Influence of Precipitation Variability on Pore Water Pressure and Surface Layer Failures in Naturally Undulating Slopes at Nau Kilo, Narayanghat–Mugling Road, Central Nepal

This study investigates the role of pore water pressure variations in slope failures within naturally undulated hill slopes characterized by topographic hollows. A representative slope with a distinct hollow and six rainfall-induced failures recorded between 2001 and 2023 was selected for detailed analysis. Seepage and slope stability modeling were conducted in GeoStudio using the July 31, 2003, precipitation event, with simulations incorporating 24-hour maximum rainfall data corresponding to 5, 10, 25, 50, and 100-year return periods. These events were normalized using 6-minute interval rainfall data, and potential seepage face boundary conditions were applied. The results demonstrated a threshold relationship between hollow area and maximum pore water pressure, indicating that larger hollows generate higher pore water pressures under extreme rainfall. Specifically, a hollow of 100 m² was found to develop a maximum pore water pressure of 6.94 kPa. The findings highlight that topographic hollows act as hydrological convergence zones, increasing subsurface saturation and instability during intense rainfall events. The developed threshold relationship offers a predictive framework for assessing slope failure susceptibility in similar geomorphic settings, contributing to more effective hazard assessment and early warning systems.