The paper deals with geotechnical study and slope stability of the Coal and overburden benches in the hanging wall of an up-throw strike fault in the dip side of NMOC-II open cast coal mine, WCL. The opencast mine is located in the Maharashtra state of India. The detailed slope stability analysis is carried out by limit equilibrium method. The overburden slope of the quarry is mainly characterized by sandstone which is fractured and weathered. The mine is mostly covered by black cotton soil. Well fractured rock mass, existing open cast working have made the geo-mining condition of the quarry to drained condition for all practical purposes after implementing an effective drainage system. The relevant strength properties were determined in the soil and rock mechanics laboratory of CIMFR and subsequently used for slope stability analyses. The rock mass rating was also used to estimate the strength properties. The detailed slope stability analysis was carried out by GALENA software based on limit equilibrium method. The optimum design parameters for final pit slopes have been recommended.
The geotechnical study was conducted for slope stability study of the Coal and overburden benches in the hanging wall of an up-throw strike fault in the dip side of NMOC-II open cast coal mine of Majri Area. The mine belongs to Western Coalfields Ltd, a subsidiary of Coal India Ltd. The opencast mine is located in the Maharashtra state of India. Total overburden and coal have been planned to be excavated by Shovel Dumper combination. The OB bench height is kept at 10m. The shovel and dumper up to 5m3 and 60 tonne have been deployed at the project. The coal and overburden waste production are 2 million tones and 12 million m3 respectively. The existing maximum depth of the pit would be 150m. The study was conducted to optimum slope design with special reference to F1 fault without sacrificing the safety of the mine operation (CSIR-CIMFR Report, 2017).
The top soil of about 10m thickness forms the top most lithology in the project area. Soil is underlain by main overburden constituted sandstone with coal seam. Floor of the seam is generally composed of sandstone. The dip of the seam in this area is about 14 degree.
The importance of safe, properly designed and scientifically engineered slope is well known. The benefit of an openpit operation largely depends on the use of the steepest slopes possible, which should not fail during the life of the mine. So, the design engineer is faced with the two opposite requirements, stability and steepness, in designing the deep openpit slopes. Steepening the slopes, thereby reducing the amount of material to be excavated, can save a vast sum of money (Hustrulid et al., 2000). At the same time excessive steepening may result into slope failure leading to loss of production, extra stripping costs to remove failed material, reforming of benches, rerouting of haul roads and production delays. The Directorate of Mines Safety, the highest statutory body, may even close the mine, in case unsafe conditions are created. Therefore, it is necessary that a balance between economics and safety should be achieved.
The mine is mostly covered by black cotton soil. The information regarding geological sequence is therefore available only from the borehole data. Table 1 furnishes summarized statement of lithological formations encountered in the boreholes drilled in the mine quarriable area. The area is structurally disturbed by fault. F1 fault is present in the present mining area.