Abstract: Rainfall infiltration is a critical factor triggering the instability of cobble-boulder mixed soil slopes in Southwest China. To reveal the infiltration characteristics and wetting front migration of such soils under rainfall conditions, in-situ artificial rainfall experiments were conducted on slopes in the Zhala hydropower station reservoir area in Xizang. The dynamic responses of volumetric water content and pore air pressure were monitored under varying rainfall intensities and durations. In combination with two-phase flow numerical simulations, inverse modeling and validation of unsaturated parameters were performed, and the spatiotemporal evolution of wetting fronts under rainfall–post-rainfall conditions was further revealed. A predictive formula for wetting front migration was then established. Results indicate that water content variations are predominantly governed by rainfall intensity, duration, and burial depth, with shallow layers responding sensitively and deep layers exhibiting lag and attenuation. The slope of the water content curve characterizes the relative magnitude of rainfall intensity and depth, surface saturation rarely occurs, and pore air pressure has negligible retardation effects on infiltration. Wetting front migration consists of a rapid advancement during rainfall phase and a slow stabilization phase after rainfall, with both the rate and depth strongly dependent on rainfall intensity and duration. The proposed predictive formula shows high accuracy across different rainfall scenarios. These findings provide theoretical support and parameter references for rainfall-induced seepage analysis and engineering design of cobble-boulder mixed soil slopes.

Key words: cobble-boulder mixed soil, in-situ rainfall infiltration test, water-air two phase flow, parameter inversion, infiltration model