Small-Strain Nonlinearity of Normally Consolidated Clay

This paper addresses the small-strain undrained shear behavior of clays based on an extensive testing program conducted on normally consolidated resedimented Boston blue clay (RBBC). The uniformity and reproducibility in the behavior of this soil permit a detailed study of the role of consolidation stress level, preshear consolidation stress path, preshear lateral stress ratio (K), rate of shear, and duration of laboratory aging on the small-strain ((epsilon)a=<0.1%) nonlinearity in undrained triaxial compression. The small-strain behavior of normally consolidated (NC) RBBC is measured using a LVDT-based device that is capable of resolving on-specimen axial strains smaller than 0.0001% strain over a 10% strain range, thus allowing full description of the small-strain nonlinearity. The testing program relies on a multistage testing approach developed to minimize specimen variability and reduce testing time. The small-strain nonlinearity of NC RBBC is found to be virtually independent of void ratio or stress level, all other testing conditions being equal. K0consolidated RBBC presents the smallest linear region and the most rapid stiffness degradation (decrease in Youngʹs modulus). Increases in K, strain rate and aging all cause a reduction in the small-strain nonlinearity of the soil.