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.
Highlights
The surficial part of the earth is mostly partially saturated except where the water table is at shallow depths
The data that was recorded by the Data Acquisition System (DAQ) was in terms of electrical voltages
The data were corrected to account for the initial stresses and strains that were in the system prior to cyclic horizontal loading
Summary
The surficial part of the earth is mostly partially saturated except where the water table is at shallow depths. The Dynamic Simple Shear (DSS) apparatus at the University of New Hampshire (UNH) has been recently modified and upgraded for unsaturated soil testing and more accurate data acquisition. The reduction of shear modulus by increasing the shear strain is typically presented using normalized shear modulus reduction functions. This modulus reduction function has been improved over the past decade starting from a simple hyperbolic form. Measuring the shear modulus and damping in medium to large strain requires accurate testing and measurement approaches that may differ between the testing apparatus such as cyclic triaxial, cyclic torsional shear, and dynamic simple shear tests. Both shear modulus and damping mainly depend on the effective stress in the soil. Increasing the suction as a result of soil desaturation increases the effective stress; according to Bishop’s effective stress formula presented in Equation 2
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