Elastic stiffness of unsaturated soil

Most soil research has dealt with dry and saturated soils, even though many soils are unsaturated. Terzaghi showed that the behavior of saturated and dry soils is dependent on one stress state variable called effective stress. The behavior of unsaturated soils has been shown to depend on two stress state variables called net normal stress and matric suction. In this research, the effect of the two stress state variables on elastic stiffness was studied. The primary goal of this research was to test the uniqueness of the relationship between elastic shear modulus and the two stress state variables. This was accomplished by independently controlling net normal stress and matric suction while measuring elastic stiffness in a fixed-free resonant column device. The uniqueness was studied by testing two samples subjected to different stress paths. Both stress paths had common starting points and common ending points, and each stress path was comprised of seven different stress combinations. The results indicate that the relationship between elastic shear modulus and state of stress is essentially unique for the unsaturated soil tested.; Another objective was to measure the soil water characteristic curve for a plastic soil and for a non-plastic soil using several different techniques. Four techniques were used: pressure plate, Tempe pressure cell, Büchner funnel, and filter paper. In the ranges where test results from different test methods overlapped, the measured values agreed within the expected scatter for such measurements. Three different models formulated to approximate the soil water characteristic curve were applied to the non-plastic soil data to determine the best fit for each model. The model by Fredlund and Xing gave the best representation of the test data. A different type of model formulated to use classification test results to predict parameters for the Fredlund and Xing model was applied to the test data for the plastic and non-plastic soils. This model appears to be very useful in conjunction with the Fredlund and Xing model in predicting the soil water characteristic curve for both plastic and non-plastic soils based on classification tests.