Acoustic emission and splitting surface roughness of sandstone in a Brazilian splitting test under the influence of water saturation

Water weakens rock masses, which induces a series of engineering geological hazards. In this study, Brazilian splitting and acoustic emission (AE) experiments were conducted on sandstone with different saturation levels (0%, 25%, 50%, 75%, and 100%), and the effect of saturation on the AE and fracture characteristics was investigated. Meanwhile, nuclear magnetic resonance (NMR) test was used to analyze the water absorption and distribution characteristics of the samples with different saturation. Three-dimensional (3D) morphological scanning and scanning electron microscopy (SEM) were performed to analyze the roughness of the splitting surface. The tensile strength of rocks decreased from 3.05 MPa to 0.98 MPa with a moisture content from dry to saturated conditions, which obeyed an exponential function. The hardness of the sandstone decreased from dry to saturated by 56.6%, whereas the plasticity of the high-saturation sandstone increased. The AE counts and cumulative energy distribution decreased from 3880 and 39,550 to 1248 and 476, respectively, from dry to saturated conditions. Based on the Ib value, the rock damage process can be divided into three stages: fluctuation, stabilization, and decline. As the saturation increased, the initial Ib value decreased from 0.1047 to 0.0755. The development of large fractures increased during the splitting of the highly saturated rocks. With increasing saturation, the fracture surface joint roughness coefficient (JRC) and fractal dimension increased. The mean JRC increased from 5.98 to 13.6025. The unsaturated JRC showed an obvious dispersion compared with the dry and saturated sandstones. Combined with micro-macro characterization, it was found that an increase in saturation caused the rock to generate a weak structure. The damage gradually evolved from transgranular to intergranular fractures, leading to the macroscopic performance of rougher splitting surfaces and lower AE.