A model is proposed to compute displacement scenarios from rainfall scenarios of a rainfall-controlled slow moving active slide along a pre-existing slip surface. The model is calibrated and validated with reference to a well-monitored landslide in stiff clays in Central Italy. The model comprises a physically based transient groundwater analysis of the slope and a kinematic analysis relating the rate of movement and the inverse of the local factor of safetyin a critical point along the slip surface. Once calibrated and validated, the model is used as a predictive tool to evaluate displacement scenarios from three rainfall scenarios. Although the reliability of the predicted displacement scenarios is not probabilisticallyevaluated, the scenarios may be considered as defining reasonable upper and lower bounds of future displacements.

Active slow-moving landslides in clayey soils exhibit continuous movements generally related to changing ground water levels. This paper highlights the importance of groundwater modelling for the prediction of these types of movements through the analysis of a well-documented case history: the Porta Cassia landslide on the Orvieto hill in central Italy (e.g. Tommasi et al. 2006). The analysis uses recorded rainfall and pore pressure data to define a reliable model of the transient groundwater regime in the slope, which is then used to derive the time-dependent shear stresses along the main slip surfaces. The displacements at selected points along the slip surface are then computed using a relationship between the local factor of safety and the displacement rate at those points.