The dynamic properties of a clean sand under different degrees of saturation were investigated using a modified custom built Direct Simple Shear (DSS) apparatus at the University of New Hampshire. The specific characteristics of the DSS were presented, and the testing procedures were discussed. The device used the axis translation and tensiometric techniques to control the matric suction in the soil specimen. The investigation on F75 Ottawa Sand showed a decrease in shear modulus and an increase in damping by increasing the shear strain over the tested range of strains for various degrees of saturation: dry, saturated, and partially saturated. The modulus reduction in the applied range of medium shear strains regardless of the degree of saturation demonstrated the capability of the DSS in consistently capturing the changes of dynamic properties. Experimental results indicated that the matric suction can have a substantial effect on the stiffness of the soil. However, the extent of this effect may depend on the induced strain level of the effective stress in unsaturated soil. In addition, partially saturated specimens resulted in lower dynamic compression.

Abstract. State variables such as confining pressures and the degree of saturation can have a profound effect on the dynamic properties of a soil. These dynamic properties are essential when performing seismic response analysis of geotechnical systems in the phreatic zone. In order to accurately obtain these properties, laboratory testing using a Dynamic Simple Shear system is often implemented. This paper concentrates on the advancements and modifications of a custom-built dynamic simple shear system at the University of New Hampshire to accommodate soils with unsaturated conditions by employing axis translation technique. Tests could be performed under drained (constant suction) or undrained (constant water content) conditions. The procedure for preparing an unsaturated soil sample and testing is discussed, followed by the methods for interpreting the data and the challenges involved. Preliminary data confirms the ability of the system to control and track suction during the cyclic simple shear test. Suction in unsaturated soil increased the shear modulus and decreased the damping ratio comparing with those in dry and saturated conditions.