Peak and Critical State Conditions for Unsaturated Sand

Abstract

The shear strength equations, based on two stress state variables, proposed by Fredlund et al. (1978) and Oberg and Sallfors (1997), respectively were compared at peak and critical states. Data of a set of drained direct shear tests for a fine sand tested under saturated and unsaturated conditions were used for the comparison. The comparison between the two equations suggests that suction contributing to the effective stress for sand used is often much smaller than predicted by the (sSr) term used in effective stress expressions of Oberg and Sallfors (1997). It is suggested that effective stress in unsaturated soils should be related to soil fabric and microstructure which caused an increase of shear strength parameters. Clusters or assemblages of particles, held by menisci, increased dilation and frictional resistance during shearing related to the different phases of water retention behaviour.