Estimation of slope annual failure probability considering uncertainty in groundwater table: a practical method based on fragility curve

High uncertainties are often observed in groundwater conditions and soil properties in slope engineering practice, leading to difficulties in choosing a representative scenario of groundwater table (GWT) and representative soil properties for slope design. A wetting band approach is recommended by the Geotechnical Manual for Slopes in Hong Kong to determine a design GWT that corresponds to a 10-year return period rainfall. Using this design GWT, a deterministic slope stability analysis is performed to estimate a factor of safety (FS) and evaluate slope safety. Although a FS value larger than 1 (e.g., 1.4) is often adopted in practice to lump the effect of all uncertainties by a single FS value, FS itself does not explicitly quantify the risk. To facilitate more effective assessment and mitigation of landslide hazards, various uncertainties shall be modelled explicitly and the occurrence of landslide hazards may be quantified by annual slope failure probability PFA. This study develops a fragility curve-based method for quantifying PFA with explicit modeling of uncertainties in groundwater conditions and soil properties. The proposed method is illustrated using a real slope in Hong Kong. The proposed method provides geotechnical engineers with a practical, reliability-based approach to slope design for landslide risk mitigation.