An experimental investigation of the mechanical behaviour of an unsaturated gneiss residual soil

The paper presents compression and triaxial tests conducted under controlled suction in horizons B and C of a residual soil gneissic rock profile from south-east Brazil. Horizon B, found at the surface, consists of a 2 to thick reddish lateritic clayey soil layer, whereas horizon C is a saprolitic soil layer that reaches great depths. The block samples representing horizons B and C were manually taken from depths of 1 m and 5 m respectively. The test programme carried out consisted of isotropic and anisotropic compression tests as well as drained and constant water content triaxial tests. Tests were performed under saturated and unsaturated conditions at controlled suction. Strength parameters c and 0 were shown to increase with matric suction. The critical state parameters were shown to vary with suction, which also expanded the yield curve. Data obtained may be useful for the development of soil constitutive models for unsaturated soils.