Acoustic emission analysis of shear processes in red sandstone with soft red bed interlayers

Rock slope instability often occurs along weak interlayers, which play a crucial role in triggering slope failures. Rainfall is a primary factor causing landslides by weakening these interlayers. This study investigates precursory signals of slip along soft rock interlayers under varying rainfall-induced water contents (10%, 20%, 30%) using shear tests combined with acoustic emission (AE) monitoring. A directional roughness coefficient, R, was introduced to quantitatively analyze the impact of structural plane roughness on shear behavior. The results indicate that increasing water content significantly decreases frictional strength, with friction coefficients dropping from approximately 0.7 to between 0.25 and 0.3. At high water contents, structural plane roughness strongly influences residual strength. AE monitoring revealed that higher water content reduces AE activity, decreases event frequency and energy release, and shifts the failure mode from tension-dominated to shear-dominated. The shear behavior demonstrates a progressive failure pattern characterized by three stages: initial deformation, peak shear strength, and residual strength. Notably, the evolution of AE b-value closely aligns with peak shear strength, providing potential early warning indicators of impending instability. Furthermore, the AE source distribution transitions from concentrated clusters to more dispersed patterns, accompanied by an increased proportion of low-energy events. The combined observations of sudden increases in AE rate, decreasing b-value, and changes in AE source distribution effectively identify precursors to rainfall-induced slope instability involving soft rock interlayers.