Experimental investigation of evolving anisotropy in unsaturated soils

This paper investigates the ‘initial’ and ‘evolving’ mechanical anisotropy of a compacted unsaturated soil. A wide campaign of triaxial compression and extension tests, involving different stress and suction paths, has been performed on both isotropically and anisotropically compacted samples of unsaturated Speswhite kaolin. The first objective is the definition of the initial yield surface of the compacted soil after suction equalisation and before any plastic loading/wetting path takes place. This is followed by the investigation of the evolution of the yield surface induced by plastic straining along different loading/wetting paths. Experimental results are interpreted by using two alternative stress variables, namely net stresses σij and Bishop's stress σ*ij = σij + δijSrs (where δij is Kronecker delta, Sr is the degree of saturation and s is suction). Constant suction cross-sections of the yield surface are represented as distorted ellipses not passing through the origin in the q:p plane of deviator stress plotted against mean net stress, and by distorted ellipses passing through the origin in the q:p* plane of deviator stress plotted against mean Bishop's stress. The inclination of these distorted elliptical yield curves evolves with plastic straining but remains the same at all suction levels for a given level of plastic deformation. The critical state lines in the planes q:p and q:p*, or in the semi-logarithmic v:lnp and v:lnp* planes (v is the specific volume), are generally independent of initial anisotropy or stress history, suggesting that fabric memory tends to be erased at critical state.