Stress, displacement, and crack evolution of top-coal layers in high-intensity repeated mining with extra-thick coal seam

Abstract

A novel two-stage mining method, involving large-height mining of the medium layer followed by top-coal caving in the lower layer, has recently been applied to extremely thick coal seams (> 20 m). Throughout the mining process, the top-coal layer undergoes two distinct stages of disturbance and failure, influenced by multiple factors that impact mining efficiency. Based on geological conditions in western China, this study employs numerical simulations to analyze the impact of coal seam burial depth, medium layer thickness, and its location on top-coal movement and pressure distribution under high-intensity repeated mining. The results reveal the formation of a strip-like stress zone and a wide-bottomed inverted funnel displacement field in the top-coal layer during medium layer mining, with maximum stress and subsidence reaching approximately 34.7 MPa and 3 m, respectively. During the lower layer mining stage, crack propagation and coal fragmentation enhance top-coal caving, forming a sawtooth-shaped displacement boundary. Additionally, maximum top-coal subsidence increases to 5.3–7.3 m as burial depth and medium layer mining height increase. These findings suggest that initiating the first mining face in the middle-lower section of the coal seam, where top-coal stress is highest, promotes efficient coal breakage and smooth caving.