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 basin19, 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 basin19, 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 thick19, 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.