Simulation experiment on the structural evolution and damage-deterioration mechanism of boundary coal pillars under immersion in abandoned mines

In coal mine production, water-proof coal pillars are reserved to prevent water hazards from old goaf areas. However, under long-term pressurized immersion of aggressive mine water accumulated in abandoned goaf areas, both macroscopic and mesoscopic structures of these coal pillars undergo continuous changes, leading to damage and deterioration. This results in reduced physical and mechanical strength, decreased stability, and ultimately instability and water inrush accidents. A self-designed high-pressure mine water-coal coupling test apparatus is used to simulate long-term immersion of coal pillar samples under different water pressures, simulated mine water, and original mine water conditions. Techniques such as computerized tomography (CT), X-ray diffraction (XRD), and a high-pressure servo-controlled compression testing system are employed to analyze structural evolution and mechanical damage degradation processes and mechanisms of coal samples under prolonged immersion. Results indicate that, under long-term immersion in aggressive mine water, coal sample structures exhibit significant irregular pore-fracture development, with porosity increasing from 0.25% to 1.2%, and the dispersion of pore development gradually decreases over time. Mechanical damage degradation effects are pronounced under long-term immersion in aggressive solutions, with immersion time and solution pH identified as the most influential factors. Mutual interaction occurs between coal samples and mine water during immersion, characterized by initial water absorption and swelling followed by dissolution and consumption in later stages. The physical-chemical coupling mechanism of structural evolution and damage degradation in boundary coal pillars of abandoned mines and goaf areas under high permeation pressure and long-term exposure to aggressive mine water is elucidated. The dynamic response process and key stages of coal pillars under pressurized immersion are revealed: the first stage is dominated by physical water absorption and swelling, resulting in an overall decrease in compressive strength and a fluctuating trend in tensile strength (initial decrease, followed by recovery and subsequent decline); the second stage is characterized by combined physical-chemical effects, where partial dissolution and depletion of clay minerals lead to increased porosity, thereby inducing physical and mechanical damage degradation. Based on the concept of the softening coefficient, the definition and calculation formula for the damage coefficient of immersed coal pillars are proposed, and empirical values for this coefficient are determined. Significant theoretical insights and practical engineering value are provided for evaluating the stability of boundary coal pillars in abandoned mines or goaf areas and for preventing and controlling water-related hazards.