A semi-empirical framework for interpreting undrained shear strength from piezocone data

Abstract

Reliable interpretation of piezocone test (CPTU) data in clays remains challenging because conventional empirical correlations with undrained shear strength often rely on cone factors that neglect stress-history effects. This paper presents a unified framework that relates CPTU results to critical state soil mechanics (CSSM) and stress history and normalised soil engineering properties (SHANSEP) concepts, providing a soil-mechanics-based expression for cone factors as functions of overconsolidation ratio and friction angle. The formulation, derived from the modified Cam Clay model, is evaluated for the main cone factors (Nkt, NΔu and Nke) and validated against an extensive database of onshore and offshore clays. The results show excellent agreement between measured and predicted trends, with low bias and limited scatter. Isotropic and anisotropic model assumptions yield comparable accuracy, while Nkt proves most robust for practical use. The proposed CSSM–SHANSEP CPTU framework offers a rational and transparent alternative to empirical approaches, enabling consistent interpretation of CPTU data and more reliable assessment of undrained shear strength in natural clays.