A fundamental interpretation of the critical state strength of unsaturated soils

Any two of the three stress state variables (p − u_a ), (p − u_w ) and s = (u_a – u_w ) may be used in describing the stress state in unsaturated soils. An equation for the deviator stress q at the critical state must reflect the duality of the stress regime implicit in this statement. In soils containing a significant percentage of fine particles, the duality arises from the bi-modal structure comprising aggregates of particles and water surrounded by air-filled macro-voids. A relationship is presented for q that links the dual stress regime to the specific volume v and specific water volume v_w . Data on kaolin, a lateritic gravel, a residual soil and a rock powder are used to validate the relationship. Consistent trends are indicated for all materials examined indicating that the strength equation provides a good basis for understanding and predicting the critical state strength of a wide class of unsaturated soils.