A method for evaluating the stability of an unsaturated slope in natural terrain during rainfall

The stability of an unsaturated slope in natural terrain was evaluated based on the variation in the suction stress in the soil layer induced by rainfall. A field monitoring site where landslides have occurred in the past was selected as the study area. To apply the concept of slope stability analysis considering the suction stress in unsaturated soil, the soil-water characteristic curve (SWCC) and the suction stress characteristic curve (SSCC) of the unsaturated soil obtained from the study area were estimated using the van Genuchten (1980) model and the Lu and Likos (2006) model, respectively. The phenomenon of ground saturation induced by the infiltration of rainfall into unsaturated soil is similar to the wetting path of the SWCC. The curve-fit parameters estimated from the SWCC wetting path were therefore used to determine the matric suction and suction stress in the unsaturated soil. The amount of rainfall and the volumetric water content of the soil were measured using a rain gauge and time-domain reflectometer (TDR) sensors, respectively, at the monitoring site. An infinite slope was chosen as representative of the slope of the natural terrain in the study area because the slope length is very long but the depth of the soil layer over the rock is very shallow. The stability of this unsaturated slope in natural terrain was evaluated based on the safety factor of an infinite slope considering the suction stress in the unsaturated soil layer. The safety factor of the slope suddenly decreased during and just after a rainfall and then recovered. Notably, the safety factor of this natural slope exhibited continuous fluctuations because of changes in the suction stress caused by the evaporation and infiltration of water in the unsaturated soil layer. The variation in the suction stress in unsaturated soil induced by rainfall can be estimated from the results of laboratory tests performed to estimate the wetting process based on the SWCC and SSCC in combination with field monitoring data collected by sensors to measure the volumetric water content or matric suction in the soil. Therefore, the infinite-slope stability of a slope in natural terrain can be evaluated in real time by estimating the suction stress in the unsaturated soil due to rainfall when the volumetric water content or matric suction in the soil is being monitored in the field.