Free fall cone penetrometer testing (FFCPT) allows for efficient site reconnaissance and soft seabed undrained shear strength (su) 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 su profiles capture different depositional environments. The derived dynamic FFCPT su 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 su; 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 su 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.

Offshore energy production increases rapidly on the Finnish west coast. Effective ground investigation methods are needed for offshore wind farm construction and cable route planning. This study identified fine-grained local subsurface sediment units based on seismoacoustic survey and sediment coring complemented with geotechnical properties from free fall cone penetrometer tests. The finegrained sediments in the study area consist of brackish-water mud, postglacial lacustrine clay and glaciolacustrine clay and silt which all show variability in physical properties and undrained shear strength profile. The study shows that free fall penetrometer tests provide a ground-truth for the seismoacoustic profile interpretation and, combined with seismoacoustic survey and sediment coring enable a fast and cost-efficient site reconnaissance.

The paper shows a moving least squares reconstruction technique applied to the B-spline Material Point Method (B-spline MPM). It has been shown previously that B-spline MPM can reduce grid-crossing errors inherent in the original Material Point Method. However, in the large deformation regime where the gridcrossing occurs more frequently, the convergence rate of B-spline MPMis lower. In this paper, moving least squares reconstruction is employed to retrieve the expected convergence rate. The proposed improvement is examined in terms of the spatial convergence using the methods of manufactured solutions for large deformations.