Bentonite swelling pressure response to temperature change

Abstract

In the proposed Swiss repository location at Nördlich Lägern, in an Opalinus Clay formation, at the depth of about 850–900 m, the ambient temperature due to geothermal gradient is expected to be about 45–50 °C.

In this study, constant-volume swelling pressure experiments were performed to assess the effect of such elevated temperatures (25 - 60 °C) on granular Wyoming bentonite performance around designed buffer dry density of 1450 kg/m³. Bentonite was hydrated with a saline solution closely simulating reference Opalinus clay groundwater, and the bentonite dry density was varied between 1300 kg/m³ and 1600 kg/m³ to evaluate density dependency. Tests included isothermal, as well as increasing (incremental) and decreasing (decremental) temperature programs following different thermo-hydraulic paths: full saturation-first and complete heating-first. The dependency of swelling pressure on both bentonite density and temperature was analyzed by pressure-temperature slope (kPa/°C) and obtained results were compared to previous similar tests with Wyoming bentonite at different bentonite densities.

Overall conclusion is that granular Wyoming-type bentonite swelling pressure response to elevated temperature at designed dry density of 1450 kg/m³ is small (−6.5 ± 2.0 kPa/°C), and robust against changes in the thermo-hydraulic path or in the density.