A GRASS GIS-based deterministic model for the analysis of deep-seated slope stability in complex geology

We present a GIS-based, three-dimensional slope stability model capable of dealing with deep-seated slope failures in complex geology. The model is developed in GRASS GIS as the raster module r.rotstab and builds on a slight modification of the three-dimensional sliding surface model proposed by Hovland and revised and extended by Xie et al. (2003). Given a Digital Elevation Model (DEM) and a set of thematic layers, the model evaluates the slope stability over a large number of randomly determined potential ellipsoidal slip surfaces. In addition, truncated ellipsoids can be used to simulate the presence of weak layers, most commonly delimited by regolith discontinuities or the interfaces between geological layers. Any raster cell may be intersected by various sliding surfaces, each associated with a computed safety factor. The lowest value of the safety factor is stored for each raster cell together with the depth of the associated slip surface. This results in an overview of potentially unstable regions without showing the individual sliding areas. We apply the model in the Collazzone area, Umbria, Central Italy. Initially, we assume homogeneous regolith. Even though the model successfully predicts much of the observed deep-seated landslides, it fails in a significant number of cases. According to field observations in the Collazzone area, morpho-structural settings - and therefore the bedding attitude of the geological layers - play a crucial role for deep-seated landslide distribution. Therefore, we prepare a 3D model of the geological substrate, based on the superficial strike and dip directions of each layer, and use this model as input for the r.rotstab model. The results for selected subsections of the Collazzone area are shown and evaluated in comparison with the results yielded with the assumption of a homogeneous substrate.