Oedometric compression and swelling behaviour of the Callovo-Oxfordian argillite

Abstract

The Callovo-Oxfordian (COx) argillite is a possible host rock for radioactive waste disposal in which the ANDRA underground laboratory of Bure (East of France) has been excavated. In this paper some aspects of the volume change behaviour of the COx argillite are investigated. To do so, high pressure oedometers with a maximum capacity of 113 MPa have been used. In a first stage, swelling tests were carried out on samples initially compressed at constant initial water content (unsaturated) that were afterwards soaked under vertical loads, respectively, smaller and higher than the in-situ vertical stress. All samples exhibited swelling, even at stress higher than the in-situ one. In a second stage, standard step loading compression tests were carried out on samples previously saturated under the in-situ vertical load, so as to investigate the volume change behaviour under load cycles. The strain-stress curves obtained appear to be different to what is currently observed in overconsolidated or cemented clays with no clear appearance of yield and pre-yield reversible behaviour. The volumetric behaviour during both compaction and swelling is interpreted in terms of damage created by the collapse of pores within a fragile matrix. The amount of swelling observed is related to the extent of damage.

A fully coupled damage-plasticity model for unsaturated geomaterials accounting for the ductile–brittle transition in drying clayey soils

“…Damaged samples are more compliant than undamaged samples: the resistance of the solid skeleton to pore filling and expansion is less in damaged materials, which tend to swell more than undamaged samples during wetting. This behaviour has been observed in oedometric swelling experiments on callovo-oxfordian argillite samples (Mohajerani et al, 2011), where it is seen that swelling capacity increases with damage.…”

The thermo-mechanical behaviour of the Callovo-Oxfordian claystone

“…33 Particular care was put in following this swelling phase to make sure that the specimen reached equilibrium in water content. It seemed that this stress path was preferable to get a relevant response, compared to the standard stress path in which the specimen would have been directly submitted to the desired stress state prior to perform the re-saturation procedure.…”

Stress release and suction generation in the Callovo-Oxfordian claystone

“…Following Mohajerani et al (10), the saturation of the sample was conducted on a high stress double lever arm oedometer (Marcial et al,11) by carefully limiting volume changes during water infiltration. To do so, the vertical displacement gauge was manually followed and the specimen was progressively loaded to compensate any swelling with a limit fixed at 2 µm.…”