On the high stress compression of bentonites

High stress consolidation tests (up to 30 MPa) were carried out on slurries made up of three bentonites considered as possible constituents for engineered clay barriers used for nuclear waste disposal at great depth (Na Kunigel, Na-Ca MX80 Wyoming clay, and Ca Fourges clay). Water retention curves (WRCs) were determined for a wide range of suctions (up to 305 MPa). A bilinear shape for the compression curves and the WRCs was observed in the three clays, with significant changes occurring in the 0.4-1 MPa suction-stress range. The permeability and the coefficient of consolidation were also determined during the compression tests. In the WRCs and compression curves, the two clays containing Na+ exhibited a similar behaviour, which was different from that of the Ca2+ clay. The coefficient of consolidation for the three clays was decreasing in the 0.4-1 MPa stress range and increasing at higher stresses. Data were interpreted in the light of existing experimental observations on the microstructural changes of similar swelling clays carried out by various authors, which obtained compatible and coherent results. The diffuse double theory, often used in the interpretation of the volume change behaviour of swelling clays, was also considered. Under higher stresses (>2 MPa), the compressive behaviour appeared to be linked to the expulsion water molecules that form the hydration shells around exchangeable cations inside the domains formed of staked clay plates. The permeability properties of the Ca2+ and Na+ clays were linked to the microstructure features and to the WRCs, whereas the changes in the coefficients of consolidation during compression were linked to the changes in soil stiffness.