Hydro-mechanical assessment of tunnel–fill slope systems under extreme rainfall: effects of foundations and slope protection

Tunnel excavation through fill slopes under extreme rainfall involves complex hydro-mechanical interactions among excavation-induced stress redistribution, rainfall infiltration, slope deformation, and adjacent infrastructure. This study investigates the response and stability of a tunnel–fill slope system using integrated physical model testing and numerical simulation. The physical experiments examined rainfall infiltration, ground settlement, stress redistribution, and pore-water-pressure evolution during excavation. The validation test simulated an upper-range heavy-rainfall event of 200 mm/day for 12 h at prototype scale, corresponding to approximately 16.67 mm/day applied for 1 h in the 1:120-scale model. Agreement between the experimental and numerical results confirmed the model’s ability to reproduce the principal hydraulic and deformation responses. Two parametric investigations were subsequently conducted. The first evaluated the effects of adjacent buildings and foundation systems under rainfall and dry conditions, focusing on building settlement and slope-supporting pile displacement. The second assessed riprap, geocell reinforcement, and vegetation under rainfall without building loads by examining slope deformation and pore-water-pressure development. The results demonstrate that rainfall intensifies hydro-mechanical responses and structural deformation, while the effectiveness of protection measures varies according to their hydraulic and mechanical functions. These findings support the selection of suitable foundation systems and slope-protection strategies in complex urban fill-slope environments.