The experimental behaviour of expansive soils has been reviewed in order to isolate its significant characterisitcs. In order to derive onstitutive models, an elastoplastic framework for unsaturated soils is first seelcted as a basic reference. The capabilities of this basic model are considerably expanded when a double structure formulation is integrated into the formulation. The two soil structures considered describe the expansion-shrinkage of the clay aggregations and the interaction among them at a larger geometrical scale. A discusseion of the role of matricand osmotic suction leads to a specific proposal to include both matricand total suction, into the constitutive model developed. Mathematical description of the model is given for isotropic stress states. The capabilities of this extended model arechecked against tests involving the application of suction cycles on sampels of expansive-collapsible soil.
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