Water is a primary factor influencing rock deformation and failure in underground engineering projects in water-rich geological environments, such as rivers, seas, and alluvial plains with a high water table. Addressing strength deterioration in rock masses caused by water–rock interaction has become a critical challenge requiring urgent resolution. Existing studies have extensively investigated changes in the physical and mechanical properties of rocks after water saturation, as well as their mechanical characteristics under different loading rates. However, analyses of damage evolution and the quantitative characterization of fractures in saturated shale under varying loading rates remain insufficient. Moreover, effective methods for quantifying changes in crack types during failure are currently lacking.

Herein, dry and water-saturated shale specimens were subjected to uniaxial compression tests at different loading rates (0.05, 0.1, 0.2, and 0.4 mm/min) to analyze the effects of water content and loading rate on the mechanical properties. The K-means clustering algorithm was used to process acoustic emission (AE) signals during loading, enabling the identification and quantification of tensile and shear crack evolution patterns. Post-failure samples were subjected to computed tomography (CT) scanning and three-dimensional reconstruction to quantitatively analyze changes in crack type, pore volume fraction, and fractal dimension. The micro-mechanisms of shale failure under different water-bearing states and loading rates were observed using scanning electron microscopy (SEM).

(1) The uniaxial compressive strength and elastic modulus of the water-saturated shale were significantly lower than those of the dry specimens, with the compressive strength of saturated shale being only 71% of that of dry shale. As the loading rate increased, the macroscopic mechanical parameters of saturated shale increased. Compared with V = 0.05 mm/min, the compressive strength at V = 0.1, 0.2, and 0.4 mm/min increased by 32.8%, 72.3%, and 126.5%, respectively. (2) During loading, the AE activity of water-saturated shale was markedly weaker than that of dry shale. However, with an increasing loading rate, the AE events in the saturated specimens gradually intensified, indicating an increasing accumulation of internal damage and a transition of the failure mode from predominantly tensile to a mixed tensile–shear mechanism. Tensile cracks were dominant during loading; nevertheless, the proportion of tensile cracks in the water-saturated specimens was noticeably lower than that in the dry specimens and decreased progressively as the loading rate increased. (3) Microscopic observations indicated that shale failure was characterized by two fracture modes: intergranular and transgranular. At low loading rates, water-saturated shale was dominated by intergranular fracture, whereas dry shale was dominated by transgranular fracture. As the loading rate increased, water-saturated shale underwent a progressive transition from intergranular to transgranular fracture, eventually becoming dominated by transgranular failure.

This study systematically reveals the influence of water saturation and loading rate on the mechanical properties, damage evolution, and fracture mechanisms of shale. The findings provide a theoretical foundation for evaluating the stability of underground engineering projects in water-rich strata and offer important insights into the coupled effects of water–rock interaction and loading rate on the evolution of rock failure.