Creep of clayey soil induced by elevated pore-water pressure: Implication for forecasting the time of failure of rainfall-triggered landslides

Many models have been proposed to forecast the time of failure of landslides, among them the Voight model ((Omega) over dot = A (Omega) over dot(alpha)) has been widely used. Values for controlling parameters of alpha and A, defined by accelerating movement, are representative of kinematic behavior prior to material failure. Nevertheless, due to the unknowns in the key parameters (alpha and A), failure-time forecast using this model remains elusive. To examine the nature of these two parameters and their physical background, we conducted ring-shear tests on clayey soil to reproduce tertiary creep through two different approaches: a pore-water pressure-controlled approach to simulate the initiation process of landslides resulting from elevated pore-water pressure during rainfall, and a shear-stress-controlled approach often used in studies of the creep behavior of clayey soils. By examining the creeping behavior observed in the pore-water pressure-controlled tests, we found a 3-stage log-linear relationship between velocity and acceleration. The sample showed drastic dilation with short shear displacement in the first stage, and the dilation decreased in the subsequent two stages with further progress of shear displacement. The smaller value of alpha in the first stage increased to another value which was larger in the third stage. This variation might result from changes in the dominant shear mechanisms, i.e., in the first stage the observed creeping displacement results from pervasive grain realignment within a relatively wide shear zone, while in the following two stages, it may arise from more localized grain slip within a narrowing domain of shear zone along the sliding surface. In contrast, no evident variations in alpha value or sample height were found in the shear-stress-controlled tests. The effects of increasing rate of pore-water pressure or shear stress on the value of alpha were not pronounced in our tests. These results provide further information for understanding the progressive failure process and the Voight model of failure-time forecast. The failure-time forecast of rainfall-triggered landslides could be fundamentally difficult because of a changeable alpha value with the intricate volumetric variation and a diverse range of operating shear mechanisms involved.