Evolution of Floor Rockburst Instability Tendency and Optimization of Borehole Pressure-Relief Horizons in Roadways with Retained Bottom Coal

To investigate the evolution of floor rockburst instability tendency and optimize borehole pressure-relief horizons in roadways with retained bottom coal, theoretical analysis and numerical simulation were employed to investigate the effects of bottom coal thickness on composite-floor stability, load-bearing and energy-storage states, and borehole pressure-relief response. Based on the equivalent transformed-section method, a stability analysis model for the bottom coal–floor composite structure was established to reveal the mechanism by which increasing bottom coal thickness reduces structural stability and influences the floor instability tendency. Numerical results show that, with increasing bottom coal thickness, the dominant load-bearing and energy-storage response progressively shifts from the floor strata to the bottom coal–floor composite structure and then to thick bottom coal. An energy–stress index K was introduced to characterize the transition in the load-bearing and energy-storage states of the composite floor. Further analysis of the effect of borehole vertical depth indicates that pressure-relief effectiveness depends on the spatial matching between the weakened zone and the main load-bearing and energy-storage zones, rather than on indiscriminately increasing borehole depth. Under the simulated conditions, the reasonable borehole vertical depth ranges from 3.0 to 4.0 m for a bottom coal thickness of 2 m and from 4.0 to 5.0 m for bottom coal thicknesses of 4, 6, and 8 m. The results provide a theoretical reference for understanding the relative evolution of floor rockburst instability tendency and optimizing borehole pressure-relief horizons in roadways with retained bottom coal.