Abstract: A large number of ancient landslides in the Three Gorges Reservoir Area present long term and slow deformation characteristics with significant creep properties when undergoing internal and external geological agents. It is commonly thought that the soil on the slip plane of a reactive ancient landslide have already reached the residual state. Mechanical properties and stress state of the soil in the rupture zone dominate the main kinematic feature of a slow-moving landslide. Creep behavior, therefore, has an important value for evaluating reactivation potential and landslide prediction. In this paper, intact soil samples are collected from a reactive ancient landslide. Residual strength of intact sliding zone soil is studied and residual-state creep test is performed afterward. Various RCSR (the ratio of the applied constant creep stress to the residual strength) levels are chosen to study the correlation between RCSR and creep rate. The results show that creep rate of soil is positively dependent on RCSR. As RCSR approaches 1, a pronounced secondary creep occurs in a short time followed by an accelerative creep stage for some tests, namely tertiary creep. At the same RCSR, soil with higher normal stress level has larger creep rate. Burger’s model is employed to simulate the creep behavior and the parameters of Burger’s model are obtained. The long-term shearing resistance is investigated by analyzing the isochronal curves, and the value of long-term shear strength of intact sliding zone soil is found to be approximately 0.95 times as much as that of residual strength. Additionally, consolidation duration can affect the creep rate, which decreases with increasing consolidation duration. This reveals that the mobilized shear strength and residual strength are regained to some extend during consolidation or with shear displacement.

Key words: intact sliding zone soil, ring shear, residual strength state, creep behavior, long term strength