Investigations on the effects of parameter selection for unsaturated slopes under Taiwan rainfall conditions

The rainfall-induced unsaturated slope failures continue to be a recurrent occurrence every year. The rise in extreme precipitation events, widespread loosened soils, effects of topography and urbanization requires that engineers understand the complex interplay of slope parameters in order to design safe structures. Whereas existing studies have provided useful insights on rainfall-induced failure mechanisms, and quantified the influence of slope parameters, the findings have been limited to certain localities and cover a narrow range of parameters. Here, we carry out a comprehensive parametric study on unsaturated silty slopes based on regional soil parameters and rainfall data to identify slope parameters of engineering interest and link unsaturated soil parameters to slope failure mechanisms. Generally, slopes with slope angle < 30° and water table below the slope toe are theoretically safe under extreme rainfall conditions. Clayey silt slopes with initial water table above and at the slope toe were most sensitive to rainfall intensity / soil permeability ratio of 0.2 – 0.6. Sandy silt slopes were less influenced by rainfall intensity / permeability ratio, exhibiting slow suction loss, negligible water table rise, and shallow failure surfaces. The proposed hydraulic parameter boundaries can be a good reference tool for slope design.