Portable free fall penetrometers are lightweight seafloor penetrometers deployed by hand and usually applied for the geotechnical characterization of uppermost seafloor surface sediments (/spl less than or equal to/ 1 m). Therefore, they are particularly suitable to measure changes in geotechnical properties from local and recent sediment remobilization processes. The so-called mobile or active seafloor layer (i.e., the seafloor surface layer actively affected by sediment transport processes) is usually very soft in the case of muddy sediments or very loose in the case of sandy sediments. Thus, the void ratios and the water contents are high, while the bulk density is low. Particularly in muddy sediments, this layer may even share characteristics with a high concentration suspension rather than with a soil. Accordingly, sediment resistance is expected to be comprised of contributions of shear resistance, drag resistance, and to some extent buoyancy and possible side friction. Furthermore, it may be argued that drag resistance may be the prevailing contributor to seabed resistance against a penetrator in the mobile seafloor layer. With increasing sediment depth, effects of drag are likely significantly reduced when the sediment hardly ?gflows?h around the probe anymore, and shear resistance is expected to start dominating. However, transitions between these processes are poorly understood, and possibly particularly complex for impacts with changing penetration velocities. In this study, the issue of forces contributing to sediment resistance against a portable free fall penetrometer at high impact velocities is revisited theoretically and through literature review, and discussed in the context of applicability for the investigation and analysis of surficial seafloor sediments.
Nina Stark, Dennis Kiptoo, Nick Brilli and Reem Jaber
Keywords: free fall penetrometers, data analysis, seabed surface
Publisher: DFI