A CONSTITUTIVE THERMOMECHANICAL MODEL FOR SATURATED CLAYS

The structural deformation in clays results from microscopic phenomena involving the mechanical contact-stress change, the physico-chemical variation of repulsive forces in expansive clays, and thermal dilatanc y of macropores. These textural strains are associated to three plasti c mechanisms represented by respectively the yield surfaces f(Tm), f(R -A) and f(T). Under a thermal cycle, the sizes of interlamellar spaces between clay platelets are not modified, hence the temperature cycle is expected to have no effect on repulsive forces and thus the second mechanism is not affected by temperature changes. This paper suggests a formulation of a model of thermo-elasto-plastic behaviour of non-exp ansive saturated clays characterised by two plastic mechanisms. The me chanical yield surface f(Tm) of the contact-stress mechanism is based on a modified cam-clay model; the thermal softening yield surface f(T) is a plane separating two thermal domains. In normally consolidated c onditions, the resulting response to an increase of temperature is com pressive. However, in highly overconsolidated conditions, a small irre versible dilative volumetric strain is observed when the temperature i s above a threshold value. In intermediate conditions, the material st arts with an expansion and tends to a compression. The constitutive mo del combines thermo-mechanical hardening, predominant in normally cons olidated states (NCS) and absent in overconsolidated states (OCS) wher e the thermal softening occurs. The characterisation of the model requ ires information about rheological parameters obtained from oedometric and triaxial paths. Lastly, some numerical simulations of thermo-mech anical tests on remoulded Boom, 'Bassin Parisien' and Pontida clays ar e presented, which show satisfactory agreement between experiments and model predictions.