Study on chain instability of residual coal pillars and overburden strata under the downward mining influence

The mining of the underlying coal seam beneath the pillar goaf can lead to dynamic mining pressure induced by the chain instability of the residual coal pillars and overburden strata. Revealing the chain instability evolution characteristics of the residual coal pillars and overburden strata induced by the downward mining is the basic premise to ensure the safe mining of the underlying coal seam. In this paper, the mining of the No. 6 coal seam under the pillar goaf of the No. 4 coal seam in Yuanbaowan Coal Mine is taken as the engineering background. The chain instability process of residual coal pillars and overburden strata in the process of downward mining is studied using 3DEC numerical simulation software. The stage division of the chain instability of residual coal pillars and overburden strata in the process of downward mining is achiored. The movement deformation,stress evolution,and plastic zone distribution characteristics of different stages of chain instability of residual coal pillars are analyzed. The results show that the evolution process of chain instability of residual coal pillars and overburden strata induced by downward mining can be divided into four stages:the gestation stage of chain instability of coal pillars,the start-up stage of chain instability of coal pillars,the expansion stage of chain instability of coal pillars,and the instability stage of coal pillars-overburden interaction. In the gestation stage of chain instability of coal pillars,the residual coal pillars and overburden strata are less affected by the downward mining,and the overall stability remains good. In the start-up stage of chain instability of coal pillars,the No. 7 coal pillar (key pillar) in front of the working face reaches an overload state,the deformation increases instantaneously, the left and right plastic zones penetrate each other, and crushing shear failure occurs first. In the chain instability expansion stage of the coal pillars,the local instability of the key pillar will induce load transfer,which will cause the stress in the advanced coal pillars to rise sharply,drive transverse and longitudinal deformation of the surrounding coal pillars. The plastic zone will penetrate,in turn,resulting in progressive expansion failure and crushing instability. In the instability stage of coal pillars-overburden interaction,the chain instability of coal pillars will induce the continuous subsidence and collapse of overburden and the collapse of overburden will promote the continuous increase of the chain instability range of coal pillars, which ults in the further increase of “domino” chain dynamic instability and overburden collapse area leading to more significant chain dynamic disaster effects. This study is expected to provide theoretical guidance and scientific basis for the prevention and control of strong dynamic mine pressure disasters during underlying coal seam mining under pillar mining areas.