Flowability of Saturated Sands under Cyclic Loading and the Viscous Fluid Flow Failure Criterion for Liquefaction Triggering

Dynamically loaded soils can exhibit large-deformation flow liquefaction or limited-deformation cyclic mobility mechanisms, depending on the initial state of the soil. Undrained cyclic triaxial tests were performed on saturated calcareous and silica sand specimens prepared with different relative densities and subjected to various effective confining pressures and cyclic stress ratios to study the flowability of viscous liquefied sand. The cyclic shear stress–strain rate relationship for calcareous and silica sands transitioned from an elliptical shape to an asymmetric Lamé curve shape as excess pore pressures accumulated under cyclic loading. The asymmetric Lamé curve–shaped relationship demonstrates that the saturated sand exhibited low shearing resistance and high fluidity under elevated excess pore pressures for the conditions evaluated. The average flow coefficient, $\overline{\kappa }$, defined as the maximum shear strain rate triggered by the unit average cyclic shear stress, and the flow curve defining the variation in $\overline{\kappa }$ with the number of loading cycles, describes the flowability of the saturated sand and is used to quantify the cyclic failure potential of the saturated sand under a proposed viscous fluid flow failure criterion. The effect of relative density, effective confining pressure, and cyclic stress ratio on the flow curves and the number of cycles to failure under the proposed viscous fluid flow failure criterion is discussed and compared with the cyclic resistance determined from widely used excess pore pressure– and strain-based cyclic failure criteria. The viscous fluid flow cyclic failure criterion is more stringent than these alternative criteria, and the corresponding axial strains are consistent with those associated with liquefaction triggering under cyclic strain approach.