Hydro-mechanical analysis of GMZ bentonite–sand mixtures in the water infiltration process as the buffer/backfill mixture in an engineered nuclear barrier

A series of unsaturated tests on the GMZ bentonite–sand mixture at different dry densities and sand ratios are analyzed. This mixture is proposed as buffer/backfilling material in the Chinese concept for radioactive waste disposal. The water retention properties were studied under both confined and unconfined conditions; the results show that the water retention capacity is independent of the confined conditions and dry density if the suction is greater that about 4–5 MPa, and by contrast, at low suctions (< 4–5 MPa) the confined conditions and dry density influence the water retention capacity. However, the water retention capacity is always influenced by the presence of sand. The unsaturated hydraulic conductivity was determined in the infiltration test by the special apparatus designed by the authors. The value of unsaturated hydraulic conductivity did not change significantly with the dry density changing from 1.50 to 1.90 Mg/m3 or sand ratio increasing from 30% to 50%, always lower than 2.0 × 10− 12 m/s. Furthermore, the swelling pressure at different position of specimen was monitored during the infiltration process, and the dry density distributions have also been determined at the end of the tests. Therefore, this method can be used for engineering purposes to predict the unsaturated hydraulic parameters at an early stage of a design to get an idea of the required design and hence, cost.

Unsaturated hydraulic conductivity: (a) affected by dry density; (b) affected by sand ratio. The estimated unsaturated hydraulic conductivity of GMZ bentonite–sand mixtures does not change significantly with dry density changing from 1.50 to 1.90 Mg/m3 or sand ratio increasing from 30% to 50%. The data show that the unsaturated hydraulic conductivity value decreases when the dry density increases or the sand ratio decreases. The permeability of bentonite–sand mixtures in this research is below 2.0 × 10− 12 m/s. Komine (2010) stated that this value is considered “safe” barrier by reducing the flowing of water, chemical species and radionuclides.Figure optionsDownload as PowerPoint slide