Swelling Pressures and Wetting-Drying Curves of a Highly Compacted Bentonite-Sand Mixture

This paper presents preliminary results of research on hydro-mechanical characterisation of a highly compacted bentonite-sand mixture for its potential use as buffer material in nuclear waste disposal system. Development of swelling pressure with change in suction is presented. Wetting curve of the material was derived from the test as suction was reduced from its initial value using vapour equilibrium technique as well as via axis-translation technique. For comparison, three-dimensional free swell tests were also performed by inducing different suction paths into several specimens such that wetting and drying curves could be obtained. No significant swelling pressure development was observed as the specimens were wetted from as-prepared and oven-dried states to about 10000 kPa suction. Comparison with the three-dimensional free swell test results leads to a conclusion that from dry state to about 10000 kPa suction, the maximum swelling pressure upon saturation is suction independent. It is also shown that swelling pressure curve, wetting-drying curves and void ratio-suction relationship generally exhibit hysteresis. Moreover, the results indicate that even at high suction, void ratio change plays an important role in determining the shape of the wetting curves.