Determination of Shear-Wave Velocities and Shear Moduli of Completely Decomposed Tuff

Based on theoretical derivations and considerations, five series of laboratory tests were planned to investigate and differentiate the degrees of inherent and stress-induced anisotropy, to study the effect of void ratio changes on shear-wave velocities and shear moduli, and to determine the relationship between shear-wave velocity and stress state on a completely decomposed tuff (CDT). Shear-wave velocities in three orthogonal horizontal and vertical planes [ $v_{s\left(\mathit{hh}\right)}$ , $v_{s\left(\mathit{hv}\right)}$ , and $v_{s\left(\mathit{vh}\right)}$ ] were measured in both vertically and horizontally cut block and Mazier specimens. Under isotropic stress conditions $(K=1.0)$ , the degrees of inherent anisotropy $[{v_{s\left(\mathit{hh}\right)}}^2∕{v_{s\left(\mathit{hv}\right)}}^2=G_{\mathit{hh}}∕G_{\mathit{hv}}]$ were 1.48 and 1.36 for the block and Mazier specimens, respectively. At the anisotropic stress state $(K=0.4)$ , the degrees of anisotropy of the block and Mazier specimens were 1.26 and 1.15, respectively, 15% reduction from the measured inherent anisotropy due to stress-induced effects. The measured higher shear-wave velocity in the horizontal plane of the CDT was confirmed by testing both vertically and horizontally cut specimens and the measured results reflect a stronger layering structure in the horizontal bedding plane of the natural material, in which $K_0$ less than 1.0 is commonly assumed in designs. Under both isotropic and anisotropic stress states, the shear-wave velocities [ $v_{s\left(\mathit{hh}\right)}$ , $v_{s\left(\mathit{hv}\right)}$ , and $v_{s\left(\mathit{vh}\right)}$ ] of the block specimens are on average about 27% higher than those of the Mazier specimens.