## 3 GHM Characterization of the Landslide Classes: A Preliminary Attempt
In particular, the values characterizing the geological (geological formations, lithological sequences, tectonic structures, landslide activity, e.g., first-time failure or reactivated landslide), hydraulic (piezometric levels and saturated permeability coefficients) and mechanical (soil granulometry, index properties, shear strength parameters) slope factors are presented in the tables, together with the boundary conditions (either hydraulic or mechanical). It is worth noting that the GHM characterization proposed here for the six classes (Tables 3-8) represents the outcome of an initial application of the methodology, and will be largely improved through a more extensive literature review and the performance of new field diagnoses, using the stage-wise methodology (Figure 5). Hence, what is discussed in the following should be considered mostly as a methodological contribution. To start with, the application of the methodology to class 1, which includes rotational and rototranslational landslides, often evolving into multiple landslides (Figure 11, box 1 in Figure 2), will be discussed. For such landslides, the sliding surface is most often from circular to composite, and develops through a relatively thin shear band. For multiple landslides, it confines a deeper body within which other rototranslational landslide bodies are included, all sharing the same toe. The case studies examined for the GHM characterization of class 1 landslides were: the London Clay landslides, due to the delayed failure of cut slopes ([31]; Figure 12 [5]; Figure 13a, the Warden Point landslide [78; 79]), some landslides in sub-Apennine Clays slopes (Figure 13b, the Petacciato landslide [80]; the Ancona landslide [81; 82]; Figure 13c, the Lucera landslide [38]; Figure 7, the Tolve slope [60]), and the Selborne landslide in a Gault Clay slope (Figure 13d [83]). The factor values and the boundary conditions recorded for these landslide case studies were shown to be very similar (Table 3). The slopes are all mainly formed of a sequence of uniform marine clays, locally weathered in the upper part and covered by a thin coarse soil deposit of either marine or continental origin. The clay is always of high consistency, since it is highly overconsolidated, and only mildly disturbed by tectonics. The more fissured portion of the clay is within the upper weathered portion of the slope. The original lateral earth pressure coefficient at rest (one-dimensional conditions), K\({}_{0}\), of the clays is generally higher than 1 [5; 38; 84; 85]. Consistent with this, the response of the clay to shearing is initially very stiff, dilative and strain hardening until peak strength, becoming strain softening after peak. The peak shear strength parameter values range from medium (Lucera sub-Apennine Clays; London Clay) to relatively high (Petacciato and Ancona sub-Apennine Clays; Gault Clay). The decay in shear strength with straining brings the clay to a critical state, and thereafter, to residual (Table 3). The saturated permeability coefficient of the clay, K\({}_{s}\), is very low, i.e., about 10\({}^{-11}\) m/s in the laboratory and about 10\({}^{-10}\) m/s at the slope scale (field value; Table 3). Due to the low permeability at the slope scale and the hydraulic boundary conditions of the slopes of reference (absence of a lateral aquifer feeding the seepage in the slope), the piezometric levels at large depths are much lower than at shallow depths. Therefore, overall, the peak strength parameter values, together with relatively low pore water pressures at depth, provide the slope locations of landslides of class 1 with a significant stability factor when the slope angle is low to medium. Accordingly, most of these landslides resulted from first-time failures triggered by slope toe excavation, conferring upon the slope a much higher inclination.