UC Davis

Liquefaction assessments require the ability to estimate triggering potential and possible consequences across a range of seismic hazard levels and in-situ soil conditions. This Dissertation aims to advance the fundamental understanding of the element level response of coarse-grained soils prior and after liquefaction triggering, and to evaluate tools currently used in engineering practice for the estimation of liquefaction effects on geosystems. This is achieved by (1) resolving experimental issues pertaining to performing Direct Simple Shear (DSS) testing, (2) establishing framework that facilitates the comparison of both triggering and post-triggering responses, and (3) performing extensive DSS investigations of coarse-grained materials encompassing a range of properties and conditions.

A series of constant-volume (CV) DSS tests show that applying a pre-conditioning sequence improves the engagement at the soil-platen interface when textured platens are used. Given the efficacy of these protocols to enhance the shear stress transfer to sand specimens, all the CV-DSS tests presented in this Dissertation consider a pre-conditioning sequence prior to constant-volume shearing. Although the pre-conditioning protocol presented herein is specific to the testing equipment and materials considered, recommendations are provided to develop pre-conditioning protocols for other soils, platens, and testing devices.

The factors and mechanisms controlling the accumulation of shear strains of clean uniform sands exhibiting cyclic mobility behavior under level-ground conditions are investigated from CV-DSS tests on Ottawa F-65 sand subjected to uniform and irregular cyclic loading conditions. Experimental data show that the rate of shear strain accumulation per loading cycle depends on the relative density, cyclic stress amplitude, and overburden stress. Mechanisms of shear strain accumulation are investigated by decoupling the shear strain developed in each loading cycle in two components: γ0, developed at near-zero effective stress, and γd, developed during dilation. Results show that γ0 mostly depends on the shear strain history, while γd depends on the cyclic stress amplitude and relative density.

The role of gradation and grain size on the liquefaction behavior of coarse-grained soils is investigated from a series of CV-DSS tests performed on seven soil mixtures with different coefficients of uniformity (Cu) and median grain sizes (D50). Empirical observations of liquefaction triggering, post-triggering shear strain accumulation, and post-liquefaction reconsolidation strains are synthesized to evaluate functional trends with Cu and D50. Results show that the effects of Cu and D50 on the liquefaction triggering resistance depend on the relative density (DR) at which they are compared, while the post-triggering shear strain accumulation decrease while increasing both Cu and D50. Results also show that an increase in Cu leads to smaller post-liquefaction reconsolidation volumetric strains, while changes in D50 have little effect on such strains.

The combined effect of gradation and sloping ground conditions on soil liquefaction is investigated from monotonic and cyclic CV-DSS tests performed on two sands with different gradations. Monotonic test results show that an increase in gradation slightly reduces the degree of strain-softening and peak shear stresses. Results from cyclic tests are synthesized to examine the effect of gradation on patterns of pore pressure generation, liquefaction triggering resistance, and sloping ground correction factors (Kα). Additional cyclic tests under irregular loading conditions are analyzed to evaluate the role of gradation in the post-triggering shear strain accumulation.

Finally, this Dissertation examines the capabilities and limitations of the PM4Sand constitutive model in capturing key features of the liquefaction behavior of two sands with different gradations under sloping ground conditions. Simulations with PM4sand successfully captured the monotonic response and liquefaction triggering resistance for both soils. Still, the calibration process required activating some of the secondary parameters of the model, and using alternative parameter sets depending on the target responses prioritized during calibration.