Investigating the time-dependent behaviour of Boom clay under thermomechanical loading

Abstract

Boom clay, a stiff clay, has been selected as a potential host formation for the geological disposal of radioactive waste in Belgium. The underground research facility HADES has been constructed to enable various in situ experiments to be performed on Boom clay so as to study the feasibility of high-level radioactive waste disposal, and to provide reliable data on the performance of Boom clay as a host formation. Among the various laboratory studies performed on samples extracted from the HADES facility to investigate the thermo-hydro-mechanical behaviour of Boom clay, relatively few were devoted to the time-dependent behaviour, limiting any relevant analysis of the long-term behaviour of the disposal facility. The present work aims at investigating the time-dependent behaviour of Boom clay under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were carried out for this purpose. The tests started with constant-rate thermal and/or mechanical consolidation and ended with isobar heating and/or isothermal compression at a constant stress rate or by step loading. The results obtained confirmed the effect of the overconsolidation ratio (OCR) on the thermal volume changes (i.e. thermal dilation under high OCRs and thermal contraction with OCR close to unity). Significant effects of temperature as well as of compression and heating rates were also observed on the volume change behaviour. After being loaded to a stress lower than the preconsolidation pressure (5 MPa) at a low temperature of 258C and at a rate lower than 0 . 2 kPa/min, the sample volume changes seemed to be quite small, suggesting a full dissipation of pore water pressure. By contrast, after being subjected to high loading and heating rates (including step loading or step heating), the volume changes appeared to be significant, particularly in the case of stresses much higher than the preconsolidation pressure. Because of its low permeability, full consolidation of Boom clay required a long period of time, and it was difficult to distinguish consolidation and creep from the total volume change with time.

Anisotropy in Oedometer Test on Natural Boom Clay

“…This may seem to be in contradiction with the expected anisotropic behaviour of Boom Clay. This may be due to the difficulty in graphically determining Cc and Cs, the perturbation of the clay around the URL (cores are taken at different distances of the gallery), the perturbation of the samples (of which the age is different) due to preparation and the difficulty of separating primary consolidation from secondary consolidation (see Cui et al 2009). More tests with better control such as oedometer test on less perturbed samples (fresher, taken at greater distance from the gallery under a more realistic range of loading), and triaxial test are needed to clarify this point.…”

Editorial: Thermal Behaviour of the Ground

“…In this issue there are nine papers covering characterisation and full-scale testing, many of which focus on storage of high-level nuclear waste. Muñoz et al (2009Muñoz et al ( ), Å kesson et al (2009 and Cui et al (2009) investigated the characteristics of host rocks for the storage of high-level nuclear waste and the bentonite barriers between the waste and host rock. Samples of rock (Muñoz et al, 2009) were subjected to heating and cooling cycles with pore pressure and temperature responses being measured within the specimen as the test proceeded for more than a year.…”

“…They found good agreement between their model simulation using the CODE_BRIGHT formulation and the experimental results. Cui et al (2009) undertook a series of tests on Boom Clay, a stiff heavily overconsolidated clay that is the host rock for a deep repository for high-level nuclear waste in Belgium. They investigated the effect of thermal consolidation/expansion created by the changes in pore pressure as a result of expansion of water under high temperatures.…”

A Transversely Isotropic Damage Model for Boom Clay

“…An important set of data about the geomechanical behavior of this argillaceous rock has been collected. Besides its thermohydro-mechanical coupling behavior (Bastiaens and Demarche 2003;Li et al 2006;Chen et al 2011a;Lima 2011) and the time dependent (Bruyn et al 1987;Giraud and Rousset 1996;Cui et al 2009;Yu 2010) behavior, lots of data (Wemaere et al 1997;and Bastiaens and Demarche 2003;Bastiaens et al 2007) indicates that Boom clay presents anisotropic mechanical properties. Both laboratory and in situ test results indicate that the stiffness in the horizontal direction (parallel to bedding plane) is higher than that in the vertical direction (Piriyakul and Haegeman 2009;Lima 2011;Chen et al 2011a).…”