Methodology to Characterize the Seismic Compression of Unsaturated Sands under Different Drainage Conditions

Abstract

This study focuses on a new experimental setup and methodology to characterize the volumetric contraction of unsaturated sands during cyclic shearing under different drainage conditions. A new cyclic simple shear device was developed with suction-saturation control using a hanging column suitable for testing unsaturated granular soils with a maximum suction of 11\xa0kPa. The pore water and pore air pressures are monitored during cyclic shearing using an embedded tensiometer and a gauge pressure transducer protected by a hydrophobic filter, respectively. In addition to describing the specimen preparation techniques and testing methodology, typical results for the seismic compression of nearly saturated, dry, and unsaturated sand specimens during cyclic shearing under different drainage conditions are compared. The differences in the response of these sand specimens were interpreted using the changes in mean effective stress and secant shear modulus during cyclic shearing. In drained conditions, the unsaturated sand specimen showed lower seismic compressions than the dry sand specimen and the nearly saturated sand specimen. In undrained conditions, a greater contraction was observed for the unsaturated conditions due to decreases in mean effective stress and secant shear modulus. The decrease in effective stress occurred due to a decrease in matric suction associated with different rates of increase in pore water and air pressure and an increase in degree of saturation with volumetric contraction.