Swelling Behaviour and Permeability of Compacted Bentonites at High Salinity as Infl uenced by Fluid Pressure Changes

This study presents results of two series of permeameter experiments with 15 bentonites compacted to a bulk density of 1.6 g/cm3 before and after one-year contact with a model clay porewater with a salinity of 155 g/l. For four original bentonites, a double-peak pattern of swelling pressure evolution was revealed, which was observed previously only at much lower salinities. For 80 % of bentonites, swelling pressure and permeability were in the range of 0.8—4.8 MPa and 7·10−20—3·10−18 m2 (for original ones) as well as 0.5—2.2 MPa and 4·10−19—6·10−18 m² (for ones after one-year contact). A fl uid pressure surge of 12.6 MPa caused no hydraulic fracturing of original bentonites. This observation challenges the validity of threshold values of a few MPa for onset of hydraulic fracturing in compacted bentonites proposed earlier using an alternative experimental method. Yet, this fl uid pressure surge and a moderate one of 0.3 MPa caused a decrease of swelling pressure by up to 66 % — an effect, which need to be accounted for when designing and assessing the performance of bentonite-based barriers in a geological repository.