Study on the Mechanical Characteristics and Microscopic Response Mechanisms of Sandstone under Pressurized Water Absorption

To investigate the characteristics of crack propagation and microscopic structure under water–rock coupling effect in deep surrounding rock, a triaxial mechanical experiment was conducted on sandstone strata in the Shandong Wanfu Coal Mine under the action of pressurized water absorption. The experimental results showed that the mass increment of the rock sample was exponentially related to the confining pressure, and the mathematical relationship curve of the mechanical strength variation of the rock sample under different confining pressures was obtained. Based on this, the strength softening formula of the rock sample with the absorption time under different confining pressures was obtained, and the softening process was divided into three stages: the relatively stable stage, the rapid softening stage and the deceleration softening stage. The T 2 spectrum curve of the rock sample was normalized, and the pore distribution and strength softening curve under different confining pressures were constructed. The speed of water molecules entering pores in different softening stages was analyzed, and it was found that in the relatively stable stage, the speed was micro-pore > meso-pore > macro-pore, in the rapid softening stage, the speed was meso-pore > macro-pore > micro-pore, and in the deceleration softening stage, the speed was micro-pore ≈ meso-pore ≈ macro-pore. According to the law of microscopic pore structure changes under water–rock coupling, a pore structure model of sandstone under different confining pressures was constructed. Based on the crack propagation model and crack propagation rate formula, it was concluded that the increase in crack strain propagation rate was the result of both water saturation and permeation water pressure, and a distribution map of the crack strain propagation rate was established. By comparing the crack strain propagation rate, it was found that under the same characteristic stress, the axial strain propagation rate of the crack was greater than the radial strain propagation rate, and the radial strain propagation rate was greater than the volume strain propagation rate. Quantitative relationships between crack propagation in sandstone and water saturation under different stress conditions were obtained.