UCLA

Centrifuge models of soft clay deposits were shaken with suites of earthquake ground motions to study site response over a wide strain range. The models were constructed in an innovative hinged-plate container to effectively reproduce one dimensional ground response boundary conditions. Dense sensor arrays facilitate back-calculation of modulus reduction and damping values that show modest misfits from empirical models. Low amplitude base motions produced nearly elastic response in which ground motions were amplified through the soil column and the fundamental site period was approximately 1.0s. High intensity base motions produced shear strains higher than 10%, mobilizing shear failure in clay at stresses larger than the undrained monotonic shear strength. I attribute these high mobilized stresses to rate effects, which should be considered in strength parameter selection for nonlinear analysis. The nonlinearity in spectral amplification is parameterized in a form used for site terms in ground motion prediction equations to provide empirical constraint unavailable from ground motion databases.

The nonlinear site response is covered by total stress simulations of centrifuge models involving soft clay, and effective stress simulations of centrifuge models including liquefiable sand layers. Primary conclusions from the total stress analysis are (1) unreasonable shear strength values may arise from extrapolating modulus reduction curves to large strains, and properly modeling the shear strength by adjusting the high-strain region of the modulus reduction curve is essential for accurate nonlinear site response modeling, and (2) the shear strength must be adjusted for strain rate effects to capture the measured ground motions. The primary conclusion from the effective stress simulations is that ground motions following liquefaction triggering are significantly under-predicted using a modeling procedure in which the backbone stress-strain behavior is degraded as pore pressures develop in accordance with a pore pressure generation function. These models fail to capture the dilatancy behavior of liquefied sand that manifests as a transient stiffening in undrained loading, and enables propagation of high amplitude high frequency acceleration pulses. Constitutive models capturing the dilatancy behavior are demonstrated to have the capability to replicate these acceleration pulses, but the resulting ground motions are highly sensitive to input parameters.