Capillary Effects on Dynamic Modulus of Sands and Silts

Resonant column tests of five fine‐grained cohesionless soils showed the influence of capillary stresses on values of the low‐amplitude shear modulus, $G_0\text{.}$ This influence was found to be greatest for the soils having the smallest effective grain diameter, $D_{10}\text{,}$ and for the lowest external confining pressure, ${\sigma }_0^{\prime }\text{.}$ The maximum capillary influence developed at degrees of saturation between 5% and 20% for the soils tested. For the soil with the smallest $D_{10}$ $(=0.0024\mathrm{mm})$ the maximum $G_0$ was about two times $G_0\left(\mathrm{dry}\right)$ for ${\sigma }_0^{\prime }=14.2$ psi (98 kN/m). This latter pressure would correspond roughly to 20 ft (6.1 m) of overburden of incompletely saturated soils. Therefore, it would be expected that capillarity would affect in situ shear wave velocity measurements required for the dynamic design of shallow footings, runways, or other structures supported by soil near the ground surface. An empirical expression was developed which permits corrections of the value of $G_0\text{,}$ measured at a particular degree of saturation, to the degree of saturation expected in prototype conditions.