DYNAMIC PROPERTIES OF SHERMAN ISLAND PEAT

The dynamic properties of peat have been identified as major source of uncertainty in the evaluation of seismic hazards throughout the Sacra mento-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 ne ar the western side of the delta. Conventional Shelby tube sampling pr ocedures were able to obtain high-quality samples because of the compa ctness of this peat layer, located between depths of 9 and 16 m. The s amples tested were very fibrous and had ash contents of 35-56%. Staged cyclic triaxial loading was used to measure the stress-strain behavio r 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 d evice, and undrained monotonic triaxial compression and extension test s. The effects of loading frequency, cyclic degradation, consolidation stress history, and structural anisotropy are evaluated. The resultin g modulus reduction and damping relationships for the Sherman Island p eat are compared with published results for other peats, solid waste m aterials, and mineral soils.