Dynamic characteristics of Old Bay clay deposits in the East San Francisco Bay area
Abstract
The localized patterns of heavy damage during the 1985 Mexico and 1989 Loma Prieta earthquakes demonstrate the importance of understanding the seismic response of deep clay deposits. The worst affected sites in the East San Francisco Bay area during the Loma Prieta event were situated atop thick strata of stiff silty clay of the San Antonio and Alameda formations (namely the Old Bay clay), which were previously not known to contribute to any significant ground amplification. The evidences of unusual ground amplification at these sites led to the initiation of this research project with the main objective of examining the hitherto unknown dynamic response of Old Bay clay deposit, and characterizing its dynamic properties (and the factors that affect them) for use in seismic site response analysis and prediction of future events. The correlations of various soil parameters (e.g., plasticity, void ratio, magnitude and duration of confinement consolidation stress history, number of loading cycles) with the Old Bay clay dynamic properties are presented, and compared with the existing database on cohesive soils. Special efforts are made to evaluate the effects of sampling disturbance on laboratory measurement of dynamic properties (e.g., shear modulus and damping ratio) of stiff cohesive soils. The laboratory measurement of low strain shear modulus and damping ratios are improved by taking the geologic time history (ageing effect) of the deposits into account. However, the effects of structural disturbance (due to sampling) can not be compensated for. Structural disturbance causes significant reduction in the Old Bay clay shear moduli, but fails to influence the damping ratios in a similar manner. Old Bay clay damping ratios are found to be insensitive to structural disturbance at both low and high strains.
Degree
Ph.D.
Advisors
Drnevich, Purdue University.
Subject Area
Geotechnology|Civil engineering|Geology|Mechanical engineering
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