The strength of unsaturated mixtures of sand and kaolin and the concept of effective stress

SUMMARY Pastes containing sand and 0, 5, 20 and 80 g kaolin kg −1 mixture were equilibrated at matric potentials of – 2, – 10, – 100 and – 1000 kN m −2 or allowed to air‐dry. The strength of cylindrical samples was determined in unconfined compression and by the indirect tensile strength test. Measured strengths are explained using the Coulomb‐Mohr theory and the concept of effective stress. At pore water tensions greater than 10 kN m −2 effective stress was the dominant factor in determining compressive and tensile strength. The contrast between this behaviour and that of friable topsoils is discussed.