Strength and Acoustic Emission Weakening Mechanism of Sandstone after Prolonged Water Immersion: Insights from Micro-Fracture

In this study, a series of uniaxial compression and acoustic emission (AE) tests were conducted on sandstone specimens immersed in water for different durations (0-720 h). The mechanical properties and AE characteristics of dry, unsaturated, saturated, and long-term saturated sandstone specimens were comparatively analyzed. The experimental results show that with the passage of immersion time, the uniaxial compressive strength and elastic modulus of the specimens both decrease, while the Poisson's ratio increase; the mechanical parameters change exponentially with the immersion time and are linearly related to the water content; the dominant failure mode evolves from tensile failure to shear failure gradually; and the AE intensity and frequency both weaken. Besides, the mechanism of long-term water-rock interaction was explored. It was proposed that the weakening of rock strength and AE signals is largely attributed to the transformation in the micro-fracture form (from transgranular fracture to intergranular fracture), which is induced by the deterioration of grain cementation. This view is supported by optical microscopic observations on micro-fracture behavior as well as AE response.