Photogrammetry and laser scanning for analyzing slope stability and rock fall runout along the Domodossola–Iselle railway, the Italian Alps

In Italy, railways crossing the alpine valleys are a vital means of civil and commercial communications with the rest of Europe. The geomorphologic configuration and the climatic conditions, especially in winter and spring, can cause rock fall events resulting in railway service interruptions and damage to infrastructure and, in the worst case, to people. There were rock fall events at a slope adjacent to the Domodossola-Iselle railway, most recently in 2004. This paper evaluates the stability of a mountain slope and maps rock fall hazards through the modeling of potential runout trajectories. Traditional geological, geomorphological and geo-engineering surveys were combined with data derived from digital terrestrial photogrammetry. Stereo photographic pairs of rocky outcrops in inaccessible areas were acquired from a helicopter. Data from photogrammetry, topographic measurements and laser scanning were then integrated to build a digital model of the slope, to characterize the rock mass and block geometry, and to define possible runout trajectories. The geomatic methods used have yielded oriented stereo-images, orthophotos and precise digital models of rocky wedges. Geometrical and structural characteristics of slopes, such as joint attitude, spacing and persistence, and block volumes, were also derived. The results were used together with a deterministic limit equilibrium method to evaluate slope stability. We assessed the probabilistic distribution of rock fall end points and kinetic energy along the rock falling paths and existing barriers, and created a hazard map based on the spatial distribution of trajectories, rock fall transit density and kinetic energy.