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Deformation control of asymmetric floor heave in a deep rock roadway: A case study
International Journal of Mining Science and Technology 24 (2014) 799–804
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Deformation control of asymmetric floor heave in a deep rock roadway: A case study Sun Xiaoming a,b,⇑, Wang Dong a,b, Feng Jili a,b, Zhang Chun a,b, Chen Yanwei a,b a b
State Key Laboratory of Geomechanics and Deep Underground Engineering, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining & Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 25 February 2014 Received in revised form 26 April 2014 Accepted 17 June 2014 Available online 14 November 2014 Keywords: Deep rock roadway Asymmetric floor heave Numerical simulation Asymmetric reinforced support
a b s t r a c t In order to control asymmetric floor heave in deep rock roadways and deformation around the surrounding rock mass after excavation, in this paper we discuss the failure mechanism and coupling control countermeasures using the finite difference method (FLAC3D) combined with comparative analysis and typical engineering application at Xingcun coal mine. It is indicated by the analysis that the simple symmetric support systems used in the past led to destruction of the deep rock roadway from the key zone and resulted in the deformation of asymmetric floor heave in the roadway. Suitable reinforced support countermeasures are proposed to reduce the deformation of the floor heave and the potential risk during mining. The application shows that the present support technology can be used to better environmental conditions. The countermeasures of asymmetric coupling support can not only effectively reduce the discrepancy deformation at the key area of the surrounding rock mass, but also effectively control floor heave, which helps realize the integration of support and maintain the stability of the deep rock roadways at Xingcun coal mine. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction There is considerable interest in stability control of deep soft rock roadways with increasing mining depth in a deep complex mechanical environment [1–6]. By entering the stage of large nonlinear plastic deformation, the failure mechanism of soft rock roadways is significantly different from that of hard rock roadways. For example, the surrounding rock mass of deep inclined rock roadways exhibits the characteristics of prone-asymmetric deformation. This unfortunately makes the support technology available for shallow roadway to be unsuitable for deep soft rock roadways [3,7–9]. It is consequently required, from theoretical and experimental standpoints, to provide alternative design rules for the deformation control of deep soft rock roadway using nonlinear continuum mechanics after analyzing in situ underground stress field and strata conditions [10–13]. The goal is to efficiently and reasonably control such asymmetric deformation, which often appears in deep soft rock roadways, in order to accommodate safe mining and ensure the stability of surrounding rock masses [14–16]. In this study, under the specific engineering geological conditions of an air-return rock roadway at the 1186 m level at Xingcun coal mine, stability control of the surrounding rock of the inclined ⇑ Corresponding author. Tel.: +86 13801166205. E-mail address: [email protected] (X. Sun).
rock roadway was investigated using the finite difference technique (FLAC3D). Suitable reinforced support countermeasures were proposed to reduce the deformation of the floor heave to eliminate potential risk during mining. The applications showed that the proposed support technology can be used to improve control of the stability of deep soft rock roadway under complex mechanical and environmental conditions. 2. Failure characteristics of the old support system 2.1. Project overview Xingcun coal mine is in the Shandong southwest rift tectonic zone, in which the air-return roadway is at the 1186 m level in the region. The length of experimental roadway is 100 m, which in turn passes through the aluminous mudstone of Shihezi Formation, hoary sandstone, and gray black sandstone of the Shanxi Formation. The dip angle of the strata is about 10°. The roadway passes through three faults including F57-2, F57, and DF22. From the viewpoint of lithological information, the geological structure at this coal mine is rather complicated. The old support system used at the coal mine lead to asymmetric heave with large deformation of the floor, which seriously influences the routine work of ventilation and pedestrian travel in the mining zone.
http://dx.doi.org/10.1016/j.ijmst.2014.10.011 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
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X. Sun et al. / International Journal of Mining Science and Technology 24 (2014) 799–804
2.2. Failure characteristics of the old support system
numerical model are given in Table 1. The mechanical response was calculated using the finite difference method (FLAC3D).
The lithological formation, structure and mechanical properties of the surrounding rock mass were not fully considered in the application of the old support scheme which was simply to adopt a symmetric support form of bolt-net-spray without any measures for the floor of the roadway, which resulted in asymmetric heave with large deformation of the floor. From the in situ investigation, failure deformation of the air-return roadway at the 1186 m level had the following characteristics under the old support scheme: (1) Serious asymmetric failure with the characteristics of heave appeared in the partial floor of the roadway. Deformation of the roadway was anisotropic in form. In the Jurassic aluminous mudstone significant asymmetric heave appeared in the floor with a maximum deformation of 1000 mm with large inclination, which seriously impacted upon the use of the roadway. (2) Serious asymmetric subsidence appeared in the partial roof of the roadway. In the aluminous mudstone, it was found that there were a series of failure phenomena such as asymmetric large downward deflection in the roadway, shear rupture of the shotcrete layer in the vault, distortion of steel net, and fracture failure of some rock bolts. (3) Extruded plastic deformation occurred locally in the spandrel and side walls of the roadway.
3.2. Parameters of the old support system The cross-section of the roadway consists of straight walls and a semicircular arch, with an arch radius of 2100 mm and straight wall length of 1600 mm. The bolt-net-spray symmetric support form was used at the start of the excavation. The support parameters used are as follows: (1) Bolt: Rebar bolt was used with a diameter of 20 mm, length of 2000 mm, and row spacing of 800 mm 800 mm, and arranged in a radial pattern. End anchorage was adopted as the fastened form of the bolt. Each hole was applied by resin anchoring of MSCK2535 and MSK2550. (2) Welded steel mesh: Welded steel was used with a diameter of 6.0 mm, grid size of 100 mm 100 mm, and hook connection. (3) Shotcrete: the thickness of the first spraying of concrete was 50 mm, and the thickness of the second spraying was 100 mm, in which the compressive strength of concrete was 20 MPa. The old reinforced support is as shown in Fig. 2. 3.3. Analysis of the numerical simulation
3. Failure mechanism of the old support system 3.1. Numerical model
40
From the engineering geological conditions and in-situ investigations, the size of the numerical model was set as length width height = 100 m 40 m 40 m. In order to simulate the initial underground stress due to the overly rock mass on the model, uniformly distributed forces of 31.6 MPa, 31 MPa, and 45 MPa along the directions of weight load, horizontal load, and roadway respectively, were applied on the corresponding surfaces of the model, except for the base which is fixed. The materials and mesh of the engineering geological model are shown in Fig. 1. The mechanical parameters of the rock masses used in the present
In the case of the old reinforced support system, after extraction for 100 m, the maximum compressive stress zone on the cross-section transfers continuously to the inner part of the floor rock mass, especially to the left side of the floor (see in Fig. 3a). Shear stress continues to develop in the deep surrounding rock mass and the area of stress concentration is further enlarged (see in Fig. 3b). Major deformation of the roadway concentrates over the floor, especially at the left part of the floor shown in Fig. 3c. From the distribution of the plastic area (Fig. 3d), it is readily observed that the plastic zones on the left corner and right side of the roadway are clearly enhanced. It is also shown from Fig. 3 that, even in the case of the old reinforced symmetric support system, stress response of the surrounding rock mass appears to emerge as an asymmetric distribution in the surrounding rock mass of the roadway.
10 0
40 Fig. 1. Materials and mesh of engineering geological model.
Fig. 2. Old reinforced support.
Table 1 Mechanical parameters of rock masses. Rock
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesive strength (MPa)
Tensile strength (MPa)
Aluminous mudstone Hoary sandstone Gray-black sandstone Sprayed concrete
2.083 5.170 5.017 1.125
1.483 3.583 4.417 1.100
20.0 21.0 26.0 22.5
0.175 0.250 0.410 0.200
4.35 6.95 8.41 0.15
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-4.9676e+007 to -4.5000e+007 -4.5000e+007 to -4.0000e+007 -4.0000e+007 to -3.5000e+007 -3.5000e+007 to -3.0000e+007 -3.0000e+007 to -2.5000e+007 -2.5000e+007 to -2.0000e+007 -2.0000e+007 to -1.5000e+007 -1.5000e+007 to -1.0000e+007 -1.0000e+007 to -5.0000e+006 -5.0000e+006 to -5.0564e+005
Interval = 5.0e+006
(a) Distribution of maximum compressive stress
-1.3106e+007 to -1.2500e+007 -1.2500e+007 to -1.0000e+007 -1.0000e+007 to -7.5000e+006 -7.5000e+006 to -5.0000e+006 -5.0000e+006 to -2.5000e+006 -2.5000e+006 to 0.0000e+000 0.0000e+000 to 2.5000e+006 2.5000e+006 to 5.0000e+006 5.0000e+006 to 7.5000e+006 7.5000e+006 to 1.0000e+007 1.0000e+007 to 1.2500e+007 1.2500e+007 to 1.3607e+007
(b) Distribution of shear stress distribution
1.8326e-003 to 5.0000e-002 5.0000e-002 to 1.0000e-001 1.0000e-001 to 1.5000e-001 1.5000e-001 to 2.0000e-001 2.0000e-001 to 2.5000e-001 2.5000e-001 to 3.0000e-001 3.0000e-001 to 3.5000e-001 3.5000e-001 to 3.5363e-001 Interval = 5.0e-002
(c) Displacement distribution of the surrounding rock
None Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p
(d) Plastic area distribution
Fig. 3. Simulation results of the cross-section after excavation for 100 m in the old reinforced support by FLAC3D.
3.4. Failure mechanism of the old reinforced support system Combining the results of numerical simulation with the in situ geological conditions and the rock mass structure, the failure mechanism of asymmetric large deformation upon the deep inclined roadway at Xingcun coal mine can be explained as follows: (1) High underground stress: the roadway is located at the 1186 m level and has relatively higher underground stress. Its depth has reached the critical value for a soft rock roadway, which shows that the roadway has been at the stage of nonlinear large deformation [17]. (2) Structure of the surrounding rock masses: the strata are inclined and the surrounding rock mass have a weak structure with feature of strong expansibility due to the many swelling minerals contained in them. (3) Support measures: the old reinforced support is traditional symmetric support, where the rock occurrence, structure and mechanical properties of the surrounding rock mass are not carefully considered in the process of support design. Also, the floor of the roadway is not reinforced. 4. Support countermeasures against asymmetric deformation 4.1. Mechanism of asymmetric deformation control Due to the large plastic deformation of the surrounding rock mass of the roadway, the deformation area between the reinforced support system and the surrounding rock mass fails first, which finally results in complete failure of the reinforced support system. Accordingly, stability control of the key zone in the roadway should be effected using the coupling reinforced support technology. In other words, based on the technology of bolt-net-spraying support, anchor cable and floor bolt combined together are used to reinforce the strength of the key zone, to accommodate and adjust the deformability between the support system and the surrounding rock mass. This will eventually exploit the advantages of the
bearing capacity of the combined support system to the full. As a result, combining the application of integrated support and homogenization of the stress state can be used to maintain the stability of the roadway [18–20]. Accordingly, the asymmetric support countermeasure of bolt-net-spray + anchor cable + floor bolt is proposed to improve and enhance the stability of the roadway at Xincun coal mine. The specific rules of the present support system are as follows: (1) Designed bolts should meet the requirements for the reserved space of the pallet (wooden pallet and steel pallet), so as to take sufficient advantage of the bearing capacity of the surrounding rock mass, fully release the strain energy stored in the high stress zone and the swelling strain energy, and at the same time reduce the stress concentration in the surrounding rock mass. (2) Coupling the support system using bolts and high rigidity steel net should enable the integrity of the support system to be enhanced and the strength of surrounding rock mass to be increased. The use of anchor cable support should reinforce the key zone, mobilize the strength of the deep surrounding rock mass, and reduce the stress concentration in the floor which is induced by the vertical stress in the roof. (3) Floor bolts with high rigidity should be used to reinforce the key area in the base, in order to prevent plastic sliding in floor heave and effectively reduce the deformation in floor heave. 4.2. Parameters of the asymmetric supportsystem The asymmetric coupling support countermeasure of blotnet-spray + cable + floor bolt was implemented at Xincun coal mine and was also calculated using the finite difference method (FLAC3D). The geometrical profile of the coupling support system is shown in Fig. 4. The parameters used in the calculation are the same as in the numerical simulation of the old support in Section 3.1.
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the inner section and 3 sticks of resin anchoring of MSK2550 in the outer section. When cable was installed whilst keeping up with the working face, the preload was 10 t. However, when cable was installed after the working face, the preload was 12 t. (4) Pallet: Bolt pallet was a compound pallet including a wooden pallet (150 mm 150 mm 10 mm) and a steel pallet (120 mm 120 mm 10 mm). The cable pallet was made of #20 channel steel and steel plate. Bolts installed in the corner of the side wall were used in a specially shaped pallet so as to ensure that one side was close to the surface of the rock wall and the other side perpendicular to the bolt. (5) Steel net: Welded steel was used with a diameter of 6.0 mm and a grid size of 100 mm 100 mm. The specification of the steel net was determined by the actual cross-section of the roadway. The overlapping length of net edge was 100 mm. 4.3. Numerical results of the asymmetric coupling support system Fig. 4. Geometrical profile of the asymmetric coupling support.
The support parameters used in the alternative support system are as follows: (1) Bolts in roof and side walls: Bolt called KMG500 was used with a diameter of 20 mm, length of 2400 mm, and row spacing of 800 mm 800 mm, and arranged in a radial pattern. (2) Floor bolts: Seam pipe bolt was adopted with a diameter of 43 mm, length of 2000 mm, and row spacing of 500 mm 800 mm. The pipe bolt was inserted by rebar with a diameter of 16 mm, length of 1800 mm. Cement paste was also injected into the inner section of the floor bolt. (3) Anchor cable: Cable was made of steel strands with the type of SK18/6.0–1700Q. The row spacing of the cable was 2400(1600) mm 2400 mm, with a parallel arrangement. When the roof was broken, roof support should be reinforced by adjusting the row spacing. Each cable was equipped with 1 stick of resin anchoring of MSCK2535 in
-6.8724e+007 to -6.0000e+007 -6.0000e+007 to -5.0000e+007 -5.0000e+007 to -4.0000e+007 -4.0000e+007 to -3.0000e+007 -3.0000e+007 to -2.0000e+007 -2.0000e+007 to -1.0000e+007 -1.0000e+007 to -4.7827e+005 Interval = 1.0e+007
(a) Distribution of maximum compressive stress
0.0000e+000 to 5.0000e-003 5.0000e-003 to 1.0000e-002 1.0000e-002 to 1.5000e-002 1.5000e-002 to 2.0000e-002 2.0000e-002 to 2.5000e-002 2.5000e-002 to 3.0000e-002 3.0000e-002 to 3.3612e-002 Interval = 5.0e-003
(c) Displacement distribution of surrounding rock
Under the new asymmetric coupling support system, after excavation for 100 m, the concentration areas of maximum compressive stress on the left and right floor are smaller and more symmetric (Fig. 5a), compared with the results of the corresponding old support (Fig. 3a). Concentration areas of shear stress (Fig. 5b) shrink, compared with the old support (Fig. 3b). The deformation in floor heave is clearly reduced and appears as approximately symmetric floor heave (Fig. 5c). The biggest value of floor heave is about 10 mm. From the plastic area distribution (Fig. 5d), it is observed that the plastic zone decreases markedly and is symmetric. It is also found from the numerical results that, under the asymmetric coupling support forms, the stress distribution in the surrounding rock mass is also symmetric, which results in symmetric deformation of the surrounding rock mass of the roadway. 5. Application of the present support system Compared with the displacements of in situ measured points located at the area of sandstone and mudstone (Fig. 6), it is found
-2.2710e+007 to -2.0000e+007 -2.0000e+007 to -1.5000e+007 -1.5000e+007 to -1.0000e+007 -1.0000e+007 to -5.0000e+006 -5.0000e+006 to 0.0000e+000 0.0000e+000 to 5.0000e+006 5.0000e+006 to 1.0000e+007 1.0000e+007 to 1.5000e+007 1.5000e+007 to 2.0000e+007 2.0000e+007 to 2.1788e+007 Interval = 5.0e+006
(b) Distribution of shear stress distribution
None Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p
(d) Plastic area distribution
Fig. 5. Results of the cross-section after excavation for 100 m in the asymmetric coupling support by FLAC3D.
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X. Sun et al. / International Journal of Mining Science and Technology 24 (2014) 799–804
40
Relative larger rate of displacement during this period Two walls shrinking
70
Displacement (mm)
Displacement (mm)
20
Rate of displacement reducing during this period Rate of displacement approaching null after 60 days
10 Floor heave
0
50
80
120 160 Time (day)
200
Floor heave
60
Roof downward deflection
30
Relative larger rate of displacement during this period
50 Rate of displacement reducing during this period
40
Roof downward deflection
30
Two walls shrinking
20
Rate of displacement approaching null after 60 days
10 240
50
0
100
150
200
Time (day)
(a) Displacement history on the monitoring points on the area of sandstone
(b) Displacement history on the monitoring points on the area of mudstone
at the roadway after using the coupling support
at the roadway after using the coupling support
Fig. 6. In situ monitoring data of displacement at the roadway after using the coupling support.
that, under asymmetric coupling supports, the deformation process in the surrounding rock mass can be divided into three stages. In the first stage, deformation is accelerated, which takes place mainly within 20 days after the completion of support. The main functions of the support system are not only to maintain the strength of the surrounding rock mass, but also to allow yielding and deformation to release the swelling strain energy and to effectively reduce support load. In the second stage, deformation slowly progresses, and mainly takes place from 20 to 60 days after placement of support. In this phase, excessive release of strain energy will greatly reduce the strength of the surrounding rock mass, which also makes the roadway lose its support function. Accordingly, after allowing some deformation of the surrounding rock mass, the support has to be reinforced to control the deformation rate of the surrounding rock mass and to maintain the strength of the surrounding rock mass so that the bearing capacity of the surrounding rock mass and the supported body can achieve the best state. Thus the active support role of the bolt-net-cable + floor bolt can be made full use of in this stage to control the deformation of the surrounding rock mass. In the third stage, stable deformation is achieved, i.e., displacement approaches a constant value, which mainly takes place about 60 days after support placement. In this stage, the surrounding rock mass of the roadway is almost in a stable state, which means that the support system and surrounding rock mass have reached the coupling state and act together as a support function for the roadway. As can be seen from Fig. 6, the deformation of floor heave in the asymmetric coupling support system is much smaller than that under the old support system. 6. Conclusions We have studied the failure mechanism of a rock roadway at Xincun coal mine using the finite difference method. The asymmetric coupling control countermeasure of asymmetric floor heave of the rock roadway has been proposed, combining numerical analysis with in-situ investigation. The following conclusions can be summarized: (1) Under high underground stresses, the phenomenon of asymmetric floor heave appeared in the deep rock roadway with the swelling soft rock at the 1186 m level at Xingcun coal mine. Simply symmetric support technology led to the failure of key area in the roadway and also resulted in asymmetric large deformation.
(2) The obtuse angle area between the cross-section of the roadway and the inclined direction of strata failed first in the deep rock roadway at the 1186 m level so that stability control of the roadway can be achieved by exploiting the coupling support technology to accommodate and adjust the deformability between the support system and its surrounding rock mass. (3) Typical application shows that the technology of boltnet-spray + anchor cable + floor bolt asymmetric coupling support can significantly reduce deformation in the key area of the surrounding rock mass of the roadway, which results in deformation of the roadway being more homogeneous and also undoubtedly enhances the stability of the roadway.
Acknowledgments The support from the National Natural Science Foundation of China (Nos. 51134005, 51374214, 41172116, and U1261212), the New Century Excellent Talents Foundation in University (No. NCET-07-0800), the Special Fund of Basic Research and Operating of China University of Mining & Technology in Beijing (No. 2009QL03), are all gratefully acknowledged.
References [1] Xie HP, Peng SP, He MC. Basic theory and engineering practice in deep mining. Beijing: Science Press; 2006. [2] He MC, Sun XM. Support design and construction guide for roadway within soft rock in China. Beijing: Science Press; 2004. [3] He MC, Xie HP, Peng SP, Jiang YD. Study on rock mechanics in deep mining engineering. Chin J Rock Mech Eng 2005;24(16):2803–13. [4] He MC, Jing HH, Sun XM. Soft rock engineering mechanics. Beijing: Science Press; 2004. [5] He MC. Present situation and prospect of rock mechanics in deep mining engineering. In: Proceedings of conference on the 8th rock mechanics and engineering. Beijing: Science Press; 2004. p. 88–94. [6] He MC, Qian QH. Summary of basic research on rock mechanics at great depth. In: Proceedings of the 9th rock mechanics and engineering conference. Beijing: Science Press; 2006. p. 49–62. [7] He YN, Han LJ, Shao P, Jiang BS. Some problems of rock mechanics for roadways stability in depth. J China Univ Min Technol 2006;35(3):288–95. [8] Sun XM, He MC, Yang XJ. Research on non-linear mechanics design method of bolt-net-anchor coupling support for deep soft rock tunnel. Rock Soil Mech 2006;27(7):1061–5. [9] Wang WJ, Hou CJ. New development of mechanism and control technique study of extraction opening floor heave. J Xiangtan Min Instit 2003;18(1):1–6. [10] He MC, Yang XJ, Sun XM. Clay mineral characteristics of soft rock in Chinese coal mine. Beijing: Coal Industry Press; 2006.
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[11] He MC, Qi G, Cheng C. Deformation and damage mechanisms and coupling support design in deep coal roadway with compound roof. Chin J Rock Mech Eng 2007;26(5):987–93. [12] Sun XM, He MC. Numerical simulation research on coupling support theory of roadway in deep soft rock. J China Univ Min Technol 2005;34(2):166–9. [13] He MC, Li GG, Wang J, Cai J. Study on supporting design for large area serious roof caving of deep soft rock roadway in Xing’an Coal Mine. Chin J Rock Mech Eng 2007;26(5):959–64. [14] Sun XM, Zhang GF, Cai F. Asymmetric deformation mechanism within inclined rock strata induced by excavation in deep roadway and its controlling countermeasures. Chin J Rock Mech Eng 2009;28(6):1137–43. [15] Sun XM, Cai F, Yang J, Cao WF. Numerical simulation of the effect of coupling support of bolt-mesh-anchor in deep tunnel. Min Sci Technol 2009;19(3):352–7.
[16] Wang J, Guo ZB, Yan YB, Pang JW, Zhao SJ. Floor heave in the west wing track haulage roadway of the Tingnan coal mine mechanism and control. Int J Min Sci Technol 2012;22(3):295–9. [17] He MC. Conception system of deep and evaluation index for deep engineering. Chin J Rock Mech Eng 2005;24(16):2854–8. [18] Qi G, Li ZJ, Guo ZB. Coupling support design for deep pit bottom in No. 5 coal mine of Hebi industry group and its application. J Min Saf Eng 2011;28(4):553–9. [19] He MC, Lv XJ, Jing HH. The characteristics of the rock mass in deep engineering and the design concept of nonlinear dynamic mechanics. Chin J Rock Mech Eng 2002;21(8):1215–24. [20] He MC, Yang J, Qi G. Optimized design and its application of coupling support for soft rock roadway at great depth. J Liaoning Tech Univ 2007;2:40–3.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Deformation control of asymmetric floor heave in a deep rock roadway: A case study Sun Xiaoming a,b,⇑, Wang Dong a,b, Feng Jili a,b, Zhang Chun a,b, Chen Yanwei a,b a b
State Key Laboratory of Geomechanics and Deep Underground Engineering, Beijing 100083, China School of Mechanics and Civil Engineering, China University of Mining & Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 25 February 2014 Received in revised form 26 April 2014 Accepted 17 June 2014 Available online 14 November 2014 Keywords: Deep rock roadway Asymmetric floor heave Numerical simulation Asymmetric reinforced support
a b s t r a c t In order to control asymmetric floor heave in deep rock roadways and deformation around the surrounding rock mass after excavation, in this paper we discuss the failure mechanism and coupling control countermeasures using the finite difference method (FLAC3D) combined with comparative analysis and typical engineering application at Xingcun coal mine. It is indicated by the analysis that the simple symmetric support systems used in the past led to destruction of the deep rock roadway from the key zone and resulted in the deformation of asymmetric floor heave in the roadway. Suitable reinforced support countermeasures are proposed to reduce the deformation of the floor heave and the potential risk during mining. The application shows that the present support technology can be used to better environmental conditions. The countermeasures of asymmetric coupling support can not only effectively reduce the discrepancy deformation at the key area of the surrounding rock mass, but also effectively control floor heave, which helps realize the integration of support and maintain the stability of the deep rock roadways at Xingcun coal mine. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction There is considerable interest in stability control of deep soft rock roadways with increasing mining depth in a deep complex mechanical environment [1–6]. By entering the stage of large nonlinear plastic deformation, the failure mechanism of soft rock roadways is significantly different from that of hard rock roadways. For example, the surrounding rock mass of deep inclined rock roadways exhibits the characteristics of prone-asymmetric deformation. This unfortunately makes the support technology available for shallow roadway to be unsuitable for deep soft rock roadways [3,7–9]. It is consequently required, from theoretical and experimental standpoints, to provide alternative design rules for the deformation control of deep soft rock roadway using nonlinear continuum mechanics after analyzing in situ underground stress field and strata conditions [10–13]. The goal is to efficiently and reasonably control such asymmetric deformation, which often appears in deep soft rock roadways, in order to accommodate safe mining and ensure the stability of surrounding rock masses [14–16]. In this study, under the specific engineering geological conditions of an air-return rock roadway at the 1186 m level at Xingcun coal mine, stability control of the surrounding rock of the inclined ⇑ Corresponding author. Tel.: +86 13801166205. E-mail address: [email protected] (X. Sun).
rock roadway was investigated using the finite difference technique (FLAC3D). Suitable reinforced support countermeasures were proposed to reduce the deformation of the floor heave to eliminate potential risk during mining. The applications showed that the proposed support technology can be used to improve control of the stability of deep soft rock roadway under complex mechanical and environmental conditions. 2. Failure characteristics of the old support system 2.1. Project overview Xingcun coal mine is in the Shandong southwest rift tectonic zone, in which the air-return roadway is at the 1186 m level in the region. The length of experimental roadway is 100 m, which in turn passes through the aluminous mudstone of Shihezi Formation, hoary sandstone, and gray black sandstone of the Shanxi Formation. The dip angle of the strata is about 10°. The roadway passes through three faults including F57-2, F57, and DF22. From the viewpoint of lithological information, the geological structure at this coal mine is rather complicated. The old support system used at the coal mine lead to asymmetric heave with large deformation of the floor, which seriously influences the routine work of ventilation and pedestrian travel in the mining zone.
http://dx.doi.org/10.1016/j.ijmst.2014.10.011 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
800
X. Sun et al. / International Journal of Mining Science and Technology 24 (2014) 799–804
2.2. Failure characteristics of the old support system
numerical model are given in Table 1. The mechanical response was calculated using the finite difference method (FLAC3D).
The lithological formation, structure and mechanical properties of the surrounding rock mass were not fully considered in the application of the old support scheme which was simply to adopt a symmetric support form of bolt-net-spray without any measures for the floor of the roadway, which resulted in asymmetric heave with large deformation of the floor. From the in situ investigation, failure deformation of the air-return roadway at the 1186 m level had the following characteristics under the old support scheme: (1) Serious asymmetric failure with the characteristics of heave appeared in the partial floor of the roadway. Deformation of the roadway was anisotropic in form. In the Jurassic aluminous mudstone significant asymmetric heave appeared in the floor with a maximum deformation of 1000 mm with large inclination, which seriously impacted upon the use of the roadway. (2) Serious asymmetric subsidence appeared in the partial roof of the roadway. In the aluminous mudstone, it was found that there were a series of failure phenomena such as asymmetric large downward deflection in the roadway, shear rupture of the shotcrete layer in the vault, distortion of steel net, and fracture failure of some rock bolts. (3) Extruded plastic deformation occurred locally in the spandrel and side walls of the roadway.
3.2. Parameters of the old support system The cross-section of the roadway consists of straight walls and a semicircular arch, with an arch radius of 2100 mm and straight wall length of 1600 mm. The bolt-net-spray symmetric support form was used at the start of the excavation. The support parameters used are as follows: (1) Bolt: Rebar bolt was used with a diameter of 20 mm, length of 2000 mm, and row spacing of 800 mm 800 mm, and arranged in a radial pattern. End anchorage was adopted as the fastened form of the bolt. Each hole was applied by resin anchoring of MSCK2535 and MSK2550. (2) Welded steel mesh: Welded steel was used with a diameter of 6.0 mm, grid size of 100 mm 100 mm, and hook connection. (3) Shotcrete: the thickness of the first spraying of concrete was 50 mm, and the thickness of the second spraying was 100 mm, in which the compressive strength of concrete was 20 MPa. The old reinforced support is as shown in Fig. 2. 3.3. Analysis of the numerical simulation
3. Failure mechanism of the old support system 3.1. Numerical model
40
From the engineering geological conditions and in-situ investigations, the size of the numerical model was set as length width height = 100 m 40 m 40 m. In order to simulate the initial underground stress due to the overly rock mass on the model, uniformly distributed forces of 31.6 MPa, 31 MPa, and 45 MPa along the directions of weight load, horizontal load, and roadway respectively, were applied on the corresponding surfaces of the model, except for the base which is fixed. The materials and mesh of the engineering geological model are shown in Fig. 1. The mechanical parameters of the rock masses used in the present
In the case of the old reinforced support system, after extraction for 100 m, the maximum compressive stress zone on the cross-section transfers continuously to the inner part of the floor rock mass, especially to the left side of the floor (see in Fig. 3a). Shear stress continues to develop in the deep surrounding rock mass and the area of stress concentration is further enlarged (see in Fig. 3b). Major deformation of the roadway concentrates over the floor, especially at the left part of the floor shown in Fig. 3c. From the distribution of the plastic area (Fig. 3d), it is readily observed that the plastic zones on the left corner and right side of the roadway are clearly enhanced. It is also shown from Fig. 3 that, even in the case of the old reinforced symmetric support system, stress response of the surrounding rock mass appears to emerge as an asymmetric distribution in the surrounding rock mass of the roadway.
10 0
40 Fig. 1. Materials and mesh of engineering geological model.
Fig. 2. Old reinforced support.
Table 1 Mechanical parameters of rock masses. Rock
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesive strength (MPa)
Tensile strength (MPa)
Aluminous mudstone Hoary sandstone Gray-black sandstone Sprayed concrete
2.083 5.170 5.017 1.125
1.483 3.583 4.417 1.100
20.0 21.0 26.0 22.5
0.175 0.250 0.410 0.200
4.35 6.95 8.41 0.15
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-4.9676e+007 to -4.5000e+007 -4.5000e+007 to -4.0000e+007 -4.0000e+007 to -3.5000e+007 -3.5000e+007 to -3.0000e+007 -3.0000e+007 to -2.5000e+007 -2.5000e+007 to -2.0000e+007 -2.0000e+007 to -1.5000e+007 -1.5000e+007 to -1.0000e+007 -1.0000e+007 to -5.0000e+006 -5.0000e+006 to -5.0564e+005
Interval = 5.0e+006
(a) Distribution of maximum compressive stress
-1.3106e+007 to -1.2500e+007 -1.2500e+007 to -1.0000e+007 -1.0000e+007 to -7.5000e+006 -7.5000e+006 to -5.0000e+006 -5.0000e+006 to -2.5000e+006 -2.5000e+006 to 0.0000e+000 0.0000e+000 to 2.5000e+006 2.5000e+006 to 5.0000e+006 5.0000e+006 to 7.5000e+006 7.5000e+006 to 1.0000e+007 1.0000e+007 to 1.2500e+007 1.2500e+007 to 1.3607e+007
(b) Distribution of shear stress distribution
1.8326e-003 to 5.0000e-002 5.0000e-002 to 1.0000e-001 1.0000e-001 to 1.5000e-001 1.5000e-001 to 2.0000e-001 2.0000e-001 to 2.5000e-001 2.5000e-001 to 3.0000e-001 3.0000e-001 to 3.5000e-001 3.5000e-001 to 3.5363e-001 Interval = 5.0e-002
(c) Displacement distribution of the surrounding rock
None Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p
(d) Plastic area distribution
Fig. 3. Simulation results of the cross-section after excavation for 100 m in the old reinforced support by FLAC3D.
3.4. Failure mechanism of the old reinforced support system Combining the results of numerical simulation with the in situ geological conditions and the rock mass structure, the failure mechanism of asymmetric large deformation upon the deep inclined roadway at Xingcun coal mine can be explained as follows: (1) High underground stress: the roadway is located at the 1186 m level and has relatively higher underground stress. Its depth has reached the critical value for a soft rock roadway, which shows that the roadway has been at the stage of nonlinear large deformation [17]. (2) Structure of the surrounding rock masses: the strata are inclined and the surrounding rock mass have a weak structure with feature of strong expansibility due to the many swelling minerals contained in them. (3) Support measures: the old reinforced support is traditional symmetric support, where the rock occurrence, structure and mechanical properties of the surrounding rock mass are not carefully considered in the process of support design. Also, the floor of the roadway is not reinforced. 4. Support countermeasures against asymmetric deformation 4.1. Mechanism of asymmetric deformation control Due to the large plastic deformation of the surrounding rock mass of the roadway, the deformation area between the reinforced support system and the surrounding rock mass fails first, which finally results in complete failure of the reinforced support system. Accordingly, stability control of the key zone in the roadway should be effected using the coupling reinforced support technology. In other words, based on the technology of bolt-net-spraying support, anchor cable and floor bolt combined together are used to reinforce the strength of the key zone, to accommodate and adjust the deformability between the support system and the surrounding rock mass. This will eventually exploit the advantages of the
bearing capacity of the combined support system to the full. As a result, combining the application of integrated support and homogenization of the stress state can be used to maintain the stability of the roadway [18–20]. Accordingly, the asymmetric support countermeasure of bolt-net-spray + anchor cable + floor bolt is proposed to improve and enhance the stability of the roadway at Xincun coal mine. The specific rules of the present support system are as follows: (1) Designed bolts should meet the requirements for the reserved space of the pallet (wooden pallet and steel pallet), so as to take sufficient advantage of the bearing capacity of the surrounding rock mass, fully release the strain energy stored in the high stress zone and the swelling strain energy, and at the same time reduce the stress concentration in the surrounding rock mass. (2) Coupling the support system using bolts and high rigidity steel net should enable the integrity of the support system to be enhanced and the strength of surrounding rock mass to be increased. The use of anchor cable support should reinforce the key zone, mobilize the strength of the deep surrounding rock mass, and reduce the stress concentration in the floor which is induced by the vertical stress in the roof. (3) Floor bolts with high rigidity should be used to reinforce the key area in the base, in order to prevent plastic sliding in floor heave and effectively reduce the deformation in floor heave. 4.2. Parameters of the asymmetric supportsystem The asymmetric coupling support countermeasure of blotnet-spray + cable + floor bolt was implemented at Xincun coal mine and was also calculated using the finite difference method (FLAC3D). The geometrical profile of the coupling support system is shown in Fig. 4. The parameters used in the calculation are the same as in the numerical simulation of the old support in Section 3.1.
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the inner section and 3 sticks of resin anchoring of MSK2550 in the outer section. When cable was installed whilst keeping up with the working face, the preload was 10 t. However, when cable was installed after the working face, the preload was 12 t. (4) Pallet: Bolt pallet was a compound pallet including a wooden pallet (150 mm 150 mm 10 mm) and a steel pallet (120 mm 120 mm 10 mm). The cable pallet was made of #20 channel steel and steel plate. Bolts installed in the corner of the side wall were used in a specially shaped pallet so as to ensure that one side was close to the surface of the rock wall and the other side perpendicular to the bolt. (5) Steel net: Welded steel was used with a diameter of 6.0 mm and a grid size of 100 mm 100 mm. The specification of the steel net was determined by the actual cross-section of the roadway. The overlapping length of net edge was 100 mm. 4.3. Numerical results of the asymmetric coupling support system Fig. 4. Geometrical profile of the asymmetric coupling support.
The support parameters used in the alternative support system are as follows: (1) Bolts in roof and side walls: Bolt called KMG500 was used with a diameter of 20 mm, length of 2400 mm, and row spacing of 800 mm 800 mm, and arranged in a radial pattern. (2) Floor bolts: Seam pipe bolt was adopted with a diameter of 43 mm, length of 2000 mm, and row spacing of 500 mm 800 mm. The pipe bolt was inserted by rebar with a diameter of 16 mm, length of 1800 mm. Cement paste was also injected into the inner section of the floor bolt. (3) Anchor cable: Cable was made of steel strands with the type of SK18/6.0–1700Q. The row spacing of the cable was 2400(1600) mm 2400 mm, with a parallel arrangement. When the roof was broken, roof support should be reinforced by adjusting the row spacing. Each cable was equipped with 1 stick of resin anchoring of MSCK2535 in
-6.8724e+007 to -6.0000e+007 -6.0000e+007 to -5.0000e+007 -5.0000e+007 to -4.0000e+007 -4.0000e+007 to -3.0000e+007 -3.0000e+007 to -2.0000e+007 -2.0000e+007 to -1.0000e+007 -1.0000e+007 to -4.7827e+005 Interval = 1.0e+007
(a) Distribution of maximum compressive stress
0.0000e+000 to 5.0000e-003 5.0000e-003 to 1.0000e-002 1.0000e-002 to 1.5000e-002 1.5000e-002 to 2.0000e-002 2.0000e-002 to 2.5000e-002 2.5000e-002 to 3.0000e-002 3.0000e-002 to 3.3612e-002 Interval = 5.0e-003
(c) Displacement distribution of surrounding rock
Under the new asymmetric coupling support system, after excavation for 100 m, the concentration areas of maximum compressive stress on the left and right floor are smaller and more symmetric (Fig. 5a), compared with the results of the corresponding old support (Fig. 3a). Concentration areas of shear stress (Fig. 5b) shrink, compared with the old support (Fig. 3b). The deformation in floor heave is clearly reduced and appears as approximately symmetric floor heave (Fig. 5c). The biggest value of floor heave is about 10 mm. From the plastic area distribution (Fig. 5d), it is observed that the plastic zone decreases markedly and is symmetric. It is also found from the numerical results that, under the asymmetric coupling support forms, the stress distribution in the surrounding rock mass is also symmetric, which results in symmetric deformation of the surrounding rock mass of the roadway. 5. Application of the present support system Compared with the displacements of in situ measured points located at the area of sandstone and mudstone (Fig. 6), it is found
-2.2710e+007 to -2.0000e+007 -2.0000e+007 to -1.5000e+007 -1.5000e+007 to -1.0000e+007 -1.0000e+007 to -5.0000e+006 -5.0000e+006 to 0.0000e+000 0.0000e+000 to 5.0000e+006 5.0000e+006 to 1.0000e+007 1.0000e+007 to 1.5000e+007 1.5000e+007 to 2.0000e+007 2.0000e+007 to 2.1788e+007 Interval = 5.0e+006
(b) Distribution of shear stress distribution
None Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p
(d) Plastic area distribution
Fig. 5. Results of the cross-section after excavation for 100 m in the asymmetric coupling support by FLAC3D.
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40
Relative larger rate of displacement during this period Two walls shrinking
70
Displacement (mm)
Displacement (mm)
20
Rate of displacement reducing during this period Rate of displacement approaching null after 60 days
10 Floor heave
0
50
80
120 160 Time (day)
200
Floor heave
60
Roof downward deflection
30
Relative larger rate of displacement during this period
50 Rate of displacement reducing during this period
40
Roof downward deflection
30
Two walls shrinking
20
Rate of displacement approaching null after 60 days
10 240
50
0
100
150
200
Time (day)
(a) Displacement history on the monitoring points on the area of sandstone
(b) Displacement history on the monitoring points on the area of mudstone
at the roadway after using the coupling support
at the roadway after using the coupling support
Fig. 6. In situ monitoring data of displacement at the roadway after using the coupling support.
that, under asymmetric coupling supports, the deformation process in the surrounding rock mass can be divided into three stages. In the first stage, deformation is accelerated, which takes place mainly within 20 days after the completion of support. The main functions of the support system are not only to maintain the strength of the surrounding rock mass, but also to allow yielding and deformation to release the swelling strain energy and to effectively reduce support load. In the second stage, deformation slowly progresses, and mainly takes place from 20 to 60 days after placement of support. In this phase, excessive release of strain energy will greatly reduce the strength of the surrounding rock mass, which also makes the roadway lose its support function. Accordingly, after allowing some deformation of the surrounding rock mass, the support has to be reinforced to control the deformation rate of the surrounding rock mass and to maintain the strength of the surrounding rock mass so that the bearing capacity of the surrounding rock mass and the supported body can achieve the best state. Thus the active support role of the bolt-net-cable + floor bolt can be made full use of in this stage to control the deformation of the surrounding rock mass. In the third stage, stable deformation is achieved, i.e., displacement approaches a constant value, which mainly takes place about 60 days after support placement. In this stage, the surrounding rock mass of the roadway is almost in a stable state, which means that the support system and surrounding rock mass have reached the coupling state and act together as a support function for the roadway. As can be seen from Fig. 6, the deformation of floor heave in the asymmetric coupling support system is much smaller than that under the old support system. 6. Conclusions We have studied the failure mechanism of a rock roadway at Xincun coal mine using the finite difference method. The asymmetric coupling control countermeasure of asymmetric floor heave of the rock roadway has been proposed, combining numerical analysis with in-situ investigation. The following conclusions can be summarized: (1) Under high underground stresses, the phenomenon of asymmetric floor heave appeared in the deep rock roadway with the swelling soft rock at the 1186 m level at Xingcun coal mine. Simply symmetric support technology led to the failure of key area in the roadway and also resulted in asymmetric large deformation.
(2) The obtuse angle area between the cross-section of the roadway and the inclined direction of strata failed first in the deep rock roadway at the 1186 m level so that stability control of the roadway can be achieved by exploiting the coupling support technology to accommodate and adjust the deformability between the support system and its surrounding rock mass. (3) Typical application shows that the technology of boltnet-spray + anchor cable + floor bolt asymmetric coupling support can significantly reduce deformation in the key area of the surrounding rock mass of the roadway, which results in deformation of the roadway being more homogeneous and also undoubtedly enhances the stability of the roadway.
Acknowledgments The support from the National Natural Science Foundation of China (Nos. 51134005, 51374214, 41172116, and U1261212), the New Century Excellent Talents Foundation in University (No. NCET-07-0800), the Special Fund of Basic Research and Operating of China University of Mining & Technology in Beijing (No. 2009QL03), are all gratefully acknowledged.
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