Causes of Subsidence In and Around the Baropukuria Coal Mining Area, Dinajpur, Bangladesh: Insight from Direct Field Investigation

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1 SUST Journal of Science and Technology, Vol. 16, No.2, 2012; P:1-8 Causes of Subsidence In and Around the Baropukuria Coal Mining Area, Dinajpur, Bangladesh: Insight from Direct Field Investigation (Submitted: April 13, 2011; Accepted for Publication: December 4, 2011) M. Farhad Howladar Dept. of Petroleum & Mining Engineering, Shahjalal University of Science and Technology, Sylhet, Bangladesh. Abstract The subsidence around the Baropukuria Coal Mine field is the most important and major hazards or threats for safely extracting the coal from underground. From this point of view, the present research mainly investigated the causes of subsidence and its possible preventive plans by direct field investigation method. The analysis shows that subsidence is caused by many factors such as applied mining method, multi-sliced ultra thick coal seams, less competency of overburden strata, depth & dip of the coal seam, existed geologic discontinuities such as faults, Joints, fissures and other inconsistencies in the overlying and surrounding strata around the mine. Investigation also implied that the subsidence cannot be eliminated thoroughly but can be reduced with completely changing the applied on going mining methods or redesigning the mining plan in the area. Keywords: Baropukuria Coal Mine, Subsidence, Field Investigation, Causes of Subsidence. 1. Introduction The Baropukuria Coal Mining Company (BCMC) is located near the Baropukuria village of Hamidpur union council under Parbatipur Upazilla, Dinajpur district. The study area and its adjoining area are situated at the northern part of Bangladesh (Fig. 1). The coalfield contains coal-bearing seams of 7 groups in 11 seams with total thickness of m. Seam-VI is the main mineable bed with thickness ranging from 29.4 to m and 36.41m on average, belonging to the regular and extra thick coal seam of the coalfield. It stretches for 4.9 km northeast, with proved area of approximately 5.8 km 2 and has an unproved possible extension area to the south of approximately 1 to 1.5 km 2. Coal mine of Baropukuria basin in Dinajpur district, Bangladesh, enters into the coal mining era for the first time. During and field seasons Geological survey of Bangladesh (GSB) drilled seven boreholes in and around Baropukuria area under Parbatipur Upazilla of Dinajpur district, Bangladesh and confirmed the presence of 157 m thick Gondwana sediment between the basement and Tertiary sediments in the area. As the country having no coal mining experience in the past, BCMC is expected to bring about a number of others mining related activities in the country. Baropukuria coal mine is promptly organized by the Jiangsu Coal Geology Company, CMC, China, under the direct supervision of Petrobangla which deals with other mining related activities in the country. Baropukuria coal mine is now under production state of which production capacity is about 1 million tones annually which are currently using to generate electricity and also other industries. It is a great achievement for the country but by now it is observed that there is severe subsidence problem occurred around the coal mine. As a result local inhabitants are very much fretful and almost against to have the full phase of production. In this case, to ensure the safe and healthy production, it is very much indispensable to figure out the causes of subsidence over there. Therefore, the objectives of the research are to clarify the possible causes of subsidence by direct field investigation and finally propose the necessary recommendations for preventing the subsidence for safely production of coal from the mine.

2 2 M. Farhad Howladar / / / / / / / / / / Fig. 1: Location map of the Baropukuria Coal Mine area, Dinajpur district, Bangladesh [4] 2. Previous Investigations and the Necessity of the Present Research Baropukuria Coal Mining Company Limited (BCMCL) formed in August 1998 to operate the Baropukuria Coal Mine Project of Petrobangla with a target production of one million ton of coal from an under construction mine. Geological investigation in the northwestern part of Bangladesh has been started in 1950's when Standard Vacuum Oil Company (SVOC) carried out seismic reflection survey in this area. In 1974, the Geological Survey of Bangladesh (GSB) first confirmed the presence of 157m thick Gondwana sediment between the Basement and Tertiary sediments in Baropukuria area. Probability of coal discovery in a faulted basin lead GSB to undertake a detailed geophysical survey and borehole drilling in the region, which was successfully confirmed by seven GSB's boreholes during and field seasons. In 1991, Wardell Armstrong [13], a British consulting firm, was carried out pre-feasibility study for the development of Baropukuria coal mine based on GSB supplied data, sampling and testing materials. Again in 1990, the same organization carried out drilling operation and completed 28 boreholes at the region for analyzing the downhole geophysical logging, laboratory analysis of coal, engineering and hydrogeological property determination of sediments. However, regarding subsidence only one M. Sc. thesis research has been performed on the prediction of possible subsidence rate in the area but not in detail and also not dealing with the causes of subsidence. Thus considering all of these previous investigations and present situation of subsidence around the area, it is cleared that the present research is very much indispensable for understanding the causes of subsidence in and around the mining area while there is no such research performed yet over there. 3. Principles of Coal Mine Subsidence Subsidence is a natural and man-made phenomena associated with a variety of processes including compaction of natural sediments, ground water dewatering, liquefaction and crustal deformation, withdrawal of petroleum and geothermal fluids, and mining of coal, limestone and metallic ores [2]. Most subsidence is either created or accelerated by humans [10]. Singh [11] defines mine subsidence as ground movements that occur due to the collapse of overlying strata into mine voids which expresses itself in three major ways such as 1. Cracks, fissures, or step fractures; 2. Pits or sinkholes and 3. Troughs or sags. There are several common misconceptions about subsidence. For example, depth of the mine (as measured by the thickness of overlying strata) is often suggested as a prevention or mitigation measure. Similarly, the extraction area is often correlated with the size of the subsidence area. However, according to Singh [11], mining at any depth can result in subsidence and the affected surface area is generally larger than the extraction area. The mining method

3 Causes of Subsidence In and Around the Baropukuria Coal Mining Area, Dinajpur, Bangladesh: Insight from Direct Field Investigation 3 is the other major component of subsidence that influences its environmental impact. The mining techniques produced surface subsidence [7] which is longwall mining, top slicing, sub level caving, room and pillar mining, block caving, stope mining and solution mining. The first four methods listed are commonly used in the coal mining industry and among these methods Longwall pillar less mining has been used in the Baropukuria coal mine. In this case of Longwall pillar less mining when an opening or a long wall face is developed in the coal seam, the support for the overlying strata is removed and hence the original equilibrium in these strata is disturbed. Under gravitational loading the overburden roof strata will deform and displace. When a long wall face of sufficient width and length is excavated; the overburden roof strata are disturbed in order of severity from the immediate roof towards the surface, forming a subsidence trough. There are three distinct zones of disturbance in the overburden strata in response to long wall mining [7]. Although each zone can be identified by the failure characteristics of the strata, its thickness is not well defined and may vary. The caving zone (A) shown in Figure 2, which is the immediate roof before it caves, ranges in thickness from 2 to 20 times the height of excavation [3]. The caving zone can be further divided into various regions of strata failure. The complete caving zone is a region of severely disturbed strata. In this zone the strata fall into the mine floor and breaks into irregular but platy shapes of various sizes. Coal seams located within this zone may not be mineable. The region from 6 to 12 times the mining height is an area of partial caving. In this zone the failure process may still be severe enough to cause the strata to be fractured into relatively large blocks. The strata may have a significant degree of bending, leading to intense fracturing, or may be displaced as a result of shear stresses [6]. Severe to moderate roof control problems are likely to be observed in this zone. The upper limit of the caving zone occupies a distance of 12 to 20 times the mining height. In this region strata may separate along bedding planes and fracture or joints may open, but the individual beds tend to remain intact. Moderate to severe ground control problems may be encountered in this region. Above the caved zone is the fracturing zone (B), in which the strata are broken into blocks by vertical and/or sub-vertical fractures and horizontal cracks due to bed separation. The adjacent blocks in each broken stratum are Fig. 2: Three zones in overburden due to long wall mining [3] contacted either fully or partially across the vertical or sub-vertical fractures [2]. Consequently, a horizontal force is transmitted through the bedding plane of the strata. In this zone the bending of the strata is not so abrupt and fractures are less pronounced. The thickness of the fracture zone ranges from 20 to 50 times the mining height. Ground control problems mainly encountered in the subsidence trough region, and coal beds within this zone may be less difficult to mine than those within the caving zone. The combined thickness of the caved and fractured zone ranges from 30 to 50 times the mining height [8]. Between the fracturing zone and the ground surface, is the continuous deformation zone, which is sometimes, called the sagging zone (C). In this zone, bending of the strata is gradual and distributed over a large horizontal

4 4 M. Farhad Howladar distance without causing any major cracks. Generally, coal mining operation in this zone may encounter no problems at all. 4. Methods of Study The direct field investigation has been performed to clear the causes of mine subsidence around the mining area. During the field investigation the Geo-Structural, Geo-hydrogeological, applied coal mining method, mine design and present situation has been investigated. For laboratory analysis, the soil samples have been collected from different locations of the field. The different research work, technical papers and analyses records were also consulted for the present research. The different investigations have been explained below, while all of the factors are directly or indirectly influence the intensity or rate of subsidence around the area. 4.1 Geo-Structural condition of the Baropukuria Coal Mine Tectonic elements play the vital role in the geological development of a region and for evaluating the economic resources of anywhere; it is essential to have a clear conception about the major tectonic zonation of the local area as well as the country. Tectonically the Baropukuria basin is located in the Rangpur saddle of the Indian Platform. The Rangpur saddle is a possible connecting block of the Indian Shield and Rajmahal Hill in the west and the Shillong Massif in the east. The block is 96 km wide and is traversed by NW-SE and N-S trending fault together with NE-SW trending faults. As evident from the regional gravity data, the area is subdivided into smaller uplifted ridges (horsts) and subsided basins [1]. The general structure of the Baropukuria Coal Mine area is a single syncline spreading along N-S direction and cut by faults [4]. The syncline is not symmetrical with a length and general axis of strike of 64 km and N 10 0 W respectively. The strata of the west limb are flat with dip angle of in general, up to The east limb is cut by fault with dip in general up to The extensive seismic and drilling survey by the Wardell Armstrong [13] during provides geological structure of the Baropukuria basin and is shown in Fig. 3. Fig. 3: Geological structure of the Baropukuria Basin, Dinajpur District, Bangladesh [4] The Baropukuria basin is an asymmetrically faulted half-graben type intracratonic basin. This half-graben structure is imparted by a major N-S trending eastern boundary fault (Fig. 2). The existence of this fault along with two other faults (NNE-SSW and NNW-SSE faults) has been proved by the borehole data of the GSB and their extents and strikes have only been predicted [4]. However, this major fault has affected the Basement Complex and thought to be plane of active subsidence along the south-eastern extremity of the basin and marked less subsidence to the north.

5 Causes of Subsidence In and Around the Baropukuria Coal Mining Area, Dinajpur, Bangladesh: Insight from Direct Field Investigation Stratigraphic condition of the Baropukuria Coal Mine The Baropukuria basin area is a plain land covered with Recent Alluvium and Pleistocene Barind Clay Residuum. The stratigraphic succession of this basin has been established on the basis of borehole data [9]. The sedimentary rocks of Gondwana Group, Dupi Tila Formation, Barind Clay Residuum, and Alluvium of the Permian, Pliocene, Pleistocene and Recent ages respectively were encountered in the bore holes which lie on the Precambrian Basement Complex. A large gap in sedimentary record is present in between Gondwana Group and Dupi Tila Formation, which is most probably happened due to the erosional or non-depositional phase exited during Triassic to Pliocene age [5]. The stratigraphic succession in the Baropukuria basin of Dinajpur district is given in Table-1. Table-1: Stratigraphic succession of the Baropukuria basin [4] Age Group Formation Member Lithology Max. thickness, m Holocene Alluvium Silty clay 1.83 Pleistocene Barind Clay Residuum Clay and sandy clay Pliocene Dupi Tila Upper Sandstone, pebbly sandstone and clay/mudstone Lower Sandstone, claystone and mudstone with silica and white clay Permian Gondwana Feldspathic sandstone, carbonaceous sandstone and shale, ferruginous sandstone, conglomerates and coal beds Precambrian Basement Complex Diorite, granodiorite, quartzdiorite, granite and diorite gneiss Hydrogeologic condition of the Baropukuria Coal Mine For any mine field operation, it is essential to have a clear conception about the hydro-geological condition of the area, for a safe and effective mine construction as well as mine development. As we know, coal mining and associated reclamation operations alter the equilibrium of ground-and surface- water flow system [9]. The type and degree of hydrologic impacts vary with the size of the operation, the method of mining, and the manner in which the site is reclaimed. The principal constraints to the design of the BCMP relate to the great thickness (average 36 m) of seam VI which contains some 90% of the reserves and the presence of massive Gondwana sandstones and unconsolidated Dupi Tila Formation [4]. The later Formation represents a major aquifer over the whole mine area and for many hundreds of square kilometers aerial extent. It is at least 100 meters in thickness reaching185m in the southern part of the mine area and extends from beneath a shallow covering of Barind clay residuum to its geologically unconformable contact with the ground water measures. The Dupi Tila Formation and Gondwana sandstone that are known to be hydraulic continuity with the coal seam VI, represent a major potential hazard to the mine from water inflow [5]. 5. Investigated causes of Subsidence and Discussion From the direct field investigation, it should be mentioned here that a number of geologic and mining parameters could affect the magnitude and extent of subsidence in and around the Baropukuria coal mine, Dinajpur. These includes the thickness of extracted materials; overlying mining areas; depth of mining; dip of mining zone; competence and nature of mined and surrounding strata; near surface geology; geologic discontinuities; fractures and lineaments; in-situ stresses; degree of extraction; ground water; mine area; method of mining; rate of advance; backfilling; time and structural characteristics.

6 6 M. Farhad Howladar 5.1 Mining method According to geological conditions of coal seam and the practical situation of Bangladesh, the method of thick seam inclined slicing long wall mining along the strike with caving is adopted. A fully mechanized coalface is arranged in the district and its average length is about 104 m. The face adopts retreating mining in district. The slice mining sequence is slicing from up to down, that is in the same district sub-level, when the first slice is mined out, then the next slice with certain interval about years will be mined and successively collapse the mine out area causing the subsidence. The subsidence formed in early stage of coal extraction, which will be increased with increasing time. 5.2 Coal seam thickness There is a direct relationship between the thickness of the extracted materials and the amount of surface subsidence that may result, making it an important factor in subsidence predictions. A greater thickness results in a greater amount of surface subsidence [12]. In the study area, coal-bearing seams of 7 groups in 11 seams with total thickness of m. Seam-VI is the main mineable bed with thickness ranging from 29.4 to m and 36.41m on average, belonging to the regular and ultra thick coal seam of the coalfield. It stretches for 4.9 km northeast, with proved area of approximately 5.8 km 2 and has an unproved possible extension area to the south of approximately 1 to 1.5 km 2. From this features, it is clear that thickness of mineable coal is very high which is playing a significant role to have subsidence over there and with time being the subsidence will be more and more. 5.3 Multiple coal seams Where multiple mining horizons exist, subsidence, which occurs in one-area increases the likelihood of similar events in other areas, because the strata have been, disturbed [12]. In the coal field, the 7 groups of coal seams exist which are very much closed to each other and their total thickness of m. From the present observation it is can be implied that multiple coal seams is one of the important cause of subsidence in the area. 5.4 Coal zone depth Singh and others [11] have determined that Subsidence is independent of depth and refute the notion that surface subsidence is prevented by leaving greater thicknesses of overburden. While this may prolong the time period before subsidence effects are observed at the surface, the total amount of subsidence is not changed [12]. The vertical height within the mining range in the study area is less than 300m, composing of soft rock as well as unconsolidated soil materials. Thus the depth of coal zone is not capable enough to carry the overburden load causing subsidence. 5.5 Dip of coal zone Where the mining horizon is inclined, subsidence becomes skewed and mitigation measures such as pillars become less effective. The dip of coal zone in the Baropukuria coal filed varies significantly from north to south. As a result the rate of subsidence will vary with time and extraction. 5.6 Competence of mine floor and roof The mine roof and floor are critical factors in the initiation of subsidence events, since they propagate from these areas. Weak roof materials permit the fall of overlying strata, and compact more easily, resulting in a greater likelihood and severity of subsidence [12]. Same condition prevails in the study area and results subsidence. 5.7 Nature of overburden The strength of the overlying strata above the mining horizon is a factor in the timing and extent of subsidence [12]. In order to understand the nature of overburden materials, the permeability of sand has been analyzed which shows that soil is more permeable and soft. Such overburden materials are very vulnerable to collapse to form subsidence. 5.8 Surface and near-surface geology Surface and near-surface soils and unconsolidated materials tend to emphasize subsidence effects, because they behave in an inconsistent manner. They are factors relative to hydrologic impacts because they affect the exchange of surface water and ground water. The Baropukuria basin area is a plain land covered with Recent Alluvium and Pleistocene Barind Clay Residuum [9]. The sedimentary rocks of Gondwana Group, Dupi Tila Formation, Barind Clay Residuum, and Alluvium of the Permian, Pliocene, Pleistocene and Recent ages respectively are soft and more viable to collapse and hence subside the mine area with coal production.

7 Causes of Subsidence In and Around the Baropukuria Coal Mining Area, Dinajpur, Bangladesh: Insight from Direct Field Investigation Geologic discontinuities Faults, folds, and other inconsistencies in the overlying and surrounding strata may increase subsidence potential. The disturbance of equilibrium forces by mining activities can trigger movement along a fault plane. Faults may also weaken the overlying strata and trigger subsidence in materials that may otherwise show desirable properties. Joints and fissures in the strata also affect subsidence but on a smaller scale [12]. In the coal field, at least nine faults have been delineated by the exploratory Jiangsu Coal Geology Company, CMC, As a result, the mine out area is subsiding and will subside gradually Degree of extraction The amount of pillar support is directly related to the timing and extent of subsidence. Lower extraction ratios result in greater thicknesses of pillars, which tend to delay and decrease subsidence [2]. As the amount of pillar support is decreased, either by mine designs or as a result of pillar extraction, subsidence occurs more rapidly and extensively. Complete removal of pillars is almost always followed by subsidence with surface manifestations being a function of upward propagation to the surface. The production capacity of 1 million metric ton annually of the Baropukuria coal industry [4], which indicates that the production is high and in future this rate will be increased, thus might play a great role to increase the subsidence Ground water The creation of a cavity by extracting the coal in the study area results in subsidence. But another miningrelated phenomenon that also created subsidence is the withdrawal of water to facilitate underground mining [2 and 3]. Water withdrawal also causes the formation of cavities (which were once filled with water) and, like cavities directly created by mining, may result in subsidence as the hydro geological properties of the associated strata are changed Water level and fluctuations Because of withdrawing the huge amount water from the mine site, the pore spaces of rocks are blank and finally they squeezed resulting subsidence in the area Time The amount of subsidence has been observed as a direct function of time. Surface effects are delayed in roomand-pillar mining for some time, unless the pillars are removed. In block caving and long wall pillar less mining method surface effects typically are immediately noticeable [12]. While in the Baropukuria coal mine, the long wall pillar less mining method has been applied the surface around the mine area subsidence are immediately noticed. 6. Conclusion Coal mine in Baropukuria basin in Dinajpur district, Bangladesh enters into the coal mining era for the first time. As the country having no coal mining experience in the past, Baropukuria Coal Mining Company (BCMC) is expected to bring about a number of others mining related activities in the country. Baropukuria coalmine is now currently under production mode and facing the very great problem on subsidence around the mining industry. Considering this point of view, the present research has been performed to understand the causes of subsidence and their preventive measures by direct field investigations. The investigation shows that the subsidence is an inevitable consequence of this mining field and which is caused by applied mining method, multi-sliced ultra thick coal seams, less competency of overburden strata, depth & dip of the coal seam, existed geologic discontinuities such as faults, joints, fissures and other inconsistencies in the overlying and surrounding strata. The subsidence around this area cannot be stopped or eliminated but reduced with changing coal extraction mining methods mainly. Moreover, in order to have more detail regarding the causes, the detail research is necessary.

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