Modelling Seasonal Hysteretic Seepage In Infinite Unsaturated Slopes And Implications For Stability

Unsaturated soil conditions strongly influence the superficial part of slopes, often resulting in shallow landslides that give rise to various challenges for environment risk management. Shallow slope failures are mostly governed by fluctuations in soil pore-water pressure and degree of saturation. In fact, unsaturated soil conditions generate a cohesive contribute of soil shear strength depending on the climate acting on the slopes. In the context of evaluating the stability of shallow slopes, neglecting these factors can results in an inaccurate assessment of the slope conditions. Recent evidence underlines the necessity of incorporating hysteretic soil water retention behaviour in the characterization of the soil behaviour, in order to highlight complex aspects of slope response. This thesis focuses on the infinite unsaturated slope hydraulic and stability responses in case of assuming hysteretic soil water retention behaviour, governed by the model proposed by Gallipoli et al. (2015). A simplified methodology for the evaluation of the two-dimensional flow regime in an infinite unsaturated slope has been analytically developed. The methodology has great practical relevance as it demonstrates that two-dimensional seepage across an infinite slope can be evaluated by solutions of onedimensional vertical infiltration across horizontal unsaturated ground. The hysteretic seepage in infinite unsaturated slopes has been evaluated by means of an algorithm which has been specifically developed for the resolution of Richards’ equation by combining the seepage decomposition method with the hysteretic retention model of Gallipoli et al. (2015). Once the flow regime has been assessed, the unsaturated infinite slope response has been studied by means of the limit equilibrium method for the evaluation of the safety factor and the sliding surface. The influence of the hysteretic flow regime has been highlighted by comparing the safety factor evolution with time and depth in the infinite unsaturated slope with the stability response obtained through the classical limit equilibrium approach and with that derived from assuming a non-hysteretic retention law. The assumption of hysteretic or non-hysteretic soil water retention behaviour leads to different slope stability assessments due to the memory effect of the former model on the evolution of hydraulic soil state. This difference underlinesthe necessity to accurately describe the soil water retention behaviour in order to catch complex aspects of both slope hydraulic and stability responses.