Effects of stress-strain history on the initial shear stiffness of an unsaturated compacted silt.

This paper is focused on the results of an experimental study of the effects of mean net stress (p-ua) and suction (ua-uw) history on the initial shear stiffness Go of a compacted silt. Isotropic tests were performed using two suction controlled devices, a triaxial cell and a Resonant Column Torsional Shear (RCTS) cell, so as to investigate the volumetric behavior of this material. In addition, by using the RCTS cell, initial shear stiffness was measured almost continuously along different isotropic stress paths in the plane (p-ua):(ua-uw), including constant suction- compressions and single stages or cycles of drying and wetting. The obtained results show a strong dependency of Go on mean net stress and suction. Cycles of suction, in particular increasing suction beyond the past maximum value, induce significant accumulation of irreversible strains and increase of stiffness, confirming that Go is not univocally linked to the stress state expressed in terms of (p-ua) and (ua-uw). The influence that mean net stress — suction history has on initial shear stiffness is analyzed and ascribed to the accumulation of irreversible volumetric strains. A model able to predict the observed behavior is proposed.