Oedometric compression and swelling behaviour of the Callovo-Oxfordian argillite

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 stress. 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.