Assessment of near-surface undrained shear strength of soft seabeds with free fall cone penetrometer testing in the northern Baltic Sea

Free fall cone penetrometer testing (FFCPT) allows for efficient site reconnaissance and soft seabed undrained shear strength (s_u) assessment in a shallow water environment. The FFCPT is a particularly useful tool for near-surface characterization for cable route assessment associated with offshore renewable developments. This study presents results from an in-situ FFCPT, seismoacoustic survey and sediment coring campaign in a nearshore site in the Gulf of Finland, northern Baltic Sea. An extended FFCPT interpretation model has been applied, including friction on the shaft (as well as tip resistance), rate dependency and soil buoyancy. The derived FFCPT profiles are repeatable at a given location, while the shapes of the s_u profiles capture different depositional environments. The derived dynamic FFCPT s_u is lower than measured in the laboratory by fall cone and triaxial tests. This is potentially due to hydrodynamic drag reducing the FFCPT terminal velocity and soil drag affecting the penetration depth and dynamic s_u; and due to the reconstituted nature of the laboratory samples and very low stress levels being considered that are difficult to achieve in the laboratory. The magnitude of derived s_u and characteristic shapes in the FFCPT profiles enable ground-truthing the interpretation of seismoacoustic profiles. This study contributes to the evidence base supporting the FFCPT as a valuable supplement, or to partially replace sediment coring and laboratory measurements, for offshore site assessment.