Physically Based Rainfall Intensity–Duration Thresholds for Multilayered Slope Stability and Early Warning Strategies

Natural slopes commonly exhibit stratified soil profiles due to depositional and pedogenic processes, and differences in hydraulic properties between soil layers can affect rainfall infiltration and slope stability. However, conventional rainfall intensity–duration (I–D) threshold curves are generally developed under the assumption of homogeneous slopes and therefore cannot adequately represent the hydraulic responses and failure mechanisms of multilayered slopes. This study evaluates the stability of multilayered unsaturated slopes using critical suction stress–depth profiles and establishes physically based rainfall I–D threshold curves to investigate the effects of permeability contrasts and interlayer failure mechanisms. The results indicate that interlayer failure mechanisms are a key factor controlling the stability of multilayered slopes. During rainfall infiltration, pore-water pressure development is strongly dependent on soil-layer configuration and hydraulic conditions, causing the distribution and relative position of rainfall threshold curves to vary with failure depth and rainfall pattern. These variations may alter the threshold criteria and lead to prediction errors in practical early warning applications. Accordingly, an adjustment framework for early warning strategies is proposed for multilayered slopes to improve the applicability and reliability of rainfall threshold curves under stratified geological conditions.