Study on the disaster caused by the linkage failure of the residual coal pillar and rock stratum during multiple coal seam mining: mechanism of progressive and dynamic failure

Multi-seam mining often leads to the retention of a significant number of coal pillars for purposes such as protection, safety, or water isolation. However, stress concentration beneath these residual coal pillars can significantly impact their strength and stability when mining below them, potentially leading to hydraulic support failure, surface subsidence, and rock bursting. To address this issue, the linkage between the failure and instability of residual coal pillars and rock strata during multi-seam mining is examined in this study. Key controls include residual pillar spalling, safety factor ( f s ), local mine stiffness (LMS), and the post-peak stiffness ( k c ) of the residual coal pillar. Limits separating the two forms of failure, progressive versus dynamic, are defined. Progressive failure results at lower stresses when the coal pillar transitions from indefinitely stable ( f s > 1.5) to failing ( f s < 1.5) when the coal pillar can no longer remain stable for an extended duration, whereas sudden (unstable) failure results when the strength of the pillar is further degraded and fails. The transition in mode of failure is defined by the LMS/ k c ratio. Failure transitions from quiescent to dynamic as LMS/ k c < 1, which can cause chain pillar instability propagating throughout the mine. This study provides theoretical guidance to define this limit to instability of residual coal pillars for multi-seam mining in similar mines.