SCPTu study of negative pore pressure and soil stratification in coastal fine-grained soils

Cone penetration testing (CPT) is a widely used, efficient and reliable in-situ testing technique. However, negative pore water pressure (-u₂) frequently develops during penetration, potentially compromising the accuracy of parameter interpretation. Understanding the mechanisms behind -u₂ and mitigating its influence are essential for improving CPT data reliability. This study utilises seismic cone penetration tests (SCPTu) to investigate the mechanism of -u₂ in fine-grained soils. A quantitative relationship between shear wave velocity (V_s) and -u₂ is established, indicating that the generation of -u₂ is only weakly dependent on the degree of soil saturation. Furthermore, CPTu conducted at varying penetration rates, in conjunction with dissipation tests, reveal that both the occurrence and magnitude of -u₂ are strongly rate-dependent. The results demonstrate that penetration parameters may exhibit pronounced time-dependent and dynamic characteristics, even under nominally static conditions. The results indicate a linear decrease in shear wave velocity (Vₛ) with increasing absolute values of pore water pressure (|u₂|). Analysis of the relationship between Vₛ and soil saturation suggests that unsaturated conditions alone cannot adequately explain the generation of -u₂. In addition, the presence of initial -u₂ may lead to the misclassification of fine-grained soils as coarse-grained layers. By incorporating the dissipation characteristics of -u₂ in cohesive soils, the proposed corrected classification method enables more accurate identification of fine-grained strata, thereby significantly enhancing the precision of stratigraphic discrimination.