Dynamic In Situ Nonlinear Inelastic Response of a Deep Medium Dense Sand Deposit

This study presents the use of controlled blasting for the determination of the in situ dynamic response of a sand deposit at a depth of 25 m under effective overburden stresses of approximately 250 kPa. The experiments were performed to establish the suitability of blasting as a seismic energy source for the quantification and evaluation of dynamic constitutive soil properties, including the coupled degradation of shear modulus, $G$, and generation of excess pore pressure, $u_e$, with shear strain, $\gamma$. The ground motion characteristics associated with controlled blasting were quantified, indicating that compression waves operate at frequencies too high to generate significant particle displacements and corresponding strains. The shear waves generated due to near- and far-field unloading of the initial compression wave were found to control the soil response, and were associated with frequencies common in earthquake ground motions. The three blast experiments provide the basis for the in situ observation of constitutive soil properties, including the threshold shear strains to trigger soil nonlinearity and residual excess pore pressure, $u_{e,r}$, as well as changes in constitutive responses as a result of alterations in the soil fabric and geostatic stress state. Field drainage during the experiments was found to exert a significant influence on large-strain $G$, and its effects distinguish the in situ response from those observed in dynamic, fully undrained or constant-volume laboratory experiments. The linear-elastic threshold shear strain, ${\gamma }_{te}$, of the natural sand deposit ranged from 0.001% to 0.002% and the threshold shear strain to initiate $u_{e,r}$, ${\gamma }_{tp}$, ranged from 0.008% to 0.01% for the intact natural deposit. Reduction in normalized $G$ of approximately $0.70G_{\max }$ was necessary to trigger $u_{e,r}$ within the intact natural sand deposit. The generation of $u_e$ in the reconsolidated sand deposit was greater than the intact deposit, with ${\gamma }_{tp}$ reducing to 0.002%–0.003%. The significantly reduced geostatic stress state inferred from shear wave velocity and settlement measurements facilitated comparison of the shear strain–excess pore pressure relationship for vertical effective stresses ranging from 44 to 256 kPa, and confirmed that such relationships are highly pressure dependent.