Unified Identification of Viscous Instability in Both Solid and Liquid Clays

Water-saturated clays are vulnerable to unstable creep if subjected to a relatively high shear stress, a phenomenon known as creep rupture for solid geomaterials and viscosity bifurcation for clay suspensions. By following Perzyna’s modeling framework and material stability theory, this work enables the unified identification of viscous instability in both solid and liquid clays. Variables reflecting the microstructure of both clay systems are considered in the adopted viscoplastic models to quantify the microstructure rearrangement mathematically. Specifically, a viscoplastic model with rotational hardening is adopted for clayed soil specimens (solid clay), while an enhanced Bingham model with a state-dependent yield stress is proposed for clay suspensions (liquid clay). Results show that the microscopic destruction dominates under high shear levels, which deteriorates the material strength and might potentially trigger the onset of material instability as marked by the violation of the proposed stability criterion, but aging governs the process under low shear levels, which leads to a stabilized flow. These analyses also show that the stability index for the corresponding inviscid, elastoplastic material defined under the same control condition could effectively detect the onset of viscous instability, which marks the transition from stable, solid-like behavior to unstable, inelastic flow for clays under various water content levels.