Dynamic Response of a Low Plasticity Silt Deposit: Comparison of In-situ and Laboratory Responses

This study compares the in-situ dynamic response of a low plasticity silt deposit subjected to multidirectional loading from vibroseis shaking and controlled blasting to a suite of element-scale, cyclic laboratory test specimens. The agreement between excess pore pressures and simple shear strain relationships over a wide range in strains is remarkable. Slightly larger excess pore pressures observed in-situ are attributed to three-dimensional loading and pore pressure migration/ redistribution in the shallower portions of the deposit. Noted differences in shear modulus, G, are attributed to strain rate effects, spatial variability in the in-situ stiffness, and hydraulic boundary conditions. The variation in in-situ G/Gmax follows the trend from torsional shear specimens up to 0.4% shear strain; larger strains in the silt deposit imposed by controlled blasting yielded a stiffer response than that from cyclic torsional shear and direct simple shear specimens due in part to field drainage for deeper portions of the deposit. The in-situ cyclic resistance ratio for the deeper portion of the deposit in which plane body waves could be assumed and for the selected excess pore pressure ratio criterion was larger than that of stress-controlled CDSS test specimens, despite the detrimental effect of multidirectional shaking in the field. The effect of strain history, spatial variability, and drainage boundary conditions to drive differences between the in-situ and laboratory test specimens is identified.