A Novel Method to Retain a Waterproof Coal Pillar Near Water-conducting Fault

Reducing the width of safe coal pillar near geological structures, such as a water-conducting fault, to improve the extraction ratio of coal seam is a major concern that needs to be addressed urgently for mining shallow coal resources in China. To tackle the contradiction between improving the extraction ratio of resources and the disaster caused by reducing the width of the waterproof coal pillar, this study focuses on retaining the coal (rock) pillar near the water-conducting fault and proposes a novel method for calculating the waterproofing of the coal pillar precisely. We consider the different location relationships between the water-conducting fault, confined aquifer and coal seam, and suggest the principle of retaining the waterproofing of the coal pillar. First, we develop a mechanical model describing a waterproof coal pillar near the water-conducting fault by using elastic–plastic mechanics theory and subsequently calculate the length of plastic zone of the coal pillar under water pressure of the fault by using the limit equilibrium method. Furthermore, we use FLAC3D software to simulate the stress distribution of the coal seam roof and floor under fault water pressure. Subsequently, based on the Mohr–Coulomb strength criterion, we propose a method to calculate the width of waterproof coal pillars by combining the plastic zone with the maximum principal stress propagation line. Ultimately, we assess and discuss the accuracy and simplicity of this method by combining it with engineering methods.