Electrical resistivity tomography technique coupled with numerical modelling: A case study for stability analysis.

Cavity due to underground old mine workings is one of the major threats to the coal mines and the overlying subsurface and surface properties, which need to be protected. The detection of old mine workings and stability assessment of overlying strata are common problems in most of the Indian coalfields. Several coal mines in India are loss‐making, mainly due to different types of mine hazards. Khandra mine is one such mine at Raniganj Coalfield, Eastern Coalfields Ltd., a subsidiary of Coal India Limited. In the present study, 2‐dimensional and 3‐dimensional electrical resistivity tomography were carried out for detailed subsurface characterization. It supports delineating underground workings, including the nature of voids/cavities (air or water‐filled). Excessive distortions were reported in electrical resistivity tomography application, especially at the near‐surface, owing to large resistivity variations. Refinement of the model by half‐unit electrode spacing was attempted here to reduce the distortions with minimum possible absolute errors. 3‐Dimensional resistivity volumetric model was also developed with the help of five electrical resistivity tomography parallel profiles for better apprehension of the subsurface. Analysis provided important inputs for stability analysis using 3‐dimensional numerical modelling. The physico‐mechanical properties of the overlying strata, pre‐excavation in situ stresses, boundary conditions and the mine geometry simulation were incorporated for understanding the stability analysis. Stability analysis was carried out using the finite difference technique. The analysis of 3‐dimensional numerical modelling indicated that two distinct layers comprising (i) laterite/part of the course to medium‐grained sandstone and (ii) developed galleries of R‐IX seam exhibited a very low safety factor below 1.0, indicating potholing/subsidence susceptibility. The other three layers comprising parts of fine‐grained sandstone exhibited a relatively higher safety factor of around 2.0, indicating moderately stable zones, but not on a long‐term basis. Parts of Siduli stream embankments need suitable retaining walls to avoid water inundation for the stability of the area. [ABSTRACT FROM AUTHOR]