Comparing the trend of V_sat similar range of saturated and unsaturated stress state variables

The effect of matric suction on S-wave velocity (Vs) of unsaturated soils has been observed to show different trends from the Vs relationship with effective confining pressure. In some soils, the increase in S-wave velocity per unit stress change in matric suction (ua-uw) is greater than the increase in S-wave velocity per unit stress change in effective confining pressure (σ3) for saturated and dry soils and vice versa for other soils. In this study, the difference was examined through the stress state variables for unsaturated soils on similar soil samples. Experiments were conducted on sand and kaolin specimens using a triaxial cell modified to include bender elements and a high-air-entry disk for the testing of unsaturated soils. The soil specimens were first fully saturated and Vs at various effective confining pressures were measured. The tests were repeated with similar soil specimens subjected to net confining pressure and matric suction for similar stress range as the previous tests. Data was also collated from the literature. Comparisons were made by plotting on two-dimensional and three-dimensional plots to understand the differences. It was found that soil type and soil fabric play important role in the differences. Soil fabric plays a more important role for compacted soils where soil fabric is affected by the compaction water content.