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.@en

This paper presents a series of laboratory free‐fall cone penetrometer (FFCP) tests conducted on marine clay samples collected from the Gulf of Finland in the Baltic Sea. Subsequently, these tests are replicated numerically with the generalized interpolation material point method (GIMP) simulations. First, the paper gives laboratory‐scale FFCP experiment results used for the validation of the numerical framework. In these experiments, a small‐scale model of a FFCP was dropped from various heights into a natural marine clay soil sample and recorded using a high‐speed camera. The tests were supplemented with a laboratory test program to determine the geotechnical properties of the clay used in the experiments. Following image processing, the tests provided data for numerical simulations: displacement, velocity, acceleration, and reaction force curves associated with the FFCP during the penetration process. The GIMP simulations shown in the paper replicate the process of penetration of the FFCP into the marine clay. The simulations used a strain‐rate dependent Tresca constitutive model, extended with strain softening that replicates the reduction of the undrained shear strength due to destructuration, an important feature of the material. The numerical simulations replicate the experiments well. The study examines the effect of cone penetrometer roughness, impact velocity, mesh density, strain rate, and strain softening on the cone penetrometer penetration process. The simulation results indicate that the presented framework can replicate the dynamic penetration process on soft and sensitive clay very well.@en

This paper proposes a numerical framework to simulate in-situ free fall cone penetration tests conducted on soft and sensitive marine clay. First, the free fall cone penetration tests are carried out at an offshore site in the northern part of the Gulf of Finland, Baltic Sea. The numerical simulation employs the Generalized Interpolation Material Point Method to replicate the process of indentation of the cone penetrometer into the clay. The clay is modelled using an advanced constitutive model that considers the effect of strain rate and strain softening associated with the dynamic penetration process. The simulation uses a friction contact model to represent the interface between the cone penetrometer and the clay. The numerical simulation accurately replicates the penetration process associated with the tests. @en

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.@en

The paper presents experimental results of investigations of offshore deposits from the Gulf of Finland in the Baltic Sea. Firstly, it briefly reviews the sedimentary history of the studied site. Then, it presents results from marine geological investigations, including seismoacoustic reflection sub-bottom profiles and long sediment cores collected by piston coring. Finally, the paper presents the physical and mechanical properties of the intact soil samples through geotechnical laboratory tests. The data show the distribution of water content, bulk density, undrained shear strength, sensitivity, and organic content of the samples at various depths up to approximately 4 meters. The undrained shear strengths, obtained by fall cone tests, are scattered and the variations are ±35% for the studied specimens. A quantitative correlation between geotechnical data and local sedimentary units and depositional history was not possible in this study. Further efforts are needed to reduce the uncertainties and measurement errors in determining the undrained shear strengths in geotechnical laboratory.@en

The paper shows a numerical simulation of a fall cone test done on a soft and sensitive marine clay sample collected from the Gulf of Finland in the Baltic Sea. The simulation involves large deformations and takes into account the destructuration of clay, strain rate effects, and non-linear material behaviour. First, the analysed fall cone test was captured with a high-speed camera (1000 frames/sec). MATLAB image processing tools helped to quantify the cone displacement, velocity, and acceleration during the indentation process. Further, we simulate the process of indentation of the cone into the soils using the Generalized Interpolation Material Point Method (GIMP). In the simulation, the undrained shear strength of soil (𝑠𝑢) is taken as a function of strain rate to consider the effect of strain rate on cone indentation, as well as a function of destructuration. The presented numerical simulation replicates well the shown experiment. The simulation results indicate that the present material framework, along with GIMP, can be used to simulate the dynamic penetration process on soft and sensitive clay.@en

In situ free-fall cone penetrometers (FF-CPTs) allow for rapid investigations of undrained shear strength of soft shallow water deposits. Often samples are also studied in geotechnical laboratory by fall cone tests. This study presents in situ FF-CPT and laboratory fall cone testing results of an offshore site in the northern part of the Gulf of Finland, Baltic Sea. Site investigations included seismoacoustic surveys and sediment coring. Based on seismoacoustic reflection profiles and long sediment cores, the local deposits were classified and divided into glaciolacustrine, post-glacial lacustrine and brackish-water stratigraphic units. FFCPT measurements provided in situ undrained shear strength and dynamic cone resistance of the seabed geological units. Laboratory fall cone tests were performed on the vertical core samples at every 0.5m. Comparisons were established between undrained shear strength from FF-CPT tests and from fall cone tests.@en