Abstract: Gravels are widely found in the sliding zone soils (SZS) of ancient landslides. It is the key to determine the mobilized strength of SZS with gravels for the stability evaluation and prevention of ancient landslides. Taking the reactivation of Jiangdingya ancient landslide in Zhouqu County, Gansu Province in 2018 as an example, the mobilized strength of SZS with gravels was studied based on the test and back analysis. The results suggested that: (1) after long-distance shearing, the cementation in the SZS has been gradually lost, and the residual strength is mainly controlled by the sliding frictional resistance and occlusion between the soil particles. When the gravel content is higher, the interaction force between the soil particles is stronger, so the residual strength is relatively larger. There is a positive linear correlation between the friction coefficient and roughness of the shear surface. The gravel changes the roughness of the shear surface, which increases the friction resistance and leads to the increase of its residual strength. (2) The statistical analysis showed that the φr of SZS with gravels is controlled by both clay and gravel content, which is significantly different from the mechanism that the φr of SZS without gravels is mainly controlled by clay content. It is suggested that the ratio of gravel content to clay content should be used as an index to estimate the φr of SZS with gravels. (3) The mobilized strength of ancient landslides is generally greater than the residual strength, but slightly less than the recovery strength. Before reactivation, the strength of SZS has gradually attenuated from the recovery strength to the residual strength. At this time, the ancient landslide is in the creeping state as a whole. Under the action of external forces, the strength of SZS tends to decay sharply, which induces the accelerated sliding of the ancient landslide.

Key words: landslide reactivation, sliding zone soils with gravels, residual strength, strength recovery, mobilized strength