This thesis examines the constitutive behaviour of high plastic clay. Three Experimental testing programs have been conducted to evaluate specific behavioural features needed to define an elastic-plastic constitutive model for unsaturated soil behaviour.

The first program evaluates the influence of suction on volume change. A series of specimens were subjected to constant suction environments under zero external pressure and the resulting volume changes were measured. The results have been interpreted and a simple model for determining the suction as a function of measured water contents is given along with the volume strains associated with changes in suction.

The second experimental program examines the behaviour of high plastic clay in quick undrained triaxial tests at varying suction levels. Specimens were a1l compacted to the same initial material conditions and then subsequently subjected to increases in suction. The results show the impact of suction and confining pressure on the undrained behaviour of high plastic clay. A simple model that can be used for predicting undrained shearing behaviour is presented.

The third and most important experimental series examines the behaviour of high plastic clay subjected to generalized (in the triaxial stress state) stress conditions including shear, isotropic and suction loading. A new apparatus and testing method developed in this program is presented and discussed along with results of tests conducted using the equipment.

The results of the three experimental programs are interpreted in an elastic-plastic framework in the final part of the document. Two specific models and their features are examined. A simple mathematical model is presented along with measured parameters taken from the experimental data set.