Investigations of Unsaturated Slopes Subjected to Rainfall Infiltration Using Numerical Approaches—A Parametric Study and Comparative Review

In the past 30 years, research on rainfall-induced landslides has grown remarkably. The contribution of matric suction to soil strength and the physics of water flow in unsaturated soils are widely accepted phenomena among researchers. However, the adoption of unsaturated soil mechanics in geotechnical engineering practice has been relatively slow, in part due to the practicality of design solutions available to the engineer. This paper conducts a parametric study on unsaturated silty slopes under a vertical steady flow rate to identify the suitable slope and hydrologic conditions to incorporate unsaturated conditions for preliminary stability analysis. Notably, the contribution of suction is most significant for silt/clay slopes with a water table located below the mid-height of the slope. For slopes with slope height ≥20 m and a fairly high water table, the slope height is a primary controlling factor of slope stability. Two case studies based on distinct failure mechanisms are presented to review the application of common geotechnical software in rainfall seepage and stability analyses of unsaturated slopes. Focus is placed on the pre-failure and failure stages of each case study. The slip surface search method, failure mode, and coupling approach integrated into each computer program caused notable differences in output results.