The objectives of this project were to investigate the dynamic behavior of peat and to evaluate the influence of that behavior on the dynamic response of peat deposits. During the course of the research, the objectives were expanded to explore the effects of strong earthquake shaking on pile foundations embedded in peat deposits.
An experimental investigation was undertaken using samples of a peat deposit typical of many large peat deposits in western Washington. This deposit, known locally as Mercer Slough in Bellevue, Washington, is crossed by a series of bridges that form Interstate 90 and its collector-distributor ramps. Test specimens were obtained by careful, undisturbed sampling of the Mercer Slough peat.
The specimens were tested in a specially modified resonant column device. The resonant column tests showed that the peat was very soft and that its stiffness increased with effective confining pressure in a manner similar to that displayed by cohesionless soils. The peat exhibited nonlinear stress-strain behavior, and the degree of nonlinearity was observed to decrease with increasing effective confining pressure. Damping was observed to increase with increasing shear strain, but at a rate that decreased with increasing effective confining pressure. Special tests designed to investigate frequency dependence of peat stiffness and damping showed that the degree of frequency dependence was relatively small.
A series of ground response analyses was performed to investigate the seismic response of peat deposits. Three vertical profiles through Mercer Slough were analyzed using both equivalent linear and nonlinear ground response analyses. The analyses showed that the soft nature of the Mercer Slough peat would produce amplification of the long-period components of an earthquake ground motion. Substantial long-period motions can produce large dynamic displacements that are potentially damaging to bridges and bridge foundations. Comparison of the equivalent linear and nonlinear ground response analyses showed that the equivalent linear model tended to oversoften the peat at low effective confining pressures, leading to underprediction of ground motion amplitudes at low periods.