Softening and instability evolution of strip coal pillar under water immersion in goaf.

The softening and instability of the coal pillar caused by water immersion is the primary factor contributing to water inrush in the goaf of the same layer. Similarly, the softening and instability of underground reservoir dam bodies due to water immersion is a critical factor leading to reservoir water failure and dam breakage. Investigating the dynamic process of coal pillar softening and instability under water immersion conditions is of paramount importance for the prevention and control of mine water hazards in mining areas and the safety assessment of underground reservoirs. This study uses the mining of the A ore strip as an engineering background, conducts theoretical analyses and laboratory tests on coal pillars subjected to varying soaking durations, and examines the macroscopic and microscopic damage and deterioration processes of coal pillars under soaking conditions from the perspectives of mechanical properties, hydrochemical characteristics, and microstructural characterization. The experimental results indicate that within 0 to 40 days of soaking, the uniaxial compressive strength of the coal pillar decreases by 50%, the pH value of the solution shifts from weakly acidic to weakly alkaline, multiple ion concentrations undergo significant changes, mineral compositions alter, and porosity exhibits a pronounced changing trend. Based on these research findings, this paper explores the physicochemical coupling mechanism of internal structural evolution and damage deterioration in soaked coal pillars, elucidating the entire process of damage-deterioration-instability. The dynamic process of water-immersed coal pillars was studied and visualized using similar materials with water-blocking additives, revealing their spatiotemporal evolution laws. Through mechanical derivations and laboratory test results, this paper identifies the key factor influencing the instability of the overlying rock mass above the coal pillar roof, termed the "effective support width," and investigates the mechanical deterioration mechanisms of water-immersed coal pillars. Finally, based on theoretical research and indoor experiments, considering the dual effects of stress and water pressure, the calculation formula for the effective support width of water-immersed coal pillars was derived, clarifying the evolution mechanism of "damage-deterioration-instability" in the overburden rock mass above the coal pillar roof. This study discusses the dynamic process of coal pillar softening-instability under water immersion conditions in goaf areas, which holds significant implications for the retention of waterproof coal pillars in coal mines, safe mining practices, and the protection and recycling of coal pillars. Additionally, research into the damage-deterioration-instability of coal pillars provides valuable guidance for the construction of underground reservoirs and the determination of appropriate widths for coal pillar dam bodies.