Experimental study on pore distribution and compression-shear failure law of coal pillar dam specimens under wet-dry cycles

• Low-field nuclear magnetic resonance tests and variable-angle shear–compression coupled tests were conducted on coal pillar dam specimens subjected to different numbers of wetting–drying cycles. • Small pores drop 5.18%, medium pores rise 4.97%; large pores hold steady. • Stiffness and peak strength decay exponentially with more cycles. • Digital image correlation shows bottom-to-top displacement gradient increases. Coal pillar dams constitute a critical component of underground reservoir dam structures in coal mines. Due to reservoir water extraction and discharge processes, coal pillar dams experience repeated wet-dry cycles (saturation-drying-saturation) throughout their operational lifespan. To investigate pore structure evolution and compression-shear failure mechanisms of coal pillar dams under different wet-dry cycles, low-field nuclear magnetic resonance (NMR) and variable-angle shear-compression coupled tests were performed on coal pillar dam specimens subjected to various cycle numbers, analyzing their pore characteristics and mechanical responses. Digital image correlation (DIC) technology was employed to capture full-field displacement evolution of the specimens. Experimental results indicate significant changes in pore structure after 10 wet-dry cycles, with micropore volume proportion decreasing by 5.183%, mesopore proportion increasing by 4.974%, negligible variation in macropore proportion, and an overall increase in total porosity by 11.631%. Moreover, average pore diameters across all pore types exhibited an increasing trend. After 4 wet-dry cycles, the compression-shear load-bearing capacity of specimens was notably reduced. Under various shear angles and wet-dry cycles, the horizontal displacement fields exhibited similar distribution patterns, with displacement values decreasing progressively from the lower-right to upper-left regions, demonstrating larger horizontal displacement at the bottom compared to the top regions. Furthermore, displacement-field heterogeneity intensified with increasing cycle numbers. The outcomes of this research provide a scientific basis for optimized design and reinforcement of coal pillar dams in underground reservoirs, thereby ensuring long-term operational safety and stability.