Characterization of Sand–Gravel Mixtures Using Shear Wave Velocity Method and Intergranular State Concept

Shear wave velocity (V_S) measurements are widely used to characterize geomaterials, evaluate small-strain stiffness, and develop indirect approaches for estimating the liquefaction resistance of various soil types. In this study, the bender element method was employed to investigate the V_S characteristics of sand–gravel mixtures (SGMs), with the aim of clarifying the combined effect of key factors such as gravel content (G_C), relative density (D_r), packing state, and soil fabric. Laboratory tests were performed on reconstituted specimens composed of two sandy soils and pea gravel with G_C of 0, 10, 25, 40, 60, 80 and 100% and D_r of 20, 30, 45 and 60%. Specimens were prepared using wet tamping (WT) and air pluviation (AP) techniques. V_S measurements were conducted under effective confining stresses (${{\sigma }^{\prime }}_0$) of 50, 100, 150 and 200 kPa. The results show that the V_S of SGMs increases with increasing D_r and ${p^{\prime }}_0$, whereas the influence of G_C depends on the limiting and threshold sand contents. The effect of soil fabric was found to be marginal. Furthermore, the combined effects of G_C and D_r on V_S can be uniquely captured using the equivalent void ratio approach for SGMs with sand-dominated microstructures, while the skeleton void ratio approach is more appropriate for SGMs with gravel-dominated microstructures.