Dynamic Characteristics of Old Bay Clay

Unusually strong ground motions at deep stiff clay sites in the East San Francisco Bay area during the 1989 Loma Prieta earthquake called for examination of the dynamic response of this clay deposit and characterization of its dynamic properties. In this paper, correlations of Old Bay clay dynamic properties with various soil parameters (e.g., plasticity, void ratio, magnitude and duration of confinement, consolidation stress history) are presented and compared with the existing database on cohesive soils. A special effort is made to evaluate the effects of sample disturbance on the laboratory measurement of dynamic properties of stiff cohesive soils. The laboratory measurement of low-strain shear modulus and damping ratios is adjusted by considering the aging of the Pleistocene era clay deposit. These adjustments, though effective, are not sufficient to account for the effects of structural disturbance. Shear modulus is reduced significantly by structural disturbance, but, surprisingly, the damping ratio is found to remain largely unaffected.