Low-strength

highly compressible soft clays are widely distributed in coastal areas of south China

where their engineering characterization is critical for infrastructure development. Although cone penetration test (CPT) efficiently acquires geotechnical parameters

data interpretation exhibits regional variability

and the applicability of existing methods requires urgent validation. Focusing on coastal clay sites in Dongguan

Guangdong

and surrounding regions

this study integrates in-situ CPT testing

consolidated undrained triaxial tests

and consolidated quick shear tests to analyze the mechanical behavior of coastal clays. Based on CPT data from Dongguan’s coastal soft clays

the modified soil behavior index (IB) is demonstrated to quantitatively characterize soft clays and enhance soil classification. A preliminary assessment of stress history is conducted using normalized cone tip resistance and the modified soil classification index

with a combined application of Mayne and Robertson’s methods recommended for determining the over-consolidation ratio (OCR) of soft clay layers. Comparative analysis of consolidated undrained triaxial tests and consolidated quick shear tests reveals that CPT-derived mechanical indices necessitate calibration against laboratory data. State parameters interpreted from CPT align with Robertson’s soil classification chart. Five methodologies are employed to evaluate the compressibility modulus

showing that CPT-derived results fall within code-recommended ranges

with the K-M1990 method shows high accuracy in coastal silt soil layers. The research outcomes provide regionally optimized solutions for CPT data interpretation in south coastal soft clays

offering practical guidance for geotechnical investigations. This work bridges the gap between conventional CPT interpretation frameworks and region-specific geotechnical conditions

advancing reliability in coastal engineering applications.