## 3 GHM Characterization of the Landslide Classes: A Preliminary Attempt
The numerical modelling performed in the stage III analyses (e.g., [5, 38, 67, 86]) demonstrated how the excavation process triggers the creation of a shear band at the toe of the slope, which propagates upslope over time (Figures 12 and 13). The morphology of the shear band and the duration of the progressive failure are controlled by various factors, such as: (1) the slope  
Figure 12: Coupled hydro-mechanical numerical modelling of the effects with time of the slope excavation in London clay: results of numerical FE analyses with negative hardening (**a**) and effect of K\({}_{0}\) on the shape of the predicted shear band ((**b**); [5]). Figure 13: Some rototranslational landslides in stiff clays (class 1 in Figure 11; Table 3; modified from [79]—(**a**) Warden Point landslide, [80]—(**b**) Petacciato landslide, [38]—(**c**) Lucera landslide, [83]—(**d**) Selborne landslide). gradient due to the excavation (i.e., by the value of the mobilized shear stresses); (2) the value of the clay coefficient of permeability and of its volumetric stiffness modulus (hence, of the consolidation coefficient); (3) the hardening law of the clay; and (4) the plastic flow of the elasto-plastic constitutive law. In general, even for homogeneous clay slopes, the modelling shows that the morphology of the shear band is not necessarily circular, but can be composite (Figure 11). [PERSON] et al. [5] and, recently, [PERSON] and [PERSON]. [67], highlighted the incidence of the earth pressure coefficient at rest, K\({}_{0}\), of the clay on the morphology of the landslide body, showing that this deepens with the increase of K\({}_{0}\). Moreover, when negative hardening becomes important, the bifurcation of the shear band occurs for high K\({}_{0}\), with the activation of multiple rototranslational mechanisms (Figure 12b). Reactivated rototranslational landslides, such as the Petacciato (Figure 13b) and Tolve landslides (Figure 7), are still part of class 1. In this case, however, the GHM study deals with the re-activation causes, which generally differ from first-failure ones. Accordingly, for the deep-seated Petacciato paleo-landslide, the current activity is due to the presence of very high piezometric heads at large depth, near the pre-existing shear band, as recognized during stage I studies (Figure 13b; [10,80]) and predicted through stage III modelling. In particular, the fissuring of the deep clay part of the pre-existing shear band determines the occurrence of the deep slope portions of higher coefficient of permeability (Table 3). Such a feature eases the presence of high hydraulic heads at large depth, which predispose the landslide to re-activation. Class 2 landslides (Figure 11) typically involve highly sensitive clays, such as quick clays [45; 89; 90; 91; 92; 93; 94; 95], which are either lateral spreads (Figure 11 and box 2 in Figure 2) or flowslides (Figure 11). The slope soils are generally glacial-marine clays characterized by high sensitivity S\({}_{t}\) > 20-30 [96; 97], which are mainly located in Northern Europe and North America (Canada, Finland, Norway, Russia, Sweden and Alaska), where they outcrop below coarse-grained moraine deposits and are overlain by either a dried fissured crust or silty-sandy soil strata [98]. The complex landslide mechanism is controlled by the peculiar hydro-mechanical properties of the quick clays, which, due to their high sensitivity, are extremely fragile under shearing, to such an extent as to become a slurry by small shear strains. In particular, in the case of the flowslides (Figure 11), the initial failure develops as a rotational landslide, and is followed by a rapid onset of retrogressive rotational landsl, as the outcome of the progression of failure upslope, in undrained conditions [99; 100]. While advancing, the clay forming the rotational landslide bodies liquefies and flows downstream rapidly.