Dynamic, in situ, nonlinear-inelastic response and post-cyclic strength of a plastic silt deposit

Abstract

This study 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. Characterization of blast-induced ground motions indicates that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low-frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high-frequency P-waves is negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual EPP ranged from 0.002% to 0.003% and 0.008% to 0.012%, respectively, where the latter corresponded to ∼70% of the maximum shear modulus, Gmax. Following pre-straining and dissipation of EPPs 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 those 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 EPP field measured using vane shear tests compared favorably with that of DSS test specimens.