Effect of Scattered Dumping of Soft Soil in Coal Mine Overburden Dumps: A Probabilistic Analysis

Energy requirement is mostly met by coal in India. Open-pit mining majorly contributes to coal production. Availability of land for waste dumping is a serious bottleneck in expansion of open-pit projects. Due to scarcity of land, mixing of some soft soil or clay within the overburden is common. In several places dump failures have occurred due to the presence of weak clay or soil mixed in the overburden dumps. In this study, numerical modelling has been conducted using a Finite difference numerical modelling software FLAC3D to assess the effect of mixing weak soil or clay of various proportions on the overall stability of an overburden dump. To incorporate the unknown random effect of weak soil placement in the overburden, a spatial probabilistic method was applied to randomise the material properties in the overburden dump. For each combination of percentage of weak material and the dump height studied, 100 models were run, and the factor of safety was analysed using probability density plots and normal distribution graphs. From the range of FOS obtained from the modelling, the lower limit of FOS with 95% reliability has been determined. The analysis revealed that the FOS dropped quite significantly due to the presence of weak soil, and in some cases, the dumps might require redesigning to avert a failure.