Comparative modelling of laboratory experiments for the hydro-mechanical behaviour of a compacted bentonite–sand mixture

Abstract

A comparative modelling exercise involving several independent teams from the DECOVALEX-2015 project is presented in this paper. The exercise is based on various laboratory experiments that have been carried out in the framework of a French research program called SEALEX and conducted by the IRSN. The program focuses on the long-term performance of swelling clay-based sealing systems that provide an important contribution to the safety of underground nuclear waste disposal facilities. A number of materials are being considered in the sealing systems; the current work focuses on a 70/30 MX80 bentonite-sand mixture compacted at dry densities between 1.67 Mg/m 3 and 1.97 Mg/m 3. The improved understanding of the full set of hydro-mechanical processes affecting the behaviour of an in-situ sealing system requires both experiments ranging from small-scale laboratory tests to full-scale field emplacement studies, and coupled hydro-mechanical models that are able to explain the observations in the experiments. The approach was to build models of increasing complexity starting for the simplest laboratory experiments and building towards the full-scale in situ experiments. Following this approach, two sets of small-scale laboratory experiments have been performed and modelled. The first set of experiments involve characterising the hydro-mechanical behaviour of the bentonite-sand mixture by means of (i) water retention tests under both constant volume and free swell conditions, (ii) infiltration test under constant volume condition, and (iii) swelling and compression tests under suction control conditions. The second, more complex, experiment is a 1/10th scale mock-up of a larger scale in situ experiment. Modelling of the full-scale experiment is described in a companion paper. A number of independent teams have worked towards modelling these experiments using different conceptual models, codes, and input parameters. Their results are compared and discussed. This exercise has enabled an improved modelling of the bentonite-sand mixture behaviour, in particular accounting for the dependence of its retention curve on the dry density. Moreover, it has shown the importance of the technological voids on the short-term behaviour of the sealing system. .

A nonlinear elastic approach to modelling the hydro-mechanical behaviour of the SEALEX experiments on compacted MX-80 bentonite

“…The gradient of the stress development matches sensor AS_000, but does not predict the non-symmetry stress development between the two sample ends. However, the physical reason for the dissymmetry in axial stress is not known (Millard et al 2016). For example, it could be an experimental difference in stress conditions at each end of the sample, such as an unaccounted for void into which the bentonite was able to expand, rather than a physical process occurring within the sample during hydration.…”

“…However, the rapid hydration observed at the lowest sensor (red) is under predicted. This is a common problem throughout the modelling teams in the DECO-VALEX-2015 task (see summary paper this issue, Millard et al 2016) and could be due to the permeability model not capturing the full range of permeability change associated with the hydration or the use of a single unique water retention curve to describe the water content-suctionstress relationship rather than specifically addressing the different scales of porosity.…”

DECOVALEX-2015: an international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems

“…Two synthesis papers provide summary findings for comparative modeling of the laboratory and mock-up studies (Millard et al 2016)…”

A method for extending a general constitutive model to consider the electro‐chemo‐mechanical phenomena of mineral crystals in expansive soils

“…The constitutive modeling of expansive soils has mainly considered bentonites as buffer materials in waste disposal facilities. Based on various experimental results assuming the condition during the in‐service period of the facilities, an empirical model in which the swelling behavior is directly given by the relationship between suction and volumetric strain has been developed.…”