Role of different suction components on swelling behavior of compacted bentonites

The paper reports results of an experimental study focused on the investigation of the development of swelling pressure in compacted bentonites during wetting under confined conditions. The main swelling pressure testing program has been performed using a new constant-volume oedometer especially designed to apply suction using the osmotic method. The device allows to continue to determine swelling pressure under constant-volume conditions during wetting close to saturation (suction below 8.5 MPa). Constant-volume swelling pressure tests were performed by decreasing the suction (or increasing the humidity) of the specimens from almost 100 MPa to 8.5 MPa and then in stepwise manner toward a zero value. In addition, a series of swelling pressure tests were performed using different experimental devices to compare test results obtained using the new device and to investigate the effect of hydration mode on the swelling pressure of compacted bentonites.Different wetting modes, water or vapor wetting, resulted different equilibrium swelling pressure values. Three stages of swelling pressure development are identified during suction reduction. In the first stage, following large suction reduction, swelling pressure increases up to a peak value; then, in second stage, in suction range comprised between 8.5 and 3.5 MPa, swelling pressure tends to decrease to a minimum value; and finally, upon further suction reduction to low values, swelling pressure increases again and eventually stabilizes. A possible interpretation of this pattern is given from micro-structural perspective.