Investigating the time-dependent behaviour of Boom clay under thermomechanical loading

The present work aims at investigating the time-dependent behaviour of Boom clay, a potential host formation for the geological disposal of radioactive waste in Belgium, under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were carried out for this purpose. The results obtained confirmed the effect of the overconsolidation ratio on the thermal volume changes (thermal dilation under high OCRs and thermal contraction with OCR close to unity). Significant effects of temperature as well as of compression and heating rates were also observed on the volume change behaviour. After being loaded to a stress lower than the preconsolidation pressure (5MPa) at a low temperature of 25.8C and at a rate lower than 0.2kPa/min, the sample volume changes seemed to be quite small, suggesting a full dissipation of pore water pressure. By contrast, after being subjected to high loading and heating rates (including step loading or step heating), the volume changes appeared to be significant, particularly in the case of stresses much higher than the preconsolidation pressure. Because of its low permeability, full consolidation of Boom clay required a long period, and it was difficult to distinguish consolidation and creep from the total volume change with time.