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Study on fault induced rock bursts
JOURNAL OF CHINA UNIVERSITY OF
MINING & TECHNOLOGY J China Univ Mining & Technol 18 (2008) 0321–0326 www.elsevier.com/locate/jcumt
Study on fault induced rock bursts LI Zhi-hua1,2, DOU Lin-ming1,2, LU Cai-ping1,2, MU Zong-long1,2, CAO An-ye1,2 1
2
School of Mines, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China State Key Laboratory of Coal Resource and Mine Safety, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China
Abstract: In order to study the rules of rock bursts caused by faults by means of mechanical analysis of a roof rock-mass balanced structure and numerical simulation about fault slip destabilization, the effect of coal mining operation on fault plane stresses and slip displacement were studied. The results indicate that the slip displacement sharply increases due to the decrease of normal stress and the increase of shear stress at the fault plane when the working face advances from the footwall to the fault itself, which may induce a fault rock burst. However, this slip displacement will be very small due to the increase of normal stress and the decrease of shear stress when the working face advances from the hanging wall to the fault itself, which results in a very small risk of a fault rock burst. Key words: fault; slip destabilization; rock burst; numerical simulation; normal stress; shear stress
1
Introduction
A rock burst caused by a fault is a phenomenon of energy being violently released due to a fault slip in coal mines[1]. It has become a serious natural disaster, affecting safe extraction of coal when the mining depth increases and geological conditions become more complex[2–3]. For example, a violent fault rock burst happened at the No.6303 working face of the Jining No.3 Coal Mine in China on November 30, 2004. About 30 m of roadway was destroyed when the working face was about 66 m away from the fault. Afterwards, several secondary rock bursts happened in the process of the No.6303 working face advancing to the fault. Up till now, many studies have been carried out on fault rock bursts and the distribution of underground pressure near the fault. Pan et al have explained some phenomena of fault rock bursts by establishing a simple fault rock burst model to analyze the effect of coal mining operation on fault stress[1]. Wang et al have proposed the instability criterion of fault rock bursts based on gradient-dependent plasticity and an energy criterion[4]. Meng et al have studied the distribution of underground pressure near the fault by using numerical simulation and analyzed the effect of faults on the stability of roofs and underground pressure by using simulation with similar material[5–7]. In another in-
stance, two large-scale rock bursts occurring in a nickel copper mine in Sudbury, Canada, in 1984, were caused by a massive rock slide along a major fault line, leading to a mine quake and the collapse of a roof[8]. Given the geological conditions at the No.6303 working face, we have used, numerical simulation and theoretical analysis to study the effect of coal mining operation on fault stresses and slip displacement during the advancement of the working face from the footwall or hanging wall to the fault itself, in order to provide guidelines for forecasting and preventing fault rock bursts.
2 Mechanism and criterion of fault rock burst A rock burst due to a fault is considered a deformation or destabilization phenomenon of the fault and its surrounding rock system. Before the coal is extracted, this system is in a stable equilibrium[1]. The rock mass in a fault zone and its effective range begins to be deformed because of stresses, referring to additional and original shear stresses, after the coal is extracted. The additional shear stress is small when the working face is far away from the fault and the system maintains a state of stable equilibrium. As the working face advances further, the distance from the working face to the fault decreases, leading to an in-
Received 12 December 2007; accepted 15 March 2008 Projects 50490273 and 50474068 supported by the National Natural Science Foundation of China, 2006BAK04B02 and 2006BAK03B06 by the Support Programs of the National Science and Technique During the 11th Five-Year Period and 2005CB221504 by the State Basic Research Program of China Corresponding author. Tel: +86-13585391209; E-mail address: [email protected]
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Journal of China University of Mining & Technology
crease of additional shear stress. At some point, the entire deformation system changes to a state of unstable equilibrium, so that a fault slip destabilization will occur, which in turn may induce a fault rock burst. A disturbance-response stability criterion is applied to study the fault rock burst. We assume that the fault and its surrounding system is in a stable equilibrium, where a is the far-field displacement and u the slip displacement of the fault. If the increment of the far-field displacement is Ƹa in Eq.(1), then the increment of slip displacement is Ƹu. When ε>0 and δ>0 are given, the fault and surrounding system is in a state of stable equilibrium when the equations ∆a ≤δ, ∆u ≤ε are satisfied. No matter how small the far-field displacement, any net increment in slip displacement will eventually become indefinitely large. Given this condition, the expression is
∆u →∞ ∆a
(1)
In this case, the fault and surrounding system is in an unstable state, which will lead to a fault rock burst caused by a rock mass slip along the fault plane. Our simulation test proves that the result of a decrease of normal stress or an increase in shear stress of the fault plane may induce a fault rock burst when coal is extracted.
3
Mechanical analysis of the effect of coal mining on fault plane stress
Given the relation between the dip direction of the fault and the direction of mining, the effect of coal mining on a fault plane stress can be analyzed by the following simple models. The first model, shown in Fig. 1a, is a working face advancing from the footwall to the fault itself (Fault model I). The second model in Fig. 1b, is a working face advancing from the hanging wall to the fault itself (Fault model Ċ) [9–10] . Direction of mining
θ
A O
B
T
O
θ
T
B
R
R T
NT
N R OF
(a) Fault model I
Fig. 1
A
O
R F
θ
(b) Fault model II
Analytical model of coal mining on fault plane stress
When the working face advances from the footwall to the fault itself (Model I), the normal force of the fault plane is N= T cos θ − R sin θ and the shear force of the fault plane F= R cos θ + T sin θ . Then a
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balanced condition can be described as: ⎧ (T cos θ − R sin θ ) tan ϕ ≥ R cos θ + T sin θ ⎪⎪ T sin(ϕ − θ ) ≥ R cos(ϕ − θ ) ⎨ ⎪ R ≤ tan(ϕ − θ ) ⎪⎩ T
(2)
where T is the horizontal force (N). R the vertical force (N), ϕ the inner friction angle of fault plane (°) and θ the angle of the fault plane (°). When the working face advances from the hanging wall to the fault itself (Model Ċ), the normal force of fault plane is N= T cos θ + R sin θ and the shear force of the fault plane F= R cos θ − T sin θ . In that case, the balanced condition can be described as:
R ≤ tan(ϕ + θ ) T
(3)
The roof rock-mass must move towards the gob after coal is extracted. As the face advances further, the support of the coal seam to the roof is reduced with the decrease in the distance away from the fault and the vertical force R of the roof rock-mass B increases. If the change in the horizontal force T is ignored. In Model I, the normal force decreases and the shear force increases when the working face advances. Under these conditions, the roof rock-mass is apt to slip along the fault plane induced by activation of the fault after coal mining, which may lead to a fault rock burst. Conversely, in Model Ċ it is easy for the roof rock-mass to become a beam balanced structure due to the increase of the normal force. Compared with Model I, the fault is difficult to activate. However, according to Eq.(3), the roof structure is difficult to balance when the vertical force R is increased. The fault begins to activate when the working face nears the fault and the risk of a fault rock burst increases.
4
Numerical simulation results and analysis
The FLAC5.02D numerical simulation software was used to establish a relevant model to study the effect of coal mining operation on fault stress and slip displacement. We refer to the geological information of drill C8-9 at the No.6303 working face of the Jining No.3 Coal Mine for strata properties, because this drill is comparatively accurate.
4.1
Model of numerical simulation
The model is 400 m wide and 99 m high, as shown in Fig. 2. The bottom boundary is fixed in the y-displacement and left and right boundaries are fixed in the x-displacement. A Mohr-Coulomb material is applied in this model. The parameters and physical characteristics of rocks in this model are listed in Table 1.
LI Zhi-hua et al
Study on fault induced rock bursts
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4.2 Effect of coal mining on fault plane stress
Fig. 2 Table 1
Numerical simulation model Strata properties of model ρ
φ
Layer
Thickness (m)
(kg/m3)
K (GPa)
G (GPa)
C (MPa)
(°)
1 2
4 3
2500 2500
8.2 9.6
1.1 3.6
1.8 2.5
30 31
3
8
2700
19.4
13.2
12.8
35
4 5
5 11
2600 2700
16.5 19.4
7.8 13.2
5.0 12.8
34 35
6 7
13 4
2700 2700
23.0 19.4
15.2 13.2
17.0 12.8
38 35
8
21
2700
23.0
15.2
17.0
38
9 10
5 5
2700 1400
19.4 1.50
13.2 0.8
12.8 1.1
35 25
11 12
2 6
2500 2700
13.1 23.0
4.2 15.2
3.0 17.0
33 38
13
12
2500
13.1
4.2
3.0
33
Note: ρ . Density; K. Bulk modulus; G. Shear modulus; C. Cohesion; φ . Inner friction angle.
The INTERFACE command in the FLAC5.02D software is used to simulate the fault and strata planes and slip or separation is allowed between two different planes. Interfaces have the properties of friction, cohesion, dilation, normal and shear stiffness, as well as tensile strength. For properties of this numerical simulation we refer to relevant references[11–13]. The interface is represented by sides C and D in the plane problem (Fig. 3).
Fault rock bursts may be induced by a decrease in normal stress or an increase in shear stress of the fault plane when coal is extracted[14]. Numerical simulation and tests in-situ have proven that roof rock-mass structures are weakened and support is decreased near the fault by activation of a fault after coal mining, so that an increase in shear stress is the reason for fault rock bursts. Besides, coal mining can make the fault plane flexible in the normal direction, while simultaneously, the friction strength decreases, so that the fault rock burst happens anyway[15]. Therefore, it is necessary to study the effect of coal mining on fault stresses in order to study the problem of fault slip destabilization caused by coal mining. The hard and thick main roof of the No.6303 working face of the Jining No.3 Coal Mine was the main reason for the fault rock burst in 2004. So, a point E in the fault plane of the main roof can be monitored during the calculation. The fault plane stress can be described by the normal stress and shear stress of point E. Table 2 shows the fault stresses as the mining progresses when the working face advances from the footwall to the fault itself (Model I). The equation F ≤ N tan ϕ must be satisfied in order to maintain the fault in a state of stable equilibrium, since the friction modulus is a constant value; the equilibrium state depends on the ratio of shear stress to normal stress at the fault. The scatter diagram of the ratio of shear-to-normal stress is shown in Fig. 4 and the negative exponential regression equation is y = 2.8835x −0.9628
where x is the distance from working face to the fault (m) and y the ratio. Table 2
Statistics of the fault plane stresses
Distance from face to fault L (m)
Sketch map of interface command
There were several rock bursts occurring near the fault during the advance of the No.6303 working face from the footwall to the fault itself. In order to understand fully the effect of coal mining on fault stability, the effect of coal mining on faults must first be analyzed when the working face advances from the footwall to the fault itself (Model I). Secondly, in order to optimize the working face design, the effect of the working face advancing from the hanging wall to the fault is also analyzed (Model Ċ). These two models of faults are analyzed in order to discover which is safer.
Test point A (footwall of fault) Normal stress (MPa)
Shear stress (MPa)
10
ˉ0.6861
0.2838
20
ˉ18.39
3.420
30
ˉ33.54
2.233
40
ˉ38.30
2.039
60
ˉ34.83
2.082
80
ˉ30.63
2.038
Shear stress / normal stress
Fig. 3
(4)
Fig. 4
Impact of mining on fault plane stress
In Fig. 4, a few changes take place about the fault
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Journal of China University of Mining & Technology
plane stresses. When the normal stress is about 30 MPa and the shear stress 2 MPa the working face is far away from the fault and the distance ranges from 40 to 80 m. The normal stress begins to decrease and shear stress increases slightly when the face is 30 m away. The shear stress increases rapidly when the face is 20 m away and the magnitude of normal stress is nearly half of the initial stress. However, when the face is 10 m away, the shear stress decreases as does the normal stress, but, the ratio of shear stress to normal stress becomes very high. Therefore, the fault slip destabilization occurs when the changes of the fault stress are caused by coal mining. Table 3 shows the fault stress as the mining proceeds when the working face advances from the hanging wall to the fault itself (Model Ċ). Fig. 5 is the scatter diagram of the ratio of shear stress to normal stress and the analytical equation is (5) y= 0.0264 ln( x) − 0.0193 Table 3
The normal stress increases gradually and the shear stress increases first and then decreases when the face advances from the hanging wall to the fault itself (Fig. 5). When the distance is over 30 m, both the normal stress and shear stress increase, but, the ratio remains almost constant at about 0.09. Thereafter, as the mining progresses and the ratio of shear-to-normal stress decreases continually with the increase of normal stress and the decrease of shear stress, this system is in a state of stable equilibrium.
4.3 Effect of coal mining on fault slip displacement The fault and surrounding system are in a state of unstable equilibrium if slip displacement is increased indefinitely, caused by coal mining (Eq.(1)), which may induce fault slip destabilization. In order to study slip displacement at the fault, two adjacent points, E and F, both of the main roof, are selected as monitor points during numerical simulation: point E comes from the hanging wall of the fault and F from the footwall of the fault. The displacements of the two points are reset to zero in the model so that only the change due to the excavation is recorded. After the calculation and the displacements of two points is recorded, the slip displacement of the fault can be obtained from Eq.(6) (we assume only sliding and no separation and rotation along the fault plane).
Statistics of the fault plane stresses Test point A (footwall of fault)
Distance from face to fault L (m)
Normal stress (MPa)
Shear stress (MPa)
10
ˉ44.72
1.737
20 30
ˉ38.65 ˉ34.00
2.216 2.460
40 60
ˉ30.46 ˉ26.25
2.587 2.463
80
ˉ24.54
2.126
L= ( X E − X F ) 2 + (YE − YF ) 2
0.10 0.08
y = 0.0264 ln(x) 0.0193 R2 = 0.9155
0.02 0
Fig. 5
(6)
When the working face advances from the footwall to the fault itself (Model I) and the mining height is 5 m, the increments in both x- and y-displacement of the monitor points E and F are shown in Table 4. The scatter diagram of slip displacement along the fault versus distance from the face to fault is plotted in Fig. 6.
0.06 0.04
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10 20 30 40 50 60 70 80 90 Distance from face to fault L (m)
Impact of mining on fault plane stress Table 4
Distance from face to fault L (m)
Statistics of the fault slip displacement 10
20
30
40
60
80
Test point E (footwall of fault)
X-displacement
– 6.053
15.10
– 7.173
– 5.784
68.26
– 0.8385
Y-displacement
– 41.25
– 94.72
– 13.55
– 3.038
– 172.8
– 64.33
Test point F (hanging wall of fault)
X-displacement
– 7.052
– 2.434
– 10.43
– 10.49
– 198.0
– 4.676
Y-displacement
– 38.90
– 168.8
– 11.24
– 3.536
– 475.6
– 78.79
403.2
76.12
14.96
2.553
3.993
4.732
Shear displacement L (mm)
Slip displacement L (mm)
Fig. 6
Impact of mining on fault slip displacement
As indicated in Fig. 6, a state of an almost constant zero slip displacement is maintained when the working face is far away from the fault, i.e., when the distance is beyond 40 m. The incremental slip displacement begins to increase when the face is less than 30 m away. The slip displacement increases rapidly when the face approaches the fault at 20 m, where the magnitude of slip displacement is 5 times higher than at 30 m. The slip displacement is 25 times higher at 10 m than at 30 m. Fault slip destabilization may oc-
LI Zhi-hua et al
Study on fault induced rock bursts
cur, induced by slip displacement approaching infinity (Eq.(1)). Fig. 7 is the displacement vector map of rock-mass caused by coal mining when the face is 10 m away from the fault. The magnitude and direction of displacement are specified by the length and direction of the arrowheads. There are different displacements between the two planes of the fault.
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of slip displacement in Fig. 6. As shown in Fig. 8, the effect of coal mining on slip displacement is very weak and the slip displacement itself is small. The slip displacement reaches its maximum regardless of whether the working face advances from the footwall or the hanging wall to the fault itself. When the face is 10 m away, the maximum is 34.13 mm, which is only 8.47 percent of the maximum in Model I. Hence, the fault and surrounding system is in a state of stable equilibrium state and the risk of a fault rock burst is very small. 50
y = 10.587 ln(x) + 52.047 R2 = 0.6078
40 30 20 10
Fig. 7
Displacement vector map 0
As Table 5 shows, when the working face advances from the hanging wall to the fault itself (Model Ċ) as the mining progresses, slip displacement of the fault occurs. This was also indicated by the scatter diagram Table 5 Distance from face to fault L (m)
Fig. 8
10 20 30 40 50 60 70 80 90 Distance from face to fault L (m)
Impact of mining on fault slip displacement
Statistics of fault slip displacement
10
20
30
40
60
80
Test point E (footwall of fault)
X-displacement
8.445
11.49
13.46
12.53
11.46
0.5438
Y-displacement
ˉ35.35
ˉ23.66
3.842
ˉ11.27
7.811
ˉ58.46
Test point F (hanging wall of fault)
X-displacement
14.59
13.40
4.239
9.097
2.067
16.24
Y-displacement
ˉ18.11
ˉ13.37
ˉ2.928
ˉ8.511
ˉ0.6435
ˉ28.15
34.13
18.30
10.46
4.404
11.43
12.63
Slip displacement L (mm)
Shear displacement L (mm)
Fig. 9 is a comparison of the fault slip displacement when the working face advances from the footwall or hanging wall to the fault itself. The results indicate that not only the slip displacement is very small, but also the effect of the dip direction of the fault on slip displacement is very low. The two curves almost coincide when the distance is beyond 30 m from working face to the fault. As coal mining proceeds, the slip displacement in both models increases gradually. Moreover, the effect of dip direction of the fault is different when the distance is less than 30 m. In model I, the slip displacement increases sharply and the fault slip increases indefinitely. Hence, the risk of fault rock burst is much higher than in Model Ċ.
Fig. 9
Comparison of fault slip displacement
5
Conclusions
1) Regardless of the effect of the decrease in normal stress or the increase in shear stress of the fault plane, fault rock bursts may be induced when coal is extracted. 2) By way of mechanical analysis and numerical simulation about fault slip destabilization, our results indicate that the slip displacement increases sharply, caused by a decrease in normal stress and an increase in shear stress of the fault plane, when the working face advances from the footwall to the fault itself. When the slip displacement increases indefinitely, fault rock bursts may be induced. 3) As the normal stress increases and the shear stress decreases, the size of the slip displacement is small when the working face advances from the hanging wall to the fault itself, which result in a very small increase in the risk of a fault rock burst. 4) To sum up, we can be assured that the risk of a fault rock burst is higher when the working face advances from the footwall to the fault itself than when the face advances from the hanging wall to the fault.
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Journal of China University of Mining & Technology
Acknowledgements The study was supported by the National Natural Science Foundation of China (Project No.50490273 and No.50474068) and also by the Jining No.3 Coal Mine, Shandong, both of which are gratefully acknowledged.
References [1] Pan Y S, Wang L G, Zhang M T, et al. The theoretical and testing study of fault rock burst. Chinese Journal of Rock Mechanics and Engineering, 1998, 17(6): 642–649. (In Chinese) [2] Dou L M, He X Q. Theory and Technology of Rock Burst Prevention. Xuzhou: China University of Mining and Technology Press, 2001. (In Chinese) [3] Dou L M, He X Q. Mining Geophysics. Beijing: China Science and Culture Press, 2002. (In Chinese) [4] Wang X B, Pan Y S, Hai L. Instability criterion of fault rockburst based on gradient-dependent plasticity. Chinese Journal of Rock Mechanics and Engineering, 2004, 23(4): 588–591. (In Chinese) [5] Meng Z P, Peng S P, Feng Y, et al. Influence of fracture structure plane on underground pressure and roof stability of working face. Coal Geology & Exploration, 2006, 34(3): 24–27. (In Chinese) [6] Meng Z P, Peng S P, Li H. Influence of normal faults on the physical and mechanical properties of coal and the distribution of underground pressure. Journal of China
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Coal Society, 2001, 26(6): 561–566. (In Chinese) [7] Peng S P, Meng Z P, Li Y L. Influence of faults on coal roof stability by physical modeling study. Coal Geology & Exploration, 2001, 29(3): 1–4. (In Chinese) [8] Dou L M. Prevention and Control of Rock Burst in Coal Mine. Xuzhou: China University of Mining and Technology Press, 2006. (In Chinese) [9] Qian M G, Shi P W. Underground Pressure and Strata Control. Xuzhou: China University of Mining and Technology Press, 2003. (In Chinese) [10] Wang L G, Miao X X. Numerical simulation of coal floor fault activation influenced by mining. Journal of China University of Mining & Technology, 2006, 16(4): 385– 388. [11] Zhou Y, Lin G, Gong F X, et al. Numerical simulations of structural deformation and fluid flow in xiangshan deposit. Journal of China University of Mining & Technology, 2006, 16(4): 404–408. [12] Wang Y X, Qi Q X, Xu G, et al. Numerical study on the protective mining in seam no 6 of Huafeng coal mine. Coal Mining Technology, 2003, 8(4): 4–7. (In Chinese) [13] Song W D, Zhao Z S, Wang H. Simulation study on interaction mechanism between fault fracture zone and rock surrounding straight work. Metal Mine, 2004, 3(32): 11– 13. (In Chinese) [14] Wang L G, Pan Y S, Liang B, et al. Analysis of rockburst process for noncontinuous area in mines. Chine Mining Magazine, 1996, 5(3): 61–64. (In Chinese) [15] Zhou G Q, Xia H C, Zhao G S, et al. Nonlinear elastic constitutive model of soil-structure interfaces under relatively high normal stress. Journal of China University of Mining & Technology, 2007, 17(3): 301–305.
MINING & TECHNOLOGY J China Univ Mining & Technol 18 (2008) 0321–0326 www.elsevier.com/locate/jcumt
Study on fault induced rock bursts LI Zhi-hua1,2, DOU Lin-ming1,2, LU Cai-ping1,2, MU Zong-long1,2, CAO An-ye1,2 1
2
School of Mines, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China State Key Laboratory of Coal Resource and Mine Safety, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China
Abstract: In order to study the rules of rock bursts caused by faults by means of mechanical analysis of a roof rock-mass balanced structure and numerical simulation about fault slip destabilization, the effect of coal mining operation on fault plane stresses and slip displacement were studied. The results indicate that the slip displacement sharply increases due to the decrease of normal stress and the increase of shear stress at the fault plane when the working face advances from the footwall to the fault itself, which may induce a fault rock burst. However, this slip displacement will be very small due to the increase of normal stress and the decrease of shear stress when the working face advances from the hanging wall to the fault itself, which results in a very small risk of a fault rock burst. Key words: fault; slip destabilization; rock burst; numerical simulation; normal stress; shear stress
1
Introduction
A rock burst caused by a fault is a phenomenon of energy being violently released due to a fault slip in coal mines[1]. It has become a serious natural disaster, affecting safe extraction of coal when the mining depth increases and geological conditions become more complex[2–3]. For example, a violent fault rock burst happened at the No.6303 working face of the Jining No.3 Coal Mine in China on November 30, 2004. About 30 m of roadway was destroyed when the working face was about 66 m away from the fault. Afterwards, several secondary rock bursts happened in the process of the No.6303 working face advancing to the fault. Up till now, many studies have been carried out on fault rock bursts and the distribution of underground pressure near the fault. Pan et al have explained some phenomena of fault rock bursts by establishing a simple fault rock burst model to analyze the effect of coal mining operation on fault stress[1]. Wang et al have proposed the instability criterion of fault rock bursts based on gradient-dependent plasticity and an energy criterion[4]. Meng et al have studied the distribution of underground pressure near the fault by using numerical simulation and analyzed the effect of faults on the stability of roofs and underground pressure by using simulation with similar material[5–7]. In another in-
stance, two large-scale rock bursts occurring in a nickel copper mine in Sudbury, Canada, in 1984, were caused by a massive rock slide along a major fault line, leading to a mine quake and the collapse of a roof[8]. Given the geological conditions at the No.6303 working face, we have used, numerical simulation and theoretical analysis to study the effect of coal mining operation on fault stresses and slip displacement during the advancement of the working face from the footwall or hanging wall to the fault itself, in order to provide guidelines for forecasting and preventing fault rock bursts.
2 Mechanism and criterion of fault rock burst A rock burst due to a fault is considered a deformation or destabilization phenomenon of the fault and its surrounding rock system. Before the coal is extracted, this system is in a stable equilibrium[1]. The rock mass in a fault zone and its effective range begins to be deformed because of stresses, referring to additional and original shear stresses, after the coal is extracted. The additional shear stress is small when the working face is far away from the fault and the system maintains a state of stable equilibrium. As the working face advances further, the distance from the working face to the fault decreases, leading to an in-
Received 12 December 2007; accepted 15 March 2008 Projects 50490273 and 50474068 supported by the National Natural Science Foundation of China, 2006BAK04B02 and 2006BAK03B06 by the Support Programs of the National Science and Technique During the 11th Five-Year Period and 2005CB221504 by the State Basic Research Program of China Corresponding author. Tel: +86-13585391209; E-mail address: [email protected]
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Journal of China University of Mining & Technology
crease of additional shear stress. At some point, the entire deformation system changes to a state of unstable equilibrium, so that a fault slip destabilization will occur, which in turn may induce a fault rock burst. A disturbance-response stability criterion is applied to study the fault rock burst. We assume that the fault and its surrounding system is in a stable equilibrium, where a is the far-field displacement and u the slip displacement of the fault. If the increment of the far-field displacement is Ƹa in Eq.(1), then the increment of slip displacement is Ƹu. When ε>0 and δ>0 are given, the fault and surrounding system is in a state of stable equilibrium when the equations ∆a ≤δ, ∆u ≤ε are satisfied. No matter how small the far-field displacement, any net increment in slip displacement will eventually become indefinitely large. Given this condition, the expression is
∆u →∞ ∆a
(1)
In this case, the fault and surrounding system is in an unstable state, which will lead to a fault rock burst caused by a rock mass slip along the fault plane. Our simulation test proves that the result of a decrease of normal stress or an increase in shear stress of the fault plane may induce a fault rock burst when coal is extracted.
3
Mechanical analysis of the effect of coal mining on fault plane stress
Given the relation between the dip direction of the fault and the direction of mining, the effect of coal mining on a fault plane stress can be analyzed by the following simple models. The first model, shown in Fig. 1a, is a working face advancing from the footwall to the fault itself (Fault model I). The second model in Fig. 1b, is a working face advancing from the hanging wall to the fault itself (Fault model Ċ) [9–10] . Direction of mining
θ
A O
B
T
O
θ
T
B
R
R T
NT
N R OF
(a) Fault model I
Fig. 1
A
O
R F
θ
(b) Fault model II
Analytical model of coal mining on fault plane stress
When the working face advances from the footwall to the fault itself (Model I), the normal force of the fault plane is N= T cos θ − R sin θ and the shear force of the fault plane F= R cos θ + T sin θ . Then a
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balanced condition can be described as: ⎧ (T cos θ − R sin θ ) tan ϕ ≥ R cos θ + T sin θ ⎪⎪ T sin(ϕ − θ ) ≥ R cos(ϕ − θ ) ⎨ ⎪ R ≤ tan(ϕ − θ ) ⎪⎩ T
(2)
where T is the horizontal force (N). R the vertical force (N), ϕ the inner friction angle of fault plane (°) and θ the angle of the fault plane (°). When the working face advances from the hanging wall to the fault itself (Model Ċ), the normal force of fault plane is N= T cos θ + R sin θ and the shear force of the fault plane F= R cos θ − T sin θ . In that case, the balanced condition can be described as:
R ≤ tan(ϕ + θ ) T
(3)
The roof rock-mass must move towards the gob after coal is extracted. As the face advances further, the support of the coal seam to the roof is reduced with the decrease in the distance away from the fault and the vertical force R of the roof rock-mass B increases. If the change in the horizontal force T is ignored. In Model I, the normal force decreases and the shear force increases when the working face advances. Under these conditions, the roof rock-mass is apt to slip along the fault plane induced by activation of the fault after coal mining, which may lead to a fault rock burst. Conversely, in Model Ċ it is easy for the roof rock-mass to become a beam balanced structure due to the increase of the normal force. Compared with Model I, the fault is difficult to activate. However, according to Eq.(3), the roof structure is difficult to balance when the vertical force R is increased. The fault begins to activate when the working face nears the fault and the risk of a fault rock burst increases.
4
Numerical simulation results and analysis
The FLAC5.02D numerical simulation software was used to establish a relevant model to study the effect of coal mining operation on fault stress and slip displacement. We refer to the geological information of drill C8-9 at the No.6303 working face of the Jining No.3 Coal Mine for strata properties, because this drill is comparatively accurate.
4.1
Model of numerical simulation
The model is 400 m wide and 99 m high, as shown in Fig. 2. The bottom boundary is fixed in the y-displacement and left and right boundaries are fixed in the x-displacement. A Mohr-Coulomb material is applied in this model. The parameters and physical characteristics of rocks in this model are listed in Table 1.
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4.2 Effect of coal mining on fault plane stress
Fig. 2 Table 1
Numerical simulation model Strata properties of model ρ
φ
Layer
Thickness (m)
(kg/m3)
K (GPa)
G (GPa)
C (MPa)
(°)
1 2
4 3
2500 2500
8.2 9.6
1.1 3.6
1.8 2.5
30 31
3
8
2700
19.4
13.2
12.8
35
4 5
5 11
2600 2700
16.5 19.4
7.8 13.2
5.0 12.8
34 35
6 7
13 4
2700 2700
23.0 19.4
15.2 13.2
17.0 12.8
38 35
8
21
2700
23.0
15.2
17.0
38
9 10
5 5
2700 1400
19.4 1.50
13.2 0.8
12.8 1.1
35 25
11 12
2 6
2500 2700
13.1 23.0
4.2 15.2
3.0 17.0
33 38
13
12
2500
13.1
4.2
3.0
33
Note: ρ . Density; K. Bulk modulus; G. Shear modulus; C. Cohesion; φ . Inner friction angle.
The INTERFACE command in the FLAC5.02D software is used to simulate the fault and strata planes and slip or separation is allowed between two different planes. Interfaces have the properties of friction, cohesion, dilation, normal and shear stiffness, as well as tensile strength. For properties of this numerical simulation we refer to relevant references[11–13]. The interface is represented by sides C and D in the plane problem (Fig. 3).
Fault rock bursts may be induced by a decrease in normal stress or an increase in shear stress of the fault plane when coal is extracted[14]. Numerical simulation and tests in-situ have proven that roof rock-mass structures are weakened and support is decreased near the fault by activation of a fault after coal mining, so that an increase in shear stress is the reason for fault rock bursts. Besides, coal mining can make the fault plane flexible in the normal direction, while simultaneously, the friction strength decreases, so that the fault rock burst happens anyway[15]. Therefore, it is necessary to study the effect of coal mining on fault stresses in order to study the problem of fault slip destabilization caused by coal mining. The hard and thick main roof of the No.6303 working face of the Jining No.3 Coal Mine was the main reason for the fault rock burst in 2004. So, a point E in the fault plane of the main roof can be monitored during the calculation. The fault plane stress can be described by the normal stress and shear stress of point E. Table 2 shows the fault stresses as the mining progresses when the working face advances from the footwall to the fault itself (Model I). The equation F ≤ N tan ϕ must be satisfied in order to maintain the fault in a state of stable equilibrium, since the friction modulus is a constant value; the equilibrium state depends on the ratio of shear stress to normal stress at the fault. The scatter diagram of the ratio of shear-to-normal stress is shown in Fig. 4 and the negative exponential regression equation is y = 2.8835x −0.9628
where x is the distance from working face to the fault (m) and y the ratio. Table 2
Statistics of the fault plane stresses
Distance from face to fault L (m)
Sketch map of interface command
There were several rock bursts occurring near the fault during the advance of the No.6303 working face from the footwall to the fault itself. In order to understand fully the effect of coal mining on fault stability, the effect of coal mining on faults must first be analyzed when the working face advances from the footwall to the fault itself (Model I). Secondly, in order to optimize the working face design, the effect of the working face advancing from the hanging wall to the fault is also analyzed (Model Ċ). These two models of faults are analyzed in order to discover which is safer.
Test point A (footwall of fault) Normal stress (MPa)
Shear stress (MPa)
10
ˉ0.6861
0.2838
20
ˉ18.39
3.420
30
ˉ33.54
2.233
40
ˉ38.30
2.039
60
ˉ34.83
2.082
80
ˉ30.63
2.038
Shear stress / normal stress
Fig. 3
(4)
Fig. 4
Impact of mining on fault plane stress
In Fig. 4, a few changes take place about the fault
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plane stresses. When the normal stress is about 30 MPa and the shear stress 2 MPa the working face is far away from the fault and the distance ranges from 40 to 80 m. The normal stress begins to decrease and shear stress increases slightly when the face is 30 m away. The shear stress increases rapidly when the face is 20 m away and the magnitude of normal stress is nearly half of the initial stress. However, when the face is 10 m away, the shear stress decreases as does the normal stress, but, the ratio of shear stress to normal stress becomes very high. Therefore, the fault slip destabilization occurs when the changes of the fault stress are caused by coal mining. Table 3 shows the fault stress as the mining proceeds when the working face advances from the hanging wall to the fault itself (Model Ċ). Fig. 5 is the scatter diagram of the ratio of shear stress to normal stress and the analytical equation is (5) y= 0.0264 ln( x) − 0.0193 Table 3
The normal stress increases gradually and the shear stress increases first and then decreases when the face advances from the hanging wall to the fault itself (Fig. 5). When the distance is over 30 m, both the normal stress and shear stress increase, but, the ratio remains almost constant at about 0.09. Thereafter, as the mining progresses and the ratio of shear-to-normal stress decreases continually with the increase of normal stress and the decrease of shear stress, this system is in a state of stable equilibrium.
4.3 Effect of coal mining on fault slip displacement The fault and surrounding system are in a state of unstable equilibrium if slip displacement is increased indefinitely, caused by coal mining (Eq.(1)), which may induce fault slip destabilization. In order to study slip displacement at the fault, two adjacent points, E and F, both of the main roof, are selected as monitor points during numerical simulation: point E comes from the hanging wall of the fault and F from the footwall of the fault. The displacements of the two points are reset to zero in the model so that only the change due to the excavation is recorded. After the calculation and the displacements of two points is recorded, the slip displacement of the fault can be obtained from Eq.(6) (we assume only sliding and no separation and rotation along the fault plane).
Statistics of the fault plane stresses Test point A (footwall of fault)
Distance from face to fault L (m)
Normal stress (MPa)
Shear stress (MPa)
10
ˉ44.72
1.737
20 30
ˉ38.65 ˉ34.00
2.216 2.460
40 60
ˉ30.46 ˉ26.25
2.587 2.463
80
ˉ24.54
2.126
L= ( X E − X F ) 2 + (YE − YF ) 2
0.10 0.08
y = 0.0264 ln(x) 0.0193 R2 = 0.9155
0.02 0
Fig. 5
(6)
When the working face advances from the footwall to the fault itself (Model I) and the mining height is 5 m, the increments in both x- and y-displacement of the monitor points E and F are shown in Table 4. The scatter diagram of slip displacement along the fault versus distance from the face to fault is plotted in Fig. 6.
0.06 0.04
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10 20 30 40 50 60 70 80 90 Distance from face to fault L (m)
Impact of mining on fault plane stress Table 4
Distance from face to fault L (m)
Statistics of the fault slip displacement 10
20
30
40
60
80
Test point E (footwall of fault)
X-displacement
– 6.053
15.10
– 7.173
– 5.784
68.26
– 0.8385
Y-displacement
– 41.25
– 94.72
– 13.55
– 3.038
– 172.8
– 64.33
Test point F (hanging wall of fault)
X-displacement
– 7.052
– 2.434
– 10.43
– 10.49
– 198.0
– 4.676
Y-displacement
– 38.90
– 168.8
– 11.24
– 3.536
– 475.6
– 78.79
403.2
76.12
14.96
2.553
3.993
4.732
Shear displacement L (mm)
Slip displacement L (mm)
Fig. 6
Impact of mining on fault slip displacement
As indicated in Fig. 6, a state of an almost constant zero slip displacement is maintained when the working face is far away from the fault, i.e., when the distance is beyond 40 m. The incremental slip displacement begins to increase when the face is less than 30 m away. The slip displacement increases rapidly when the face approaches the fault at 20 m, where the magnitude of slip displacement is 5 times higher than at 30 m. The slip displacement is 25 times higher at 10 m than at 30 m. Fault slip destabilization may oc-
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Study on fault induced rock bursts
cur, induced by slip displacement approaching infinity (Eq.(1)). Fig. 7 is the displacement vector map of rock-mass caused by coal mining when the face is 10 m away from the fault. The magnitude and direction of displacement are specified by the length and direction of the arrowheads. There are different displacements between the two planes of the fault.
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of slip displacement in Fig. 6. As shown in Fig. 8, the effect of coal mining on slip displacement is very weak and the slip displacement itself is small. The slip displacement reaches its maximum regardless of whether the working face advances from the footwall or the hanging wall to the fault itself. When the face is 10 m away, the maximum is 34.13 mm, which is only 8.47 percent of the maximum in Model I. Hence, the fault and surrounding system is in a state of stable equilibrium state and the risk of a fault rock burst is very small. 50
y = 10.587 ln(x) + 52.047 R2 = 0.6078
40 30 20 10
Fig. 7
Displacement vector map 0
As Table 5 shows, when the working face advances from the hanging wall to the fault itself (Model Ċ) as the mining progresses, slip displacement of the fault occurs. This was also indicated by the scatter diagram Table 5 Distance from face to fault L (m)
Fig. 8
10 20 30 40 50 60 70 80 90 Distance from face to fault L (m)
Impact of mining on fault slip displacement
Statistics of fault slip displacement
10
20
30
40
60
80
Test point E (footwall of fault)
X-displacement
8.445
11.49
13.46
12.53
11.46
0.5438
Y-displacement
ˉ35.35
ˉ23.66
3.842
ˉ11.27
7.811
ˉ58.46
Test point F (hanging wall of fault)
X-displacement
14.59
13.40
4.239
9.097
2.067
16.24
Y-displacement
ˉ18.11
ˉ13.37
ˉ2.928
ˉ8.511
ˉ0.6435
ˉ28.15
34.13
18.30
10.46
4.404
11.43
12.63
Slip displacement L (mm)
Shear displacement L (mm)
Fig. 9 is a comparison of the fault slip displacement when the working face advances from the footwall or hanging wall to the fault itself. The results indicate that not only the slip displacement is very small, but also the effect of the dip direction of the fault on slip displacement is very low. The two curves almost coincide when the distance is beyond 30 m from working face to the fault. As coal mining proceeds, the slip displacement in both models increases gradually. Moreover, the effect of dip direction of the fault is different when the distance is less than 30 m. In model I, the slip displacement increases sharply and the fault slip increases indefinitely. Hence, the risk of fault rock burst is much higher than in Model Ċ.
Fig. 9
Comparison of fault slip displacement
5
Conclusions
1) Regardless of the effect of the decrease in normal stress or the increase in shear stress of the fault plane, fault rock bursts may be induced when coal is extracted. 2) By way of mechanical analysis and numerical simulation about fault slip destabilization, our results indicate that the slip displacement increases sharply, caused by a decrease in normal stress and an increase in shear stress of the fault plane, when the working face advances from the footwall to the fault itself. When the slip displacement increases indefinitely, fault rock bursts may be induced. 3) As the normal stress increases and the shear stress decreases, the size of the slip displacement is small when the working face advances from the hanging wall to the fault itself, which result in a very small increase in the risk of a fault rock burst. 4) To sum up, we can be assured that the risk of a fault rock burst is higher when the working face advances from the footwall to the fault itself than when the face advances from the hanging wall to the fault.
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Acknowledgements The study was supported by the National Natural Science Foundation of China (Project No.50490273 and No.50474068) and also by the Jining No.3 Coal Mine, Shandong, both of which are gratefully acknowledged.
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