Mining-induced subsidence prediction by Displacement Discontinuity Method: A case from Barapukuria longwall coal mine, Bangladesh

doi:10.21203/rs.3.rs-1654901/v1

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Mining-induced subsidence prediction by Displacement Discontinuity Method: A case from Barapukuria longwall coal mine, Bangladesh

https://doi.org/10.21203/rs.3.rs-1654901/v1

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It is essential to predict the mining-induced subsidence for sustainable mine management. The maximum observed subsidence having a noticeable areal extent due to Northern Upper Panels (NUP) and Southern Lower Panels (SLP) at the Barapukuria longwall coal mine is 5.8 m and 4.2 m, respectively, after the extraction of a 10 m thick coal seam. The mining-induced subsidence was simulated by the Displacement Discontinuity Method (DDM). The numerical model considered the effects of the ground surface, mining panels, faults, and the dyke. The predicted and the observed subsidence due to the mining of NUP and SLP were compared varying Young's modulus, and the 0.10 GPa Young's modulus was found to be the best match in the geo-environmental condition. The effects of the faults and the dyke in the calculation were negligible. Future subsidence was predicted by considering 30 m extraction of the thick coal seam as 15.7–17.5 m in NUP and 8.7–10.5 m in SLP. The vulnerable areas demarcated considering the tilt angle and extensile strain might extend up to the coal mine office area and some villages.

DDM

Young’s modulus

Longwall coal mining

Faults

Dyke

Subsidence is allowed in the longwall coal mining method, and because of that, the stress-induced accidents are lower in this mining system, having a higher production rate. As subsidence is a must in a longwall coal mine without stowing, it is essential to predict the mining-induced subsidence for sustainable mine management. During material extraction by longwall mining may induce several types of ground movements such as vertical ground displacements, ground curvature (tilt angle), and horizontal ground strain (extensile strain) at the surface¹. Buildings and infrastructures on the surface might be damaged² depending upon the position. Surface subsidence was first observed in 2006, evident from cracks in the surface structures of the Barapukuria (Bangladesh) mining area (Fig. 1), and the government has acquired 2.61 km² of the affected land area^{3, 4}.

Several prediction methods have been developed and classified into empirical, semiempirical, and numerical methods and are the graphical methods, profile function methods, and influence function methods. Graphical methods are derived from extensive field data by the NCB⁵ concerning a particular geo-environment context. The profile function method follows a curve-fitting procedure to match the predicted profile with observed profiles by mathematical functions⁶. These methods suffer from the same disadvantage as the graphical methods having many profile functions⁵: they can be used for a particular geo-environmental condition and are developed to predict a two-dimensional subsidence profile. The superposition principle is being used in the Influence function methods were developed by Ren, Reddish, and Whittaker⁷ and are used extensively⁸ to predict mining subsidence. Different coefficients are suggested to adjust the superposition with exact questionable influence and application fields⁸. The subsidence due to longwall coal mining at Barapukuria, Bangladesh, was tried to predict by the empirical methods, but the observed and the predicted subsidence showed noticeable miss-match and might not be suitable for the geo-environmental condition⁹.

Numerical methods like the finite element method¹⁰, the distinct element method or the finite differences method^{11, 12} physical models^{13, 14} and, GIS and remote sensing¹⁵ have been used for subsidence prediction in the different mines and can be very accurate when validated. In this research, we have tried to predict the mining-induced subsidence of the Barapukuria longwall coal mine by the Displacement Discontinuity Method (DDM). DDM was originally developed by Grouch and Fairhurst¹⁶ as a kind of the boundary element method (BEM) and especially aiming for the application to tabular excavations. They presented algorithms to effectively obtain elastic solutions for mine-wide stress change due to the mining of parallel ore seams. In the algorithm, parallel ore seams are divided into square displacement discontinuity (DD) elements, and boundary conditions are assigned according to the mining indices, which were unmined, mined, or closed. Simultaneous equations, each representing stress change in an infinite elastic body by a DD element, are solved to obtain the elastic solution. The current authors modified the above method so that the ground surface, mining panels, faults, and dykes at any orientations could be divided by rectangular DD elements and used here. The predicted and the observed subsidence were compared and, the future subsidence and the approximate vulnerable areas are demarcated for the geo-environmental condition.

Tectonically Bangladesh can be broadly subdivided into two zones (i) Stable Platform (SP) (ii) Geosynclinal Basin (GB) that are separated by a narrow northeast-southwest trending shelf edge/ slope break known as Hinge Zone (HZ)¹⁷ (Fig. 2). The SP is relatively geologically stable and is situated in the northern part of the HZ. The GB is in the south, characterized by thick sedimentary rock layers resulting from rapid subsidence and sedimentation in a relatively short span of geological time.

The SP and GB can be sub-divided into two subzones each; Dinajpur shield and Bengal shelf in SP; Folded belt and Foredeep in GB. The Dinajpur shield¹⁸ has a thin sedimentary cover above the Precambrian basement rock, whereas the Bogra shelf has moderately thick sedimentary rock layers gently dipping towards the HZ. Folds characterize the Folded belt, and the intensity of the folding is greater in the eastern part in comparison to the western part of thick sedimentary rock layers. The Foredeep zone is characterized by horizontal to sub-horizontal relatively thick sedimentary rock layers without major tectonic deformation.

Five coal basins, namely Barapukuria, Phulbari, Khalashpir, Dighipara, and Jamalgonj, have been discovered in the SP of Bangladesh¹⁷. Among four of them, including the Barapukuria coal basin where the only coal mine is being operated is situated in the Dinajpur shield, where the coal seams are in relatively shallow depth starting from 131 m to 328 m depending upon the coal basins. The Jamalgonj coal basin is in the Bogra shelf, where the coal seam is encountered at a relatively deeper depth of 640 m.

Barapukuria coal basin is a graben, an asymmetrical faulted syncline (Fig. 3), with an approximately N-S axis¹⁹. The rock sequence of the coal basin consists of the following five units¹⁹. (1) Madhupur Clay Formation (MC) (2) Upper Dupi Tila Formation (UDT) (3) Lower Dupi Tila Formation (LDT) (4) Gondwana Formation (GW) and (5) Basement Complex (BC).

The MC is Holocene to recent in age and about 1–15 m thick²⁰. The MC is underlain by DT, mainly a Late Miocene –Middle Pliocene aged layer. The UDT is mainly an unconsolidated to partly consolidated sand layer; with medium to coarse-grained, occasionally gravelly with bands of silt with an average thickness of about 94 to 126 m in the basin^{19, 20}, having a thickness of almost 100 m in the mine area (Fig. 2). The LDT consists of sandstone, silt, and white clay, and the thickness varied from 0 to 80 m in the basin^{19, 20}, which is 0 to 60 m in the mine area (Fig. 2). The DT is underlain by GW, a Permian-aged coal-bearing rock layer unconformable on the Basement Complex. This rock sequence is up to 390 m thick^{19, 20} in the basin, which is about 150–300 m in the mine area (Fig. 2), consisting of predominantly arkosic sandstone with subordinate siltstones, shales and, breccia-conglomerates with occasional interbedded siltstone, sandstones²⁰. The coal seams are found in the GW. The average thickness of the thickest coal seam of the basin is about 36 m. The coal seam has a gentle slope of 13° to 19°, dipping towards the east. The BC is mainly a layer of diorite, meta-diorite, ophlitic gneiss, and granite rock²⁰.

The western part is more faulted than the southern part of the Barapukuria coal basin²¹ (Fig. 4). Faults bound the basin east by Eastern Boundary Fault (EBF) and west by numerous. The faults within the basin can be divided into (i) intra-basinal faults and (ii) boundary faults. The EBF is downthrown at 70°–75° in the west and having a vertical displacement of about 200 m is around 5 km in length with NNW-SSE and N-S strike²¹. The faults of the west have the strike mainly of NNW – SSE and in some portion of about NNE-SSW. There are several intra-basinal faults with the throw about 10 m within the coal-bearing rock layer in the mine area. A dyke, an igneous intrusion, has been detected in the northern mining panels with a strike of around NEE- SWW.

The uniaxial compressive strength (UCS) of the coal-bearing rock (GW) (Fig. 5) is relatively high 35.61 ± 17.08 MPa (n = 3) with a bulk density of 2.30 ± 0.20 g/cm³ (n = 3) in DOB 5, which is the southern up-dip portion of the basin. The UCS is moderately ranged from 20.91 ± 11.22 MPa (n = 10) with bulk density 2.22 ± 0.15 g/cm³ (n = 10) in DOB 4 to 21.57 ± 8.98 MPa (n = 7) with bulk density 2.17 ± 0.62 g/cm³ (n = 7) in DOB 10 which represents the almost central and southern down-dip portion of the basin. In central up-dip portion represented by DOB 8, the UCS is the lowest of 12.34 ± 6.61 MPa (n = 6) with a bulk density of 2.02 ± 0.36 g/cm³ (n = 6)¹⁹.

Many faults (Fig. 4) with weaker rocks (Fig. 5) are in the central up-dip portion than those of the central and southern portion of the basin, where few faults with stronger rocks are. The ground in the south is relatively stronger than that of the central part of the basin. The central up-dip ground is weaker than that of the central portion of the basin.

3.1 The observed subsidence

From the bird’s eye view of the Barapukuria coal mine area, the subsided area can be divided into two regions (Fig. 6a), i.e., the northern and southern parts considering the subsidence epicenters²². The subsidence in the north is just above the Northern Upper Panels and named as NUP, and it is above the Southern Lower Panels in the south named as SLP. The observed subsidence is shown as a contour map in Fig. 6b ²³. The subsidence in the north can be further subdivided into North-Western and North-Eastern zones. The maximum subsidence in the North-Western and the North-Eastern zones is 5.8 m and 4.6 m, respectively (Fig. 6b), whereas; it is 4.2 m in the southern part. The observed subsidence of the contour map was converted to grid values having a specific range in the modeled grid area to compare the observed and the predicted subsidence.

3.2 Modification of the DDM method

The algorithm by Crouch and Fairhurst¹⁶ focuses on effectively obtaining mine-wide stress distribution by mining parallel tabular ore seams with limited computer resources at the time of publication. We modified the algorithm so that non-parallel rectangular elements could be used.

3.3 The model and the simulation

The ground surface of 2500 × 1560 m² was divided by 40 × 25 DD elements, and the free surface condition was assigned. Each mining panel, fault, or dike was approximated by a rectangular plane (Fig. 7) divided into 4–22 DD elements. The element division is not fine enough due to the memory limitation. One of the reasons is that the matrix to be solved BEMs, including DDM, is not sparse, and techniques such as the band matrix method for FEM cannot be used. Also, the used compiler (Microsoft FORTRAN Power Station, ver.4.0) is not the latest version and generates only 32-bit executables. This problem should be solved in the future.

The mining height was assigned as 10 m on average, the first 3 m slice of coal was extracted by conventional longwall mining, and the next 7 m slice was extracted by the longwall top coal caving (LTCC) method. A friction angle of 30° was assigned to the faults and the dyke.

The calculation should be carried out for the case in which the ground surface, mining panels, faults, and the dyke existed (Case1) and the case without mining panels (Case2), and subsidence for Case2 was subtracted from that for Case1 to obtain subsidence by mining panels. However, calculation with the ground surface and mining panels (Case3) was carried out first for simplicity. The calculated results show a peak at NUP and another peak at SLP for lower Young's modulus, and only one peak at NUP for higher Young's modulus (Fig. 8).

4.1 The selection of the best value for Young’s modulus

The Young's modulus of rock, rock-like material, and rock mass varies with environmental conditions^{24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34}. It is also known that Young's modulus of the rock mass is much smaller than Young's modulus of intact rock specimens. In other words, it is not easy to deterministically fix Young's modulus value. The selection of Young's modulus was performed by back analysis. The peak values are saturated by the closure of the mining panels for lower Young's modulus and decrease with Young's modulus (Fig. 9). As a result of comparing the calculated results with the observation, Young's modulus of 0.1 GPa was selected as the best value. The predicted subsidence distribution (Fig. 8, 0.1 GPa) well simulated the observed one (Fig. 6) with a slightly different areal extent.

4.2 Effects of the faults and the dyke

The subsidence due to mining panels, faults, and the dyke (Fig. 10a) is almost the same as the subsidence without faults and the dyke (Fig. 8, 0.1 GPa). The contribution by the faults and the dyke is almost negligible (Fig. 10b).

4.3 Future subsidence and vulnerable area

The future subsidence was predicted by considering a 30 m thick coal extraction of the thickest (36 m) coal seam. The maximum subsidence of 15.7–17.5 m in the NUP and 8.7–10.5 m in the SLP is predicted in the mining area (Fig. 11). The effects of the faults and dykes were not included because the effect was expected to be negligible. For proper/sustainable mine management, the mining authority should count on this subsidence issue. The future vulnerable areas plot (Fig. 12), considering the 0.3% tilt angle and 0.2% extensile strain on the mine area to demarcate the potential danger area might extend up to the coal mine office area and some villages (Fig. 13) considering LTCC without backfilling. Backfilling or other mining methods might be considered for the total extraction of the 30 m coal seam. They might be able to reduce the areal extent of the potential damage zone on the surface.

The maximum observed subsidence having a noticeable areal extent due to Northern Upper Panels (NUP) and Southern Lower Panels (SLP) at the Barapukuria longwall coal mine is 5.8 m and 4.2 m, respectively, after the extraction of a 10 m thick coal seam (Fig. 6). The mining-induced subsidence was simulated by the Displacement Discontinuity Method (DDM). The numerical model considered the effects of the ground surface, mining panels, faults, and the dyke. The predicted and the observed subsidence due to the mining of NUP and SLP were compared to varying Young's modulus, and the 0.10 GPa Young's modulus was found to be the best match (Fig. 8, 0.1 GPa). The effects of the faults and the dyke in the calculation were negligible (Fig. 10b). Future subsidence was predicted by considering 30 m extraction of the thick coal seam as 15.7 – 17.5 m in NUP and 8.7 – 10.5 m in SLP (Fig. 11), and the potential vulnerable future zone due to the extraction might go up to the mining office area and some villages (Fig. 12). For the total extraction of the 30 m coal seam, the mining authority might need to count on this subsidence issue and adopt backfilling or other mining methods to avoid the damage of the surface structures and the land area. The method and the outcomes of the research will be helpful for proper mine management of the other coal basins considering the geo-environment conditions.

Acknowledgements

The authors would like to thank the mining authority of Barapukuria Coal Mining Company Limited (BCMCL), Bangladesh, for providing us with the necessary information. Special thanks to AKM Badrul Alam and Mohammad Kamal Uddin, Manager, BCMCL, for their sincere cooperation.

Author contributions

A.B. and F.Y. contributed to the conception of the study; wrote the main manuscript text; modified the fundamental DDM; E.S., B.S., R.A contributed to data analysis and in improving the manuscript; All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article. The raw data that support the findings of this study are available from the corresponding author, upon reasonable request.

Additional information

Correspondence and requests for materials should be addressed to A.B.

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No competing interests reported.

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You are reading this latest preprint version

Mining-induced subsidence prediction by Displacement Discontinuity Method: A case from Barapukuria longwall coal mine, Bangladesh

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Version 1

Abstract

Figures

1. Introduction

2. Structural Framework And The Characteristics

3. Materials And Methods

3.1 The observed subsidence

3.2 Modification of the DDM method

3.3 The model and the simulation

4. Results And Discussions

4.1 The selection of the best value for Young’s modulus

4.2 Effects of the faults and the dyke

4.3 Future subsidence and vulnerable area

5. Concluding Remarks

Declarations

References

Additional Declarations

Status:

Version 1