The combined effect of clay and moisture content on very small strain stiffness of compacted sand-clay mixture

The very small strain shear modulus (stiffness) of soils, G_max, is one of the most important parameters for predicting ground movements and dynamic responses of geo-structures. In this study, the combined effect of clay fraction and moisture content on shear stiffness of an unsaturated sand-clay mixture at very small strains was investigated using bender elements. Compacted soil specimens were prepared at three different clay contents of 10, 20, and 30%, and at four different initial moisture contents of 3, 6, 9 and 12%. Bender element tests were carried out under isotropic and constant moisture content conditions and inside a modified triaxial testing system equipped with a pair of piezoelectric bender-extender elements. G_max was calculated based on the velocity measurement of shear waves propagated through the specimen. The tests results showed that G_max decreases approximately linearly with an increase in moisture content, and non-linearly with an increase in clay content. A basic empirical equation was derived from an examination of trends in evolution of G_max with clay and moisture content. Additional empirical correlations were also derived for estimation of moisture content and degree of saturation based on the compression wave velocity measurements.