-rock combined body and reduces the overall stability of a coal body. Therefore, the whole failure behavior of a rock-coal-rock combined body in large cutting height working faces is controlled by these mechanisms. Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction In order to ensure continuous, rapid and healthy development of the economy in our country, the demand for energy must also increase significantly. Since the quantity of shallow natural resources is limited, energy development in China has increasingly focused on the resources available at greater depth in the earth. However, in deep coal mining operations, a number of technical problems need to be solved, including roof falls, floor heave, large deformation of gateways and coal or gas outburst etc. Most of the rock strata are comprised of sandstone, mudstone and shale in alternating layers; the strengths of these rocks are normally low, so that under the combined effects of gravity and tectonic stress, deformation or failure of surrounding rocks is inevitable, especially in rock masses with weak intercalated layers [1]. While investigating the stability of underground tunnels in a layered rock mass using the finite element method (FEM), Wang et al. considered that a layered rock mass or a weak layer can be regarded as a transverse isotropic material and the interlayer contact might be selected as Goodman contact elements with rotational degrees of freedom [2]. The influence of a weak interlayer (such as a fault, spandrel
⇑ Corresponding author. Tel.: +86 10 62331358.
and the sidewall) on the stability of the rock around an underground cavern and the behavior of a spray anchor bracing structure were theoretically analyzed by Zhang et al. [3]. Zhao et al. studied the initiation, propagation, connection, and closure of sub-cracks in rock materials. Research on cement mortars including weak fillers under uniaxial compression was carried out by Zhao et al. [4]. Combining the results from indoor mechanical tests on weak structural planes in soft rocks with damage criteria, a damage evolution model for weak intercalated layers was established and applied to stability analysis of the excavation of a deep soft rock roadway in Huainan coal mine by Yang et al. [5]. Utilizing knowledge of rock mechanics, material mechanics, a theoretical approach, and an analysis of the hinge rock beam stability of layered composite rock beam, Ma et al. studied the effect of the location and thickness of a soft interlayer on roadway roof stability [6]. Analyzing the characteristics of ground stress in two models, with and without a weak interlayer, Wang et al. concluded that both jointing and the weak interlayer contributed to the deformation and damage of the rock mass [7,8]. With large-scale high intensity coal mining now being carried out with a continuing improvement in coal exploitation technology, the design of roadways is gradually being transferred from rock roadways to coal roadways, from rectangular shapes to arch shapes, and from small cross-section to large-cross section. These large cross-section, rectangular-shaped coal roadways which
E-mail address: [email protected] (J. Zuo). 2095-2686/$ - see front matter Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.11.005
908
J. Zuo et al. / International Journal of Mining Science and Technology 23 (2013) 907–912
are under high ground stress and having low intensity between the coal seam roof and floor strata, are prone to undergo large deformation or instability, which greatly affects the stability of deep coal roadways. As shown in Fig. 1, the stability of roadways in coal seams at great mining depth depends greatly on the overall failure properties of the coal-rock combined body under high stress conditions. In addition, stability is controlled by the distribution and mechanical characteristics of the weak interlayer coal.
2. Preparation and testing of coal-rock specimens In order to investigate the mechanisms of coal mine disasters such as roof falls and coal-rock bursts, a series of experiments have been carried out to study the overall failure characteristics of a rock-coal-rock combined body in a typical coal layer containing a weak intercalation. From the results, three different failure mechanisms and strength characteristics have been analyzed. Samples were taken from 2071 working face in Hebei Kailuan coal mine, at a depth of approximately 850 m, the thickness of the coal seam being 3.5 m. Generally speaking, the coal seam is soft and friable, resulting in large deformations in the rock around the roadway. Under the coupled action of high gravity and tectonic stress, large contractions occur between the two sides of the coal floor resulting in serious floor heave and a high risk of dangerous coal bursts. Fig. 2 shows typical coal burst and floor heave in Kailuan Qianjiaying coal mine caused by the softness of the coal seam and the coupled effects of gravity and high ground stress. The figure illustrates the deformation and failure characteristics of both coal and rock. Since coal is a comparatively weak and broken material, coal samples should be cut into small pieces. Two rock samples are then used to clamp the coal sample forming a rock-coal-rock combined body which approximately meets the recommended 2:1 ratio of height to diameter in accordance with international standards for rock mechanics testing. To further comply with international rock testing standards, both ends of the rock and coal samples must be
Roof Coal
Roadway
Floor
(a) Geological engineering model
(b) Indoor model
Fig. 1. Rock-coal-rock combined body concept model.
Lateral wall
Coal burst and floor heave
Fig. 2. Coal burst and floor heave in Kailuan Qianjiaying coal mine.
polished to ensure parallelism of the specimen faces such that the non-parallelism should be less than 0.01 mm and the end diameter deviation is less than 0.02 mm. 3. Test system and loading mode All tests were carried out using the advanced MTS 815 testing machine at Sichuan University. The machine has a maximum axial load capacity of 4600 kN, a uniaxial lateral extensometer range of 4–4 mm, and an axial stroke of 0–100 mm [9–15]. The precision of each sensor is 0.5% of the range for the current geometric calibration point. Due to the relative weakness of a coal or coal-rock combined body, the displacement loading mode was used in these tests. 4. Uniaxial failure experiments on a rock-coal-rock combined body containing a weak coal layer For a rock-coal-rock combined body, a specimen test identifier RMR-A-B is applied. The first R refers to the roof rock, M refers to the middle weak intercalation coal, and the second R represents the floor rock. A defines the confining pressure in MPa and B represents the test number under the same confining pressure. For example, RMR-5-1 indicates the first test of a rock-coal-rock combined body using a confining pressure of 5 MPa and a loading rate of 3 MPa/min. The displacement loading mode was applied in all RMR experiments. First, a loading rate of 0.06 mm/min is applied until failure of the specimen occurs. Then, after failure of the specimen, displacement control mode is applied using a loading rate of 0.1 mm/min. The stress value used is the overall stress of the rockcoal-rock combined body and the axial strain is the deformation of the whole coal, half of roof rock and half of the floor rock. A lateral strain measuring device was installed on the coal body, as the strain is mainly the lateral strain in the coal. Uniaxial compression experiments for three groups of RMR specimen were carried out. For specimen RMR-0-1, the initial load was 0.15 kN, and the maximum peak load of the combination was 33.69 kN. As the rock was being crushed, noise was emitted from the specimen accompanied by the formation of rock fragments. Finally, the rock failed by crushing of the bottom end of the sample, with the formation of two connected axial cracks. Though the coal was soft and broken, the energy of the high speed propagating crack was large enough to destroy the hard rock. After all the coal was crushed, rock failure occurred from the bottom section, as shown in Fig. 3a. Two cracks are found in the upper section of rock, which were caused by the two end faces not being flat. Vertical cracks were observed in the failed coal, together with several horizontal cracks in which crack bifurcation phenomena were observed. Additionally, four larger, approximately parallel, cracks appeared at an angle of 60° to the horizontal. For specimen RMR-0-2, the initial load was 0.3 kN. With increasing load, micro cracks first appeared in the middle of the weak intercalation. When the load reached 41.26 kN, the rockcoal-rock combined body suddenly failed with many micro cracks evident in the coal body, as shown in Fig. 3b. Similarly, the highspeed propagation of micro cracks in the coal led to roof rock fracture. The brittle nature of this specimen is apparent when compared with specimen RMR-0-1. After peak load had occurred, the load rapidly dropped accompanied by noise. Exfoliation occurred in both rock and coal. For specimen RMR-0-3, the initial load was 0.16 kN, and the maximum peak load of the combination was about 38.53 kN. The central coal section failed massively and dilatancy in the coal was highly evident. There were also many small coal fragments produced, as shown in Fig. 3c.
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Fig. 3. Stress–strain curves and failure images of coal-rock combined body under uniaxial load.
As shown in Fig. 4, the stress-strain curves for specimen RMR-0 under uniaxial load, show clearly that a compressive process is taking place during initial loading resulting in deformation of the central weak coal. With increasing load, the stress-strain curve transforms into a linear elastic phase. When the load reaches 70– 90% of the peak load, nonlinear behavior is seen on the stressstrain curve because of micro fractures or macro fractures inside the coal. After reaching peak load, the coal-rock combined body suffered complete failure. Lateral deformation was far greater than axial deformation, and coal dilatancy was also evident. Mixed cracks occurred mainly in the coal. Under uniaxial load, hard rock failure due to high speed crack propagation of the coal and coal burst due to mixed crack development, are likely to be the main mechanisms of coal-rock burst after excavation, which coincides with experience from mining engineering.
5. Triaxial failure experiments on a rock-coal-rock combined body containing a weak coal layer In order to study the failure behavior of RMR specimens under different depth conditions, confining pressures of 5, 10, 15 and 20 MPa were respectively applied to the combination of coal and rock specimens.
5.1. Experiments under 5 MPa confining pressure
5.2. Experiments under 10 MPa confining pressure For the RMR-10-3 specimen, peak load was 56.69 kN. As shown in Fig. 7, lateral deformation occurred in the coal which fractured at a roughly 20° angle to the vertical axis. In addition, several micro cracks appeared in the coal besides the obvious large crack. Basically, rock failure was not observed. The group of experiments indicated that the peak strengths were greater than uniaxial strengths, and showed some evidence of ductility damage. Due to the restraining effect of confining pressure, the crack changed from a mixed crack to a parallel crack at an angle to the vertical direction. Experiments indicated that the failure mechanism would turn from brittle failure to shear failure with an increase in confining pressure, as shown in Fig. 8. 5.3. Experiments under 15 MPa confining pressure For the RMR-15-1 specimen, peak load was 101.19 kN, and the residual load was 68.21 kN. Damage occurred mainly in the coal body which showed a small amount of dilatancy. Compared with the experiments carried out at low confining pressure, the dilatancy was clearly reduced. Local damage occurred on the interface between the coal body and the rock body, as shown in Fig. 9. In the case of specimen RMR-15-2, since there were original microcracks distributed randomly in the coal body, the peak load at failure was low-approximately 49.2 kN. Damage also occurred
Stress
(MPa)
For the RMR-5-1 specimen, the peak load was 62.09 kN, and the residual load was 31.4 kN. Two vertical cracks were observed in the floor rock, and were interconnected at the base. Many micro cracks appeared in the coal at the same time. This indicates that rock failure is affected by micro crack development and propagation. In this case, high speed crack propagation in coal also caused failure of the hard rock, which is similar in behavior to combined body failure under uniaxial load. Due to the effects of confining pressure, both
peak values of stress and corresponding strain values increased (Fig. 5). From the stress-strain curves under 5 MPa, as shown in Fig. 6, the failure of the specimen was still predominantly brittle failure, as. However, compared with the uniaxial experiments, micro fracturing decreased, the distribution of micro cracks having a similar distribution to the approximately parallel vertical cracks orientated at an angle of 0–10° with the vertical axis. Few mixed cracks appeared.
Fig. 4. Stress–strain curves of RMR-0 under uniaxial load.
Fig. 5. Stress–strain curve and failure image of RMR-5-1.
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Fig. 6. Stress–strain curves under confining pressure 5 MPa.
Fig. 10. Stress–strain curve and failure image of RMR-15-2.
From the stress-strain curves for the specimen, the failure mode displayed typical ductility characteristics. Due to the restraining effects of a high confining pressure, the number of cracks in the coal is significantly reduced, with few parallel cracks, highlighting the overall instability of coal and rock, as shown in Fig. 11. 5.4. Experiments under 20 MPa confining pressure
Fig. 7. Stress–strain curve and failure image of RMR-10-3.
Fig. 8. Stress–strain curves under confining pressure 10 MPa.
For specimen RMR-20-1, the lower section of the specimen was very non-homogeneous. The peak load was 70.39 kN, and the residual load was 60 kN. Damage occurred mainly in the coal, which divided into two parts. In the coal, a vertical crack could be observed accompanied by several small inclined cracks, as shown in Fig. 12. For the RMR-20-3 specimen, the peak load was 72.86 kN, and the residual load was 46.6 kN. The specimen emitted loud noises on two occasions, corresponding with the two sharp declines seen in the stress-strain curve. A 45° shear crack developed in the coal, which led to the formation of an approximately parallel crack in the rock, inclined at 45°. The lower section of the rock also displayed a triangular destruction belt, as shown in Fig. 13. From the stress-strain curves produced in the whole failure process of the specimens, a high degree of ductility was also evident. One failure mechanism in coal and rock was, in part, overall buckling failure resulting from the use of high confining pressures; another mechanism was shear failure, also resulting from the high confining pressures used, as shown in Fig. 14. 6. Failure analysis of coal-rock combination with a weak coal interlayer Based on all the RMR uniaxial and triaxial failure experiments, the physical and mechanical parameters of the specimens were derived, as shown in Table 1. Wave velocity tests on coal and rock are used mainly for the evaluation of the integrity of the coal-rock combined body. Generally, the lower the wave velocity, the greater
Fig. 9. Stress–strain curve and failure image of RMR-15-1.
mainly in the coal body. There were also some parallel inclined cracks and a large crack through the entire coal body. However, the specimen showed high ductility under the high confining pressure, as shown in Fig. 10.
Fig. 11. Stress–strain curves under 15 MPa confining pressure.
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Fig. 12. Stress–strain curves and failure image of RMR-20-1.
Fig. 13. Stress–strain curve and failure image of RMR-20-3.
Fig. 14. Stress–strain curves under 20 MPa confining pressure.
the number of internal fractures in the coal or rock body. Conversely, the higher the wave velocity, the fewer the number of internal cracks. With increasing confining pressure, the number of internal cracks in the combined body reduces. This is mainly because the confining pressure restrains crack production. With
Fig. 15. Peak strengths of coal, rock, and coal-rock combined body.
increasing confining pressure, the overall elastic modulus of the coal-rock combined body also has a tendency to increase. As the confining pressure increases from 0 to 10 MPa, the elastic modulus increases rapidly. However, as the confining pressure increases from 10 to 20 MPa, the elastic modulus increases more slowly. Poisson’s ratio for the RMR combined body does not change significantly with increasing confining pressure, and is generally about 0.45. The failure strength of the RMR combined body increases slowly with increasing confining pressure. When the confining pressure was increased from 0 to 10 MPa, the failure strength rapidly increased, whilst from 10 to 20 MPa, the failure strength increased slowly. The main reason for this behavior is the low failure strength of the coal body; at high confining pressure, the overall failure of the RMR combined body is caused by failure of the coal body. However, at low confining pressure, the failure mechanism of the RMR combined body is structural failure. We also analyzed the peak failure strength of the coal and rock samples described in reference, for comparison with the tests on the rock-coal-rock combined body with a weak coal interlayer, with damage behavior in coal and rock, and with their strength characteristics [9]. These comparisons are illustrated in Fig. 15. It can be seen that the failure strength of the combined body is greater than that of coal, but less than that of rock. Experimental results from uniaxial and triaxial testing of the RMR combined body indicate that brittle failure occurred frequently in coal due to its low strength properties, which thereby affects the stability of roadways. The failure mode was also influenced by the confining pressure.
7. Conclusions Using the MTS 815 testing system, the deformation and failure behavior of a rock-coal-rock combined body containing a weak
Table 1 Physical and mechanical parameters of RMR combined body with a weak coal interlayer under different confining pressures. Specimen code
Wave velocity V (m/s)
Elasticity modulus E (GPa)
Poisson’s ratio l
Peak strength rc (MPa)
RMR-0-1 RMR-0-2 RMR-0-3 RMR-5-1 RMR-5-2 RMR-5-3 RMR-10-1 RMR-10-2 RMR-10-3 RMR-15-1 RMR-15-2 RMR-20-1 RMR-20-2 RMR-20-3
1812.9687 901.7530 1373.7589 1327.8677 1298.0021 1321.9590 1409.5094 1375.0605 598.9874 715.1626 884.2235
7.724866 5.795695 9.962877 9.330667 8.206638 9.873470 10.22807 11.17452 10.67823 11.86291 8.013938 10.63099 12.39705 11.52835
0.415305 0.486088 0.458710 0.407667 0.427589 0.476940 0.427756 0.409306 0.499431 0.439825 0.474579 0.477872 0.457807 0.457150
35.90809 43.67907 40.54892 72.43493 69.48120 50.74946 86.70815 93.64627 60.21350 106.1981 51.42236 74.47479 105.2938 77.56996
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coal interlayer was experimentally studied and described in this paper. The following conclusions can be drawn: (1) The results of tests on RMR combined body with a weak coal interlayer subjected to uniaxial load indicate that uniaxial loading leads to mixed cracking in coal, which leads to instability of the coal. This is the main mechanism controlling coal bursts. If the energy of high speed crack propagation is sufficient to destroy roof rock, then further rock and coal bursts may occur. (2) With increasing confining pressure, the failure strength of an RMR combined body has a tendency to increase. The failure mechanism of the coal interlayer changes from a mixed crack condition under low confining pressures, to a parallel crack condition under medium confining pressures and to a single shear crack condition or integral cross section damage under high confining pressures. (3) In general, the existence of the weak coal interlayer changes the overall failure mechanism and strength in an RMR combined body, thereby reducing the overall stability of a coal roadway. Furthermore, it should be noted that the deformation and failure of an RMR combined body has an important influence on the design and excavation of coal roadways in large cutting height coalfaces. Acknowledgments The work was supported by the Special Funds for Major State Basic Research Project (Nos. 2011CB201201 and 2010CB732002), the National Natural Science Foundation of China (Nos. 11102225 and 51374215) and the National Excellent Doctoral Dissertation of China (No. 201030).
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