Investigation the Risk of Spontaneous Combustion in Barapukuria Coal Mine, Dinajpur, Bangladesh

Spontaneous combustion of coal is one of the major problems in the coal mine. The fire may occur due to exogenous and endogenous causes, by which coal liberated heat to the air or heat absorbed into it. It causes loss of production, as well as economical or financial losses and polluted the environment. If, the heat liberated during this process is allowed to accumulate, the rate of reaction increases exponentially and there is a further rise in temperature that generates the flame and produce CO, CO₂, CH₄, N₂, O₂ etc. In addition, the heat generated within coal affected by different factors such moisture, ash, volatile matter etc. of coal. This paper deals with the oxidation and spontaneous combustion risk in Barapukuria underground longwall coal mine, Dinajpur, Bangladesh. In this study, the laboratory analyses (proximate analyses) shows the inherent and the total moisture content value is average 2.73% and 5.82% to 12.75%, respectively. It indicates that these moisture contents are moderately liable to self heating. The less ash content value (av.13.2%) shows, it is less liable to spontaneous combustion. In addition to this, the temperature and concentration of some mine gases (CO, N₂, O₂) were monitored to calculate the Graham’s ratio. According to Graham’s ratio, the longwall faces have high oxidation risk and medium combustion risk. Therefore, the actual control of spontaneous combustion of coal is important to save coal mine from mine fires and also provides a real opportunity to improve the financial performance of the overall organization.

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Coal & Coal Mine in Bangladesh (Barapukuria Coal Mine)

Student group are collecting coal for lab experiment

Coal is an important energy resource produced by one of the hydrocarbon group. In the early of 20th century coal was used more than 65% of the total energy resource of the world. During 1960-1970's the uses of coal decreases, when cheaper and efficient oil and natural gas were available and utilized. But since 1980's the proportion has increased again and it takes about 30% at the present world. Although Bangladesh has a substantial quantity of fossil fuel, in this regard it has to pay attention through coal mining. Barapukuria coal mine is the first developed mine in Bangladesh and now passes its trial production period, with a production capacity of 1 million metric ton annually.

To extract coal from Barapukuria Coal Mining Company (BCMC) us

ing the method of Inclined Slicing Roof Caving Long Wall Mining along the Strike, and the sequence of slices mining from top to bottom. Mining of 1101 coal face initiates caving from the lowest strata in the immediate roof and propagates upward into the Gondwana Formation and up to the base of lower Dupi Tila. 

The redistribution of stresses analyze by the Peng (1986) and Wilson (1981) method, and the result shows that the stresses are relatively large; of which affect the surrounding rock mass and the next slice solid coal measure. The main concern of caving process relating to subsidence occurrence at the ground surface of the BCMC site is the development of subsidence troughs. By applying NBC (England, 1975) method, it is estimated that at around 0.75 m ground subsidence may occur for the mining of 1^st face, and successively for the mining of 5^th slice the ground subsidence may 2.25 m occur, of which is relatively difficult to control the ground response and a violent interaction effects may anticipated.

Filling process can not eliminate subsidence but reduce it if the operation is carried out to a higher standard and to allow an increase in the percentage of recovery of the coal over the caving mining methods. The hydraulic filling materials can reduce the effects of subsidence at around 8% from its total caving of 30% in a non filling condition, also the degree of curvature on the subsidence profile and consequently result in a reduction of tensile and compressive strain on the ground surface. However it is recognized that implication of hydraulic filling operation may eventually necessary to control strata movements and reducing of mining problems may arise from multi level mining conditions in the very thick seam like Barapukuria condition. Again, such a high risk mining methods must be avoided because failure of the project would be totally unacceptable for economic, political and social grounds of the country. Its failure would seriously jeopardize any future mining prospects in the country. Incorporation of this thesis work to the mine authority will facilitate guideline and provide an integrated tool for future longwall planning and design of the mine

BARAPUKURI COAL MINE AREA

Location .

Maddhapara hardrock Mine is located in Maddhapara , Dinajpur , Bangladesh. Its geographical coordinates are 25˚ 33΄ 15΄΄ N to 25˚34΄ 15΄΄ N latitude and 89˚ 3΄ 30΄΄E to 89˚ 4΄ 53΄ E longitude (see map 1.) Maddhapara hardrock Mine is 330km away from Dhaka, the capital of Bangladesh and 14km away from phulbari Dinajpur.

Fig.2.1Location map of the Barapukuria in Bangladesh showing probable depth of the coal

Climate :

As Bangladesh belongs to the tropical zone, the climate in the mine area is characterized by hot season in summer, moderate in monsoon and cool in winter.

Average temperature in June is the highest, with a maximum of 38.8˚c and average temperature in January the lowest, with a minimum of 7˚c. (Table 1-1)

Monthly average temperature based on data available with Dinajpur Meteorological observation Station is given in Table 1-1.

Monthly Average Temperature in 1998 & 1999 (Unit : Degree Celsius)

Table 2-1

YearJan Feb Mar Apr May June July Aug Sep Oct Nov Dec 1998Min 25.529.732.637.037.839.834.84.035.534.831.229.24Max 7.08.512.016.021.824.024.824.823.819.414.210.21999Max 27.032.736.036.536.837.835.735.734.033.531.229.2Min 7.08.213.518.020.423.224.024.021.221.514.210.2

Bangladesh has a heavy rainfall and the mine area has rainfall equivalent to 85 percent of annual rainfall in the monsoon season between June and October and little rainfall in the dry season from November to May the following year. Monthly rainfall based on data available with Dinajpur Meteorological Observation Station is given in Table 1-2

Monthly rainfall in 1998 & 1999 (Unit mm)

Table 2-2

Year Monthly rainfall (mm)Total annual rainfall (mm)Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 19982.017.024.0146.0162.6251.8722.6523.0315.0380.0002354.8199900068.0368.0274.0394.0727.0586.4234.54.002589.6

Humidity

The highest annual humidity was at the year 1987(84) and the lowest was at the year 1981(69). A correlation between precioitation and humidity values of the study area can be found. Higher humidity has occurred in the low precipitation year.

Geomorphology :

Topography :

Bangladesh is dominantly a plain and so is Barapukuria Mine area which is table land . this table land extends across Northern Bangladesh FROM Comilla in the east through Rajshahi division to West Bengal on India in the west. Historically known as the Barind Tract , contains the BARAPUKURIA COAL MINE site and extends over an area of more then 7800 sq km . In the area elevation generally varies from 10m to a little over 40m above sea level and depicts more or less a flat topography. Elevation of the Barapukuria coal mine area varies from 28m to 30m above see level.

Physiography :

Barapukuria coal mining area and its environs are formed by the deposition of three different materials, e.g. Piedmont Alluvium, Tista Alluvium and Barind Tract . The Piedmont plains occur throughout most of Dinajpur District and of Rangpur District.

The old Himalayan Piedmont plain is a part of a gigantic allucial fan formed by the Tista River, before it abandoned the landscape. The generally consist of gently undualing ridges with intervening broad sometimes narrowly leveled depression. The Tista Alluvium is underlain by Recent to Sub-Recent and older Floodplain. Barindra Tract presents somewhat alluvium termed the Mdupur Clay.

Soil condition :

Alluvium deposit has not taken place in and around the site of drilling operation at Barapukuria . The Madhupur clay is either exposed at the surface or concealed under a thin soil layer less then one m thick, formed from the weathering of the red clay. Due to the removal of the clayey materials, the soil has become more sandy than the parent rock.

s A limited quantity of ferruginous concretions still remain mixed in the soil. In and around the study area two main types of soil have been identified, which are piedmont and Tista floodplain soil and Barindra Tract soil.

Drainage :

The entire Tableland is surrounded by two major river system, Jamuna and Ganges. An impervious red clay layer covers most of the tableland area and limits direct seepage of rainwater. This causes the water to drain along shallow streams that flow from north to south. Fragmented agricultural land hold some rain water for rice irrigation. A major part of this water is ultimately evaporated.

The regional rivers mainly drain the local runoff. Stream flow is the major component of regional surface water. Kharkharia and Tulshiganga rivers floeing from north join the Jamuna river. Two other rivers, Nagor and Bhadal passing through the southeastern part meet the Karatoa-Atrai-Gur-GUMAIN-Hurasagar river flowing through the southern boundary. Jamuneswari-karatoa is the principal river in the area.

This river is the downstream portion of Deonai-Charalkata river. It has two other smaller tributaries, but these tributaries carry small amount of water during monsoon only. Jamuneswari enters the area at Badarganj in Badarganj Upzila and it leaves through Mithapukur Thana. Spills from the Brahmaputra river into are controlled by the Brahmaputra right Embankment. The Barind Tract is crossed by a number of rivers occupying faults troughs.

Most of the rivers within the study area are small in size and flow. Phase 2 falls within a boundary formed by the Chirnai and Kala river is distributary of the Chirnai and separates at Kisamat Union. But it again coalesces with the river as a tributary at Debipur Union. The river runs for about 10kms, independently . no significant drainage system is available within Phase 3. there are many ponds within the area.

Mostly are local few naturals . surface area of standing water bodies, such as, haors, baors and beels excluding ponds and tanks was surveyed by SPARRSO (1986). In the study area there are 5 standing water bodies, beels and blocked river channels. Total surface area of standing water bodies are 254 km( MPO, 1985)

GEOLOGY

Tectonic elements play the vital role in the geological development of a region and for evaluating the economic resources of Bangladesh. It is essential to have a clear conception about the major tectonic zonation of the country.

The geotectonic of Bangladesh has been suffering both from oversimplification and over-complication i.e. the geology of Bangladesh is a complex one and also of the study area. A series of articles worked out concerning the tectonic framework of Bengal Basin and its adjoining areas by several workers regarded in the course of Morgan and mclnter, 1966; and follow the same way in a very recent time by Islam, 2002.

Bangladesh constitutes the major part of the Bengal Basin where immense thickness of sedimentary formations rest on the ancient igneous and metamorphic pre-Cambrian Archaen Basement. These formations range in age from Permo-Carboniferous, through Jurassic and Cretaceous, to Tertiary and Quaternary and are up to 20 km thick in the vicinity of Dhaka. In the north-western part of the country (out side of the Bengal Basin) the basement is present at much shallower depth where Archean rocks are covered directly by late-Tertiary sediments, as little as 128 m thick at Madhyapara Hardrock Mine area. Within the basement, it has been postulated from regional geophysical surveys that a number of small basins occur - probably faulted or half-faulted grabens - where sediments of the coal bearing Gondwana Formation is present, concealed by the ubiquitous cover of Tertiary sediments. These mostly continental Gondwana sediments were deposited within the ancient southern super-continent of Gondwanaland when the Indian sub-plate was directly ‘attached’ to Southern Africa, Antarctica and Australia.

The ancient Gondwanaland mass was broken up in Cretaceous time when the Indian sub-plate commenced it’s global wandering from near the South Pole to where it is today, colliding in Tertiary time with the relatively stable landmass of Eurasia, the northern super-continent, leading to the creation of the massive crustal upheaval which formed the

REGIONAL TECTONIC SETTING

Bangladesh largely covers two tectonic elements: a) Indian Platform- a stable shelf with reduced thickness of sediment; and b) Bengal Foredeep- a deeper part of the basin with rapid increase of individual formation thickness. The Indian Platform or Stable Shelf part in Bangladesh constitutes the southwestern part of the Platform and covers almost two-third of the Rajshahi Division that is conventionally subdivided into three tectonic events (Haque, 1975; Guha, 1978) as: (a) Dinajpur Slope (northern slope of the Rangpur Saddle); (b) Rangpur Saddle; and (c) Bogra Slope (southern slope of the Rangpur Saddle). Tectonic map of Bangladesh and its adjoining area is shown in Fig. 4.1

In the recent year, on the basis of gravity field analysis and bore hole data, Khan (1991); and Khan & Rahman (1992) have proposed revised tectonic classification of the region (Fig. 4.2) as: northern slope of the Platform; Stable Platform; Nowabgonj-Gaibandha-Intra-cratonic High; and southern slope of the Platform.

The Rangpur Saddle is a possible connection between Indian Platform and Shillong Massif with the thinnest sedimentary cover over the basement is about 128m at Madhyapara. The width of the Saddle is 97 km, which slopes both sides towards north and south forms an oval shaped body (Zaher and Rahman, 1980). The northern slope of the Rangpur Saddle (Dinajpur Slope) is about 64 km wide and slopes towards the Sub-Himalayan Foredeep and this part is sseperated from the stable platform by a series of faulting. Two prominent of them trends towards east-west and the others takes a sudden south-eastward swing (Khan and Rahman, 1991, 1992). However presences of intrusive bodies are inferred from records of few small magnetic anomalies in the area. The southern slope of the Rangpur Saddle (Bogra Slope) is 64 to 129 km wide and extends up to Hinge Zone. The inclination of basement is gentle up to Bogra, which increases further southeastwards. In this area Gondwana sediment were deposited in the faulted troughs or subsiding basins in the Basement Complex (Zaher and Rahman, 1980).

The saddle area has severely suffered from faulting of different age, as it is evident from Geophysical survey (Fig- ). Multiple faulting along the Jamuna and Ganges rivers down thrown the region, make it's a graben type structure corresponding to the horst structure of Shillong Massif and Mikir Hills (Khan, 1991). Networking of this fault ultimately made up as many as five fault bounded Gondwana basins in the area (Uddin and Islam, 1992; Islam, 1993; Khan, 1996). Moreover the aeromagnetic map shows that the frequency of faulting is low over the basement of the stable platform. The faults observed within these tectonic elements are actually existed during the Gondwana period formed intrabasinal horsts and graben. This is the characteristics feature of the Gondwana basins of Peninsular India (Datta et.al, 1983; Mitra and Rao, 1987).

STRUCTURE AND TECTONICS OF THE STUDY AREA

Tectonically the Barapukuria 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 (graben and hlf-graben) (Bakr et.al., 1996; Islam,1993, 2002).

The coal bearing Gondwana intra-cratonic basins (graben and half-graben) have been discovered in many gravity lows within the basement of Rangpur saddle and adjoining areas (Islam and Uddin, 1993; Islam, 1994 ) i.e, Kuchma, Jamalgonj, Barapukuria, Khalispir, Dighipara, Phulbari etc (fig.-). It is believed that such types of graben were formed as a result of post-Gondwana rifting in the Gondwanaland. These types of basis are thought to be down warped blocks of the platform into the basement complex and are found to contain coal bearing sediments in different continents, like Australia, Africa and Antarctica. The Barapukuria basin is thought to be one such subsided basins or graben within the basement (Islam, 1993, 1994, 2002).

The general structure of the Barapukuria Coal Mine area is a single syncline spreading along N-S direction and cut by faults (CMC, 1994). The syncline is not symmetrical with a length and general axis of strike of 64 km and N 10⁰ W respectively. The strata of the west limb are flat with dip angle of 7⁰-16⁰ in general, up to 27⁰. The east limb is cut by fault with dip 30⁰-40⁰ in general up to 50⁰. The extensive seismic and drilling survey by the Wardell Armstrong during 1988-1990 provides geological structure of the Barapukuria basin and is shown in Fig. 4.3.

The Barapukuria 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. 4.3). 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. However this major fault has affected the Basement Complex & thought to be plane of active subsidence along the south-eastern extremity of the basin and marked less subsidence to the North.

At least nine faults have been discovered by the exploratory Jiangsu Coal Geology Company, CMC; 1994. All the faults are thought to be of normal type, as demarcated by Fa, Fb to Fi respectively. The major faults as observed in the basin are described below:

Fa fault or eastern boundary fault (N-S fault): This is a major normal fault with high-angle dip towards the west, which has marked the eastern boundary of this basin providing the half-graben structure (Fig. 3.3). Due to this faulting, the western block is down thrown by at least 518 m, as measured from the variation of abrupt depth the basement in boreholes GDH-40 and GDH-44. The deepest part of the basin is located near this fault (around GDH-40).

Fb fault or NNE-SSW Intra-basinal fault: This fault has been traced in between boreholes GDH-39 and GDH-43 from the discontinuity of the coal seams and depth variation of basement in these two holes. Instead of western block, the eastern one is down thrown by at least 152 m as estimated from the borehole data. This fault is limited within the basin and is of normal type, which dips towards east (Fig. 3.3).

Fc fault or NNE-SSW trending fault: This is also a normal fault and is present in the southwestern part of the basin as inferred from the variation in depth of basement and major coal seam-VI in bore holes GDH-38 and GDH-42. The southwestern block is down thrown by at least 182 m. This fault has disrupted the western boundary of the basin (Fig. 3.3). The southward extension of this fault is uncertain but appears to dissect the southern basin.

Besides this Wardell Armstrong has identified another small fault in the southwestern corner of this basin, which has dissected the basin and pushed northward by a reasonable amount. Due to this faulting, the structure becomes complex (Fig. 3.3). These faults along with the NNW-SSE and NNE-SSW faults are considered to be sympathetic faults (Bakr et. al., 1996). Although five major unconformities have been noticed in the basinal area, each represents a major period of erosional or non-depositional phase of which is shown in Table.3.1 (Islam, 2002). A local unconformity is also identified between lower and Upper Dupi Tila Formation.

STRATIGRAPHY

The Barapukuria 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 bore hole data (Bakr el al.,; 1986; Islam, 1987). 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 Pre-cambrian Basement Complex. A large gap in sedimentary record is present in between Gondwana Group and Dupi Tila Formation, which are most probably happened due to the erosional or non-depositional phase exit during Triassic to Pliocene age (Islam, 1994). The stratigraphic succession in the Barapukuria basin of Dinajpur district is given in Table: 4.1.

Table-4.1 Stratigraphic succession of the Barapukuria basin.

AgeGroupFormationMemberLithologyMax. thickness, mHolocene Alluvium Silty clay 1.83Pleistocene Barind Clay Residuum Clay and sandy clay10.36Pliocene Dupi Tila Upper Sandstone, pebbly sandstone and clay/mudstone 126.82LowerSandstone, claystone and mudstone with silica and white clay Permian Gondwana Feldspathic sandstone, carbonaccous sandstone and shale, ferruginous sandstone, conglomertes, and coal beds. 457.32PrecambrianBasement Complex Diorite, granodiorite, quartzdiorite, granite, and diorite gneiss 14.32+

Basement complex: It is the oldest rock sequence and conjectured the age as Archaen, of which occurs at different depths in different boreholes are faults controlled (Bakr et. al., 1996). The rocks are mainly granodioritic and quartz dioritic, which are light grey to light greenish grey; holocrystalline, coarse grained and equigranular. They are composed mainly of plagioclase feldspar, hornblende, quartz and biotite. The diorite is dark grey to greenish grey, medium to fine grained. The rocks are fractured and the fracture planes are filled up with reddish (siderite?) and white (calcite) secondary minerals.

The granite is light colored (pinkish, greenish or greyish), holocrystalline, medium to coarse grained, anhedral, and porphyritic. It is composed of feldspar, mica, and dark minerals. The rocks are highly fractured and the fracture planes contain iron oxides, calcite, and other greenish secondary minerals. The diorite gneiss and schist are grey to greenish grey coloured, medium grained and equigranular. The rocks are obliquely fractured which are filled with white colored secondary mineral. The fracture planes are inclined from 45⁰ to 60⁰ (Bakr et. al., 1996).The above said rock will be exploited from the adjacent Madhyapara Hard rock mining project, Dinajpur of which mainly used as building materials.

Gondwana group: The Gondawan group unconformably overlies the Archean Basement Complex and consisted of feldspathic sandstone, carbonaceous sandtsone, ferrugineous sandstone, carbonaceous shale/mudstone, conglomerte and coal. In the Barapukuria basin, the depth of the Gondawana sediments ranges from 118 to 190 m below the surface and the thickness of which varies from 117 to 475 m. Deposition of Gondawana sediments commenced from the Early Permian time with a well-marked unconformity at the top. Based on lithology and sedimentary structure, the Gondwana rocks are divided into five distinct lithofacies (Bakr, et. al., 1996) as: a) conglomerate facies, b) sandstone with mudstone facies, c) coal facies, d) sandstone facies, and e) sandstone with coal, mudstone and conglomerate facies.

The coal in the coal facies in Barapukuria basin is dull black to shiny black in color and vitreous to subvitrious in luster. In most cases, the coal is banded, and at places the coal grades into shaly coal and carbonaceous shale. Thin carbonaceous bands are also present in the coal beds. Glossopteris and Gangamopteris flora are common in the coal beds. The coal is fractured and blocky in nature and others are conchoidal. Slickensides, microfaults and slope failure features are present in this coal facies.

Extensive deposits of coal of early Permian age (Islam, 1994, 1996, 2002) have been discovered in the Gondwana sediments of the Barapukuria basin. Seven coal seams have been identified in this Formation, extended over an area of 1.5 sq. km (proved) and the inferred area is about 3 sq. km. The maximum and minimum composite thickness of coal seams is about 79.57m and 25.62m respectively. In terms of coal reserve, which is potentially available to the Barapukuria basin in seam-VI, Contributes over 90% of it’s total proved reserve because of its extreme thickness (avg.36m) and its presence over the greater aerial extent. The project is based on mining this seam only, detailed lithological description of this seam is given below:

(a) The section of coal seam-vi floor: The rock strata under the coal seam, called floor. It is composed of low strength rock strata, such as grayish black carbonaceous siltstone, containing plant fossil fragments and medium to fine grained feldspathic sandstone. This section mainly composed of quartz, mica and dark colored minerals. The phenomena of slip, expansion, upheaval and extruding supports may happen to affect the roof control in this section.

(b) The section of coal seam-vi: It is the target horizon that would be mined out, which comprises of Semi-dull coal, bandy texture, pitch luster and mostly dull coal, interbedded with thin beds of bright coal and vitrain bands of carbonaceous mudstone. In some cases horizontal bedding and well-developed internal fissures with steep angles contains pyrite film.

(c) The section of false roof of seam-vi: It’s a thin rock-stratum sticking to the coal seam, which is very easy to collapse. The thickness of the roof is generally less than 0.5m. During mining, it falls simultaneously with the coal seam. This section belongs to the range of rock with lower stability and consists of bad integrated rock type of lower RQD (Rock Quality Designation) value, which indicates that the stability of seam-vi roofs decreases markedly.

The false roof mainly composed of carbonaceous mudstone and plant fossil, of higher proportion of carbonaceous matter than other section. The rock sequences are of virtain types and represents horizontal bedding structure.

(d) The section of coal seam-vi roof: The rock strata overlying the coal seam, called roof rock. It may be whether immediate or main roof, the immediate roof will collapse immediately after prop drilling but the main roof remains a long unsupported length without taking in gob. In Barapukuria coal mine the immediate roof mostly absent.

However the main roof mainly consists of greish white,coarse grained sandstone and gravel deposits, at the lower part a little siderite is present in some places and also interbedded carbonaceous and vitrain bands of wavy bedding.

Dupi Tila Formation: The Dupi Tial Formation unconformably overlies the Gondwana Group. The depth of the Dupi Tila Formation ranges from 9 to 12m below the surface with a thickness from 108 to 188 m. On the basis of lithology, the Dupi Tila Formation is divided into two members with a local unconformity between as:

a) Lower member: Depth of this member ranges from 114 to 135 m with a thickness varies from 15 to 67 m. In the northern part of the basin, this member is absent. Based on the lithology, the lower Dupi Tila member is divided into two facies as i) sandstone and ii) sandstone and mudstone facies. The sandstone is assumed to be the channel deposits. The coarse and fine clastics in this facies indicate the deposition by over-bank flooding of a stream that indicates that the stream was often laterally shifted.

b) Upper Member: The thickness of this member ranges from 104 to 126 m. This member is divided into three major lithofacies: sandstone facies, pebbly sandstone facies, and clay with sandstone facies. The characters of the lithofacies correspond to the concentrated fluid flow on the transverse bars of a shallow river to floodplain deposits. The alteration of sandstone and clay may be due to lateral migration of the depositing media (Bakr, et. al., 1996).

Barind Clay Residuum: In this formation, only the red clay facies is recognized and unconformably overlies the Dupi Tila Formation. The thickness of this litho-facies ranges from 8 to 10 m. The clay is yellowish brown to reddish brown and mottled but soft and very sticky when wet. Prolong exposure of the Dupi Tila sediments during Pleistocene time is considered to be the prime factor for the formation of the clay facies. In this region, it bears the characteristics of an ideal paleosol.

Alluvium: This is the top most formation of the area, lies unconformably over the Barind Clay Residuum and composed of light grey and silty clay. This formation includes the silty clay facies with average thickness of about one meter resulting from residual processes, which is intermingled by the present day fluvial sediments.

HYDROGEOLOGY

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, the stratum temperature at different level (originated by geothermal heat and ground water flow) and the hydro geological condition would be favorable. As we know, Coal mining and associated reclamation operations alter the equilibrium of ground-and surface-water flow system. 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 V1 which contains some 90% of the reserves, and the presence of massive Gondwana sandstones and unconsolidated Dupi Tila Formation .The later Formation represents a major aquifer over the whole mine area and for many thousands 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 is known to be hydraulic continuity with the coal seamV1, represent a major potential hazard to the mine from water inflow. The hydrogeology of the Barapukuria coal deposit has accordingly been the subject of an extensive investigation from exploratory drilling work to still today mine trail basis production period.

The current exploration activity in the Barapukuria coal mine field represents a much more complex hydro-geological condition than that described in the original exploration report. It suffers variations of water inflow and difficulties in terms of mining technology. Maintaining the basic design of the mine development plan, unfortunately (5^th April 2005) severe water inrush occurred in the central district of the mine and water logged condition arises. Therefore, the basic design of the mine, especially the parts regarding to mining system and facilities for provisions against flooding would be modified and readjusted.

Barapukuria coal mine is an independent Gondwana coal bearing basin which is controlled by half-fault graben, and unconformably laid on the denuded Archean Basement Complex. Drilling data shows that strata can be divided in to four units, such as Basement complex, Gondwana group, Dupi Tila formation and Barind Residium Clay.

HYDROSTRATIGRAPHY

A number of exploratory boreholes have been drilled in and around the Barapukuria coal Mine area and location of which is shown in Fig.1.1. According to the lithological characteristics of strata obtained during exploration activities and correlation of stratigraphy with aquifer characteristics, the hydrostratigraphic succession of the study area as described by CMC (1994) is given in Table 5.1.

AgeLithologic unitHydrostratigraphic unitsLithologyAverage thickness, mPleistoceneBarind Clay ResidiumAquicludeClay and Sandy clay 10PlioceneUpper Dupi TilaAquiferMedium sand interbeded with fine sand, pebbly grit and thin clay.104Lower Dupi TilaAquicludeWeathered residual clay, clay silts, sandstone interbeded with silty mudstone and course grain quartz.80PermianGondwana

I) Sandstone of

Seam VI Roof

II) Sandstone of Seam VI Floor

Aquifer

Aquifer

Aquifer

Medium to coarse-grained sandstone and Pebbly sandstone, interbeded with coal seam I to V; also siltstone and mudstone.

Medium to coarse grained Sandstone, Grit stone, interbeded with thin medium to fine grained Sandstone, Siltstone and Mudstone are some times

Fine grained sandstone, Medium to fine grained sandstone interbeded with siltstone, carbonaceous mudstone and 2/3 beds of tuffy siltstone.

156

140

67

ArcheanBasement Complex

a) Upper Section

b) Lower Section

Relatively

Aquiclude

Aquiclude

Sedimentary, igneous, and metamorphic rocks with sandy and muddy fragments interbeded with fine grained sandstone, Carbonaceous mudstones and molted mudstones at bottom.

Granodiorite, quartz, diorite and diorite gneiss.

53

31

Table 2.3 Hydrostratigraphic succession of the Barapukuria coal Mine area.

DESCRIPTION:

Barind Clay Residium: Hydrostratigraphically this formation is called aquiclude, which has an average thickness of 10m.This aquiclude is thick in the northwestern part and thin in the southeastern part and mainly consisted of brown yellow sandy clay with a infiltration rate of about 1.5mm/d.

Upper Dupi Tila (UDT): The Upper Dupi Tila is an aquifer constituting the major ground water reservoir in the mine area with an average thickness of about 104 m and depth of the floor varies from 102 to 136 m. Its thickness is low in the southern and northern side while thick and deep to the center eastwards and mainly consisted of medium sand beds interbedded with fine sand, pebbly grit and thin clay beds. Pumping test data during dry season shows that elevation of water level and specific discharge values are +23.55 and 7.65 L/s respectively indicating relatively well watery condition. The hydraulic gradients, average transmissivity, permeability, storage coefficient and specific yield values of UDT are 0.0004-0.0006, 12000 m²/d, 0.004 and 25-30% respectively (CMC, 1994). The movement of groundwater in the area is from the northeast to southwest direction.

Lower Dupi Tila (LDT): It is an aquiclude and consisted of grayish white weathered residual clay and clayey silt of thickness 80 m where the depth of the floor varies from 115 to 118m. The horizontal and vertical permeability values of LDT are 5.15 ´ 10E-4 to 9.5 ´ 10^-5 m/d, and 2.0 ´ 10^-5 respectively indicating comparative aquiclude and poor watery nature.

Gondwana Sandstones: It represents completely an aquifer system and the coal seam VI divides this in to upper and lower sections as:

a) Upper section: Sandstone aquifer of upper section of thickness 156 m is located in the center of the basin and consisted of medium to coarse-grained sandstones and pebbly sandstone interbedded with seam I to V, siltstone and mudstone. The rocks are friable or loose and feldspars are generally kaolinized. The vertical tensile cracks are well developed and mostly filled with mud or pyrite films. The elevation of the water level, specific discharge and permeability values are +27.22m, 0.051 l/s and 158 m/d respectively indicating poor watery condition of this section. For relatively far from seam VI, this aquifer has comparatively poor effect on the development of coal (CMC, 1994).

b) Lower section: The thickness of this section varies from 107 to 244 m and becomes thicker from northwestern and southeastern parts towards the central part of the basin and consisted of medium to coarse grained sandstone, gritstone and conglomerate, interbedded with mudstone and siltstone. The specific discharge, hydraulic gradient, average transmissivity, permeabilities, average porosities, specific yield and storage coefficient values are 0.0929 l/s, 0.002, 25 m/d, 0.0001 to 0.4 m/d, 15%, 6%, and 0.001 to 0.0001 respectively indicating poor watery condition. The movement of groundwater is from southeastern part towards center of the basin.

Basement Complex: It is divided as follows:

a) Upper section – Breccia aquiclude: It has a thickness of 23 to 84m and consisted of light grey, grayish green sedimentary rocks, igneous rocks and metamorphic rocks with sandy and muddy fragments, interbedded with light grey fine grained sandstones or carbonaceous mudstones and mottled mudstones at the bottom. The permeability values vary from 0.001 to 0.006 m/d and relatively aquiclude.

b) Lower section - Basement aquiclude: It is mainly consisted of light green to grayish green granodiorite, quartz diorite and diorite gneiss of thickness ranges from 9 to 53m and is characterized by a relatively aquiclude.

HYDROGELOGICAL CHARACTERISTICS OF AQUIFERS

In Barapukuria Coal Mine area, the watery properties of the different aquifers become poor downwards. The upper sand horizon of UDT is very well watery while the lower is moderate with apparent decrease.

The water levels were measured at 22 boreholes in the Barapukuria Coal Mine area by W.A. Company in 1991 for the study of hydrogeological characteristics, water level fluctuation and hydraulic relationships between aquifers. The results of pumping test data analysis by W.A. Company reveals that the upper and the lower sand horizon of UDT aquifer are thick and its recharge condition is good. The pumping test data analysis of sandstones of seam-vi roof and floor also reveals that the transmissivity of the aquifer is good, storage quantities and recharging source are available. A good hydraulic relationship has been indicated between the lower section of UDT and the sandstone of seam-vi roof (CMC, 1994).

Rainfall is the main source of recharge to each aquifer. Water levels during dry season and monsoon change as a cycle. Average monthly rainfall varies from 6 to 59 mm in the dry season between the month of November and April. The water levels fall gradiently during this period and minimum elevations vary between +22.60 to +23.89 m on the month of March and April. On the other hand, average monthly rainfall is from 236 to 482mm in the monsoon between the month of May and September, which is about 87% of total annual rainfall falls. During that time water levels keep rising and the maximum elevations are ranges between +30.83 and +30.11m. Range of water level fluctuations is 7.04 to 7.70m, no variation occurring in the relative aquicludes (CMC, 1994).

A number of faults are reported in the coal mine area by seismic survey. Because of small thrown, no apparent fracture zone and poor watery base rocks, water-filled spaces are poorly developed and transmissivities are generally poor. But lithology and degree of cement, which control transmissivities of faults, are poorly distributed in the mine area (CMC, 1994).

The Barapukuria Coal Mine area is an incomplete syncline basin, which elongates along north-south direction. The boundary of this basin in the east and the west are faults and sub-crops of coal seams are occurred in the extremely in the south and the north. Impervious boundary is consisted by the boundary fault, which makes the coal series, contact with the metamorphic rocks of the basement. The coal basin is overlain by sandy clay layers of Quaternary, which have a thickness of 4 to 16m and limited infiltration of rainfall and ground surface water. An aquiclude of the LDT clay, which has a significance thickness, varies up to 80m in the south. The basement of the seams is a water-resisting layer also. Therefore, the coal mine area could be described as an incomplete syncline hydrogeological unit, which receives some recharge and is semi-confined. The seam roof is directly water-filing aquifer and is poorly watery. The lower sands of UDT as directly water filling aquifer is comparatively well watery, but it is confined by LDT clay aquiclude.

WATER INFLOW ESTIMATION OF 1101 COAL FACE DEVELOPMENT

The total water inflow from 1101’s development roadway i.e in the Track gate and Belt gate road way is recorded by CMC at 10 day intervals by a flow meter, in the drainage channel in 1101’s Track Cut Through roadway near the inclined connection to the belt Dip Drive Chamber. Formulae for calculation of the flow rate are:

V=C₁Ín+C₂ m/sec

Where, V= velocity of water flow

n= time of one flow meter revolution;

C₁ and C ₂ are calibration constants for the flow meter in use.

Q = w ÍdÍvÍ3600 m³/h

Where, Q= flow rate;

W = width of drainage channel

D = Depth of flow

The result of these flow measurements from 6^th November 2005 are given in Table. 4.2 And presented graphically in Figures 5.1. It should be noted that the flows are inclusive of water from the de- watering drill chambers located in 1101`s Track cut Through roadway, estimated at approximately 50 m³/h.

Table.2.4 Water inflow rate in the 1101 coal face roadway development. (IMCL, 2003, Unpublished report)

Date Flow (m³/h)Date flow (m³/h) Date Flow(m³/h)6-Nov-0260.0565-Feb-03173.6255-May-03361.37515-Nov-0254.89615-Feb-03183.62415-May-03365.19225-Nov-0270.00025-Feb-03201.24425-May-03369.8905-Dec-0264.5945-Mar-03221.7145-Jun-03341.98115-Dec-0270.00015-Mar-03245.68515-Jun-03365.42015-Jan-03 64.5945-Mar-03221.7145-Jun-03341.98125-Jan-03152.84725-Apr-03335.00025-Jul-03383.556

Both the Track and Belt roadways have been driven at, or very close to the top of Seam VI and it is one of the least permeable coal zones of the seam, characterized by localized lenticular mudstone horizons and typically high ash content. Despite this, the greatest proportion of the inflowing water issues from the coal seam, with only minor or negligible quantities form the roof sandstone. The large volume of water flowing through the connection originated from the coal exposed in the Belt Gate up dip. This appears to contradict the hypothesis that the roof sandstone is the source of recharge of the Seam VI aquifer (IMCL, 2003).

Fig.5.1. 1101’s coal face development Total Water Inflow/ Time.

The total volume of water flowing into 1101’s developments has consistently increased as the roadways have been extended, as shown in Figures 4.1 and 4.2. It is concluded, therefore that the bulk of the flow is into the floor of the roadways via the more permeable zone. This is supported by the fact that flow rates in the Track Gate have shown a steady in crease generally in proportion to roadway length. Flow rate in the Belt Gate increased significantly since driving the inclined section of roadway up through the different seam zones until the roof sandstone was reached. Again it is to be expected that flow rates will be somewhat higher in he Belt Gate as this is at a higher elevation, and also nearer to the sub crop of Seam VI beneath the Lower Dupi Tila formation in the west.

Total water inflow from 1101’s developments is currently estimated at 383 m³/h (+10% for measurement error). When the face commences production, this figure will increase very significantly as mining-induced strains open natural jointing and fractures in the Gondwana sandstone sediments above the seam. Critical stages will be reached with the initial “cave” and when the face has retreated by about 100m; in other words, the waste has attained a “square plan” (IMCL, 2003). The result of these flow measurements from 5^th January, 2005 is given in Table. 4.3 And presented graphically in Figures 5.2 below.

Table.2.5 Water inflow rate in the 1101 coal face roadway development. (IMCL, 2003, Unpublished report)

Date Flow (m³/h)DateFlow (m³/h)DateFlow (m³/h)26-July-05465.39916-May-005461.49406-Feb-005412.64816-July-05464.58505-May-005464.67216-Feb-005419.26206-July-05461.76725-Apr-005460.0525-Feb-005421.84325-June-05463.88815Apr-005454.90526-Jan-005411.18516-June-05461.2425-Apr-005464.67215-Jan-005423.7706-June-05467.14125-Mar-005459.72205-Jan-005441.66325-May-05470.54915-Mar-005444.49925-Dec-005431.663

Fig .2.5 In 1101’s Long wall coal face Total water Inflow/ Time.

As the face will retreat to the dip, roof water flowing into waste will run forward onto the face, and hence down dip to the Track Gate. At the present time, it is uncertain what volume of water will be released by induced fractures and caving in the roof, but it will be essential to monitor this carefully as 1101’s retreats.

FACTORS AND SOURCES OF MINE WATER FILLING

The Barapukuria Coal Mine area is a semi-confined and incompleted syncline with some recharge characteristics. Here the Tertiary strata overlie the coal series as unconformity, and the lower sand horizons of UDT aquifer overlie the sandstones of seam VI roof in the north. The deposits of LDT aquiclude are stable in the southern part, and they cut down the direct recharge from the lower sand horizons of UDT to the base rock aquifer. The sandstone of seam VI is a directly water-filling aquifer, which has an affect on mining. The lower UDT is indirect water filling bed.

The groundwater level shows that rainfall is the main source of the water filling in the coal mine area. Rainfall infiltrates to the confined porous medium aquifer of the lower section of UDT. Through the Quaternary sandy clay layer, then penetrates into the lower confined pores-medium aquifer of UDT and forms the source of recharge to the weathered zone of base rocks, coal sub-crops and the confined fissured medium aquifer of seam VI roof. The recharge has an apparent variation with the seasons.

So we may conclude about the hydrogeological condition of the Barapukuria Coal Mine area as follows

¨ Since the LDT is the main aquiclude in the study area and the lack of this layer within the 0.29km² towards the northern region causes a great influence in to the recharge condition of the lower aquifer.

¨ There is a vertical recharge from UDT to the Gondwana aquifer at the northern part of the mine area.

¨ Because of relative homogeneity, big thickness well developed vertical and horizontal fissures also good recharge condition (due to absent of LDT aquiclude) from UDT to Gondwana formation the roof sandstone of coal seam VI is a good aquifer which is affecting the mining condition (CMC, 2000).

COAL MINING METHOD

Recovery of mineral from subsurface rock mass involves the development of physical accesses to the mineralized zone, extraction of the ore from the enclosing host rock and transport of this material to the mine surface. Excavation of various shapes, sizes, orientations and duty functions are required to support the series of operations which comprises the complete mining process. A mining method consists of a sequence of production unit operations, which are executed continually in and around the production blocks into which an ore body is divided. The operations of ore mobilization, extraction, and transportation are common to all mining methods, while other operations may be specific to a particular method. Hence, differences between mining methods involve different techniques of performing the unit operations.

During geological history, the large coal bearing area, which was formed by deposited carbonaceous material, called coalfield. The social combination for developing a coalfield stated as mining area. It can be seen that a mining area is composed of many underground or surface (Open-pit) structure. So, It is necessary to develop the whole mine area plane fully, step by step and reasonably. In order to meet mine construction and production, a set of auxiliary enterprise, traffic transportation, civil and other corresponding enterprises and city buildings are constructed. Therefore, before developing a mining area, a careful plan, a research on feasibility and a general design of the mining area should be worked out as a basis of development and construction of the mine. In the same way, for any engineering research work, it is essential to get details knowledge about the geometry and relevant issues of the studied site. In such demands, this chapter mainly concerns with the geometric layout of the mine from extensive mine visits, literature reviews, discussion among researchers and practitioners.

In April 1985; in order to find out economical valuable minerals in the northern part of Bangladesh, Geological survey of Bangladesh (GSB) carried out some gravimetric and magnetic survey, and drilled boreholes GDH-26. Since then, the Gondwana Coal-bearing basin in Barapukuria area was discovered and which outlined the structure of the basin. The contract of Barapukuria coal mine Development project number BCMP-77 was signed by China National Machinery Import and Export Corporation (CMC) and Bangladesh oil, gas and minerals corporation (BOGMC) in February, 1994. Barapukuria mine is the first developed and constructed coal mine in Bangladesh, which is a modern and large one with a production capacity of 1 Million metric ton per year. The basic design for this mine was completed in 1995. The mine commenced its construction in June 1996; thereafter in December 1996 and April 1998 started the development of main and service shaft respectively.

In the mean while, the 1101 coal face is going to be prepared for full scale mine production and it is now under trail basis production mode.

MINE DEVELOPMENT PATTERN

After the coalfield is divided into minefield, a series of main shafts and roadways should be excavated from the surface to underground, in order to access easily to or near to the predetermined position of coal seam of the preparation of mining districts. The arrangement and the excavation engineering of main shafts and roadways, which serve for mining level stated as mine development. Again the main shafts and roadways excavated for mine development, called development roadways, such as shafts, shaft bottom, main haulage roadways, main return air way, main cross-cut etc. However, the rational arrangement methods of minefield development roadways mainly depend upon certain conditions of minefield geology and accessible mining technique.

The occurrence of coal seam in the Barapukuria minefield is a syncline basin, deep in the middle part and shallow in the surrounding parts, the strike length of north-south is 4.9 km, spans of 0.3-9.1 km east-westward, with an area of 5.8 km². The effective mining area after deducting the safety pillars for mine site, faults and preventing against water is only about 3 km². The vertical height within the mining range is less than 300m. A single level mining method is applied in this minefield, in order to ensure the life of the mine enough and the capital investment of basic construction is reduced because of the smaller mine field area and smaller vertical height of the mining range. So, only a mining level is used and the level elevation is determined as -260 m by analysis and comparison for this minefield. 

The basic design divided the minefield into 3 mining districts. Because of the adjustment to the angle of coal seam and some correction in terms of hydrogeology in the supplementary exploration report, the design combines the original central mining district and southern mining district of the design modification. Therefore, the whole minefield is divided into two mining districts, as No. 1 (Southern) mining district and No. 2 (Northern) district. The current development works run through the No. 1 (southern) mining district by long wall face slicing method and its is to be taken over the owners i.e. the Petrobangla after completion of construction. The northern district will be excavated later by adopting room and pillar mining method i.e. which is not suitable for long wall face method, due to its location within the open window area. The proposed succession and developing plan of the Barapukuria coal mining project at a glance shown in Fig. 6.1

However the development method of Barapukuria coal mining project is vertical shaft single level down-dip development. The arrangement manner of mining level main roadways is double main roadways in each coal seam floor. According to the layout of the surface production system, combining the relative position in relations within the main roadways and the upper yard of the central mining district proposed in the basic design. A pair of vertical shafts is laid in the central shallower part of the basin, on north-south direction. The main shaft is used for coal hosting and air return of the whole mine, where as the auxiliary shaft is used for supplementary hoisting i.e. rocks, materials, equipment’s and men, also uses as a function of fresh air inlet in the mine.

ROADWAY SYSTEM FOR THE MINE

A mining district is a comprehensive section with independent production system divided along the strike in horizon or mining level. A series of preparation roadways and entries must be driven to form a perfect system in a mining district and ensure coal conveying, materials and equipment hauling, ventilation, drainage and power supply to go smoothly. Preparation roadways are the main roadways served for the preparation of mining district, such as district rise or district dip, district station, district sub-level cross-cut and district rooms i.e. transformer station, winder chamber and coal bin (storage). Whether, the roadway arrangement system of a mining district is proper or not will directly affect the production capacity of the working face, the district and as well as the mine, and all the techno-economic indexes. Therefore, the roadway arrangement system of the district must be designed reasonably based on geological and mining technical conditions of each district. The arrangement patterns and the position determination of main roadways in the BCMC based on the fact that the main mine able coal seam (seam-vi) of the coal mine belongs to extra large thickness of coal seam, is about 36.14 m and the distance between the seam-vi and other mine able seam is large (more than 100m). So, the in-seam roadways are used in order to reduce the engineering quantity of rock roadways (crosscut) and production cost. Again, considering the fact that the underground temperature of Barapukuria coal mine is very high and comprises of complicated hydrogeological condition, the double-entry layout is adopted for roadway arrangement, which is very much favorable for temperature controlling, ventilation and construction safety. Taking consideration in all of the above stated factors, CMC comparing the three options for selection the position of main haulage roadways, namely-

The roadway arrangement

1) In the roof seam-vi

2) In seam-vi, and

3) In the seam-vi floor.

After long deliberation and analyzing a brief, the mine authorities adopted the parallel arrangement of the roadways in the harder rock bed of seam-vi floor. The three dips are laid out in medium hard sand stone bed of seam-vi floor and the horizontal distance among dip is 30 m form each other. However both the Belt dip and track dip entry rood way tunnel has a net cross-sectional area of 13.7 m² and total length of 800 m. The tunnel is started from the -260m level pit bottom station and completed at -450m level, with an angle of 12⁰ respects to the Global horizontal axis.

� Track dip entry (a=12⁰), is used for the elevation of face materials, Men and equipment, driving of coal and waste and as air out take concurrently.

� Belt conveyor dip entry (a=12⁰), is used for coalface hoisting.

In the southern mining district, the road way arrangement pattern for the development of the Barapukuria coal mine project is shown in fig.6.2

SLICE GEOMETRY

According to geological conditions of coal seam-vi 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 1.0-1.5 years will be mined.

The first coalface is remarked as 1101, which denotes the first number ‘1’ as No. 1 mining district, the second number ‘1’ as the first slice and the last two digits refers to No. 1 coal face. The numbering of south wing of mining district is descending as 1101, 1103, 1105........and similarly, the numbering of north wing of district is descending as 1102, 1104, 1106....and so on. The No. 1101 coalface is located at upper ends of district south wing, shown in fig. 5.3. In the basic design of the mine, total 11 slices will be extracted, of which seven from the southern district and the rest from the northern. The working face lengths both for fully-mechanized faces are of 120m, the mining heights are 2.5 m for the first slice i.e. slice No. 1101 and 3.0 m for the other slices. The 1101 working face lies at the top of the south of the district, where 100 m fault coal pillar will left from the belt gate to the Fb fault, 50 m from open-off cut to DOB 5 hole, 50 m coal pillar from ending line to GDH 43. Again it is strictly controlled the track elevation is higher than 257 m, in order that the water from the crossheading and the working face can flow naturally from cutoff to track roadway to the –260 shaft pump station.

The sub-level haulage entry (belt conveyor gate) and air return entry (track gate) are laid out in parallel to keep the length of the working face to be 104 m for the requirements of fully mechanized coal face. The single entry layout is adopted between two sub-levels while the double entry layout and the roadway driving along the gob are used for the successive sub-level mining. The slice geometry and the whole mine plane at a glance is shown in Fig 6.3.

Table. 2.6 Revised Long wall face geometry of BCMC (CMC, 2004)

Description Face Run (m)Face Length (m)Long wall Team 11101 Face4801041103 Face12501041105 Face12001041107 Face10501041109 Face 8901111 Face730100

Long wall Team 2

1102 Face630601104 Face6201201106 Face5501201108 Face6001201110 Face4801201112 Face5501201114Face5501201116 Face5501201118 Face400104

UNDERGROUND LONG WALL LAYOUT

By definition, a thick seam is one that can not be mined by one slice mining method. The highest ‘one slice mining method’ is presently applied up to about 4.5 m. Accordingly, any coal seam above 4.5 m thick (above 3.5 m in some countries) is regarded as thick seam. In BCMC, seam VI has a thickness ranging from 28 to 42 m with an average of 36 m. It is therefore regarded as a very thick or ultra thick coal seam.

Considering the geological, hydrogeological, and other technical parameters, long wall mining method is applied in the Barapukuria underground mine. Long wall mining involves removal of coal from single faces, generally 80 to 200 m long with the working area protected by moveable roof supports. As the coals are extracted, these supports are moved forward so that the roof behind them collapses to form an extensive abandoned area called “gob”. Since coal seam VI is the only seam to be extracted in Barapukuria, is very thick, a multi slice long wall mining method would be adopted.

SLICING MINING METHOD IN THICK COAL SEAM

As for thick coal seams, using full-seam extraction will be difficult and complicated in technique. If the thickness of coal seam is over 5m, the support technique and equipment of working face could be very difficult to control. Therefore, under general condition, in order to overcome the difficulties of extra thick coal seam of full-seam extraction can be divided into three types, that is, inclined slice, horizontal slice and diagonal slice.

Inclined slice – Coal seam is divided into some slices which are parallel to the bedding plane of coal seam, and the face advances along the strike or dip.

Horizontal slice – Coal seam is divided into some slices which are parallel to the level, and the face commonly advances along the strike.

Diagonal slice- Coal seam is divided into some slices which have a certain angle with the level, the face advances along the strike.

Two main types of long walls are being used throughout the world: advancing and retreating long walls. Both methods have advantages and disadvantages, but retreating long wall mining has proven to be more conducive to the current mining experience and geological conditions found in the Gondwana sediments (CMC, 2002).

Advancing long wall mining requires no pre-development around the perimeter of the long face; the gate roads are developed as the long wall face advances. Therefore the initial investment is much smaller when compared to retreating long wall mining, and the coal can be produced for sale much sooner (Peng and Chiang, 1984).

The retreating long wall mining method requires that roadways be driven around the length and width of the face before the long wall face begins to retreat, thus initial investment is higher than the advancing long wall, as the roadways must be driven by continuous mining equipment, and the coal in the long wall face cannot be extracted until the roadways are completed. The entry arrangement of the retreating long wall method is somewhat more complex than that of the advancing method since additional roadways must be created for ventilation and transportation. However, the retreating long wall provides first-hand information about geological conditions along the entire length and width of the face, which is very important if adverse geological conditions are discovered, such as faults or pinch-outs, etc. Entry maintenance is easier and lasts longer in the retreating long wall since the roadways are developed in solid coal (Peng and Chiang, 1984). As the mining technical condition for the BCMC is much more complex, the selection of retreating mining method is one of the principal reason, shown in fig 6.4

The faces are blocked out by developing face roadways, which are excavated in-seam, perpendicular to the main roadways on both sides of the main roadways. When the face roadways on both sides have been developed to the designed length, they are connected by the crosscut roadways, which are parallel to the main roadways. Usually, the face width varies from 100 to 150 m, and the face length varies from 600 to 900 m. After getting available mining experiences in the Gondwana sediments, the trend to increase face width and length to achieve a higher extraction ratio and to extend the time and production between long wall face moves.

MINING SEQUENCES

In a multi slice long wall mining environment, four different mining sequences are possible regardless of the mining method employed, as shown in Figure 6.5 (After Chekan, 1993).

(1) The upper seam is mined out prior to the lower seam; that is, the seams are mined in descending order. Mining of the lower seam does not commence until operations in the upper seam are complete. Mining procedures in this order are usually called undermining.

(2) The lower seam is mined out prior to mining the upper seam; that is, the seams are mined in ascending order. Mining of the upper seam does not commence until operations in the lower seam are complete. Mining procedures in this order are usually called over mining.

(3) Mining of the upper and lower seam is carried out simultaneously, with development and mining being kept in advance in the upper seam. Mining procedures in this order are usually called simultaneous mining.

(4) The upper seam may be partially developed and mined, followed by extraction of the lower seam under the development section of the upper seam (Lazer, 1965).

To avoid potential multiple seam interaction problems, the preferred mining order should be descending mining, in which only the high stress areas under the abutments will need to be accommodated for or avoided in underlying mining operations (Peng, 1984). In the past, coal beds were mined in no particular order with regard to controlling interactions and reducing ground control problems. Again seam sequencing was based mostly on economics, availability, and ownership.

Coal mine ground control problems associated with interaction effects when mining in ascending order, namely over mining, were reported about half a century ago. Hasley (1951) observed that the extraction of the coal from the lower seam would inevitably cause subsidence, fracture, and parting of the overlying strata and beds of coal, and that large and extensive remnant pillars of coal left in the lower seam, or non-uniform extraction of the lower seam, would cause bumps in the floor, crushing, and fractures of the coal in the upper bed, endangering the mining activities of the superjacent seam.Extracting order and the way of handling gob have very close relation. When descending order is used, gob can be handled by caving or filling method, while ascending order is used, gob is commonly handled by filling method

In Barapukuria, Coal seam VI will be extracted in multi-slice method in descending order at the upper part of the seam. In the mine plan, there are 11 slices to be worked in seam VI, each 2 to 3 m thick. Each slice will be separated vertically from the next one by sections (septa) of unmined coal of approximately 3 m. Fig 6.6

Fig. 3.2 Schematic diagram showing multi-slice long wall mining of thick seam of Barapukuria coal mine.

COAL MINE FACE OPERATION

In a retreating long wall, mining starts from the crosscut roadways and proceeds toward the main roadways. Fig. 6.7 is a cutaway view showing the setup in a retreating long wall face. All of the face equipment is assembled in the setup room where the long wall begins. There is generally a barrier pillar between the crosscut roadways and the setup room. When the long wall face line reaches the designed termination point on the main roadways side, a recovery room is established from where all face equipment is recovered, moved, and assembled at the setup room f the next face. The face roadways consist of two, three, four, or sometimes five roadways. The most common is the three-entry system which is the minimum number of roadways required by law for developing the face roadways (Luo, 1998). After the face begins to retreat, the immediate roadways on both sides of the face serve special functions. The track gate roadway, which will be used as the belt gate roadway after the face moves to the next 1103 coal face, is used for transporting coal, personnel, and supplies to and from the face and for the passage of fresh air. The belt gate roadway on the other side of the face is used for the passage of return air.

The stability of the roadways plays a very important role in retreating long wall mining. If any one of them is blocked or fails to function normally due to ground control problems such as pillar failure, roof fall, or floor heave, coal production may be disrupted or the escape way for personnel and route of air circulation could be cut off. Repair work not only costs money but also time, which in turn dramatically decreases the efficiency. Sometimes when the blockage is severe, a large quality of reserves must be abandoned. Hence, in order to keep retreating long wall mining safe, continuous, and efficient, maintaining the stability of both \track gate road way and belt gate road way becomes a major concern in fig. 6.8. The precise, though not conservative, arrangement and design of the roadways system, which takes into account various geological and mining conditions and interaction effects for multiple seams, is a positive and effective way to maintain face road way stability and can reduce labor and materials for primary and supplemental support.

Mine safety.

Mine safety are as follow’s

personal protective equipment

1.head protection

2. Eye and face protection

3. hearing protection

4.Hand foot, and leg protection

5.Protection clothing

6. respiratory equipment

Sanitary facilities .

Suitable sanitation and hygienic facilities must be provide at mine and properly maintained.

Appropriate toilet facilities must be provides within a reasonable distance from each workplace at the mine.

Mine drys.

Potable water.

Mine climate.

The climate of the mine is determined by the temperature and humidity of the mine air.

Mine danger’s

i) Toxic gases

ii) Caving

iii) sliding

ECONOMIC ASPECT .

The coal of BARAPUKURIA COAL MINE is economically very important. The coal is used for produces Heat Electricity in Varamara. The coal is also used in berning bricks, road construction .