Mathematical interpretation of delayed instability in viscous unsaturated soil

Fluid-infiltrated soils are vulnerable to wetting and often exhibit delayed deformation characterised by acceleration stages. In this paper, mathematical tools are developed to diagnose delayed failure in unsaturated materials subjected to saturation. For this purpose, stability criteria for saturated viscous soils are extended to account for hydraulic state variables (e.g. suction and water content), which may provoke an unexpected increase of the creep rate. A simple one-dimensional constitutive law is used to test the proposed theory and assess its capability to distinguish stable and unstable creep. Numerical simulations revealed that, although the mathematical conditions associated with a loss of stability resemble those of rate-independent models, high viscosity delays strain acceleration and accentuates suction dependence (i.e. drier states are less susceptible to tertiary creep than wetter states). Most importantly, the analyses indicate that a violation of the stability criteria is a precursor of sharp suction loss and consequent fluidisation.