Continuous evaluation of marine clay strength via an I c -unified CPTu framework

The undrained shear strength (Su) of marine soft clays is a paramount parameter in the foundation design of offshore engineering projects. Owing to its capability for rapid, continuous characterization, the piezocone penetration test (CPTu) is widely adopted for Su assessment. Conventional CPTu-based interpretations of undrained shear strength are commonly constrained by method-specific parameter requirements: the empirical cone factor method depends on site-specific Nkt selection, CSSM-based interpretation requires reliable effective stress strength parameters, and the SHANSEP framework relies on stress-history parameters such as OCR or apparent yield stress. To reduce these separate dependencies within a unified interpretation structure, this study introduces the soil behavior type index (Ic) as a bridging variable linking CPTu response, soil behavior type, effective stress state, and stress history. On this basis, three Ic-unified models are developed: CSSM-Ic for state-dependent interpretation, SHANSEP-Ic for stress-history effects, and Nkt-Ic for rapid engineering profiling while preserving the simplicity of the conventional cone factor approach. Validation against multi-site experimental benchmarks indicates that the proposed models accurately capture the measured Su profiles, thereby providing a robust framework for in-situ strength characterization of marine soft clays.