Assessment of undrained cyclic resistance of sand with non-plastic fines under sustained shear stress using a critical state interpretation

There are many geotechnical applications involving dams, embankments and slopes where the presence of an initial static shear stress prior to the cyclic loadings plays an important role. The current paper presents the experimental results gathered from undrained cyclic simple shear tests carried out on non-plastic silty sand with fines content in the range 0-30% with the consideration of sustained static shear stress ratio (α). Two distinct parameters, namely the conventional state parameter Ψ, and the equivalent state parameter Ψ*, are introduced in the context of critical state soil mechanics (CSSM) framework to predict failure mode and undrained cyclic resistance (CRR) of investigated soils. It is proved that the failure patterns for silty sands are related to (a) the initial states of soils (Ψ or Ψ*) and (b) the combination of initial shear stress with respect to cyclic loading amplitude. At each α, the CRR-Ψ (or Ψ*) correlation can be well represented by an exponential trend which is practically unique for both clean sands and silty sands up to a threshold fines content (fthre≅24.5%). Varying α from low to high levels simply brings about a clockwise rotation of the CRR-Ψ (or Ψ*) curves around a point. This CRR-Ψ (or Ψ*) platform thus provides an effective methodology for investigating the impact of initial shear stress on the cyclic strength of both clean sands and silty sands. The methodology for estimating Ψ (or Ψ*) state parameters from in-situ cone penetration tests in silty sands is also discussed.