Water Inrush and Failure Characteristics of Coal Seam Floor Over A Confined Aquifer

Abstract

Abstract Failure behaviors of the floor rocks under coal seam mining in the conditions of hard magma rock roof and confined aquifer are studied. Based on the theory of rock stresses and elasticity mechanics, the combined effects of the abutment pressure induced by the hard roof and by the water pressure under the thin aquicludes of the floor rocks were considered, and a mechanical model was constructed along the strike of the working face. An analytical solution of stress distribution was derived in the floor rocks, the distributions of vertical, horizontal and shear stresses were calculated. In combination with the in-situ measurement, the results show that: 1) when the periodic pressure caused by the roof collapse occurs on the working face, and the maximum stress concentration in the floor appears at the elastic-plastic junction in the direction of the strike of the working face. With the increase of the depth of the floor, the horizontal stress coefficient tends to decrease, and the corresponding shear stress coefficient isoline shows a “symmetric spiral” distribution and propagates downward to the floor at a certain angle with the vertical direction. This causes the floor rocks to generate compression and shear or tension and shear failure. 2) when the immediate roof of coal seam is the magma rock, the abutment pressure shows a trend of a slow increase initially and then a rapid increase later. The peak value of abutment pressure appears at the location of 4 - 6 meters from the coal wall of the working face, and the concentration coefficient of the abutment pressure is between 1.4 and 1.8. 3) according to the measurement and calculation of the failure depths of the floor at different positions under the same coal seam, it is found that the maximum failure depth appears near the coal wall of the working face. The failure depth reduces by 11.6% after the floor goes through “the roof caving and re-compaction”, which causes the fractures in the floor to close and the thickness of the effective aquiclude increases. In the un-mined area of the working face, the failure depth is 55% of the maximum failure depth. 4) both the theoretical calculation and the numerical simulation show that the failure depth of the floor increases obviously under the combined action of high vertical stress and the water pressure. Under the condition that the thickness of the aquiclude is relatively thin, the water pressure of the floor and pressure intensity of the roof are the sensitive factors to affect the maximum failure depth of the floor.