Mechanical Deterioration and Micro-macro Damage Evolution Mechanisms of Underground Reservoir Coal-Pillar Dams Subjected to Drying-Wetting Cycles

<title>Abstract</title> <p> Utilizing underground mine reservoirs for pumped storage power stations represents a highly promising approach for large-scale energy storage. However, the long-term exposure of coal pillar dams to drying-wetting cycles (DWC) poses a severe risk of structural instability. This study investigated the mechanical degradation and damage mechanisms of coal specimens, leveraging triaxial loading coupled with acoustic emission (AE) and nuclear magnetic resonance (NMR) monitoring. The results indicated that the compressive strength and cohesion underwent non-uniform, multi-stage degradation with increasing DWC, with the most acute deterioration occurring after the first cycle. AE monitoring revealed that with increasing DWC, the frequency of high-amplitude AE events escalated, and their peaks shifted forward. Concurrently, the overall <italic>b</italic> -value exhibited a downward trend. Analysis of the RA-AF characteristics confirmed that the proportion of shear fracture events escalated significantly with increasing DWC, triggering a failure mode transition from micro-tensile cracking to macro-shear sliding. NMR testing showed that the <italic>T</italic> ₂ spectra exhibited a predominantly unimodal distribution, with micropores dominating the pore network. Under the action of DWC, the dissolution and spalling of hydrophilic minerals drove the transformation of internal micropores and mesopores into macropores. These findings provide a theoretical foundation for the stability assessment of underground reservoir-based PHES systems. </p>