## 3 Steady-State and Reactivated Residual Strength
The Calatabiano landslide occurred the 24 October 2015, in a flysch formation, as a consequence of heavy rainfall in the centre of Calatabiano town, Sicily. Figure 3: Dotted line showing a schematic representation of the residual strength trend in a ring shear test at constant vertical stress: shearing at constant displacement speed followed by relaxation/ageing and then by reactivation at constant displacement speed. Figure 4 shows for both Caltavuturo (Figure 4a) and Calatabiano clays (Figure 4b) the trends of \(\tau_{0}\), \(\tau_{\text{r}}\)(v), and \(\tau_{\text{reac}}\), for vertical stresses of 27.6 kPa, 52.1 kPa, and 101.1 kPa, shearing speed 0.089 mm/min, and ageing of one hour, while Figure 5 shows, for both Caltavuturo (Figure 5a) and Calatabiano clays (Figure 5b), the trend of \(\tau_{\text{r}}\)(v) and \(\tau_{\text{reac}}\), for shearing speeds of 0.018 mm/min, 0.089 mm/min, and 0.445 mm/min, vertical stress of 27.6 kPa, and ageing of one hour. From Figures 4 and 5, both referring to the same ageing time, the influence of both vertical stress and reactivation speed on the reactivated strength is discernible, as is how the increase in strength, \(\Delta\tau_{1}\), rapidly decays towards the stationary value \(\tau_{\text{r}}\)(v). For comparison purposes, the secant strength angle was evaluated: as shown in Figure 6, the secant angle of residual strength, \(\varphi_{0}\), corresponds to the stress state \(\tau_{0}\) and \(\sigma_{\text{v}}\), while the increments, \(\Delta\varphi_{1}\) and \(\Delta\varphi_{2}\), are related to the increments in shear stress \(\Delta\tau_{1}\) and \(\Delta\tau_{2}\). The increments of \(\Delta\varphi_{1}\) after one hour of ageing and with varying vertical stresses and reactivation shearing speeds are reported in Figure 7 for both Caltavuturo (Figure 7a) and Calatabiano clays (Figure 7b). It is possible to notice how the reactivated strength grows in direct proportion to the shearing speed, whereas the relat  
Figure 4: Influence of vertical stress on \(\tau_{0}\), \(\tau_{\text{r}}\)(v), and \(\tau_{\text{reac}}\) for a shearing speed of 0.089 mm/min and ageing of one hour: (**a**) Caltavuturo clay; (**b**) Calatabiano clay. stress is of inverse proportionality; this follows from the fact that the reactivated strength envelope slightly shifts upwards, with the result that \(\Delta\varphi_{1}\) increases more as the vertical stress decreases. Figure 5: Influence of the shearing speed on \(\tau_{\mathrm{r}}(\mathrm{v})\) and \(\tau_{\mathrm{reac}}\) for vertical stress of 27.6 kPa and ageing of one hour: (**a**) Caltavuturo clay; (**b**) Calatabiano clay. Figure 6: Schematic representation of the secant angle of residual strength, \(\varphi_{0}\), and its increments, \(\Delta\varphi_{1}\) and \(\Delta\varphi_{2}\), depending on vertical stress, ageing time, and displacement speed. At the minimum speed of 0.018 mm/min, vertical stress of 27.6 kPa, and one hour of ageing, the increase in the strength angle at reactivation, \(\Delta\up_{1}\), was about 2.5\({}^{\circ}\) for Caltavuturo clay and about 3.5\({}^{\circ}\) for Caltabiano clay. At the maximum speed of 0.445 mm/min, this increase was about 5.5\({}^{\circ}\) for Caltavuturo clay and about 6.5\({}^{\circ}\) for Calatabiano clay. The increase in strength at reactivation versus displacement speed is also highlighted in Figure 8 for both Caltavuturo (Figure 8a) and Calatabiano clays (Figure 8b), aged one hour under the vertical stress of 101.1 kPa. Both the magnitude of the reactivated strength and the displacement necessary to degrade the aged structure up to the steady condition depend on the shearing speed and the soil plasticity. The more plastic Catavuturo clay seems to require larger displacements to reach the steady residual condition, whereas the non-plastic Calatabiano clay shows smaller displacements under the same conditions. Moreover, the percentage increase in the reactivated strength angle (Figure 8) seemed to be greater in the more plastic samples.