Sea Technology

FEB 2013

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In-Situ Geotechnical Investigation Of Sediment Remobilization Processes Utilizing Geotechnical Measurement Techniques To Detect, Quantify and Describe Sediment Erosion and Deposition By Nina Stark ��� Achim Kopf T he increasing usage During sediment of coastal zones for transport, areas of sedirecreational and indusment erosion will be trial purposes, as well distinguishable from aras the need for coastline eas of deposition. Freshconservation, highlight ly eroded seafoors will the importance of mabe characterized by a rine sediment remobihard surface due to the lization processes. Refact that looser particles search has been carried were removed, whereas out using numerical in areas of sediment models, laboratory exdeposition, a loosely periments and feldwork, packed layer will be with regard to naturally found on top of the origevolving processes and inal sediment surface. Different modes of sediment remobilization and their correspondence to geotechnical marine engineering. Measurements in such properties. Measuring Sediment areas are diffcult due to Remobilization In Situ strong hydrodynamic forcing and an easily disturbed seafoor One option to get a better idea of the friction angles of surface. Geotechnical methods have been used to investigate surfcial seafoor sediments are laboratory measurements sediment remobilization processes, with the deployment of and shear tests conducted under water-saturated conditions the small-scale dynamic penetrometer Nimrod developed by and low normal stresses. However, it is diffcult to simulate the Center for Marine and Environmental Sciences (MARUM) the sensitive and quickly varying packing conditions at the at the University of Bremen. seafoor sediment surface. As such, in-situ methods are required but face a numSediment Properties Versus Remobilization Processes ber of challenges. Devices used in situ must be operable in The seafoor can be characterized by a number of geohighly turbulent waters, easily deployable from small and technical properties, such as cohesion, friction angle and navigable platforms, and able to provide vertical profling at pore pressure. high resolution of loosely packed layers (e.g., fuid mud), as To initiate sediment movement, the shear stress applied well as spatial resolution as active layers are often in a range by currents and waves has to overcome a threshold, which of a few centimeters. depends on the friction angle and cohesion. Approaches In-situ vane shear apparatus (often diver operated) and to assess the threshold shear stress predominantly use the standard engine-driven cone penetration testing are often grain size to estimate the friction angle. However, the fricnot applicable due to vigorous wave and current environtion angles of surfcial seafoor sediments are also highly ments. The latter also faces diffculties in the case of shallow infuenced by particle packing, grain shape and protrusion waters, which do not allow navigation of large platforms. of particles. Dynamic penetrometers are a promising technology that are As soon as sediment transport is initiated, particles in the easily deployable and can estimate geotechnical properties, so-called active layer will be entrained and moved. Thus, such as sediment strength and pore pressure, with high vertithe existing particle fabric will be broken up, and propercal resolution. ties such as friction angle, pore pressure and density will change within the range of the active layer, allowing observDesign of the Dynamic Penetrometer Nimrod ers to distinguish the active layer from the stable underlying The Nimrod follows the measurement strategy of existseabed. ing small-scale dynamic penetrometers: It free-falls through www.sea-technology.com FEBRUARY 2013 / st 53

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