Probabilistic fragility analysis of unsaturated soil slope under rainfall infiltration considering stress-dependent water retention behaviour

Fragility analysis is a key technique for probabilistically assessing slope performance and potential damage under various external loadings. Although previous studies have mainly focused on seismic scenarios, rainfall is also a significant trigger of slope failure, yet its impact on fragility has received limited attention. Stress states in the field affect soil hydraulic properties but are rarely included in rainfall-related fragility studies. To address these issues, this study conducts fragility analyses of an unsaturated soil slope subjected to various rainfall scenarios, explicitly incorporating stress effects. A stress-dependent water retention model is utilised to capture stress effects on soil hydraulic properties, while spatially variable soil hydraulic and mechanical parameters are modelled by multivariable cross-correlated random fields. The return period of rainfall (T) is used as an indicator of rainfall magnitude. It is found that ignoring the effects of stress on soil hydraulic properties significantly underestimates the probability of failure (p_f) and associated risk of the slope. For T ≥ 50 years, this underestimation can reach up to 83% for p_f and be as much as 6.7 times for failure risk, especially at shorter rainfall durations. This highlights the significance of incorporating stress effects, as the increasing frequency of short-duration extreme rainfall events under climate change. Different criteria for determining damage states influence the development of fragility curves, but minor damage is predominant. Rainfall events with larger T (i.e., more extreme rainfall) cause higher probabilities of slope failure and damage, which become even greater with longer rainfall durations at the same T. However, these rainfall events must occur first for the effects to be realised. This indicates that, apart from focusing on extreme rainfall, attention should also be directed toward prolonged rainfall durations with relatively low intensity, as their high likelihood of occurrence can also increase the probabilities of slope failure and damage.