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, G0, is one of the most important parameters for predicting ground movements and dynamic responses of geo-structures. Piezoelectric elements are the most common transducers used for laboratory measurement of wave velocity and determination of G0 in soil and rock specimens. Many factors including stress history, current stress state, soil suction, and soil’s physical properties can influence the measured wave velocity and subsequently determination of G0 values. 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 a modified triaxial testing system equipped with a pair of piezoelectric bender-extender elements accommodated in the top cap and pedestal. Compacted soil specimens of 50 mm diameter and 100 mm height 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. Sinusoidal shear (S) and compression (P) pulses were triggered at frequencies between 1 and 10 kHz and wave velocities were calculated based on identification of wave arrival time in time domain. The tests results revealed non-linear variation of G0 with clay fraction and moisture content. Basic empirical laws were derived from an examination of trends in evolution of G0 with clay and moisture content.