## 1 Introduction  
When a landslide undergoes large displacements along a sliding surface, the available soil strength reaches a minimum value, which is the residual strength. Under this condition, quiescence or reactivation may occur, depending on the extent the available soil strength is able to balance the increase of destabilising forces. However, predicting the future behaviour of a quiescent landslide is very complex: the landslide could reactivate withuniform or accelerated motions or with both, could creep, stop, and then reactivate again. Under this scenario, the recovered strength during quiescence could play a key role, especially if it could be different from the value reached in the previous active phase. At the present time, the existence of a reactivated strength different from the residual one is still a matter of discussion, even though some researchers have highlighted the importance of the quiescence time, the stress history, the stress level, and the speed of reactivation on the amount of the mobilised strength upon reactivation. For example, [PERSON] and [PERSON] [1] hypothesised that ageing could cause the progressive increase in strength during the stability periods. In contrast, [PERSON] [2,3] stated that if a failure has already occurred in clayey soils, any subsequent displacement along the slip surface will be controlled by the residual strength; therefore, an old quiescent landslide could only reactivate by mobilising again the residual strength along the pre-existing sliding surface. To confirm [PERSON]'s hypothesis, [PERSON] and [PERSON] [4] presented a comparison between secant residual strength angles determined from both back analyses of reactivated landslides and laboratory tests: this comparison did not show any strength regain at the reactivation. Many other authors [5-12] expressed different opinions supporting the fact that the strength at reactivation could exceed the steady residual strength, in both laboratory tests and full scale observations. [PERSON] et al. [13] found that the mobilised strength for a West Virginia landslide (U.S.A.) was greater than the laboratory residual strength, and [PERSON] and [PERSON] [14] stated that, in the Appalachian Plateau (U.S.A.), the strength gains are likely to be within the range of 20% to 50% of the shear strengths measured in laboratory tests of practical duration on small specimens. The reactivation mechanism of ancient landslides in Southwest China was examined by [PERSON] et al. [15] and a healing mechanism was considered a plausible explanation for the stable status of these landslides for a long period. The self-healing phenomenon of a red-bed landslide was studied by [PERSON] et al. [16]: the relationship between the recovery strength of the shear surface and the healing time was analysed by reversal shear tests, and the law of the recovery strength was then analysed under different vertical stresses. The self-healing of a landslide using laboratory ring shear tests with different shear rates and grain sizes was studied by [PERSON] et al. [17]. The authors found that the peak strength of the landslide that had undergone accelerated creep was greater than the previous peak strength. [PERSON] et al. [18] and [PERSON] et al. [19] showed that the strength recovery increases as both the vertical stress and the healing time increase; however, the rate of growth decreases and ultimately approaches zero as the recovery time increases. [PERSON] et al. [20] and [PERSON] and [PERSON] [21] suggested that the residual shear strength of the soils within the sliding zone in the Jurassic red strata in Three Gorges Reservoir (China) could have had a certain recovery as a result of the secondary compression of the shear zone. Soils with a high clay content generally do not exhibit a significant strength recovery, while soils with a low clay content clearly recover their residual strength due to the volume change of the shear zone caused by secondary compression after a long period of reconsolidation.