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.