Hydro-Mechanically Coupled Numerical Modelling of Protective Water Barrier Pillars in Underground Coal Mines in India

Protective water barrier pillars (PWBPs) are inter-mine barrier pillars. An adequate PWBP can protect active mine workings from the danger of inundation from adjoining inundated workings. This paper discusses a hydro-mechanical coupled numerical modeling approach for the design of PWBPs, considering different flow regimes. The coupled model considers the effect of seepage through the roof, pillar, and floor on the mechanical strength of the rock mass and vice-versa. A statistical model based on the extent of positive volumetric strain zones (ZoPVS, %) was formulated to assess the mechanical stability of the pillar. The model considers the pillar width, cover depth, strength, modulus of elasticity of coal and roof/floor, and the extraction ratio as the input parameters. Based on the results of numerical modeling-based parametric studies, statistical models were developed to estimate the seepage rate for assessing the hydraulic performance of the pillar. The seepage rate of water through a PWBP was estimated in terms of the cover depth of the seam, compressive and tensile strengths and modulus of elasticity of the pillar system, mean in-situ horizontal stress, pillar width, extraction ratio, and permeability of the rock mass. A ZoPVS of 95% represented unstable behavior and piping failure of the pillar. The maximum seepage rate of 315 L/s/km (5000 GPM/km) was reckoned as the acceptable hydraulic performance of the PWBP based on the field experience in Indian geo-mining conditions. The validation of the model for two case studies with minimum pillar widths of 30–60 m at a cover depth of 84.5–134.5 m was in agreement with the field experiences.