Hydro-mechanical properties of a water unsaturated sodium bentonite. Laboratory study and theoretical interpretation

Sammanfattning: Highly compacted bentonite has been proposed as the buffer material in the Swedish concept for the disposal of nuclear waste. To increase the knowledge of the hydro-mechanical behaviour of unsaturated swelling bentonite a series of laboratory experiments was carried out. The main purpose was to conceptually model the hydro-mechanical behaviour of the bentonite. The influence of testing techniques was also studied so that the laboratory tests could be properly evaluated. The investigated bentonite is a commercial bentonite, MX-80, which is available from American Colloid Co. The focus of the laboratory program was to investigate the influence of confinement on the water retention properties, the development of swelling pressure during a decrease in suction and the effect of external pressure and swelling pressure on suction. The majority of the test types involved a stepwise increase in relative humidity under free swelling and under constant volume conditions with measurement of swelling pressure. In addition, tests with constant water content and externally applied pressure were performed. In those tests relative humidity was measured. The measurement of relative humidity through narrow channels was investigated and the influence of testing technique was investigated with regard to gradients in temperature and air pressure over the equipment. The results show that swelling pressure develops during water uptake from water in vapour phase when samples are tested under constant volume conditions. The results also show that if a pressure is applied to a sample with a constant water content the result will be an increase in the measured relative humidity and vice versa. Retention curves for absorption and desorption were determined under free swelling conditions at different relative humidity. The appearance of the retention curves was dependent on the initial water content. The results of the laboratory tests were used to find a relation between water content, void ratio, swelling pressure and suction. In the analysis of the test results the interpretation was made under the assumption that equilibrium had been reached. The measured axial swelling pressure that developed during water uptake under constant volume condition was normalised to a pressure corresponding to swelling pressure at saturation to be independent of void ratio. A relation between the normalised swelling pressure and the degree of saturation is suggested. A second relation based on a thermodynamic relationship for saturated conditions was compared to the laboratory results. In the second relation the swelling pressure is related to the actual relative humidity in the clay and the relative humidity according to the retention curve for the current water content. The correspondence between the second relation and the measured test results was found to be good. The analyses are mainly based on results from the tests conducted with an increasing degree of saturation under constant void ratio. Based on the mentioned relations a model is proposed which can be used e.g. to evaluate field measurements and model the late stage of the wetting process.