Abstract: The process of unsaturated seepage in expansive soils is accompanied by the generation of expansion strains. The expansion strains under the coupling of multiple fields are crucial for investigating the variation of unsaturated rainfall infiltration in expansive soil slopes. By applying C++ and FISH language to programming, we introduced expansion strain into the fluid-structure interaction model of unsaturated soils, and proposed a multi-field coupling analysis method for unsaturated expansive soil considering expansion strain and stress strain. By using this model, we simulated numerically the unsaturated seepage flow of expansive soil slope in the presence of rainfall infiltration, and furthermore explored the influences of expansion strain on saturation degree and transient area ratio. Research results suggest that expansion strain has an evident impact on the process of unsaturated rainfall infiltration in expansive soil slopes. The depth of rainfall infiltration, the expansion speed of wetting front, and the temporal and spatial distribution of transient saturation region are controlled by the expansion strain. After rainfall, the time lag of transient area ratio is extended when expansion strain is considered.

Key words: expansive soil, multi-field coupling, unsaturated seepage, rainfall infiltration, transient saturation zone