Geotechnical study and optimal slope design at Lajkura open cast coal mine, India

Abstract

The paper deals with the geotechnical study and slope stability of the dragline highwall slope at Lajkura open cast coal mine, Orissa state, India (Fig. 1). At the mine, 22m of overburden immediately above the Lajkura coal seam (having a thickness of 18 m) is being removed by deploying a 10/70 dragline machine. Initially benches are being left in the coal seam. During the final cut, the total height of the dragline bench and coal seam becomes 40 m. The 30–50m of overburden above the dragline bench is being removed by shovel and dumper combination in 3–5 benches. The Lajkura open cast mine is mainly characterized by sandstone, shale and coal. The point load testing of all the formations was done in the field. The bulk density and direct shear tests were conducted at the Rock Mechanics Laboratory of CMRI on the samples collected from the field. The geotechnical mapping was conducted on the exposed benches of the quarry as per the norms of the International Society for Rock Mechanics. Kinematic analysis was included to determine the critical orientation of structural discontinuities. After identifying the kinematically possible failure modes, detailed slope stability analysis was carried out using the GALENA software based on the limit equilibrium method. Based on the present study, the 40m high dragline highwall is likely to be safe with a 70° slope angle in the drained condition. The sensitivity analysis shows that the groundwater is highly detrimental to the slope stability for this high and steep slope. So, a good drainage system in and around the mine was also recommended to keep the slopes in a drained condition. The slope monitoring is being done with the help of the Total Station method to detect any instability in and around the mine. To date no movement has been recorded.