Model-based interpretation of undrained creep instability in loose sands

Evidence shows that in the presence of extended stages of undrained creep, loose sands may approach liquefaction instabilities with a non-negligible time lag with respect to the application of loading. In this paper, a mechanical interpretation of such delayed failure events is provided by using stability criteria for rate-dependent materials. For this purpose, a viscoplastic constitutive law for sands has been calibrated to replicate delayed failure processes documented in the literature. To explain the origin of the transition from stable to unstable creep, the model predictions have been inspected from a mathematical standpoint and a strategy to evaluate the time required for the initiation of failure has been provided. The analyses show that the acceleration of the creep strains anticipates the sharp increase in the rate of pore water pressure, thus constituting a precursor to runaway failure. Furthermore, the computed stresses at which the two variables accelerate are located in proximity of the instability line for static liquefaction, with a shift from it that depends on the rate of loading prior to creep and the soil viscosity. These findings provide support to understand the interplay between rate-dependent soil properties and delayed liquefaction by offering a new conceptual platform to interpret the temporal evolution of flow failures observed under field or laboratory conditions.