This dissertation deals with the prediction of volume changes in unsaturated soils. First, the theoretical density and compressibility relationships of air-water mixtures are developed. Throughout the testing program, experimental evidence was obtained which confirms the compressibility equation.

Four pieces of apparatus were developed to test unsaturated soils. Two employed modified Anteus oedometers for one-dimensional conditions. The other two allowed isotropic volume change testing conditions in modified triaxial cells. The diffusion of air through the water in the high air entry disc was an impertant design consideration. A new diffused air volume indicator was built to measure and correct the water volume change for the diffused air volume change.

The unsaturated soil is considered as a four phase system;​ two solid phases that come to equilibrium under the application of stresses (i.e., soil particles and contractile skin) and two fluid phases that flow under the application of stresses (i.e., air and water).

Each phase is considered as a continuum with an independent force equilibrim equation. Two independent stress state tensors are extracted from the equilibrium equations. The stress state variables are verified experimentally by means of null tests, for soils with varying degrees of saturation. They are verified to within 1 part of volume change per 10,000 parts of volume.

Independent displacement fields are written for each phase of an unsaturated soil. Unifying the deformations is the continuity requirement that states that the sum of the volumetric deformations of each phase is equal to the volumetric deformation of the overall element.

A constitutive relationship for the soil structure and the contractile skin is developed. These two equations link the stress and deformation state variables and allow the evaluation of all volumeweight soil properties. The uniqueness of the proposed constitutive relationship was proven for small deviations from a stress point. The experimental results show a correlation coefficient for the predicted versus measured volume changes which is in excess of the critical correlation coefficient for a 1 percent level of significance.

Hysteresis destroys the uniqueness of the constitutive surfaces and is the result of a reversal in the direction of deformation of either the soil structure or the contractile skin.

A one-dimensional volume change analysis for an unsaturated soil is presented. All volume-weight soil property changes, in response to stress changes, can be predicted provided the constitutive equation soil parameters are known.

A one-dimensional theory of consolidation is derived on the basis of the continuity requirement. It consists of two partial differential equations that must be soived simultaneously. The initial pore air and pore water pressures, as a result of a change in total pressure, are also predictable by solving two simultaneous equations. No attempt has been made to solve or experimentally verify these equations.