The paper focuses on the delayed behaviour of marls induced by a change in stress state and by the effect of water. Seven experiments were conducted in oedometric cells on marl from the Arbus tunnel (France). The samples were subjected to a single loading step (i.e one-stage creep test) or to successive loading steps (i.e. multi-stage creep test) followed, after hydration, by several unloading stages. For the creep behaviour, Lemaitre's law is found adequate to model the viscoplastic strain, but its parameters are different for one-stage and multi-stage creep tests. As for the swelling related to the unloading stages after samples hydration, an empirical hyperbolic law provides very good results. A global model that partitions the deformation during the creep and swelling stages allows to well reproduce the measured axial deformation.
Studies on the delayed behaviour of rocks are performed since the end of the 19th century and they keep developing nowadays. This is because of the damages and the considerable disorders caused by the effect of time on civil engineering works and underground constructions. The delayed mechanical behaviour of geomaterials distinguishes itself from the classic behaviour (elasticity and elasto-plasticity) by taking into account time in an explicitway (Boidy, 2002).This behaviour concerns phenomena such as creep, swelling, consolidation, relaxation, alteration, ageing and healing. This paper considers creep and swelling. Creep is defined as the time-dependent deformation of a material subjected to a constant stress. It is conditioned by the testing conditions (mechanic, hydric, thermal) and the nature of the material (mineralogy, texture, porosity, degree of saturation). Creep tests aim at studying the time dependence on the rock behaviour. They generally consist in imposing a constant deviatoric stress on samples under controlled hydric and temperature conditions and in recording their deformation with time.