Dynamic, In-situ, Nonlinear-Inelastic Response of a Plastic Silt Deposit

This presentation presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit at the Port of Portland. The experimental program, instrumentation, and subsurface investigation is described, including laboratory investigations on intact specimens for comparison to the in-situ response. Characterization of blast-induced ground motions revealed that the maximum particle displacements, shear strain, and corresponding residual excess pore pressures developed in the silt deposit are associated with the low frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the higher frequency compressive P-wave did not produce residual excess pore pressure. Three blasting programs conducted at the test sites were used to develop the initial and pre-strained relationships between shear strain, excess pore pressure, and nonlinear shear modulus degradation, revealing the initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual excess pore pressure ranging from 0.002 to 0.003% and 0.008 to 0.012%, respectively, where the latter corresponded to a reduction of the maximum shear modulus of approximately 30%. Drainage and excess pore pressure migration appears to have contributed to a stiffer large-strain response than otherwise expected from laboratory-derived shear modulus reduction curves Following pre-straining and dissipation of excess pore pressures within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in-situ compared to that of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in-situ undrained shear strength within the blast-induced excess pore pressure field measured using vane shear tests compared favorably with that of DSS test specimens.