Rain-induced slope instabilities in unsaturated soils with transmission tower pile foundations: a parametric study with two-phase flow analysis

Rainfall-induced slope failures pose a significant threat to infrastructure, including transmission towers. While rainfall effects on slopes are well-studied, research on multi-hazard effects involving wind-induced loads from superstructures like towers is lacking. This study addresses this gap by developing a water-air two-phase flow model for simulating rainfall infiltration and wind-induced loads for pile-slope systems. This model is validated against a rain-induced landslide at Mountain Umyeon, Seoul. Extensive parametric studies are carried out to analyse how pile position, length and combined wind-gravity loads influence slope stability during rainfall. Results provide new insights. Specifically, it is shown that horizontal forces can transform stabilising piles into destabilising elements when they exceed critical thresholds. Short piles, in particular, may initially improve stability but can destabilise the slope under large lateral loads and increased pore pressure. Toe-positioned piles generally improve slope stability, but their effectiveness decreases with length under large horizontal forces and during rainfall. Vertical forces have limited effects on stability, but when combined with horizontal forces, they amplify instability and lead to shallow failure patterns. Rainfall increases pore pressure, further degrading stability, and the study examines localised failure patterns around piles, shifting failure surfaces closer to the slope's top.