Numerical simulation on the risk of roof water inrush in Wuyang Coal Mine

International Journal of Mining Science and Technology 22 (2012) 273–277

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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Numerical simulation on the risk of roof water inrush in Wuyang Coal Mine Yao Banghua ⇑, Bai Haibo, Zhang Boyang State Key Laboratory of Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221008, China

a r t i c l e

i n f o

Article history: Received 2 August 2011 Received in revised form 10 September 2011 Accepted 25 October 2011 Available online 26 March 2012 Keywords: Fracture zone Numerical simulation Water inrush Wuyang coal mine

a b s t r a c t Water-inrush in mine is one of the mine disasters caused by mining. In order to assess the risk of roof water-inrush in Wuyang Coal Mine based on the geological material of the coal mine, we built numerical models for the roof fracture and seepage development rule by using RFPA2D and COMSOL respectively, to analyze the changes in fracture zone, stress, water pressure and seepage vector with the advancement of working face, and compared the results with the field investigated data. The numerical simulation results indicate that: (1) with the advancement of the working faces, the stress relief range and fracture zone in the overlying strata increased rapidly up to about 90 m, and then tended to remain constant, reaching a final height of about 95 m which agrees with the field investigation; (2) the seepage flow constantly increased with a larger flow volume both in the front and rear area, where the stress concentration are the most serious. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction With the coal mining extending to deeper ground, people pay more and more attention to the production safety of coal mining. Water-inrush is one of the mine disasters caused by mining. According to statistics, in the past 20 years, water-inrush disasters resulted in the highest economic loss in China, and ranked the second with the highest toll deaths among accidents in coal mining. Many researchers home and abroad have done much work to study the fracture zone of overlying strata. At the aspect of theoretical analysis, based on the theory of key strata, Miao et al. and Mao et al. put forward the concept of water-resistant key strata and built a mechanical model [1,2]. In the side of numerical calculation, Wang et al. simulated the stress and displacement fields after excavation of the mine by using FLAC3D, and analyzed the law of fracture zone caused by mining excavation [3]. Chen et al. simulated the possibility of water-inrush under the condition of thick loose bed and thin rock strata by using UDEC (Universal Distinct Element Code) [4]. Other research achievements also provided useful theoretical and simulation methods for this paper [5–15]. This paper uses software of RFPA and COMSOL to investigate the fracture zone and seepage rule respectively, analyzes the change rule of plastic region, water pressure and seepage vector, and estimates the risk of water-inrush in Wuyang Coal Mine. The conclusion of the numerical simulation is very important for water-inrush and other predictions in Wuyang Coal Mine.

⇑ Corresponding author. Tel.: +86 13813284099. E-mail address: [email protected] (B. Yao).

2. Numerical simulation study on the permeable fracture zone of Wuyang Coal Mine 7803 working face 2.1. Software selection This paper first uses RFPA2D to simulate the fracture zone caused by excavation of 7803 working face in Wuyang Coal Mine. The RFPA2D software simulates nonlinear material by taking into account the heterogeneity of materials, and simulates the nonlinear behavior such as deformation and failure by decreasing the strength of elements. Based on the damage mechanics and classical Boit seepage coupling equation, the nonlinear constitutive equations and permeability model of the rock were established to study the full process for rock materials from the microscopic damage to the macroscopic view failure. Therefore we choose the RFPA2D rock failure process analysis system to simulate the development rules of overlying strata fracture zone.

2.2. Theoretical basis of the numerical simulation (1) Rock mass deformation equations. The equation of motion of rock mass expressed by deformation is:

ðj þ GÞ  uj;ji þ Gui;jj þ F i ¼ 0

ð1Þ

where G, j are the shear modulus and Lame constant respectively, u the displacement in direction i; j ¼ 1; 2; 3, and F i the volume force. (2) Rock mass damage evolution equation.

2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. doi:10.1016/j.ijmst.2012.03.006

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Damage deformation appears under the effect of mining, and the elements begin to get damaged when either the stress state or the strain state of microscopic elements satisfy a given damage threshold. Elastic modulus of the damaged elements is expressed by:

E ¼ ð1  DÞE0

q=6 MPa Aquifer

ð2Þ Working face

where D is the damaged variable and E and E0 the damaged element and nondestructive unit of elastic modulus. The element failure criterion adopts the Mohr-coulomb criterion, i.e.:

F ¼ r1  r3

1 þ sin u P fc 1  sin u

ð3Þ

Fig. 1. Mechanics mode breaking law numerical simulation of three coal seam mining.

where u is the internal friction angle and fc the uniaxial compressive strength. When the shear stress reaches the damage threshold, the damaged variable D can be expressed as:

D¼

0

ðe < ec0 Þ

Fracture height (m)

(

100

ð4Þ

1  Efcr0 e ðec0 6 er Þ

where fcr is the compressive residual strength, compressive strain and er the residual strain.

ec0 the maximum

60 40 20 0 50 60 70 80 90 100 110 120 130 140 150 Working face advancement distance (m)

2.3. Model establishment According to the drilling columnar figure of south 31 in the fully-mechanized working faces and the mechanic tests of the rock material, we obtained the mechanical parameters of the rock mass presented in Table 1, and established the numerical model (Fig. 1). The coal seam is 480 m deep with thickness of 6 m. The model is 15 beds including the coal seam. The horizontal and vertical directions of the numerical model are 300 and 200 m, respectively. This numerical model is divided into 1 meter square grid with a total number of 300  200 = 60,000. The working face in this model advances 150 m from left to right with a 10 m length of each excavation step. We set different bedding strengths between different rock strata. In the model, the brighter the stratum is, the larger the elastic modulus will be.

80

Fig. 2. Overlying fracture height development curve as the advancement of three coal seam working face.

above the coal seam. Since then the fracture height tended to remain constant, but it reached to about 95 m when working face advanced to 150 m. Figs. 2 and 3 illustrate the overlying strata curves drawn for changes of fracture height with the working face advancement. After the working face advanced to 130 m, the variation curves of fracture height tended to be constant with advancement. This is because the falling strata filled the mined-out area of three coal seams and gradually gained supporting capacity. (2) The stress variation process of three working faces. Fig. 4 is the support pressure curves of the working face. It can be seen from the graphs that the support pressure peak of coal seam is slightly increased (about 23 MPa in 110 m and 25 MPa in 150 m) with the advancement of working face. This is because the advancement range of mined-out area causes an increase of the stress concentration degree. With the advancement of the working face, the support pressure peak point

2.4. Simulation results (1) The roof fractures evolution process of three working face. Through numerical simulation, we can observe that: with the advancement of working face, roof fracture zones constantly grew upwards. When the working face advanced to 50 m, the first weighting happened. When the three working faces advanced to 100 m, the fracture heights reach nearly 90 m

Table 1 Mechanical parameters and lithology for rock strata in Wuyang Coal Mine. Layer

Rock lithology

Thickness (m)

Buried depth (m)

Elastic modulus (MPa)

Compressive strength (MPa)

Poisson’s ratio

Density (kg/ m3)

Friction angle (°)

1 2 3

Shaly sand Sandstone Sandy mudstone Mudstone Sandy mudstone Sandstone Mudstone Coal seam Sandy mudstone

32 14 43

250–282 296 339

10,000 22,000 16,000

40 60 45

0.25 0.25 0.25

2200 2500 2400

31 26 32

16 22

355 377

12,000 16,000

30 45

0.25 0.25

2200 2400

31 27

14 9 6 44

391 400 406 450

22,000 16,000 3000 16,000

60 30 10 45

0.25 0.25 0.25 0.25

2500 2200 1500 2400

33 36 32 33

4 5 6 7 8 9

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(a) Working face advance to 110 m

(b) Working face advance to 150 m

Fig. 3. Overlying fracture height development as the working face advances. (a) Working face advance to 110 m. (b) Working face advance to 150 m.

30

Vertical stress (MPa)

Vertical stress (MPa)

25 20 15 10 5 0

150

100 50 0 50 100 Advancement distance (m)

150

(a) Working face advance to 110 m

25 20 15 10 5 0

150

100 50 50 100 0 Advancement distance (m)

150

(b) Working face advance to 150 m

Fig. 4. Vertical compressive stress distribution curve of three mined-out areas. (a) Working face advance to 110 m. (b) Working face advance to 150 m.

moved forward with stress concentration coefficient of about 2.0.

q=6 MPa

3. Numerical simulation study on 7803 working faces strata seepage properties 3.1. Software selection In this part, we selected COMSOL Multiphysics to carry out seepage and displacement simulation. It is a professional finite element numerical analysis software package with an interactive development environment system for calculations and simulations based on the multi-physical model of partial differential equations. It is also a professional finite element analysis software package for describing and simulating various physical phenomena, making the various established physical mathematical phenomena be easy and possible. The advantages of this software lie in its multi-physical coupling simulation.

Fig. 5. COMSOL numerical model diagram.

cf the fluid of isothermal compression coefficient, c/ the pore compression coefficient, ca the acceleration coefficient, u the fluid dynamic variable degree, j the permeability, b the non-Darcy and F the fluid quality force.

3.2. Theoretical basis of numerical simulation 3.3. Establishment of model The Rock Ahmed-Sunada type of non-Darcy gap seepage system equations is controlled by three parts including quality, momentum and state equations. They are classified as: @ð/qÞ þ  ð VÞ ¼ @t ca @V ¼  p  uk V @t @ðq/Þ ¼ 0 /0 ct @p @t @t

r

q

q

r

q

qq þ bqVV þ F

9 > = > ;

ð5Þ

where p is the fluid pressure, V the seepage velocity, / the porosity, q the mass density of fluid, q the source (remit) intensity, /0 and q0 the mass density and porosity under the corresponding reference pressure p0 , respectively, ct ¼ cf þ c/ the fluid compression coefficient,

In order to research on the influence of aquifer in shallow depth of coal seam three mining in Wuyang Coal Mine, we established a COMSOL numerical model (Fig. 5) to study the development of roof fracture of coal seam three after mining according to the mechanical parameters for the surrounding rock of coal seams presented in Table 1. The model size is the same as the above RFPA2D model, and the working face advances 150 m. The first three steps are 10 m, and then each step increased by 20 m until the working face excavation distance reach 150 m. According to the mechanical parameters for the surrounding rock of coal seams presented in Table 1, we established a numerical

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Max:10×105 10 9 8 7 6 5 4 3 2 1 0 Min: 7.406×10

Max :10×10 5 10 9 8 7 6 5 4 3 2 1 0 Min: 3.704×10

8

(a) Stope advancement distance is 110 m

8

(b) Stope advancement distance is 150m

Fig. 6. Stope pressure and seepage vector in different advancement distance. (a) Stope advancement distance is 110 m. (b) Stope advancement distance is 150 m.

increase steadily before the working face advances to 90 m, then it same to be constant until the working face advances to about 110 m, and from then on, the water inflow volume continuously grow with the advancement of working face. This is because roof fracture zone height and range develop as the advancement of working face, which causes the increase of water inflow. We can see from the Fig. 8 that, with the advancement of the working face, the pressure relief range became larger, which can be attributed to the enlargement of the displacement of overlying strata with the advancement of the working face. The displacement of overlying strata increased with the advancement of the working face and then tended to be constant, which indicates that the fracture zone height could not grow when it reach to a certain height.

Water inflow (m3/min)

1.5 1.2 0.9 0.6 0.3 0

30 50 70 90 110 130 150 Working face advancement distance (m)

Fig. 7. Stope water inflow volume as the advancement of working face.

computation model by using COMSOL as presented in Fig. 5. The model size is the same as the above RFPA2D model, and the working face advances 150 m. The first three steps are 10 m, and then each step increased by 20 m until the working face excavation distance reach 150 m. 3.4. Simulation results We drew the stope seepage field and displacement distribution rule after mining by using the numerical simulation software COMSOL, which were shown in Figs. 6–8. The simulation results show that the seepage flow constantly increased with a larger flow volume both in the front and rear areas (Fig. 6), this is because the stress concentration happened and the failure of rock mass was more serious in this zone. Fig. 7 illustrates the stope water inflow volume as the advancement of working face, and we can find that the water inflow volume Max: 0.0243 0.02 0 − 0.02 − 0.04 − 0.06 − 0.08 − 0.10

Min: - 0.116

4. Field measurement inspection Wuyang Coal Mine has not been conducting technology research of coal mining under water body previously. At home and abroad, the observation and research results of the fracture zone are less under the condition of fully mechanized and top coal caving mining. Therefore, this is often based on observation results of the middle and thick coal seam slice mining. In adjacent area of Wangzhuang Coal Mine, researches were carried out on the fracture zone observation under fully-mechanized caving mining with drilling. Since Wuyang Coal Mine and Wangzhuang Coal Mine have similar geological conditions, this research on the fracture zone can be used in Wuyang Coal Mine for reference. The actual observation results of the fracture zone exploration for 6206 working faces in Wangzhuang (Table 2) showed that, for numerical simulation results, the permeable fracture zone height of about 95 m in Wuyang and mining height of 6 m is reliable.

Max: 0.0220 0.022 0.008 − 0.006 − 0.020 − 0.034 − 0.048 − 0.062 − 0.076 − 0.090 − 0.104

Max: 0.0223 Max: 0.0191 0.02 0.019 0 0 − 0.02 − 0.019 − 0.039 − 0.04 − 0.058 − 0.06 − 0.077 − 0.08 − 0.096 − 0.10 − 0.116 − 0.12 − 0.135 − 0.14 − 0.154 − 0.16

Min: - 0.113

Min: - 0.170

(a) Stope advancement distance is 110 m

Min: - 0.167

(b) Stope advancement distance is 150 m

Fig. 8. Stope displacement in different advancement distance. (a) Stope advancement distance is 110 m. (b) Stope advancement distance is 150 m.

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Table 2 Wangzhuang 6206 face guide water fracture zones exploration results. Drill hole

Thickness of coal seam mining (m)

Fracture zone height (m)

Proportion of thickness and height

K1 K2 K3

5.9 5.2 5.7

94.67 82.27 94.87

16.046 15.82 16.64

5. Conclusions (1) According to the actual investigation, using COMSOL and RFPA2D, we analyze the changes of roof fracture zone height, water inflow volume, water pressure and seepage vector and obtains the coal seam mining overlying strata fracture evolution and seepage rule. The calculation results agree with the field investigation. (2) This study showed that the pressure relief range of roof strata constantly expands and the plastic zone continually increases with the advancement of working faces, and then generally begins to become constant. This indicates that the overlying strata fracture height no longer increases until it reaches a certain height. (3) The numerical simulation results indicate that the Wuyang Coal Mine has no risk of roof water inrush with good geological conditions. However, some measures need to be taken to deal with geological defects, such as faults and Karst collapse columns, to prevent water inrush accidents in Wuyang Coal Mine.

Acknowledgments Project supports from the National Basic Research Program of China (No. 2010CB226800), the 111 Project (No. B07028), and the National Natural Science Foundation of China (Nos. 50974115 and 41002087) are acknowledged. References [1] Miao XX, Chen RH, Bai HB. Fundamental concepts and mechanical analysis of water-resisting key strata in water-preserved mining. J China Coal Soc 2007;32(6):561–4.

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