Mechanisms of support failure induced by repeated mining under gobs created by two-seam room mining and prevention measures

Failure of supports often occurs in fully-mechanized longwall (FML) panels under gobs that are created by two-seam room mining (TSRM). Theoretical and numerical models are constructed to investigate the mechanisms responsible for support failure and to find prevention measures. First, the overall stability of the pillars is determined by using a combination of safety factor thresholds, numerical simulation and field experimentation of topsoil stripping. The spatial relationships between the upper and the lower pillars were classified as: well aligned, partly staggered and completely staggered. This study found that the distributions of stress concentration and stress relief zones in the floor depended only on the locations of the lower pillars: stress concentration is the most intense when the upper and lower pillars are well aligned and the least intense when they are completely staggered. Temporally, the lower pillars failed simultaneously with the upper pillars or earlier. In the case of simultaneous failures, the pattern of strata behavior occurring in the FML panel is similar to that under rooms and pillars left by single-seam room mining (SSRM). In the case that the lower pillars collapse earlier, dynamic load coefficients are high during periodic weighting and support resistance suddenly increased when the face exited the regions under pillars or periodic weighting took place. Furthermore, the cause of support failure in the #131210 panel of the Shigetai coal mine is analyzed and two kinds of solutions have been proposed.