Experimental investigation of rainfall-induced shear behavior and instability precursors in soft rock interlayers using acoustic emission

Rock masses in rock slopes often slip along weak interlayers, which play a key role in the instability mechanism. Rainfall is one of the main factors triggering landslides. This paper investigates the precursory characteristics of soft rock interlayer slip under rainfall conditions through shear tests and acoustic emission (AE) monitoring. We studied the mechanical response and failure evolution under different water content conditions (10%, 20%, and 30%). A directional roughness coefficient R was introduced for quantitative analysis of structural plane roughness influence on shear behavior. Results show that increased water content significantly reduces friction strength, with the friction coefficient decreasing from approximately 0.7 to 0.25–0.3. At high water content, structural plane roughness has an important influence on strength retention. AE characteristics reveal that increased water content leads to weakened AE activity, reduced number of events and energy release, and a transformation of shear failure mode from tension-dominated to shear-dominated. Shear behavior of structural planes with soft rock interlayers exhibits a progressive failure process without obvious peak values. The evolution of b-value highly corresponds with the moment of shear peak, which can serve as a warning signal for impending instability. The distribution of AE sources transforms from concentrated to dispersed, with an increased proportion of low-energy events. By combining sudden changes in AE rate, decrease in b-value, and evolution of source distribution, the precursory characteristics of slope instability with soft rock interlayers under rainfall conditions can be effectively identified.