Yongqiang Zhang

To reveal the influence of water accumulation in open pits on the stability of boundary coal–rock pillars, this study investigates a boundary coal–rock pillar between an underground coal mine and an adjacent open pit in western China. Coal–rock physical property tests, hydrochemical analysis, permeability tests, and theoretical calculations of water-resisting coal–rock pillars were conducted to examine seepage channel formation, physical property changes, and stability evolution under long-term water accumulation. The results show that the mechanical strength of coal and rock specimens decreases under the saturated state. The uniaxial compressive strength of rock specimens decreases by 8.75–50.64%, while that of No.2⁻² and No.3⁻¹ coal specimens decreases by 17.72% and 25.01%, respectively. The tensile strength decreases by 24.59–59.11%, and the shear strength decreases by 4.36–45.96%. The hydraulic conductivity of intact specimens is mostly 10⁻⁴~10⁻³ m/d, whereas that of fractured specimens increases to 10⁻³~10⁻² m/d. The calculated width of water-resisting coal pillars increases by 19.7~21.9% under long-term water accumulation. Long-term water accumulation in the open pit changes the external hydraulic boundary of the boundary coal–rock pillar, allowing water to migrate inward along bedding planes, joints, primary fractures, mining-induced fractures, and coal seam pores. This process promotes the connection of pre-existing pore–fracture structures and seepage channel formation, weakens particle cementation and structural-plane shear resistance, and reduces the structural integrity, bearing capacity, and water-resisting capacity of the coal–rock pillar. Therefore, the stability deterioration of boundary coal–rock pillars is a continuous process involving channel formation, sustained seepage, strength degradation, enhanced pore–fracture connectivity, permeability enhancement, and further stability reduction.

To reveal the influence of water accumulation in open pits on the stability of boundary coal-rock pillars, this study investigates a boundary coal-rock pillar between an underground coal mine and an adjacent open pit in western China. Coal-rock physical property tests, hydrochemical analysis, permeability tests, and theoretical calculations of water-resisting coal-rock pillars were conducted to examine seepage channel development, physical property changes, and stability evolution under long-term water accumulation. The results show that the mechanical strength of coal and rock specimens decreases under the saturated state. The average uniaxial compressive strength reduction of rock specimens exceeds 40%, while that of coal specimens is 7.6~18.2%. The tensile and shear strengths decrease by 30.0~57.1% and 7.5~34.6%, respectively. The hydraulic conductivity of intact specimens is mostly 10-4~10-3m/d, whereas that of fractured specimens increases to 10-3~10-2m/d. The calculated width of water-resisting coal pillars increases by 19.7~21.9% under long-term water accumulation. Long-term water accumulation in the open pit changes the external hydraulic boundary of the boundary coal-rock pillar, allowing water to migrate inward along bedding planes, joints, primary fractures, mining-induced fractures, and coal seam pores. This process promotes pore-fracture connection and seepage channel formation, weakens particle cementation and structural-plane shear resistance, and reduces the structural integrity, bearing capacity, and water-resisting capacity of the coal-rock pillar. Therefore, the stability deterioration of boundary coal-rock pillars is a continuous process involving channel formation, sustained seepage, strength degradation, fracture development, permeability enhancement, and further stability reduction.

To mitigate the threat posed by accumulated gob water to underlying coal seams during multi-seam mining, this study investigates the mechanism of water inrush induced by repeated mining and its control through roof cutting pressure relief. The 31110 panel of the Holowan Coal Mine is taken as an engineering case, where the 3⁻¹ coal seam is threatened by gob water from the overlying 2⁻² coal seam. The mechanisms of interlayer rock mass damage accumulation, fracture interconnection, and water-conducting channel formation were systematically analyzed using a combination of theoretical analysis, numerical simulation, and field tests. The results indicate that the superimposed mining-induced failure zones of the upper and lower coal seams significantly exceed the interlayer spacing of 46.5 m. This condition promotes through-going damage of the interlayer strata and facilitates the downward migration of gob water. Without roof cutting, the main roof fractures toward the solid coal side of the 31110 auxiliary headgate, resulting in full connectivity of the overburden plastic zones and the formation of a continuous water-conducting channel. Roof cutting pressure relief, achieved by pre-inducing artificial weak planes, effectively guides roof fracturing toward the gob side, alleviates stress concentration on the solid coal side, and suppresses the expansion of interlayer damage. When the roof cutting height exceeds 35 m, plastic connectivity between the water-resisting coal pillar and the underlying mining-induced damage zone is interrupted, preserving the integrity of the key aquiclude. Field application of directional hydraulic fracturing roof cutting confirms the formation of continuous weakened fracture planes and controlled roof caving along the designed trajectory. The overburden caving angle increases from 70° to approximately 90°, effectively blocking water-conducting pathways and eliminating the risk of gob water inrush. These findings not only deepen the understanding of water inrush mechanisms under repeated mining disturbances but also establish a proactive fracture-regulation framework for gob water hazard control, providing broadly applicable design criteria and technical references for safe and efficient multi-seam mining in water-threatened coalfields.