## 5 Conclusions  
In this paper, the drained residual strength at reactivation has been discussed on the basis of laboratory tests carried out with the Bromhead ring shear apparatus in some normally consolidated saturated samples, both of natural soils and of clays provided by industry (bentonite and kaolin). Two different classes of phenomena may account for changes in the residual strength, the first related to the interaction between the soil and the physical environment [24; 25; 26], while the second related to the mechanics of the particle medium. The aim of this research was to assess the coupling effect of ageing time, vertical stress and reactivation speed on the mobilised residual strength upon the reactivation of displacement. Ten samples were subjected to different ageing cycles at various time amplitudes and vertical stress levels; the samples were then sheared after each ageing cycle. For two soil samples, the effect of different displacement rates was also investigated. The following conclusions were drawn:  
* The stress condition, \(\tau_{0}\), reached after relaxation under zero displacement speed should identify a reference condition for which shear creep can attenuate within the primary phase;
* After quiescence at \(\tau_{0}\) stress, the strength at reactivation could exceed the residual value, \(\tau_{\tau}(\
u)\), depending on the ageing time, the shearing speed, and the vertical stress. This could be the consequence of some causes, such as viscosity, volumetric creep, and thixotropic hardening, which can account for an initial reactivated strength proportional to the ageing time and the reactivation speed;
* The reactivated strength rapidly falls towards \(\tau_{\tau}(\
u)\) as the displacement increased at constant speed. This would mean that the aged soil fabric undergoes destructuration;
* If reactivation occurred at a displacement speed approaching zero, the reactivated shear strength approached the reference value,\(\tau_{0}\), irrespective of the amount of time spent in the previous ageing phase; this could occur if the strength at reactivation depended only on the viscous reaction of the particle edge-to-face hinges. The mechanisms discussed in this work outline how a landslide that is already moving, even at very low speed, could not develop an age-induced strengthening along the sliding surface, so that no further recovered strength would be opposed to a new destabilising event. This latter point suggests how knowing the real kinematics of a landslide is strongly necessary; in fact, shear creep displacements can occur with velocities even lower than \(3\times 10^{-5}\) mm/min (16 mm/year) [33], difficult to highlight but capable of preventing the benefit of volumetric creep along the sliding surface. However, if it is certain that the quiescent landslide is stable, the experimental results of this research can provide methodological guidance on the design of reinforcement works to counteract a rather rapid increase in destabilising actions, such as in the case of seismic stresses, morphological changes of the slope, or as a consequence of a rising water table. Furthermore, the triggering speed of a landslide may also depend on the landslide morphology and on the boundary conditions, both difficult to reproduce in the laboratory. Beyond the geotechnical parameters studied in this research, it may be important to further investigate the influence of both the stress history (OCR) and the partial saturation on the mobilised strength upon reactivation. Partial saturation, in fact, could arise from the interaction of the landslide body with the meteorological environment of the area. Despite the limitations of this research, which certainly require further investigation and confirmation, the important role of reactivation speed in the kinematics of quiescent landslides is emphasised. This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.