Rockburst mechanism and control of staggered roadways beneath upper-slice residual coal pillars in extra-thick coal seams: a case study

Abstract Upper-slice residual coal pillars in extra-thick coal seams can form local high-stress structures and strongly affect lower-slice staggered roadways. To clarify the rockburst mechanism beneath such a pillar, this study investigated the 250101-2 lower-slice working face by integrating field damage investigation, microseismic monitoring, coal-pillar mechanical analysis, FLAC3D simulation, and destressing verification. During retreat, 5737 microseismic events (MS) were recorded, including 56 high-energy events with energy not lower than $$\:1.0\times\:{10}^{4}$$ J. These events were mainly distributed within approximately 200 m ahead of the working face and concentrated on the haulage roadway side. The 20 m residual-pillar zone accounted for 41% of the high-energy event frequency and 37% of the total high-energy release, consistent with repeated field manifestations such as rib deformation, support damage, mesh failure, coal-rock collapse, and floor heave. Mechanical analysis shows that the residual pillar may behave as a narrow pillar with peak-stress superposition or evolve into a wider composite bearing structure after goaf recompaction. Numerical simulation further indicates that, during lower-slice mining, the original 20 m pillar and adjacent compacted coal-rock mass formed an effective bearing zone of approximately 54 m, enhancing stress accumulation and downward transfer. The LW250101-2 haulage roadway was located near the boundary between the high-stress zone and the stress-relieved zone, where a sharp stress gradient promoted asymmetric deformation and dynamic instability. Coordinated destressing by roof deep-hole blasting, rib deep-hole blasting, and ultra-deep large-diameter boreholes transformed microseismic activity from concentrated high-energy release to more dispersed low-energy release, reducing rockburst risk in the residual-pillar-affected roadway.