Dynamic Properties of Sherman Island Peat

Abstract

The dynamic properties of peat have been identified as major source of uncertainty in the evaluation of seismic hazards throughout the Sacramento–San Joaquin Delta in northern California. This paper summarizes the results of a laboratory study of the dynamic properties of a layer of peaty organic soil underlying the south levee on Sherman Island near the western side of the delta. Conventional Shelby tube sampling procedures were able to obtain high-quality samples because of the compactness of this peat layer, located between depths of 9 and 16 m. The samples tested were very fibrous and had ash contents of 35–56%. Staged cyclic triaxial loading was used to measure the stress-strain behavior of several specimens under cyclic shear strains ranging from about 0.0005% to 10%. Other tests included piezo-ceramic bender element tests to measure the shear wave velocity of specimens within the triaxial device, and undrained monotonic triaxial compression and extension tests. The effects of loading frequency, cyclic degradation, consolidation stress history, and structural anisotropy are evaluated. The resulting modulus reduction and damping relationships for the Sherman Island peat are compared with published results for other peats, solid waste materials, and mineral soils.