Implication of subsurface flow on rainfall-induced landslide: a case study

Post-failure field investigation, instrumentation, monitoring, and numerical simulation were performed to give insights into the failure mechanism of a 13-h-delayed rainfall-induced landslide. A conceptual hydrological model was postulated based on the findings obtained from the investigation works. The results showed that subsurface flow was recharged by intense and prolonged rainfall through outcrops of fissured bedrock. The recharged water was mounded in the moderately weathered granite layer and caused an increase in hydraulic head. The groundwater seeped gradually upward into the overlying fill layer even after the rain has ceased, and eventually triggered the landslide when the water table was raised to a critical state. As most of the existing hydrologic-slope stability models were developed on the basis of soil-impermeable bedrock model, this could result in great discrepancies between the simulated results and the real hydrological responses of the slope. The findings from the present study highlighted the importance of considering subsurface flow and hydro-geological features in assessing the mechanism of rainfall-induced landslide.