Study on Microscopic Mechanism and Failure Mode of Water–Rock Coupling in Fine Sandstone under Uniaxial Compression

The large deformation of nonlinear soft rock caused by the interaction between water and rock is a prominent problem faced in engineering rock masses. Furthermore, the strength softening mechanism of rocks under the action of water is relatively complex. Based on this, this study considered the deep sandstone strata of the Shandong Wanfu coal mine to investigate the strength softening and microscopic mechanism under the action of different water contents. In addition, it explored the pore structure, mineral composition, and internal fracture expansion in sandstone water absorption. Experimental studies were performed using nuclear magnetic resonance, scanning electron microscopy (SEM), X-ray diffraction (XRD), and computerized tomography scanners (CT). The experimental results showed that with the increase of water absorption time, the increment of mass and spectral area of fine sandstone increased and the compressive strength of specimens decreased. Based on the mathematical expressions between water content of specimens and water absorption time and compressive strength, the exponential relationship between compressive strength of fine sandstone and water absorption time was established. The stress change stages of fine sandstone were divided into relatively stable stage, rapid softening stage, and decelerating softening stage. Due to the dissolution of hydrophilic minerals and adherents in water, the percentage of micropores increases and then decreases, and the percentage of mesopores and macropores decreases and then increases, based on which the model diagram of the change of water absorption microstructure of the rock is constructed. According to the change law of mechanical strength of fine sandstone under the action of different water content, the mathematical expression of compressive strength and water content is established. Based on the results of water absorption stage division of fine sandstone, the fracture porosity is introduced, and it is concluded that the decrease of three-dimensional fracture porosity of fine sandstone is mainly concentrated in the rapid water absorption stage. The failure mode gradually changed from tensile–shear composite failure to single shear failure with an increase in water content. The fractal characteristic analysis of the fractures in the horizontal and vertical sections showed that the fractal dimension gradually decreased with an increase in water content. Finally, a curve relationship between the fractal dimension and water content was established.