## 2 General Setting of the Study Area
### Geomorphological and Engineering-Geological Setting of the Landslide
Specifically, the landslide thickness was found to progressively increase in the downslope direction, from approximately 9 m at the upper edge to approximately 27 m at the landslide foot. However, much more thickness fluctuations were observed laterally, at short range as well. This was pointed out by concave-shaped low-velocity anomalies of seismic waves and, subsequently, by the similar geometrical configuration of low-resistivity anomalies (<100 Ohm m) detected by means of electrical resistivity tomography (ERT) surveys, which were performed along the former seismic alignments (Figure 5a,b). These anomalies mainly correspond to depressions filled by low-stiffness and porous landslide deposits [55]. Some minor discrepancies between the seismic and ERT sections are likely due to the different response of the two distinct physical parameters to the same subsoil variations in porosity, lithology, and degree of alteration. The lateral changes in the geometry of the landslide accumulation depend on the complex architecture of the underlying bedrock, and they were correlated to buried hollows/channels likely inherited from the ancient pre-failure slope morphology (Figure 5c). The core samples extracted from stratigraphic boreholes along with geotechnical laboratory analyses showed that the landslide-forming materials are heterogeneous in terms of grain size [55]. The landslide deposit is broadly made up of clayey sands and clayey-silty sands (SM-SC and SC soil groups according to the USCS classification), irregularly alternated with clayey-silty gravels and clayey gravels (i.e., GC and GM-GC groups). Coarser soils typically consist of gravel contents ranging between 30 and 45% and sand percentages approximately between 25 and 35%, while the percentage passing through sieve number 200 (i.e., particle size <75 \(\upmu\)m) ranges between 25 and 45%. Fine-grained soils consist of gravel and sand percentages as high as approximately 15% and 55%, respectively, with silt and clay amounts varying between 30 and 50%. Soil materials are characterized by low plasticity (plasticity index < 15%), whereas the bulk unit weight determined on partially undisturbed specimens is between 19.4 and 21.5 kN/m\({}^{3}\). The landslide accumulation overlays the firm bedrock that consists of rocks belonging to the FBS Fm. Drilled boreholes reveal that the rock mass is lithologically heterogeneous and sometimes intensely fractured, resulting in a very poor to fair geomechanical quality. Rock materials can include clayey shales, clayey-arenaceous shales, silty-clayey shales and  
Figure 4: Longitudinal engineering–geological cross-section of the Fontane di San Bernardo landslide (see Figure 3 for location; redrawn and modified from [55]). mary shales, typically thinly bedded (from a few centimetres to decimetres thick). Owing to the intrinsic fissility of lithotypes and the tectonic disturbance, the RQD index ranges between 0 and 55%. The outcropping rock masses can be described as either structurally complex (A2 and B2 groups) or heterogeneous and tectonically disturbed (rock mass Type VIII), according to [64; 65], respectively. Rock core samples result from grey-black to brownish coloured according to the weathering degree and exhibit a weak strength. The average unconfined compressive strength is 15 MPa while the natural unit weight is around 26 kN/m3. Bedrock horizons directly underlying the landslide deposit can be highly weathered, weakened, and fractured, sometimes locally crushed and de-structured, with litho-relicts immersed in a soil matrix.