On the THM behaviour of a sheared Boom clay sample: Application to the behaviour and sealing properties of the EDZ

In this study the effect of the temperature increase on the hydro-mechanical properties of sheared Boom clay samples is investigated. Two samples of Boom clay are resaturated in a new hollow cylinder triaxial cell with a short drainage path and then sheared by performing an axisymmetric triaxial loading. The effect of the undrained heating under deviatoric stress is studied on the first sample. It is shown that undrained temperature increase leads to an increase of pore water pressure and consequently to a decrease of the effective mean stress which brings the sample to failure. For an initially sheared sample, the failure occurs along the existing shear band which behaves as a weakness plane in the sample. The responses of the local strain measurement transducers clearly show the sliding of rigid blocks when failure occurs in the sample.The effect of the presence of a shear band on the permeability of the other Boom clay sample is investigated at ambient temperature and at 80 °C. It is shown that the presence of a shear band does not affect significantly the permeability. These results confirm the good self sealing properties of Boom clay at ambient and at high temperature.     

The Thermal Volume Changes of the Callovo–Oxfordian Claystone

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.     

Temperature and Damage Impact on the Permeability of Opalinus Clay

The effects of temperature on the water transport properties of intact and damaged Opalinus (OPA) clay are investigated by using a recently developed hollow cylinder triaxial cell [Monfared (Int J Rock Mech Min Sci 48:637–649, 2011b)] that allows full saturation and drainage conditions in low-permeability clays and shales. The volumetric response of saturated OPA clay sample during a drained heating test shows an irreversible contraction after a temperature threshold. The permeability tests which are performed before and after the heating test show that the induced irreversible sample contraction by thermal loading reduces the permeability of OPA clay sample. In order to study the effect of temperature on the permeability of a damaged sample of OPA clay, the permeability tests are performed on a saturated sample previously sheared by a standard drained triaxial loading. The test results show no significant effect of shear-type damage on the permeability of the sample at 25 °C and 80 °C. The experimental results presented in this paper show the crucial role of the thermally induced strains on sample permeability. Thermo-elastic dilation leads to a slight increase of the permeability, whereas thermoplastic contraction leads to a reduction.     

A Laboratory Investigation on Thermal Properties of the Opalinus Claystone

Some aspects of the thermal behavior of the Opalinus claystone are investigated through laboratory tests conducted on a new hollow cylinder triaxial apparatus specially designed for studying the thermo-hydro-mechanical behavior of very low permeable materials. Two hollow cylinder samples are first resaturated under isotropic stress state equal to the mean effective in situ one in order to minimize swelling and induced damage during the resaturation phase. Two drained heating–cooling cycles are performed on the first sample of Opalinus claystone. During the first cycle, a thermo-elasto-plastic response similar to that of plastic clays with low overconsolidation ratio is obtained. The thermal hardening of the sample is demonstrated by the quasi-reversible behavior of the sample during the second heating–cooling cycle. An undrained heating test performed on the second sample of Opalinus claystone induces an excess pore pressure in this sample. This induced pore pressure is attributed to the higher thermal expansion coefficient of pore water compared to that of the solid phase. It is shown that the excess pore pressure generated in the sample by undrained heating cannot be modeled by considering the free water thermal expansion coefficient. The thermal expansion coefficient of the Opalinus claystone water is back-analyzed from the experimental results which show a higher value than free water.