Rainfall-induced landslides are among the most recurrent hazards around the world. Their spatial and temporal frequency poses considerable risk in urban areas, thus requiring the development of spatially-distributed models to assess the susceptibility to such events. Such methodologies usually rely on a Mohr-Coulumb criterion to define slope-failure thresholds, regardless of the complex variability of soil failure mechanisms well-known in the geotechnical literature.;This thesis focuses on the use of physically-based models for the assessment of landslide susceptibility at regional scale. In particular, emphasis is given to novel principles of unsaturated soil mechanics and plasticity theory. The objective is to establish a modelling framework for the simultaneous assessment of two types of rainfall-induced landslides: frictional slips of low mobility and liquefaction-induced flowslides. For this purpose, a methodology is proposed to express the onset of failure by introducing the concept of the instability modulus, i.e. a constitutive index able to reflect the role of hydromechanical constraints. A constitutive model for unsaturated soils based on a non-associated flow rule, density-dependent hardening and suction-dependent yielding is presented. To address the problem of rainfall-induced shallow landslides, closed-form expressions of safety factors for unsaturated shallow slopes are then derived by incorporating such thresholds into the kinematics of an infinite slope. The model parameters are then calibrated from laboratory experiments, and the performance of the safety factors is tested against a series of highly-instrumented flume experiments. Potential uses of the theory are outlined, such as the real-time assessment of the margin of safety through monitoring data and the derivation of stability charts.;Hydrological analyses are performed through the implementation of a fully-coupled, nonlinear finite element program. A complete description of its implementation is presented. The code has been validated against available analytical solutions and it has the ability to simulate feedbacks between transient infiltration and plastic deformation processes. Simulations are performed to model the infiltration patterns measured in highly-instrumented flume experiments demonstrating a good agreement with the experimental data.;Lastly, the model is applied to a series of landslides occurred in 1998 in Campania (Italy). Input datasets, implementation procedures in a Geographic Information System (GIS), and analyses of the obtained results are described. The analyses provide spatial and temporal patterns of landslide initiation consistent with field evidences, in that they capture most of the affected landslide source areas and predict the temporal evolution of instabilities in agreement with reported failure times. The performance of the model is discussed, focusing on the evolution of indices quantifying the success and error of the predictions. It is shown that the proposed model provided improved performance metrics when compared to existing models, thus suggesting that principles of unsaturated soil mechanics can be effectively used to augment current methodologies.
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