Dynamic Properties of Highly Organic Soils from Montezuma Slough and Clifton Court

The nonlinear dynamic properties of highly organic soils from two levee sites in the Sacramento-San Joaquin Delta in California are described. Cyclic triaxial, resonant column and torsional shear tests were performed on thin-walled tube samples obtained from beneath levee crests, beneath adjacent berms, and in the free field such that the in situ vertical effective stresses $\left({\sigma }_{vo}^{\prime }\right)$ ranged from about $16\text{to}67\mathrm{kPa}$ . These highly organic soils had considerably different organic characteristics from those used in previous studies of dynamic properties. The tested samples had organic contents of 14 to 61%, initial water contents $\left(w_o\right)$ of 88 to 496%, shear wave velocities $\left(V_s\right)$ of $20\text{to}130\mathrm{m}∕\mathrm{s}$ , and organic components that ranged from highly fibrous to highly decomposed and amorphous. Secant shear modulus $\left(G\right)$ , normalized secant shear modulus $(G∕G_{\max })$ , and equivalent damping ratio $\left(\xi \right)$ versus cyclic shear strain amplitude $\left({\gamma }_c\right)$ relations are presented, and their dependence on variables such as consolidation stress, organic content, prior loading history, testing device, and loading frequency are illustrated. Findings are compared to previously published results.