## 3 Steady-State and Reactivated Residual Strength  
Generally, the effect of the displacement speeds on the steady residual strength has been subjected to extensive studies throughout the last few decades, considering both low [2,3,28] and high displacement speeds [29, 30, 31, 32]. Referring exclusively to the effect of the low reactivation speed, which concerns the object of this study, it was deemed essential to recall some results available in the scientific literature: [PERSON] [3] stated that, at a sliding speed of less than 0.01 mm/min, the residual strength can be considered as drained, i.e., not influenced by neutral pore pressure. At greater displacement speed, but not exceeding 1.0 mm/min, the residual strength can oscillate between \(\pm 10\%\) of the drained residual value obtained at a shearing speed of 0.01 \(\div\) 0.025 mm/min [30]. Further authors [33, 34, 35] suggested that, irrespective of the clay fraction content, the kinematics of a landslide could occur basically drained, if the displacement speed is less than 0.50 mm/min along the failure surface. Figure 2: Secant residual strength angle for the 26 samples tested at 3 vertical stresses and reactivation speed of 0.089 mm/min: (**a**) versus clay fraction CF and the dashed Skempton [2] limits; (**b**) versus (CF + PI)/w\({}_{\rm P}\) with the upper and lower boundaries dashed. As sketched in Figure 3, the drained residual strength, \(\tau_{\rm r}({\rm v})\), comes out as a reaction to a viscous-frictional behaviour, in which the viscous component of strength is related to the sliding speed, \({\rm v}\). If the relative displacement is stopped, a progressive decay of the residual strength is observed over time until its stabilisation on \(\tau_{0}\), a value for which no further stress decay should occur. [PERSON] and [PERSON] [33] pointed out that shear creep may occur even at rates as low as 0.001 mm/min, thus requiring a very long time of observation; however, these observations were omitted from this research and the strength decay was considered concluded within the 30 days of the relaxation stage. Keeping this simplification in mind, the parameter \(\tau_{0}\) corresponded to a zero displacement speed and the shear creep was considered almost completely attenuated. Later, \(\tau_{0}\) was used as a reference value in the quantification of the strength recovery upon reactivation. The time spent in the relaxation stage represented an ageing period, \(t_{\rm age}\), during which the sample could have experienced variation in strength. Actually, ageing may occur under shear stresses also below or slightly above \(\tau_{0}\), as investigated by other researchers [36; 37; 38; 39]; in this research, only ageing at \(\tau_{0}\) stress was considered. When reactivation started at a constant displacement speed, an almost instantaneous growth in strength, \(\tau_{\rm react}\), occurred, followed by an equally rapid decay up to \(\tau_{\rm r}({\rm v})\), as soon as the displacement proceeded. The transient increase in strength, \(\Delta\tau_{1}=\tau_{\rm react}-\tau_{0}\), configured rigid-fragile behaviour, which, in normally consolidated samples, could depend on three main factors: the vertical stress, \(\sigma_{\rm v}\), the sliding speed at reactivation, \({\rm v}\), and the time spent during the quiescence phase, \(t_{\rm age}\). The increase in strength, \(\Delta\tau_{2}=\tau_{\rm r}({\rm v})-\tau_{0}\), could be considered a viscous strength component, related to both the vertical stress, \(\sigma_{\rm v}\), and the sliding speed, \({\rm v}\). The behaviour outlined above is discussed now with reference to the shear strength tests carried out for Caltavuturo and Calatabiano clays, for which the influence of the displacement speed upon reactivation was also investigated. The Caltavuturo landslide occurred on 10 April 2015, in a flysch formation, as a result of heavy rainfall in locality Favara, Sicily.