Interpretation of Piezocones in Silt, using Cavity Expansion and Critical State Methods

Silt sediments are frequently encountered in the coastal areas of the North Sea. Evaluation of the silt behaviour must ideally rely on in-situ tests, in particular the piezocone test. However, interpretation of piezocone data in silt sediments is problematic and derived parameters can seem to differ significantly from those obtained by investigating intact samples in the triaxial apparatus. Use of the state parameter ψ in conjunction with the critical state line provides a precise definition for the state of silt, and thereby important aspects, such as resistance to cyclic liquefaction. The objective of this paper is to provide a site-specific correlation between the dimensionless group of piezocone parameters (qt-u2)/p’ and the in-situ state parameter ψ0, while accounting for partial drainage during penetration. Silt from the North Sea was used as a case study. The process of penetration was simulated using cylindrical cavity expansion in conjunction with a plasticity model formulated within the framework of critical state soil mechanics. The results readily explain the low cone tip resistance measured in silt sediments; this is a derived effect of the silt having a large slope of the critical state line, resulting in rather weak and compressible behaviour at high mean effective stresses.