[THE INFLUENCE OF THE SUBGRADE ON THE STRUCTURAL PERFORMANCE OF THE ROAD: A STUDY OF ROADS ON SOFT CLAY]

The aim of this thesis is to increase the understanding of the interaction between the unbound material and the clay subgrade, i.e. in what way the plastic and elastic properties of the subgrade influence the structural condition of the pavement. Furthermore the objective has been, if possible, to form the base for a methodology for pavement performance taking the subgrade into account. The study comprised of a) field tests to study the importance of the elastic properties, of the subgrade and for defining the course of damage; and b) dynamic triaxial test to study the deformation properties and pore pressure build up for three different types of clay as well as whether or not the clay may fail during a simulated traffic load. The field tests results show that the rut depth can be assumed to decrease when the shear modulus in the subgrade increases. Depending on testing method, the shear modulus of the soft clay subgrade may vary as much as 25 MPa or 400 per cent. If the shear strain level at the subgrade level is of the magnitude of 0.1 per cent to 0.2 per cent, as in the triaxial tests, then the shear modulus becomed more or less the same, independent of the clay. (A)