In order to understand the mechanisms that govern the development of lateral stresses in overconsolidated clay deposits, specimens of an undisturbed sensitive clay from Lachute (P.Q.) were tested in a triaxial chamber under K₀-conditions. The results of these tests show that the response of the clay can be divided into three distinct phases of deformation. At low stress levels, the clay behaves as an elastic material. At intermediate stress levels, the clay behaves as a plastic material. At high stress levels, the clay becomes normally consolidated.

The experimental data are used to establish a model describing the mobilization of lateral stresses, and it is shown that the in situ coefficient of earth pressure at rest cannot be determined by laboratory testing. The theory of anisotropic elasticity is used to predict the development of lateral stresses and pore water pressures at low stress levels. Predicted values are compared with those measured underneath the St. Alban (P.Q.) test fills.