Damage evolution in coal specimens treated by variable durations of water infiltration

• Established a multiscale damage mechanism in coal samples spanning meso- and macroscales. • Revealed transformation in coal's pore structure under negative pressure infiltration. • Demonstrated progressive mechanical weakening of coal samples under negative pressure infiltration. The continuous degradation of the mechanical properties of coal-rock may result in borehole collapse and instability during drilling in water-bearing coal seams. The progressive damage characteristics of coal specimens under different water-infiltrated durations were investigated. The coal specimens were prepared via negative pressure water infiltration of 1, 5 and 10 days. Subsequently, the nuclear magnetic resonance (NMR) tests were conducted on both natural and water-infiltrated coal specimens. Based on nuclear magnetic resonance technology and fractal theory, the time decay of fractal dimension is correlated with mechanical degradation, and a complete damage evolution path from the aging damage of pore structure to deterioration of macroscopic mechanical properties is established. A progressive dynamic failure test was also performed to examine the degradation of macroscopic mechanical properties. The results indicate that the saturation of coal specimens exhibits a periodic change with increasing of the infiltrated duration - first rapid and then stable. In the first stage, the saturation increases sharply. Water seeps rapidly in the primary pore network of the coal body, destroying the microporous structure and promoting the gradual expansion and penetration of micro-pores, forming medium and large-sized pores, which causes damage to the coal body. The compressive strength and elastic modulus of the coal sample decreased rapidly, and the macroscopic mechanical properties were significantly reduced. Subsequently, the deterioration rate slows down. At this point, the hydration reaction of mineral fillers such as montmorillonite further intensifies the damage, ultimately leading to a significant weakening of the macroscopic strength of the coal body.