Analytical Effective Stress Piezocone Methods for Assessing Geoparameters of Offshore Clay-Paper

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The paper presents an effective-stress approach to interpreting piezocone penetration tests (CPTU) in offshore clay. It combines a spherical cavity expansion–critical state soil mechanics model (SCE-CSSM) with an NTNU limit-plasticity method to estimate yield stress ratio (YSR), undrained rigidity index (IR), undrained shear strength (su), and effective friction angles at two strength criteria: peak deviator stress and maximum stress obliquity.<br /><br />The methods are demonstrated using CPTU data from the upper, highly plastic clay layer at the Troll East offshore field. The SCE-CSSM analysis uses the friction angles derived from the NTNU method and CPTU cone resistance and pore-pressure measurements. For the Troll clay, the estimated rigidity index was about 63. The resulting YSR and undrained-strength profiles agreed well with independent laboratory consolidation and anisotropically consolidated triaxial-compression tests. The NTNU method estimated friction angles of approximately 31–33° using the original formulation and 27–28° after accounting for stress history, broadly consistent with laboratory values of 33° at maximum obliquity and 29° at peak strength.<br /><br />The study also describes estimating clay unit weight from the increase in cone resistance with depth, with estimates comparable to laboratory measurements. Overall, the authors conclude that these analytical methods can provide useful CPTU-based profiles of key strength and stress-history parameters, reducing reliance on empirical correlations. The approach may require iteration because the friction-angle estimate at peak strength depends on YSR.

Keywords

piezocone penetration test

CPTU

offshore clay

effective stress

spherical cavity expansion

critical state soil mechanics

yield stress ratio

undrained shear strength

friction angle

Troll East field