Residual state rate effects of shear-zone soil regulating slow-to-fast transition of catastrophic landslides

Catastrophic landslides with basal shear zones usually involve a number of mechanisms that are poorly understood. One mechanism issue is the transition from slow sliding to fast movement, leading to catastrophic failure. We perform laboratory experiments on landslide shear-zone materials to investigate how rate effects influence the slow-to-fast transition. Two types of ring-shear tests are carried out, namely residual state shear tests involving a wide range of shear rates and residual state creep tests with the applied shear stress close to the residual strength. Our tests show that the shear-zone soil exhibits rate-weakening behaviour at low shear rates and rate-strengthening behaviour at high shear rates. These two opposite mechanisms are distinguished by a transition rate, which also regulates whether the creep motion falls into the viscous flow regime or returns to the slow sliding regime. • We provide insight into the rate-dependent behaviour of landslide material from a catastrophic landslide. • The landslide movement undergoes a complicated process from creep to fast flow. • Residual state ring-shear tests are carried out to explore the rate effect of shear-zone soil. • A transition rate is observed to regulate the slow-to-fast transition of catastrophic failure.