Prediction of mining-induced subsidence at Barapukuria longwall coal mine, Bangladesh

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. 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 by 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.

www.nature.com/scientificreports/ Barapukuria longwall coal mine by the Displacement Discontinuity Method (DDM) because it can easily handle the effect of the ground surface, closure of mined-out areas, and deformation of faults and dykes. DDM was originally developed by Crouch and Fairhurst 16 as a boundary element method (BEM), especially for applying 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, 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.

Structural framework and the characteristics
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 shield 18 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 compared 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 Bangladesh 17 . Among them, 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 depths starting from 131 The MC is Holocene to recent in age and about 1-15 m thick 20 . 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 94-126 m in the basin 19,20 , having a thickness of almost 100 m in the mine area (Fig. 3). The LDT consists of sandstone, silt, and white clay. The thickness varied from 0 to 80 m in the basin 19,20 , which is 0 to 60 m in the mine area (Fig. 3). 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, about 150-300 m in the mine area (Fig. 3), consisting of predominantly arkosic sandstone with subordinate siltstones, shales, and breccia-conglomerates with occasional interbedded siltstone, sandstones 20 . 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 13-19°, dipping towards the east. The BC is mainly a layer of diorite, meta-diorite, ophlitic gneiss, and granite rock 20 .
The western part is more faulted than the southern part of the Barapukuria coal basin 21 (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 has a vertical displacement of about 200 m is around 5 km in length with NNW-SSE and N-S strike 21 . The faults of the west have the strike mainly of NNW-SSE and 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/cm 3 (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

Materials and methods
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 epicenters 22 . The subsidence in the north is just above the Northern Upper Panels and is named NUP, and it is above the Southern Lower Panels in the south, named 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.
Modification of the DDM method and assigned boundary conditions. The algorithm by Crouch and Fairhurst 16 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 nonparallel rectangular elements could be used.
Boundary conditions are as follows.
x-and y-axes are in the strike and dip directions. z-axis is normal to the seam, fault, or dyke. b is the displacement discontinuity. b x and b y represent slip along x-and y-axes. Positive or  (1) τ zx = τ xy = σ z = 0 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).

Results and discussions
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.
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).
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 without backfilling, half-strike length, and backfilling (Fig. 11). 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 without backfilling. The effects of the faults and dykes were not included because the effect was expected to be negligible (Fig. 9). For proper/sustainable mine management, the mining authority might need to 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. www.nature.com/scientificreports/ Shorter panels (half-strike length) and backfilling by fly-ash slurry (Fig. 11) are considered a subsidencecontrolling approach to reduce the vulnerability from the total extraction of the 30 m coal seam. It could reduce the areal extent and magnitude of subsidence with reduced potential damage zone on the surface. The half-strike length approach shows lower subsidence and affected areas than the half-strike approach. Moreover, the production becomes half for the half-strike approach. Backfilling might be a better option (lowest subsidence with higher production) in the geo-environmental condition. Fly ash from nearby coal power plants can be used for backfilling, reducing the amount of fly ash as waste. Moreover, there is a potential to mix CO 2 from the power plants [35][36][37] for a more sustainable solution in the future.

Concluding remarks
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  www.nature.com/scientificreports/ 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). The potential vulnerable future zone due to the extraction might go up to the mining office area and some villages (Fig. 13). For the total extraction of the 30 m coal seam, the mining authority might need to   www.nature.com/scientificreports/ 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 research method and outcomes of the research will be helpful for proper mine management of the other coal basins considering the geo-environment conditions.