Swelling behaviour of compacted Ca/Mg bentonite during hydration on different scales

A mock-up test on decimetric scale in an innovative column test device and small-scale element tests were carried out on a German Ca/Mg bentonite, which is investigated as a suitable material for engineered barrier systems in nuclear waste disposal. Key hydro-mechanical parameters such as relative humidity, water content and swelling pressure were monitored and spatially resolved over the 30 cm height of the column in the mock-up test. The test was hydrated from the bottom and ran for 370 days. To compare the material behaviour at different scales, swelling pressure tests and water retention measurements were performed at the element-test scale. The results of the column test showed a heterogeneous evolution of swelling pressure and hydration state variables along the column height. Simultaneous measurement of vertical and radial swelling pressure indicated swelling anisotropy. Transient measurement of relative humidity and water content revealed a redistribution of porosity within the sample, which was confirmed by sampling after completion of the test. Comparison with the results of the element test showed that the two test scales are comparable in similar hydraulic states. The column test is therefore suitable as a precursor to the field test and provides valuable insights into the spatially and temporally resolved hydration behaviour of a buffer material, which helps for the design of in-situ experiments, the validation of numerical models and supports the overall process understanding. • Hydro-mechanical behaviour of a Ca/Mg bentonite in laboratory scale mock-up test and small-scale element tests. • Time domain reflectometry measurement in mock-up test to evaluate water content upon swelling. • Heterogeneous hydration, porosity redistribution and anisotropic stress field were observed in mock-up test.