Suctions, stresses and strengths in unsaturated sand–bentonite

This paper presents suction and strength characteristics for a dense, compacted, unsaturated sand–bentonite mixture under a variety of preparation and stressing conditions. Suctions were determined experimentally using thermocouple psychrometers and the filter paper method. They are shown to be related to water contents, saturation, dry densities and osmotic agents. The influence of the initial suctions on strength was evaluated using quick undrained triaxial compression tests, here called ‘constant-mass’ tests. Examination of the suction response to applied external stresses was carried out using stress-controlled triaxial tests along selected stress paths. These tests measured suctions in tests where the mean stress p and deviator stress q were changed systematically to give a series of constant values of Δq/Δp. The results showed that suction decreased as mean stress loading increased. Suction changes appear to be produced only by the mean stress component of the stress tensor and not by the shear stress component. In this compacted sand–bentonite material, for the pressure range at which the tests were performed (p′≤3 MPa), changes in suction produced by changes in mean stress are largely reversible.