Multidirectional Vibroseis Shaking and Controlled Blasting to Determine the Dynamic In Situ Response of a Low-Plasticity Silt Deposit

In this paper, efforts to characterize and compare the full-scale in situ three-dimensional (3D) dynamic response of a low-plasticity silt deposit to multidirectional loading from two different sources, a vibroseis shaker named T-Rex and controlled blasting, are presented. Horizontal vibroseis shaking at a frequency, $f$, of 10 Hz, produced dynamic responses in the silt that ranged from linear-elastic to nonlinear-inelastic, inducing maximum equivalent direct simple shear (DSS) shear strains, ${\gamma }_{\mathrm{DSS},\max }$, up to 0.15% and residual excess pore pressure ratios, $r_{u,r}$, of 14.1%. Blast-induced shear waves with predominant frequencies ranging from 9.6 to 14.6 Hz excited nonlinear-elastic and nonlinear-inelastic responses in the silt deposit, with ${\gamma }_{\mathrm{DSS},\max }$ of 1.14% and maximum $r_{u,r}$ of 61%. Importantly, these responses were observed to be minimally influenced by high frequency compression waves. Multidirectional loading, and excess pore pressure, $u_e$, migration and impedance were identified as the predominant factors for achieving the large $r_{u,r}$ in the silt deposit from these two in situ testing techniques. The cyclic threshold shear strain, ${\gamma }_{tp}$, to trigger $r_{u,r}$ observed from the T-Rex shaking equaled 0.007% to 0.011% and varied with the initial soil stiffness. The two testing techniques demonstrated that the in-situ shear modulus, $G$, reduced to 90% of the maximum shear modulus, $G_{\max }$, at ${\gamma }_{\mathrm{DSS},\max }\approx {\gamma }_{tp}$, whereas by ${\gamma }_{\mathrm{DSS},\max }\approx 1\%$, $G$ further reduced to 10 to 30% of $G_{\max }$ corresponding to $r_{u,r}$ of $\sim 60\%$. Changes in soil fabric were quantified using small-strain shear-wave velocity measurements performed before and/or upon initiation and after each stage of dynamic testing, and were linked to the observed increase and decrease in ${\gamma }_{tp}$ for the shallower and deeper 3D elements, respectively, following T-Rex shaking. The side-by-side comparison of the dynamic responses and soil properties derived from these two distinctly different field-testing techniques validate the use of controlled blasting for quantifying in situ dynamic soil properties and responses.