Stability Failure Characteristics of Hanzhong Granitic Residual Soil Slopes Under Coupled Effects of Rainfall Intensity and Slope Inclination

Hanzhong granitic residual soil exhibits a pronounced rainfall-sensitive response, but the evolution of hydraulic participation and failure pathways under different rainfall intensities and slope inclinations remains insufficiently understood. Physical model tests and coupled Geo-Studio simulations were conducted under three rainfall intensities (40, 60, and 110 mm·h−1) and two slope inclinations (45° and 60°). For the 45° slope, increasing rainfall intensity from 40 to 110 mm·h−1 shortened the overall failure time from approximately 1.5 h to 0.35 h and changed the failure process from toe-initiated retrogressive deformation to rapid stepped overall failure. For the 60° slope, high-intensity rainfall triggered instability before the deeper soil developed a hydraulic response comparable to that observed under lower rainfall intensity, indicating that failure development can outpace deep wetting. Numerical analyses further showed that the major reduction in factor of safety was concentrated within the stage of rapid hydraulic adjustment rather than occurring uniformly throughout rainfall. These results indicate that rainfall intensity affects not only the rate of wetting, but also the timing and depth of soil participation in the pre-failure process of Hanzhong granitic residual soil slopes.