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Numerical modeling study of asymmetry in the induced stresses over coal mine pillars with advancement of the goaf line
ARTICLE IN PRESS
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
Technical Note
Numerical modeling study of asymmetry in the induced stresses over coal mine pillars with advancement of the goaf line A. Jaiswal, S.K. Sharma*, B.K. Shrivastva Department of Mining Engineering, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India Accepted 12 January 2004
1. Introduction The bord and pillar system of mining is still the predominant method of mining underground coal in India. Even after adoption of the longwall method of mining, bord and pillar has an edge—due to less roof control problems. However, strata control is still a major problem in underground mining. With each passing day, mining operations are becoming more and more difficult as the mining becomes deeper, resulting in more adverse geo-mining conditions. This situation requires the simulation of mine and structure prior to extraction, as well as advanced instrumental monitoring of the strata control parameters for calibrating the numerical model. Designing a coal pillar is a key issue for every bord and pillar mining operation. Mine planners have a variety of modeling methods, both empirical and numerical, for analyzing pillar stresses and determining safe pillar sizes for the various mine geometries and geotechnical conditions. Empirical methods [1–4] emphasize the collection and interpretation of case histories of pillar performance. However, it is difficult to apply these empirical methods to mining situations beyond the scope of the original site characteristics. Therefore, when complicated stress conditions arise due to complex single seam or multiple seam mining geometries, numerical modeling techniques based on finite element, boundary element, discrete element, or finite difference, etc. attain superiority over the empirical methods, providing that they can be adequately validated. Many researchers have contributed in the field of numerical methods for simulating coal panels [5–9]. These numerical or analytical design methods are based *Corresponding author. Tel.: +91-542-236-9442; fax: +91-542-2368174. E-mail address: [email protected] (S.K. Sharma). 1365-1609/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.ijrmms.2004.01.007
on the fundamental laws of force, stress, and elasticity. Their primary advantage is that they are flexible and can quickly analyze the effects of numerous geometrical and geotechnical variables on the mine design. The ability of the three-dimensional (3-D) boundary-element method to model large mine areas with complex geometries has enabled successful simulation of mining conditions. This is also helpful in identifying potential solutions to ground control problems in mines.
2. Simulation of the sub-panel This Technical note deals with the simulation of bord and pillar sub-panel (Fig. 1) of a mine using the Blasting Gallery method (BG) requiring assessment of the stability of pillars until the first major fall. The coal measure formations observed in a typical borehole within the selected mine area are shown in Fig. 2. The thickness of number 3 seam is about 11 m, with an average gradient of 1 in 5.2. The strata overlying the coal seam is comprised of massive sandstone of thickness 22.45 m. A large mine panel of about 1000 m 120 m to 175 m is divided into sub-panels of 160 m 120 m to 175 m by driving main rises. A barrier of about 30 m is left in between the sub-panels. It is necessary to adopt this sub-paneling to restrict the size of the panel so that the extraction is completed within the incubation period. After extraction of the coal, the sub-panel is sealed off. In the present study, the sub-panel of dimensions 160 m 160 m (see Fig. 1) is divided into 16 pillars each with dimensions 40 m 40 m by driving level and dip galleries 4.0 m width and 3 m height. The sub-panel has been simulated using the 3-D boundary element method. Thereafter, the induced stress patterns in the pillars have been determined. The impact of the face advance on pillars has also been determined in terms of changing the
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A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
Fig. 1. Plan of sub-panel.
induced stress pattern. Though the complete stress profile was determined, only the induced stress is presented in this paper. These results have been compared with field instrumentation results. For the simulation work, 14 stages of the line of extraction have been taken (the first, seventh and final stages are shown in Fig. 1)—at a regular interval of 4.4 m. The pillar under specific consideration for detailed observation has been shown highlighted in Fig. 1. Induced stresses were also determined at the middle level of this pillar for comparison with the induced stress at roof level (eight points were taken for comparison). A stress monitoring capsule was installed in the pillar for validating the model. The salient features of the selected sub-panel are summarized in Table 1. Table 2 gives the physicomechanical properties of the coal. 2.1. Discretization of the model Fig. 3 shows the discretization of the gallery around one pillar for clarification purposes. However, in the actual simulation process all voids (i.e. goaf and gallery) were discretized with triangular elements of variable
sizes for simulation of the structure by the threedimensional BEM.
3. Results and discussion By gray shading, Fig. 4 (stages 1, 8, 11, 14) shows the induced stress on the pillar with advancing line of extraction. The magnitude of the induced stress is also indicated by different gray shades in Fig. 5 for the pillar highlighted in Fig. 1 (stress distribution patterns on the specific pillar for all fourteen stages) ranging from 5 to 30 MPa (5 MPa being the in situ vertical stress). Fig. 4 shows the induced stress when the line of extraction was the A1-B1 (stage 1, see Fig. 1). It shows that the induced stress is symmetrical around the center of the pillar. The corners are more stressed compared to other locations. The edges also experience higher stress value in comparison to the central portion of the pillar, but less than the corner points. The induced stress values at the corners are about 21 MPa. As the line of extraction starts advancing towards the pillar under consideration (Fig. 1), the induced stress pattern on the pillar changes from symmetrical to
ARTICLE IN PRESS A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
861
Table 1 Salient features of the selected sub-panel Gallery width Gallery height Pillar width (Center to Center of the Gallery) Panel size Goaf height Angle of extraction of line Depth of working seam
4m 3m 40 m 160 m 160 m 11 m 65 253 m
Table 2 Physico-mechanical properties of coal Young’s Modulus of the coal Poisson’s ratio Specific density
5 GPa 0.19 2.01
completely, indicating that the induced stress on the pillar has exceeded 5 MPa. Then the entire pillar is under the influence of the goaf. Later in the eleventh stage, when the line of goaf just touches the pillar’s corner, that corner experiences maximum stress in comparison with that of the remaining pillar—as is clear from gray shaded pattern alterations which can be observed in Fig. 5, stage 14. The induced stress values at the roof level were also compared with the middle level at the selected eight points indicated in Fig. 1 for the highlighted pillar. Fig. 6 depicts the variation of induced stress value with the advance of the line of extraction (distance from point to line of extraction); the induced stress value at the roof level was higher compared to the middle level. When the goaf was beyond the 30–32 m (points 1, 4, 6) the increments of the induced stress from stage 1 to stage 14 were negligible, i.e. just 0.016–0.019 MPa/m. When the line of extraction came nearer to the point between the range 10–30 m, the rate of increasing of the induced stress was significant, i.e. 0.1 MPa/m. When the line of extraction came within 10 m from the point, the induced stress increased rapidly, about 0.6–0.7 MPa/m.
4. Validation
Fig. 2. Bore hole section of the mine.
asymmetrical. The central portions of the pillar in Figs. 4 and 5 depict stress value of 5 MPa. The dark patch diminishes and gradually disappears as excavation proceeds. In the eighth stage, the dark patch vanishes
A comparison of results of the simulation work with the actual results obtained from the field was made at point ‘I’ (see Fig. 1). As is evident from Fig. 7, there is not too much discrepancy between the two curves, indicating that the results obtained were acceptable. The slight irregularity in the field results curve is likely to have been due to local coal/rock inhomogeneity, as evidenced by the fact that the areas beneath the two curves (representing the applied force) are similar.
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Fig. 3. Discretization of single pillar.
Fig. 4. Induced stress pattern on pillar.
ARTICLE IN PRESS A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
863
24 23 22 21 20 19 18 17 16 15 14
Induced Stress (Mpa)
Induced Stress (Mpa)
Fig. 5. Induced stress pattern on the specific pillar in all stages.
20
30
40
50 Distance (m)
60
70
32 30 28 26 24 22 20 18 16 14 0
10
20 Distance (m)
30
20
30
40
50
16 15 14 13 12 11 10 9 8 20
40
60 Mid Level Roof Level
Point 2
30
Mid Level Roof Level
Point 3
40
50 60 Distance (m)
70
80 Mid Level Roof Level
Point 4 24 Induced Stress (Mpa)
24 22 20 18 16 14 12 10 8
22 20 18 16 14
0
10
20 30 Distance (m)
40
30
50
Induced Stress (Mpa)
15 14 13 12 11 10 9 8 20
30
40
Distance (m)
Point 7
50
50
60
70 Mid Level Roof Level
60 70 Distance (m)
80
90 Mid Level
Point 6
16
10
40
Mid Level Roof Level
Point 5 Induced Stress (Mpa)
10
Distance (m)
Induced Stress (Mpa)
Induced Stress (Mpa)
0
80 Mid Level Roof Level
Point 1
Induced Stress (Mpa)
18 17 16 15 14 13 12 11 10 9 8
Roof Level
28 26 24 22 20 18 16 14 0
10
20
30
Distance (m)
Point 8
Fig. 6. Induced stress versus distance from goaf edge at different points.
40
50
60 Mid Level Roof Level
ARTICLE IN PRESS 864
A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
instrumentation for strata monitoring (the stress capsule) at the Mine.
References
Fig. 7. Verification of results of simulation work with the field results.
5. Conclusion On the basis of the present study, the following conclusions can be made. The central portion of the pillar at the roof level experiences the least induced stress; whereas the pillar corners experience the highest stress. As one goes away from the goaf edge, the induced stress decreases. It was also seen, that the induced stress on the pillar was greater at the roof level, as compared to the middle level. Also, the change in magnitude of the induced stress was almost negligible up to a distance of 30–32 m from the line of extraction. However, the change in the induced stress level within a range 10–30 m was significant, while the change in induced stress level was very rapid, when the distance was less than 10 m. The percentage difference of the induced stress at the roof level and at the mid-level of the pillar reduces when the distance between the observed points and the goaf edge decreases. The results of this numerical model were acceptable as they tally well with the results obtained from the
[1] Bieniawski ZT. Improved design of coal pillars for US mining conditions. In: First Annual Conference on Ground Control in Coal. Morgantown, WV: West Virginia University; 1981. p. 13–22. [2] Obert L, Duvall WI. Rock mechanics and the design of structures in rock. New York: Wiley; 1967 p. 542–45. [3] Salamon MDG, Munro AH. A study of the strength of coal pillars. J S Afr Inst Min Metall 1967;68:55–67. [4] Wilson AH, Ashwin DP. Research into the determination of pillar sizes—Part I. An hypothesis concerning pillar stability; Pat Ii, measurements of stress in two pillars at Lea Hall Colliery. Min Eng 1972;141:409–30. [5] Singh R, Sheorey PR, Singh DP. Stability of the parting between coal pillar working in level contiguous seams. Int J rock mech Min Sci 2002;39(1):9–39. [6] Singh UK, Godara BR. The numerical simulation of failure in rib pillar between longwall panels in Moonidih mine. Minetech 2002;23(3):29–36. [7] Karabin GJ, Evanto MA. Experience with the boundary-element method of numerical modeling to resolve complex ground control problems. In: Proceedings of the Second International Workshop on Coal Pillar Mechanics and Design. Pittsburgh, PA: US Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 99-114, IC 9448, 1999. [8] Adhikary DP, Shen B, Fama MED. A study of highwall mining panel stability. Int J rock mech Min Sci 2002;39(5):643–59. [9] Crouch Research, Inc. The BESOL system: boundary element solutions for rock mechanics problems: user’s guide, version 2.01. St. Paul, MN: Crouch Research, Inc; 1998. p. 5–19.
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
Technical Note
Numerical modeling study of asymmetry in the induced stresses over coal mine pillars with advancement of the goaf line A. Jaiswal, S.K. Sharma*, B.K. Shrivastva Department of Mining Engineering, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India Accepted 12 January 2004
1. Introduction The bord and pillar system of mining is still the predominant method of mining underground coal in India. Even after adoption of the longwall method of mining, bord and pillar has an edge—due to less roof control problems. However, strata control is still a major problem in underground mining. With each passing day, mining operations are becoming more and more difficult as the mining becomes deeper, resulting in more adverse geo-mining conditions. This situation requires the simulation of mine and structure prior to extraction, as well as advanced instrumental monitoring of the strata control parameters for calibrating the numerical model. Designing a coal pillar is a key issue for every bord and pillar mining operation. Mine planners have a variety of modeling methods, both empirical and numerical, for analyzing pillar stresses and determining safe pillar sizes for the various mine geometries and geotechnical conditions. Empirical methods [1–4] emphasize the collection and interpretation of case histories of pillar performance. However, it is difficult to apply these empirical methods to mining situations beyond the scope of the original site characteristics. Therefore, when complicated stress conditions arise due to complex single seam or multiple seam mining geometries, numerical modeling techniques based on finite element, boundary element, discrete element, or finite difference, etc. attain superiority over the empirical methods, providing that they can be adequately validated. Many researchers have contributed in the field of numerical methods for simulating coal panels [5–9]. These numerical or analytical design methods are based *Corresponding author. Tel.: +91-542-236-9442; fax: +91-542-2368174. E-mail address: [email protected] (S.K. Sharma). 1365-1609/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.ijrmms.2004.01.007
on the fundamental laws of force, stress, and elasticity. Their primary advantage is that they are flexible and can quickly analyze the effects of numerous geometrical and geotechnical variables on the mine design. The ability of the three-dimensional (3-D) boundary-element method to model large mine areas with complex geometries has enabled successful simulation of mining conditions. This is also helpful in identifying potential solutions to ground control problems in mines.
2. Simulation of the sub-panel This Technical note deals with the simulation of bord and pillar sub-panel (Fig. 1) of a mine using the Blasting Gallery method (BG) requiring assessment of the stability of pillars until the first major fall. The coal measure formations observed in a typical borehole within the selected mine area are shown in Fig. 2. The thickness of number 3 seam is about 11 m, with an average gradient of 1 in 5.2. The strata overlying the coal seam is comprised of massive sandstone of thickness 22.45 m. A large mine panel of about 1000 m 120 m to 175 m is divided into sub-panels of 160 m 120 m to 175 m by driving main rises. A barrier of about 30 m is left in between the sub-panels. It is necessary to adopt this sub-paneling to restrict the size of the panel so that the extraction is completed within the incubation period. After extraction of the coal, the sub-panel is sealed off. In the present study, the sub-panel of dimensions 160 m 160 m (see Fig. 1) is divided into 16 pillars each with dimensions 40 m 40 m by driving level and dip galleries 4.0 m width and 3 m height. The sub-panel has been simulated using the 3-D boundary element method. Thereafter, the induced stress patterns in the pillars have been determined. The impact of the face advance on pillars has also been determined in terms of changing the
ARTICLE IN PRESS 860
A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
Fig. 1. Plan of sub-panel.
induced stress pattern. Though the complete stress profile was determined, only the induced stress is presented in this paper. These results have been compared with field instrumentation results. For the simulation work, 14 stages of the line of extraction have been taken (the first, seventh and final stages are shown in Fig. 1)—at a regular interval of 4.4 m. The pillar under specific consideration for detailed observation has been shown highlighted in Fig. 1. Induced stresses were also determined at the middle level of this pillar for comparison with the induced stress at roof level (eight points were taken for comparison). A stress monitoring capsule was installed in the pillar for validating the model. The salient features of the selected sub-panel are summarized in Table 1. Table 2 gives the physicomechanical properties of the coal. 2.1. Discretization of the model Fig. 3 shows the discretization of the gallery around one pillar for clarification purposes. However, in the actual simulation process all voids (i.e. goaf and gallery) were discretized with triangular elements of variable
sizes for simulation of the structure by the threedimensional BEM.
3. Results and discussion By gray shading, Fig. 4 (stages 1, 8, 11, 14) shows the induced stress on the pillar with advancing line of extraction. The magnitude of the induced stress is also indicated by different gray shades in Fig. 5 for the pillar highlighted in Fig. 1 (stress distribution patterns on the specific pillar for all fourteen stages) ranging from 5 to 30 MPa (5 MPa being the in situ vertical stress). Fig. 4 shows the induced stress when the line of extraction was the A1-B1 (stage 1, see Fig. 1). It shows that the induced stress is symmetrical around the center of the pillar. The corners are more stressed compared to other locations. The edges also experience higher stress value in comparison to the central portion of the pillar, but less than the corner points. The induced stress values at the corners are about 21 MPa. As the line of extraction starts advancing towards the pillar under consideration (Fig. 1), the induced stress pattern on the pillar changes from symmetrical to
ARTICLE IN PRESS A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
861
Table 1 Salient features of the selected sub-panel Gallery width Gallery height Pillar width (Center to Center of the Gallery) Panel size Goaf height Angle of extraction of line Depth of working seam
4m 3m 40 m 160 m 160 m 11 m 65 253 m
Table 2 Physico-mechanical properties of coal Young’s Modulus of the coal Poisson’s ratio Specific density
5 GPa 0.19 2.01
completely, indicating that the induced stress on the pillar has exceeded 5 MPa. Then the entire pillar is under the influence of the goaf. Later in the eleventh stage, when the line of goaf just touches the pillar’s corner, that corner experiences maximum stress in comparison with that of the remaining pillar—as is clear from gray shaded pattern alterations which can be observed in Fig. 5, stage 14. The induced stress values at the roof level were also compared with the middle level at the selected eight points indicated in Fig. 1 for the highlighted pillar. Fig. 6 depicts the variation of induced stress value with the advance of the line of extraction (distance from point to line of extraction); the induced stress value at the roof level was higher compared to the middle level. When the goaf was beyond the 30–32 m (points 1, 4, 6) the increments of the induced stress from stage 1 to stage 14 were negligible, i.e. just 0.016–0.019 MPa/m. When the line of extraction came nearer to the point between the range 10–30 m, the rate of increasing of the induced stress was significant, i.e. 0.1 MPa/m. When the line of extraction came within 10 m from the point, the induced stress increased rapidly, about 0.6–0.7 MPa/m.
4. Validation
Fig. 2. Bore hole section of the mine.
asymmetrical. The central portions of the pillar in Figs. 4 and 5 depict stress value of 5 MPa. The dark patch diminishes and gradually disappears as excavation proceeds. In the eighth stage, the dark patch vanishes
A comparison of results of the simulation work with the actual results obtained from the field was made at point ‘I’ (see Fig. 1). As is evident from Fig. 7, there is not too much discrepancy between the two curves, indicating that the results obtained were acceptable. The slight irregularity in the field results curve is likely to have been due to local coal/rock inhomogeneity, as evidenced by the fact that the areas beneath the two curves (representing the applied force) are similar.
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Fig. 3. Discretization of single pillar.
Fig. 4. Induced stress pattern on pillar.
ARTICLE IN PRESS A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
863
24 23 22 21 20 19 18 17 16 15 14
Induced Stress (Mpa)
Induced Stress (Mpa)
Fig. 5. Induced stress pattern on the specific pillar in all stages.
20
30
40
50 Distance (m)
60
70
32 30 28 26 24 22 20 18 16 14 0
10
20 Distance (m)
30
20
30
40
50
16 15 14 13 12 11 10 9 8 20
40
60 Mid Level Roof Level
Point 2
30
Mid Level Roof Level
Point 3
40
50 60 Distance (m)
70
80 Mid Level Roof Level
Point 4 24 Induced Stress (Mpa)
24 22 20 18 16 14 12 10 8
22 20 18 16 14
0
10
20 30 Distance (m)
40
30
50
Induced Stress (Mpa)
15 14 13 12 11 10 9 8 20
30
40
Distance (m)
Point 7
50
50
60
70 Mid Level Roof Level
60 70 Distance (m)
80
90 Mid Level
Point 6
16
10
40
Mid Level Roof Level
Point 5 Induced Stress (Mpa)
10
Distance (m)
Induced Stress (Mpa)
Induced Stress (Mpa)
0
80 Mid Level Roof Level
Point 1
Induced Stress (Mpa)
18 17 16 15 14 13 12 11 10 9 8
Roof Level
28 26 24 22 20 18 16 14 0
10
20
30
Distance (m)
Point 8
Fig. 6. Induced stress versus distance from goaf edge at different points.
40
50
60 Mid Level Roof Level
ARTICLE IN PRESS 864
A. Jaiswal et al. / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 859–864
instrumentation for strata monitoring (the stress capsule) at the Mine.
References
Fig. 7. Verification of results of simulation work with the field results.
5. Conclusion On the basis of the present study, the following conclusions can be made. The central portion of the pillar at the roof level experiences the least induced stress; whereas the pillar corners experience the highest stress. As one goes away from the goaf edge, the induced stress decreases. It was also seen, that the induced stress on the pillar was greater at the roof level, as compared to the middle level. Also, the change in magnitude of the induced stress was almost negligible up to a distance of 30–32 m from the line of extraction. However, the change in the induced stress level within a range 10–30 m was significant, while the change in induced stress level was very rapid, when the distance was less than 10 m. The percentage difference of the induced stress at the roof level and at the mid-level of the pillar reduces when the distance between the observed points and the goaf edge decreases. The results of this numerical model were acceptable as they tally well with the results obtained from the
[1] Bieniawski ZT. Improved design of coal pillars for US mining conditions. In: First Annual Conference on Ground Control in Coal. Morgantown, WV: West Virginia University; 1981. p. 13–22. [2] Obert L, Duvall WI. Rock mechanics and the design of structures in rock. New York: Wiley; 1967 p. 542–45. [3] Salamon MDG, Munro AH. A study of the strength of coal pillars. J S Afr Inst Min Metall 1967;68:55–67. [4] Wilson AH, Ashwin DP. Research into the determination of pillar sizes—Part I. An hypothesis concerning pillar stability; Pat Ii, measurements of stress in two pillars at Lea Hall Colliery. Min Eng 1972;141:409–30. [5] Singh R, Sheorey PR, Singh DP. Stability of the parting between coal pillar working in level contiguous seams. Int J rock mech Min Sci 2002;39(1):9–39. [6] Singh UK, Godara BR. The numerical simulation of failure in rib pillar between longwall panels in Moonidih mine. Minetech 2002;23(3):29–36. [7] Karabin GJ, Evanto MA. Experience with the boundary-element method of numerical modeling to resolve complex ground control problems. In: Proceedings of the Second International Workshop on Coal Pillar Mechanics and Design. Pittsburgh, PA: US Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 99-114, IC 9448, 1999. [8] Adhikary DP, Shen B, Fama MED. A study of highwall mining panel stability. Int J rock mech Min Sci 2002;39(5):643–59. [9] Crouch Research, Inc. The BESOL system: boundary element solutions for rock mechanics problems: user’s guide, version 2.01. St. Paul, MN: Crouch Research, Inc; 1998. p. 5–19.