The Thermal Volume Changes of the Callovo–Oxfordian Claystone

Abstract

There are very few data on the thermoplastic behaviour of claystones, while their knowledge is necessary in understanding long-term behaviour of high-level radioactive waste confining systems. In this paper, a number of thermomechanical parameters of the Callovo–Oxfordian claystone, the possible host rock of radioactive waste in France, are presented. These parameters were obtained from experiments performed on a hollow cylinder cell. The short drainage path of this device allows a good saturation in a reasonably short time; also, the good drainage conditions and reasonable homogeneity of pore pressure during the tests in such a low-permeability material. The saturation procedure was performed under in-site stresses prior to testing to reduce the swelling effect during hydration. The thermomechanical experimental programme conducted here evidenced a plastic contraction of the claystone during drained heating under in-situ stress conditions, like in normally consolidated soils. Previous loading induced a dilating–contracting response comparable to that of overconsolidated clays. Finally, whereas the elastic response appeared to be temperature independent, the plastic compressibility investigated through a drained isothermal isotropic compression test at 80 °C increased compared to that at 25 °C. These first results should be confirmed by further thermomechanical investigation on claystones.