The determination of the monotonic undrained shear strength profile from piezocone (CPTU) tests relies upon the selection of the cone factor N/sub kt/ that is based on the site-specific laboratory tests results. In overconsolidated clays, the selection of N/sub kt/ is challenging because of the dilative stressstrain behavior. Therefore, the N/sub kt/ should reflect the strain level at which the undrained strength is selected in the laboratory tests. In addition, for sites where the overconsolidation ratio (OCR) varies significantly with depth (e.g. soil deposits subjected to ice-loading in the past), the N/sub kt/ should be chosen to reflect the stress history of the deposit. This paper discusses the applicability of the SHANSEP method to describe N/sub kt/ in overconsolidated clays. The SHANSEP method may define a variable N/sub kt/ with depth as a function of the OCR. Moreover, the SHANSEP parameters can be calibrated to account for the dilative behavior at large OCRs. The usability of the method is illustrated through a case study from a heavily overconsolidated clay site in the North Sea. The method is compared to the traditional approach, where a constant N/sub kt/ is generally used to define the undrained shear strength profile. Unlike the more classical approach, the combined SHANSEP-N/sub kt/ method is able to capture the trend indicated by the laboratory tests. The case study demonstrates how using a constant N/sub kt/ may result in an unconservative shear strength profile, especially at shallow depths. This may have severe consequences on the behavior of shallow foundations (e.g. skirted spudcans or gravity base structures) subjected to combined static and cyclic loading.
Marco D'Ignazio, Knut H. Andersen, Harun Kürsat Engin, Sivasithamparam Nallathamby, Hans Petter Jostad and Gülin Yetginer
Keywords: CPTU, interpretation, overconsolidated clays, SHANSEP
Publisher: DFI