## 2 The Database  
Italy is particularly affected by slope instabilities in proximity of the Alpine and the Appennine chains. The samples under investigation were taken from landslides that occurred in 14 sites located on the slopes of these two long chains. The sites of Recoaro, Fantoni, Tessina, Cortina, Rosazzo, and Montona fall into the south-eastern Alps, an area characterised by overlapping sedimentary layers delimited by the Padana plain at the southern margin. The site of Arqua Petrarca belongs to the complex of the Euganean hills, an area characterised by eruptive bodies surrounded by sedimentary profiles gradually becoming part of the Padana plain. The sites of Val Tidone, Castiglion de Pepoli, Val di Sambro, Buttoli, and Cecina fall into the Tosco-Emilian Apennines, while the sites of Calatabiano and Caltavuturo fall into the Sicilian Apennines. The location of the aforementioned sites along the Italian peninsula is shown in Figure 1, along with the Italian landslide hazard map compiled by ISPRA [27]. At present, the ISPRA database reports about 630,000 landslides, many of which reactivated after several years, decades, or even centuries of quiescence. The geotechnical data of 24 natural soils tested in this research are summarised in Table 1; for comparison purposes, the data of two clays, a sodium bentonite and of a kaolin both provided by industry, are also shown. The residual strength was measured with the standard Bromhead ring shear apparatus with outer and inner diameters of 100 mm and 70 mm, respectively. To avoid excessive settlement during consolidation, reconstituted specimens were made by hydrating at a water content sufficiently below the liquid limit. After consolidation, the reconstituted specimens had an initial thickness of about 5 mm. In order to generate the sliding surface  
After pore overpressure dissipation, the stabilised residual strength was evaluated for all the samples at a constant rate of 0.089 mm/min. The 26 mean secant residual strength angles, \(\overline{\varphi}_{\mathrm{r}}\), reported in Table 1 were evaluated by averaging the results obtained for three vertical stresses, \(\sigma_{\mathrm{v}}\), of 27.6 kPa, 52.1 kPa, and 101.1 kPa at the sliding speed of 0.089 mm/min. The effect of ageing was studied for 10 samples (Sodium Bentonite, Buttoli 1, Val di Sambro 1, Fantoni S10, Rosazzo, Rosazzo 1, Montona, Castiglion de Pepoli, Caltavututuro, and Calatabiano) for which reactivations were performed after cycles of ageing ranging from fifteen minutes to, at most, one month. Finally, the effect of three rate of shearing, v, of 0.018 mm/min, 0.089 mm/min, and 0.445 mm/min was investigated for Calatabiano and Caltavuturo clays only. In order to draw a general picture of the 26 samples under study, all the secant residual strength angles have been reported in Figure 2 in relation to the clay fraction, CF% (Figure 2a) and by using a new comparison parameter, (CF + PI)/w\({}_{\mathrm{P}}\) (Figure 2b), being w\({}_{\mathrm{P}}\) (%) the plastic limit and PI% the plasticity index. All the data in Figure 2 refer to the three aforementioned vertical stresses and to a rate of shearing of 0.089 mm/min. Compared with the representations making use of CF or PI alone, it was found that the parameter (CF + PI)/w\({}_{\mathrm{P}}\) involves less dispersion of the data. Looking at Figure 2b, all the data are placed within a band limited by a lower and an upper envelope: the upper envelope delimits the samples with the largest residual strength angle, characterised by an interplay between lower plasticity, lower clay fraction, and lower vertical stress, whereas the lower envelope delimits the samples with the smallest residual strength angles, characterised by an interplay between greater plasticity, greater clay fraction, and greater vertical stress. The evidence that the secant residual strength angle decreases as the vertical stress increases proved that the residual strength envelopes are affected by non-linearity, especially at a low stress level.