## 3 Steady-State and Reactivated Residual Strength
From both Figures 7 and 8, it can be deduced that quiescent landslides reactivated at very slow displacement speed may not show a significant increase in residual strength at reactivation; however, a quiescent landslide subjected to a seismic event or quick destabilisi  
Figure 7: Influence of both vertical stress and reactivation speeds on \(\Delta\up_{1}\) after one hour of ageing: (**a**) Caltavuturo clay; (**b**) Calatabiano clay. ing forces could mobilise a reactivation strength higher than that mobilised at very slow reactivation speed. Figure 9 shows, for both Caltavuturo (Figure 9a) and Calatabiano clays (Figure 9b), the increase in strength, \(\Delta\tau_{1}\), for a reactivation speed of 0.089 mm/min by considering the influence of both the vertical stress and the quiescence time, the latter extended up to one month. The equivalent representation in terms of secant strength angle, \(\Delta\varphi_{1}\), is shown in Figure 10 for both Caltavuturo (Figure 10a) and Calatabiano clays (Figure 10b). Most of the time-dependent strength recovery takes place within the first 48 h, whereas the strength recovery for longer ageing periods appears to be slower. After one month of ageing, the increase in the reactivated secant strength angle for a reactivation speed of 0.089 mm/min and vertical stress of 27.6 kPa was about 7.5\({}^{\circ}\) for Caltavuturo clay and about 8.5\({}^{\circ}\) for Calatabiano clay. As already stated, passing from the rep  
Figure 8: Influence of both shear displacement and reactivation speed on the strength increase and subsequent degradation after one hour of ageing at vertical stress of 101.1 kPa: (**a**) Caltavuturo clay; (**b**) Calatabiano clay. \(\Delta\tau_{1}\) to that one in terms of \(\Delta\varphi_{1}\), the role of vertical stress reversed: due to the envelope curvature, more accentuated at low stress, the secant strength angle increases more as the vertical stress decreases. Figure 11 summarises the secant strength angles at reactivation, \(\varphi_{\text{react}}\), for the 10 samples tested up to one month of ageing at shearing speed of 0.089 mm/min and vertical stress of 101.1 kPa. In the same Figure 11, the reactivated strength angles are compared with the corresponding steady residual, \(\varphi_{\text{r}}\)(v), also obtained at the same shearing speed and vertical stress as the first ones. As already stated by [PERSON] and [PERSON] [21], it was observed that less plastic samples tend to develop a slightly larger increase in the reactivated strength angle in comparison to the more plastic ones. This fact is better highlighted in Figure 12, which shows the increase in the secant strength angle at reactivation, \(\varphi_{\text{reac}}-\varphi_{\text{r}}\)(v), as a function of clay content, CF, and considering both the minor and the major vertical stresses employed in the experiments, it is possible to note how the recovered strength angle seems to increase as the clay fraction decreases. Figure 9: Increment in shear stress at reactivation, \(\Delta\tau_{1}\), versus vertical stress and ageing time for a reactivation speed of 0.089 mm/min: (**a**) Caltavuturo clay; (**b**) Calatabiano clay. Figure 11: Comparison between the residual strength angle and that reactivated after one month of ageing; reactivation speed of 0.089 mm/min and vertical stress 101.1 kPa. Figure 10: Increment in the secant strength angle at reactivation, \(\Delta\varphi_{1}\), versus vertical stress and ageing time for a reactivation speed of 0.089 mm/min: (**a**) Caltavuturo clay; (**b**) Calatabiano clay.