Shear strength of GMZ07 bentonite and its mixture with sand saturated with saline solution

An experimental study involving direct shear, compression and swelling tests is carried out to investigate the effects of salt solution concentration on mechanical behaviour and microstructural features of a sodium bentonite (GMZ07 bentonite) and its mixture with sand. GMZ07 bentonite is a potential buffer/backfill material for deeply buried geological disposal system to isolate high-level radioactive waste (HLW) in China. Compacted unsaturated GMZ07 bentonite and its mixture with sand saturated with distilled water and sodium chloride solutions at different concentrations (0.2, 0.5 and 1.0 mol/L) were first consolidated to a specified stress and then sheared at a constant displacement rate. Test results indicate that shear strength or shear resistance has a noticeable improvement with the increase of salt solution concentration. The test results of GMZ07 bentonite reveal that friction angle increases fairly with the salt solution concentration but the cohesion intercept doesn't. Results of compression tests performed with different salt solution concentrations showed almost the same compressibility for GMZ07 bentonite specimen and its mixture with sand. At last, the microstructure changes associated with variations in the salt solution concentration have been photographed by the Scanning Electron Microscopy (SEM). The photographs show that the increase of salt solution concentration would change the surface roughness and specific surface area of aggregates, which will produce greater interaction among aggregates.