Vapour transfer in unsaturated compacted bentonite

Results of an experimental and theoretical investigation of heat and moisture movement in unsaturated MX-80 bentonite are presented. A thermo-hydraulic cell that allows measurement of transient temperatures and facilitates the determination of pseudo-transients of moisture content, dry density and chemical composition has been used to perform thermal gradient tests. Results of a number of tests are presented, and observation of the accumulation of chloride ions near the hot end clearly indicates that there is a cycle of vapour and liquid moisture movement, with vapour moving from hotter to cooler regions, condensing, and then moving as liquid towards the hotter regions. An empirical method is applied to calculate approximate vapour fluxes using measured variations in chloride ion concentration and moisture content with time. The vapour fluxes calculated empirically are found to be lower than those determined by some existing vapour flow theories. Subsequently, an existing vapour flow model is modified to represent the observed vapour fluxes more closely

Les matériaux argileux gonflants des ouvrages de fermeture de Cigéo

National audienc

Laboratory studies of the swelling behaviour of a bentonite material considered for andra underground repository engineered barriers

International audienc

Bentonite THM behaviour at high temperatures: experimental and numerical analysis

The Temperature Buffer Test is a heated full-scale field experiment carried out at the Äspö Hard Rock Laboratory in Sweden, simulating repository conditions for radioactive waste. The initial thermo-hydro-mechanical (THM) evolution in the clay barrier was investigated in a separate mock-up test. The paper describes this laboratory experiment and the corresponding numerical simulations. Most of the related work refers to THM analyses of bentonite barriers well below 100°C, but here higher temperatures are considered. A 20 cm closed specimen of compacted MX-80 bentonite was subjected to a temperature gradient (84°C and 120°C at the end points). The evolution of temperature, relative humidity, pore pressure and stresses was monitored at several points. The test was allowed to reach steady-state conditions. The specimen was then sampled and analysed in terms of water content and bulk density. Several finite element analyses considering different coupled THM interactions were performed, and compared with measurements. Bentonite properties were obtained from independent tests. Additionally, retention properties were also obtained from measured saturation ratios and steady-state suction values. For the mechanical problem the Barcelona Expansive Model was used, which includes explicitly the two structural levels that actually exist in expansive clays (macro- and microstructure). This model made it possible to simulate the evolution of stresses as well as the expansion of bentonite at the 'cold' side and the compression at the 'hot' side, using a single set of parameters