Physically – Based critical rainfall thresholds for unsaturated soil slopes

In this work, a physically based model for the definition of the critical rainfall thresholds for shallow landslide initiation at regional scale is presented. The model is capable of considering unsaturated conditions in the soil volume, by taking into account the effect of partial saturation in: a) the balance of mass for the pore water; b) the deformability of the solid skeleton; and, c) the soil shear strength. Starting from the simplified hypothesis of infinite slope, a series of numerical simulations was conducted in parametric form to determine the functional relationship between the critical rainfall intensity leading the slope to failure to rainfall duration, in terms of dimensionless variables. This has allowed to identify the functional dependence of the critical rainfall intensity on: a) event duration; b) slope geometry; c) mechanical properties of the soil cover; and, d) initial conditions in terms of pore water pressure distribution. The function thus obtained can be easily and efficiently implemented in GIS–based codes for the evaluation of physically–based, spatially–distributed critical rainfall thresholds.