Thursday, October 28th, 2010
No reason for Bangladesh to go for open-pit mining
OPEN-pit coal mining generates multi-dimensional problems, the most important of which is drop of groundwater table. Other hazards recognised worldwide include acid mine drainage, desertification, replacement of infrastructures, air pollution due to huge amounts of suspended dust particles from the pit area, noise pollution, destruction of valuable agricultural lands, very large holes, high pit slopes steep and not stable which cannot be maintained, may fill with acid-mixed water, and loss of valuable top soil, which is essential for agricultural purposes.
Open-pit mining in the Phulbari and Barapukuria coal deposits has increasingly come under public attention. In this article, boundary element method numerical modelling has been applied to recognise in situ stress state around the proposed open-pit mine of the Phulbari coal deposit. The major objective of the present article is to clarify the rock failure (tensional) state around the proposed open-pit excavation and to illustrate the geological and hydro-geological problems linked to the in situ stress state of the surrounding open-pit dimension and how extensively the groundwater table would be affected due to the mine dewatering.
Several models pertaining to hydro-geological aspect of the proposed Phulbari open-pit mining have been prepared by the AEC’s promoters. However, these models are vague to scientific and general societies of the country. In the present study, sophisticated mining software has been applied to simulate the open-pit dimension of the Phulbari coal deposit and its surrounding rock failure conditions.
The present numerical simulation reveals that high tensional failure (see figure) within rock strata would be extended up to 27 kilometres to both sides from the centre of the open pit. The lateral tensional effect within the strata would be around 54 kilometres. This means that the natural porosity and permeability within the rock strata of regional water-bearing formation would be damaged and groundwater inflow would be increased rapidly with the progress of mining activities. More clearly, continuous dewatering would cause rapid drawdown of regional water table and instability of the regional aquifer system.
The drawdown of groundwater table due to open-pit dewatering process creates a serious problem in Germany. For example, Lusatia, one of the Germany’s driest regions, faces one of the most serious water scarcity problems in German history. Dewatering from open-pit mine, which involves pumping of six tonnes of groundwater for each tonne of lignite extracted, has led to a serious drop in the groundwater table in the region. An area of more than 80,000 hectares is affected by open-cast lignite-mining activities in Lusatia. Above that an area of 2,100 square kilometres is affected by groundwater lowering due to large-scale pumping measures before the opening of a mine creating a groundwater shortage of 9 billion cubic metres in the region. Once the groundwater level is lowered beneath the depth of the lignite seam (around 70 metres below surface in Lusatia) huge amounts of overburden have to be translocated. The dumping of these overburden substrates result in largely devastated landscapes with disturbed or destroyed soils, flora, and fauna.
The crucial problem of open-pit mining regarding north-western Bangladesh is the thick (about 95-130 metres) and highly permeable, unconsolidated sedimentary pile, which is called the Dupi Tila aquifer that extends all over the northern districts of the country. The Phulbari coal deposit is located between 150 and 350 metres depths. Movement of stable and unstable open-pit slope is directly connected to the drop of water table. The movement of stable and unstable open pit slope due to high water table is a common phenomenon in Germany. For the case of the Phulbari open-pit mine, extensive tensional failure (see figure) into the rock strata up to 54 kilometres would create numerous fractures and tensional shear stress that will lead to the movement of the stable and unstable open-pit slope, as experienced in the Lusatia region of Germany.
In a rational sense, when a mine dewatering is continued, groundwater is released continuously from geological formations and the water level doesn’t rebound. Recovery of surrounding water level due to continuous dewatering from an open-pit mine is impossible. The present study clarify that the cone of depression (see figure) will be extended up to 54 kilometres and local tube-wells, shallow machines, and deep tube-wells will not get groundwater for agriculture and domestic purposes within a radial distance of up to 27 kilometres from the centre of the mine. The AEC has proposed that they will establish some injection wells into the aquifer and will try to continue to rebound the water table, which is an imaginary story. As an inexperienced mining company, the AEC has no such types of practice elsewhere.
Anyway, selection of mining methods doesn’t depend on emotion. It always depends on some important factors, like (1) physical and mechanical characteristics of the coal deposits such as rock material shear strength, natural fractures and discontinuities shear strength, length, spacing and location of major geologic structures, especially faults, joints, etc, in situ stress, hydro-geological conditions, etc, (2) technical factors, (3) productivity factors, and (4) economic factors. The argument for open-pit of the Phulbari and Barapukuria coal deposits hinge upon only coal recovery rate (over 90 per cent), i.e. only on the economic factor. However, the present study illustrates a serious technical issue regarding geological and regional hydro-geological aspects of the northwest Bangladesh. The government of Bangladesh, scientists, and policymakers must consider these factors before selecting an environmentally favourable and sustainable mining method.
Dr Md Rafiqul Islam is a geoscientist and mining expert at the Department of Petroleum and Geo-resources Engineering, Shahjalal University of Science and Technology, Sylhet.
http://www.newagebd.com/2010/oct/28/edit.html