A framework for the estimation of coseismic deformations in the postliquefaction regime is developed based on an extensive database of available cyclic undrained stress-controlled tests on clean sand samples without static shear bias, covering a wide range of relative densities. Based on fundamental experimental observations, a compliance rate is defined as the postliquefaction shear strain rate per cycle over the shear stress amplitude. Semiempirical relationships of the compliance rate as a function of relative density are developed to provide guidance for estimating postliquefaction shear strains. The proposed framework provides a basis for the calibration of advanced constitutive models capable of capturing postliquefaction strain accumulation. A calibration methodology is proposed using both existing liquefaction resistance curves and the newly developed semiempirical relationships for estimating postliquefaction shear strain accumulation. The validity of the proposed methodology is demonstrated by numerical simulations, using the PM4Sand model, of two well-documented centrifuge tests focusing on liquefaction-induced demands on engineering structures.

Textured platens are often used to improve the transfer of shear stresses from the platens to the soil specimen during direct simple shear (DSS) tests. However, constant-volume DSS tests when textured platens are used can be affected by inadequate engagement of the soil at the platen-soil interface, leading to large reductions in the vertical stress at the start of shearing.The application of a preconditioning sequence involving small-strain drained cycles prior to constant-volume shearing can improve engagement at the platen-soil interface, but when excessively implemented, it can also have adverse effects on the measured soil behavior (e.g., strength, stiffness). A series of constant-volume cyclic DSS tests preceded by different preconditioning sequences was performed to evaluate the effect of preconditioning on the engagement of sand specimens at the platen-soil interface and the stress-strain response of these specimens. Results showed that textured platens that are properly engaged with sand specimens can reduce slippage at the platen-soil interface. This engagement can be achieved by applying a limited number of small-strain drained cycles at a low vertical stress while still obtaining representative soil behavior during the subsequent equivalent undrained constantvolume cyclic loading. Although the preconditioning protocol presented herein is specific to the testing equipment and materials considered, similar procedures may be adopted to develop preconditioning 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 examined. This phenomenon is investigated through a series of constant-volume cyclic direct simple shear (DSS) tests subjected to uniform and irregular loading conditions, and undrained cyclic element tests collected from the literature. Experimental data show that the rate of shear strain accumulation per loading cycle depends on the relative density, cyclic stress amplitude, and effective 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 γ_d, developed during dilation. Results show that γ₀ mostly depends on the shear strain history, while γ_d depends on the cyclic stress amplitude and the relative density. These dependencies of γ_d and γ₀ are used to provide an explanation for the gradual decrease of the rate of shear strain accumulation that is observed while increasing the number of post-triggering loading cycles in tests performed on dense specimens.

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