Mechanical deterioration and damage evolution of coal specimens under drying–wetting cycles

Utilizing underground mine reservoirs for pumped hydroelectric storage (PHS) 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 b -value exhibited a downward trend. RA-AF analysis showed an increasing fraction of shear-related AE signals with increasing DWC. NMR testing revealed predominantly unimodal T 2 spectra dominated by small pores. With increasing DWC, the relative fraction of small pores decreased, whereas those of medium and large pores increased, indicating a shift toward larger pore structures. These findings provide laboratory evidence of DWC-related coal deterioration relevant to further material assessments for underground reservoirs and potential PHS applications.