Nonlinear Dynamic Properties of a Fibrous Organic Soil

The nonlinear dynamic properties of a fibrous peaty organic soil beneath a levee in the Sacramento–San Joaquin Delta in California are described herein. Thin-walled tube samples were obtained from four locations between the levee crest and the free field such that the in situ vertical effective stresses $\left({\sigma }_{\mathrm{vo}}^{\prime }\right)$ ranged from about 12 kPa in the free field to about 135 kPa beneath the levee crest. The peaty organic soil was very soft and highly compressible in the free field with initial water contents $\left(w_o\right)$ of 236–588% and shear wave velocities $\left(V_s\right)$ of typically 22–27 m/s, and moderately firm beneath the levee crest with $w_o$ of 152–240% and $V_s$ of typically 88–129 m/s. Stress–strain measurements in a cyclic triaxial device showed that the normalized secant shear modulus $(G/G_{\max })$ and equivalent damping ratio (ξ) versus cyclic shear strain amplitude $\left({\gamma }_c\right)$ relations were dependent on the consolidation stress $\left({\sigma }_{\mathrm{vc}}^{\prime }\right)\text{.}$ Tests involving prior overstraining followed by reconsolidation showed that the effects of sample disturbance were likely small. Stress history, creep, and loading frequency effects were also examined. Tests on reconstituted specimens provided supplementary data on the functional relation between maximum shear modulus $\left(G_{\max }\right)$ and consolidation stress conditions. Summary relations are provided for $G/G_{\max }$ and ξ versus ${\gamma }_c$ and for $G_{\max }$ versus ${\sigma }_{\mathrm{vc}}^{\prime }\text{.}$