Refined Interpretation of Plasticity Index of Marine Soft Clays from CPTu Data for Offshore Site Characterization

The plasticity index (I p ) is a critical indicator of the deformation and strength behaviour of fine-grained soils, closely linked to the compression coefficient, internal friction angle, and earth pressure coefficient. Accurate evaluation of I p is essential for offshore foundation design and marine site characterization. However, existing CPTubased approaches for estimating I p suffer from limited applicability and significant prediction errors when applied to marine deposits. This study investigates the correlations between I p and CPTu normalized parameters (B q , F r , and Q tn ) with reference to soil classification charts, focusing on coastal and offshore fine-grained soils. A refined CPTu-based model for predicting I p across various soil types is proposed. The new model can apply different calculation methods for different soil types. Results indicate that B q , F r , and Q tn effectively characterize soil plasticity, and their combinations (B q -Q tn and B q -F r ) exhibit strong potential for distinguishing marine soil types. The proposed method improves I p prediction accuracy within a 30% error margin from 56.9% to 89.4%, offering a more reliable tool for the interpretation of CPTu data in offshore geotechnical investigations and foundation engineering.