DOI 10.1007/s11204-016-9375-7 Soil Mechanics and Foundation Engineering, Vol. 53, No. 2, May, 2016 (Russian Original No. 2, March-April, 2016)

DESIGN COMPRESSION RATIO DESIGN AND RESEARCH ON LOWER COAL SEAMS IN SOLID BACKFILLING MINING UNDER URBAN AREAS

Li Huai-zhan, Zha Jian-feng, Guo Guang-li, Zhao Bin-chen, Wang Bing

UDC 624.131.439.4

School of Environment Science and Spatial Informatics; NASG Key Laboratory of land Environment and Disaster Monitoring, China University of Mining and Technology, Xuzhou, China.

Large coal resources occur under buildings in the Iron Third District of Tangshan Mine, which not only greatly reduces the service life of the coal mine, but also wastes the coal resources. To ensure the safety of ground buildings and maximize exploitation of the coal resources, solid backfill mining technology is proposed. This paper studies the compression ratio of the coal seam during backfill mining with a global numerical model based on the finite difference software, FLAC3D. The compression ratio of the coal seam was also similarly studied based on theoretical analysis and the local model. Introduction

The Iron Third District of Tangshan Mine is located in the east-central district of Tangshan City, and the surface buildings and structures are dense and crowded. The Nos. 5, 8, and 9 coal seams have been exploited by strip mining. This can guarantee the safety of the surface buildings but will cause great deal of wastage of the coal resources and will conflict with the requirements of national coal resource exploitation. On the other hand, a new green coal mining technology, the gangue backfilling technology, has been widely applied in China [1-7], which can dispose of waste and effectively control surface subsidence. This technology has been used in coal mines in Xing Tai, Yan Zhou, and Xin Wen and has achieved success. In the background, in order to maximally exploit coal resources and improve the service life of coal mines, fully mechanized backfilling mining technology is proposed to exploit the No. 9 coal seam, but it is doubtful whether this mining scheme could cause instability of the No. 8 coal pillar and surface subsidence. The basic approach of the strip mining is to divide the areas to be mined into formal stripped shape. The mining operation exploits one strip at a time and keeps one strip. The remaining strip coal pillars can usually support the load of the overlying strata and cause slight and even surface movement and deformation, which can recover some coal resources and control surface subsidence. Some scholars have studied the surface subsidence of strip mining [8-14]. The key to safe operation of strip mining is the stability of the partitioning coal pillars. The instability of partition coal pillars will seriously threaten the safety of surface buildings and structures. For example, between 1980 and 1990, the U.S. has had many failures and instability of the coal pillars during room and pillar mining. These examples occurred suddenly without clear signs. Most of the examples have caused storm and destroyed the underground ventilation systems [15]. Cui Xi-min has analyzed the cases and mechanism of larger area of surface subsidence caused by instability of the coal pillar in the Da Tong area of Shan Xi [16]. At present, the main evaluation methods of strip coal pillar stability are the safety factor method, the numerical simulation method, and the A. H. Wilson theory. The safety factor is the ratio of the maximum loads that a strip coal pillar can withstand to the actual load. When the safety factor exceeds 1,

Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, p. 20, March-April, 2016. 0038-0741/16/5302-0125

©

2016 Springer Science+Business Media New York

125

the coal pillar is stable; otherwise, the coal pillar is unstable [17-18]. The numerical simulation evaluation method seeks to evaluate the stability of coal pillars based on strip coal pillar deformation conditions and stress distribution. Mahdi Shabanimashcool has used the numerical simulation method to study the stress changes in boundary pillar mining and their impacts on the stability of neighboring coal pillars [19]. The A. H. Wilson theory states that when the stress of coal pillars exceeds the yield point and flows to the gob area from the area of peak stress to the boundary pillar, forming the yield zone (also called the plastic zone). The width of the strip coal pillar's yield zone Y = 0.0049 mH. Guo has analyzed the failure and instability mechanism of horizontal strip coal pillars by using the A. H. Wilson theory and deduced the requirements for strip coal pillar failure and instability. He has pointed out that strip pillar failure and instability are likely when the ratio of the yield zone's width to the coal pillar's width is no less than 88.08%, or when the core region ratio of strip coal pillars is no more than 11.92% [20]. The stability of coal pillars is related to geological factors, mining factors, and mechanical properties. The geological factors include the nature of the geological structure, mining depth, density of the overlying strata, coal seam dip angle, wall rock conditions, ground stress, and groundwater effects. Mining factors mainly include the mining width, coal pillar width, recovery ratio, strip coal pillar height, disposal methods for the gob area, and underground mining methods. The mechanical properties of coal pillars include uniaxial compressive strength, elasticity modulus, cohesive force, internal friction angle, internal structure, weak plane, and cohesive force between the coal pillar and roof. Shabanimashcool has pointed out that no theory can take all the factors into consideration, while numerical simulation method provides an access to consider all the factors [19]. To address concerns, this paper studies the compression ratio of the No. 9 coal seam during backfill mining by theoretical analysis and three-dimensional numerical simulation, with the objectives of ensuring the safety of the surface buildings and structures, maximize exploitation of coal resources, and minimize use of gangue material. The research results are of vital realistic significance to the exploitation of coal resources exploitation in the Tangshan mines. The Iron Third District is a built-up area. Most buildings are one- or two-tiered brick and concrete structures. The quality of the buildings is different and is better than rural houses. According to the Regulations of Buildings, Water Bodies, Railways, and Main Shaft and Tunnels Pillar Design and Compressed Coal Mining, surface buildings protection standards of this district require that horizontal deformation and surface inclination deformation should be no more than 1.5 mm/m. The maximum surface subsidence should be set at 1000 mm considering that it is easy to cause invalidation of the urban drainage system when the surface subsidence is too large. Safety Analysis of Surface Buildings

Due to the complex surface movement and deformation when mining the Nos. 5 and 8 coal seams by strip mining and mining the No. 9 by backfill mining, it is hard to predict surface subsidence by the probability integration method [21-22]. Thus, a global model of this area is established with FLAC3D software based on the geological conditions so as to reasonably determine the No. 9 coal seam's compression ratio during the backfilling mining according to the protective indicators of ground buildings. The mining thickness of the Nos. 5, 8, and 9 coal seams is 2.3, 3.57, and 6 m, and dip angle is 3°, which can be considered to be horizontal seams. The mining and pillar width of the Nos. 5 and 8 is 60 and 90 m, respectively. The simulated rock strata simplified the synthesis column map of the mine. The rock mechanical parameters were initially determined according to laboratory testing and then further adjusted based on field survey data after the strip mining of the Nos.5 and 8 (Table 1). The rectangular coordinate system is used in the model. Its size is 1,8001,500 m with 144,000 units and 189,771 nodes. The boundary conditions for the model are: horizontal displacement constraints in the sides, vertical displacement in the bottom, and free boundary at the top. To study the impact of different compression ratios on surface movement during gangue backfill mining, the filling bodies elasticity modulus is changed in the experiment in order to change their rigid126

TABLE 1 Rock

Thickness, m

Topsoil Mudstone2 Medium sandstone Coarse sandstone Fine sandstone 2 Sandy mudstone 2 No.5 coal seam Sandy mudstone 1 Fine sandstone 1 No.8 coal seam Mudstone 1 No.9 coal seam Siltstone

120 54 84 70 20 84 3 20 12 4 34 6 30

Density, Elasticity mod- Poisson's kg/m3 ulus, GPa ratio 1,800 2,200 2,661 2,680 2,630 2,200 1,427 2,200 2,630 1,427 2,200 1,427 1,970

0.01 5.5 10.5 10.9 10.09 5.7 1 5.7 10.09 1 5.5 1 9

0.3 0.19 0.25 0.2 0.26 0.2 0.36 0.2 0.26 0.36 0.19 0.36 0.25

Cohesion, Tensile MPa strength, MPa 0.02 1.56 4.25 3.27 3.47 3.27 0.53 3.27 3.47 0.53 1.56 0.53 1.56

0.01 1.21 2.65 1.71 1.8 1.7 0.43 1.7 1.8 0.43 1.21 0.43 1.21

Internal frictional angle, deg 20 27 34 32 32 30 25 30 32 25 27 25 27

TABLE 2 Elasticity Bulk mod- Shear modumodulus, MPa ulus, MPa lus, MPa 150 102.39 36.39 17.58 5.4 4.5

100 68.26 24.26 11.72 3.6 3

60 40.96 14.6 7.03 2.16 1.8

Cohesion, MPa 1 1 1 1 1 1

Tensile Internal frictionstrength, MPa al angle, deg 1 1 1 1 1 1

30 30 30 30 30 30

Density, kg/m3

Compression ratio (%)

1700 1700 1700 1700 1700 1700

97 90 87 80 70 68

ity and to further adjust the filling bodies' compression ratio. By adjusting the filling bodies' elasticity modulus to simulate the compression ratio, this experiment has six backfilling schemes with compression ratio of 68%,70%, 80%, 87%, 90%, and 97%. Each scheme's mechanical parameters of filling bodies for each scheme are shown in Table 2. Based on the above six schemes, the surface subsidence, inclination, and horizontal deformation of each scheme were calculated. Each scheme's maximum surface movements and deformations were calculated by numerical simulation. As shown in Fig.1, the maximum surface subsidence decreases as the compression ratio η increases: w = 37,672exp(-0.05η) and the correlation coefficient R2 = 0.9934. For the maximum surface inclination deformation the equation is i = 152.17exp(−0.058η): R2 = 0.9957. For the maximum horizontal surface deformation the equation is ε = 82.173exp(−0.052η): R2 = 0.9833. The protective indicators of the buildings are that the surface horizontal deformation should not exceed 1.5 mm/m, the surface inclination deformation should not exceed 1.5 mm/m, and the maximum surface subsidence should not exceed 1,000 mm. As shown in Fig.1, when η > 72.55%, the maximum surface subsidence is no more than 1,000 mm; when η < 80%, the maximum surface inclination deformation is no more than 1.5 mm/m; when η < 78%, the maximum surface horizontal deformation is more than 1.5 mm/m. Therefore, the compression ratio of the No. 9 coal seam during backfilling mining should exceed 80% to ensure the safety of the surface buildings. Theoretical Analysis of Strip Pillar Stability During multiple coal seam mining, the stability of the upper coal pillars must ensure the stability of the pillar itself and not destroy the interlayer rock body. The safety factor method can be used to evaluate the stability of the coal pillar itself. The mining and retaining pillar width of the No. 8 coal seam are 60 and 90 m. Without the influence of exploitation of the No. 9 coal seam, the No. 8 coal pillar has good stability. If the mining depth, interlayer spacing, interlayer lithology, upper coal seam mining, and retaining pillar width are assured, lower coal seam mining should be controlled to ensure the stability of the interlayer rock body. The corresponding mechanical model was established to evaluate the stability of the interlayer rock mass during lower coal seam backfilling mining. Given that mechanical models are symmetric, half of the model was chosen to conduct the analysis and the unit length along the z direction 127

a mm 1,400 1,200 1,000 800 600 400 200

b

65 70

Fig.1.

75

80

85

90

95 %

c mm 3.0 2.5 2.0 1.5 1.0 0.5 0 65 70

75

80

85

90

mm 3.0 2.5 2.0 1.5 1.0 0.5 0 65 70

95 %

75

80

85

90

95 %

Curves of surface movement and deformation change with compression ratio: a) maximum surface subsidence, b) maximum surface inclination deformation, c) maximum surface horizontal deformation. Y surface

H q upper coal seam h lower coal seam

2l

X

Fig. 2. Model of stability evaluation model of the interlayer rock body in backfilling mining of the lower coal seam.

was selected. The starting point of the left beam represents the coal wall of the working face, and the length of beam l refers to half of the backfilling zone. Meanwhile, the filling body is assumed to be an elastic foundation and can support the interlayer rock body; the support force is p(x), as shown in Fig. 2. According to Winkler's assumption, the subsidence of any arbitrary point on the surface is proportional to the stress per unit area at that point: p = kw.

(1)

Because the model is a finite long beam, the deflection equation of the finite elastic foundation beam under a uniformly distributed load can be obtained by analyzing the beam: w( x ) = w0ϕ1 + θ

ϕ 1 − ϕ1 ϕ2 ϕ − M 0 3 2 − Q0 4 3 + q0 , k β EI β EI β

(2)

where k = E1/h1; (3) I = 1/12h3; q0 = γH; β = 4 k / 4 EI ; ϕ1(βx) = chβxcosβx; ϕ2(βx) = 1/2(chβxsinβx + shβxcosβx); ϕ3(βx) = 1/2shβxsinβx; ϕ4(βx) = 1/4(chβxsinβx − shβxcosβx). According to the beam's boundary conditions ⎧w ( x ) x = 0 = 0 ⎪ ⎨ dw( x ) ⎪⎩θ ( x ) x = 0 = dx

x=0 = 0

;

(4)

⎧Q (x )x =1 = 0 ⎪ . ⎨ (5) dw( x ) ⎪θ ( x ) x = l = x =l = 0 dx ⎩ The deflection equation and bending moment equation for the finite elastic foundation beam under a uniformly distributed load can be obtained. According to the beam's deformation characteristics 128

under stress, the maximal bending moment is in the right of the beam with the maximum bending stress, given by: M (l ) = −

2 EI β 2 q0 sh(2l β ) − sin(2 l β) 4 EI β 2 q0 ch(2 l β) − cos(2 l β) ϕ1 (lβ ) − ϕ 2 ( lβ ) + k sh(2l β ) + sin(2 l β) k sh(2 l β) + sin(2 l β)

EI β 2 + 4 q0 ϕ 3( l β). k

(6)

According to the beam's strength theory, we have

σmax = M(l)/W; W = 1/6h2,

(7) (8)

where h is the height of the beam and W is the modulus of the bending section. Because the tensile strength of the rock mass is less than the compressive and shear strength, the tensile damage constitutes the main damage in the interlayer rock mass. To ensure no damage in the interlayer rock mass, the followings formula should be followed:

σmax < [σ],

(9)

where [σ] represents the average tensile strength in the interlayer rock mass, tons/m2. The actual height of the filling body h1 can be calculated according to formulas (3), (5), (6), (7), and (8), and the compression ratio η can be obtained based on the mining thickness of the coal seam m.

η = (m − h1)/m.

(10)

Based on the rock parameters and the corresponding data for Tangshan Mine's, the stability of the No. 8 coal seam can only be ensured if the compression ratio of the No. 9 coal seam during backfill mining should not be less than 85%. Numerical Analysis of Coal Pillar Stability

The average distance between the No. 9 coal seam and the No. 8 strip coal pillar is only 34 m in the Third District of Tangshan Iron Mine. The mining thickness of the No. 9 coal seam is 6 m. The key to designing the compression ratio is whether the backfill mining of the No. 9 coal seam will influence the stability of the No. 8 strip coal pillar. Therefore, this area's local model is established based on the geological conditions, using FLAC3D software. The compression ratio of the No.9 coal seam during backfill mining is determined reasonably based on the stability of the No. 8 strip coal pillar. Based on the geological conditions in the Third District of Tangshan Iron Mine, the model was also established by using finite difference software FLAC3D. Given that the local model aims to analyze the stability of the Nos.5 and 8 strip coal pillar, only the Nos. 5, 8, and 9 coal seams and their roof and floor were established in the local model. Meanwhile, a stress of 14.6 MPa is exerted on the immediate roof of the No.5 coal seam, which was obtained from the global model. The similar rectangular coordinate system was used for this local model. The model size was 600600 m with 612,000 zones and 930,25 nodes. The boundary conditions of the model are: horizontal displacement constraints at the sides, vertical displacement at the bottom, and a free boundary at the top. To evaluate the stability of the coal pillar with different η during solid backfilling mining, the elasticity modulus of filling bodies is changed in the experiment to simulate the compression ratio of the filling bodies. By adjusting the elasticity modulus of filling bodies in the local model, the simulated compression ratio is 89.35%, 88.30%, 86.39%, 85.23%, and 81.71%. Figure 3 shows the distribution of the plastic zone of the No.8 coal seam for different compression ratios. 129

a

b

Fig.3.

c

d

Distribution of the plastic zone in the No.8 coal seam with different compression ratios: a) without mining No.9 coal seam; b) η = 89.35%; c) η = 86.39%; d) η = 81.71%.

TABLE 3 η, %

89.35 88.30 86.39 85.23 81.71

Percentage of the plastic zone in the total zone for No. 8 coal seam, % left coal pillar right coal pillar 55.17 53.40 55.56 53.47 70.30 66.98 70.68 68.63 73.00 73.15

As shown in Fig.3, without mining the No. 9 coal seam, the plastic zone of the No. 8 coal seam is mainly near the mining area, and there is a wider range of the core zone between pillars, and the pillars are stable. When using the backfill mining technology to mining the No.9 coal seam, the plastic zone range of the No. 8 coal seam zone expands. As η decreases, the plastic zone range of the No. 8 coal seam increases gradually, which is consistent with the actual situation. Therefore, reasonable design of the compression ratio for the No. 9 coal seam is an important guarantee of the strip pillar stability of the No. 8 coal seam. From the plastic zone of the No.8 coal seam in the above five models, the percentage of the plastic zone in the total zone can be obtained with different compression ratios, as shown in Table 3. This paper evaluates the stability of strip pillars according to Guo's conclusions [20], which were obtained by combining the A. H. Wilson's theory with catastrophe theory. It is found that failure and instability of the strip pillars can occur when the ratio of the yield zone width to the coal pillar width is no less than 88.08%, or the core zone ratio of the strip coal pillar is no more than 11.92%. That is to say, it is likely that instability of the coal pillar will occur when the No. 8 coal seam plastic zone's percentage in the coal pillar unit exceeds 88.08%. Considering that a certain safety factor will be chosen in a project, 1.5 is taken as the safety factor in this paper. That is, the compression ratio of the No. 9 coal seam should be that the percentage of the plastic zone in the coal pillar unit should be no more than 58.72%. The relationship between the compression ratio and the percentage of the plastic zone in the total zone is not linear, and as η decreases, the percentage of the plastic zone in the total zone increases. When η = 87.7%, the stability of the No. 8 strip coal pillar can be ensured during backfilling mining of the No. 9 coal mine. Above all, in order to ensure the stability of the No.8 strip coal pillar, the compression ratio of the No. 9 coal seam during backfill mining should not be lower than 87.7%. Conclusions

Considering the limitations of the probability integral method in predicting surface subsidence in multiple coal seam mining, we used theoretical analysis and numerical simulation to study the feasibility of backfill mining of the No. 9 coal seam and the design of the compression ratio. The following conclusions can be drawn: 1) Based on the beam theory, the evaluation method of the interlayer rock mass stability in backfill mining was established and used in the Third District of Tangshan Iron Mine to design the com130

pression ratio of the No. 9 coal seam in backfill mining. It is found that the compression ratio should exceed 85% to ensure the stability of the interlayer rock mass. 2) According to the global model, the compression ratio should not be less than 80%, so that the impact of mining damage on ground buildings does not exceed the protective indicator. 3) According to the local model, the compression ratio of the No. 9 coal seam during backfill mining should not be less than 87.7% so as to ensure the stability of the No. 8 strip coal pillars. Therefore, if the compression ratio of the No. 9 coal seam in backfill mining is 87.7%, the security of surface buildings, the maximum exploitation of coal resources, and the minimum usage of gangue can be ensured and realized. Acknowledgements

This work was funded by the Fundamental Research Fund for the Central University (No. 2013QNB07). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

X. X. Miao, J. X. Zhang, and G. L. Guo., "Study on waste-filling method and technology in fullymechanized coal mining," J. China. Coal. Soc., 35(1), 1-6 (2010). Y. L. Huang, J. X. Zhang, and Q. Zhang, "Strata movement control due to bulk factor of backfilling body in fully mechanized backfilling mining face," J. Min. Safe. Eng., 29(2), 162-167 (2012). J. X. Zhang, Y. J. Zhou, and Y. L. Huang, "Integrated technology of fully mechanized solid backfill mining," Coal Sci. Technol, 40(11), 10-13 (2012). Y. L. Huang, "Ground control theory and application of solid dense backfill in coal mines," Chin. Univ. Min. Technol., (2012). X. X. Miao, Y. L. Huang, and Ju. F, "Strata movement theory of dense backfill mining," J. China. Univ. Min. Technol., 41(6), 863-867 (2012). J. F. Zha, "Study on the foundational problems of mining subsidence controlled in waste stow," Chin. Univ. Min. Technol. (2008). G. L. Guo, X. X. Miao, and J. F. Zha, "Priliminary analysis of the effect of controlling mining subsidence with waste stow for long wall working face," Chin. Sci. Pap. Online., 3(11), 805-809 (2008). W. B. Guo, "Surface movement predicting problems of deep strip pillar mining," J. China. Coal. Soc., 33(4), 368-372 (2008). W. Guo, Q. Hou, and Y. Zou, "Relationship between surface subsidence factor and mining depth of strip pillar mining," Trans. Nonferrous. Metal. Soc., 21, s594-s598 (2011). L. Zhang, K. Z. Deng, and C. Zhu, "Analysis of stability of coal pillars with multi-coal seam strip mining," Trans. Nonferrous. Met. Soc., 21, s549-s555 (2011). G. Wei, "Study on the width of the non-elastic zone in inclined coal pillar for strip mining," Int. J. Rock. Mech. Min., 72, 304-310 (2014). G. L. Guo, J. F. Zha, and W. U. Bin, "Study of "3-step mining" subsidence control in coal mining under buildings," J. Chin. Univ. Min. Technol., 17(3), 316-320 (2007). G. Liu, H. X. Zhang, and N. Z. Xu, "Coal pillar stability of deep and high seam strip-partial mining," J. Chin. Coal Soc., 33(10), 1086-1091 (2008). L. M. Yuan, and J. Z. Wang, "Analysis for strata movement in strip-and-pillar mining," Chin. J. Rock. Mech. Eng., 9(2), 147-153 (1990). R. Karl, J. Zipf, and C. Mark, "Design methods to control violent pillar failures in room and pillar mine," Trans. Inst. Min. Metall., 106, 124-131 (1997). X. M. Cui, Y. Gao, and D. Yuan, "Sudden surface collapse disasters caused by shallow partial mining in Datong coalfield, China," Nat. Hazards, 74(2), 911-929 (2014). G. Q. He, Lun. Yang, and G. J. Ling, "Mining subsidence," Chin. Univ. Min. Technol., (1991). Y. f. Zhou, K. Z. Deng, and W. M. Ma, "Mining subsidence engineering," Chin. Univ. Min. Technol., (2003). M. Shabanimashcool and C. C. Li, "A numerical study of stress changes in barrier pillars and a border area in a long wall coal mine," Int. J. Coal Geol., 106, 39-47 (2013). W. B. Guo, "Research on the nonlinear theoretical models and their applications in strip-partial mining," Chin. Univ. Min. Technol., (2004). L. Y. Zhang, and K. Z. Deng, "The law of surface movement for multi-coal seam strip mining," J. Chin. Coal Soc., 33(1): 28-32 (2008). K. Z. Deng, H. D. Fan, and Z. X. Tan, "Research on pillar stability evaluation method of multiple coal seam strip mining," Chin. Sci. Pap. Online, 4(11), 824-829 (2009). 131