Experimental investigation of primary-wave velocities and constrained moduli of quartz sand subjected to extender element tests and stress anisotropy

The measurement of primary wave velocities (V_p) of geo-materials is an essential step for the geophysical and dynamic characterization of sediments. Advances in experimentation have established in recent years the use of the extender element test method in laboratory research and practice. This method is applicable for the measurement of V_p of geo-materials applying a high-frequency pulse excitation, typically between 5 and 20 kHz. In the study, extender elements inserted in a stress path triaxial apparatus were used for the measurement of primary-wave velocities of quartz sands of variable grain morphologies. During the dynamic tests, the samples were subjected to a deviatoric compressive load, which allowed the study of the effect of stress anisotropy, expressed through the stress ratio, on V_p. To overcome difficulties of poor signal output of P-waves, which may occur when the sample is fully saturated, the samples were prepared and tested in a dry state. Measurements of sample volumetric strains were conducted from local strain sensor attached to the sample which captured radial deformations during the isotropic and anisotropic stress paths and a vertically positioned LVDT which captured the axial strains of the samples. Accordingly, based on the measured P-wave velocities and the density of the samples, the constrained modulus (M_max) was quantified. The results indicated that for a given mean effective confining pressure, the sensitivity of V_p and M_max to the magnitude of the applied stress ratio was controlled, markedly, by the morphology of the grains; for sands of fairly rounded and spherical grains, this effect was relatively small, but for sands of irregular in shape grains, the stress ratio affected markedly the behavior of the samples.