Centrifuge modeling and numerical analysis on seismic site response of deep offshore clay deposits

Nonlinear site response of deep offshore clay deposit plays an important role in changing the characteristics of ground motions when subjected to strong shaking, especially when different amplification behaviors of acceleration and displacement are considered. The paper describes a series of centrifuge shaking table tests and numerical simulations to investigate the behavior of deep offshore clay deposits subjected to earthquake loadings. The centrifuge model tests were performed on a slightly overconsolidated clay deposit at UC Davis. A suite of shaking tests were conducted including impulse step wave testing, frequency sweeps, a small “elastic” earthquake event and a “ductility” level large earthquake event. Accelerations, displacements and pore pressures were measured in the soils (free field) throughout the test program. For the elastic level excitations, about a 33% amplification was observed between the input acceleration at the base and the measured accelerations near the top of the clay deposit. However, for the ductility-level earthquake excitation, which had peak input acceleration at the base of 0.46g, acceleration near the top of the deposit was reduced to 0.07g. These observations are correctly simulated by DEEPSOIL with proper soil models and parameters back analyzed from the centrifuge model test. Then systematic 1-D site response analyses are performed on synthetic clay deposit models with 30 to 120 m depths using the validated DEEPSOIL program, and the amplification characteristics of acceleration and deformation induced by base excitation with different intensities and frequencies are analyzed in both time and frequency domains. The results reveal that for deep soft offshore clay deposits subjected to large earthquakes, significant acceleration attenuation may occur near the top of deposit due to soil nonlinearity and even local shear failure; however, significant amplification of displacement at low frequencies is expected regardless of the intensities of base motions, which suggests that for displacement sensitive offshore foundations and structures, such amplified low-frequency displacement response will play an important role in seismic design.