Geotechnical Properties of Sandy Seafloors and the Consequences for Dynamic Penetrometer Interpretations: Quartz Sand Versus Carbonate Sand |
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Authors: | N Stark R Wilkens V B Ernstsen M Lambers-Huesmann S Stegmann A Kopf |
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Institution: | (1) MARUM, University of Bremen, Leobener Str., 28359 Bremen, Germany;(2) Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822, USA;(3) Department of Geography and Geology, University of Copenhagen, ?ster Voldgade 10, 1350 Copenhagen K, Denmark;(4) Bundesamt f?r Seeschifffahrt und Hydrographie, Bernhard-Nocht-Str. 78, 20359 Hamburg, Germany;(5) Centre de Brest, IFREMER, 29280 Plouzan?, France |
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Abstract: | The societal usage of coastal zones (including offshore wind energy plants, waterway deepening, beach conservation and restoration)
is of emerging importance. Sediment dynamics in these areas result in sandy deposits due to strong tidal and wave action,
which is difficult to simulate in laboratory geotechnical tests. Here, we present data from in situ penetrometer tests using
the lightweight, free-fall Nimrod penetrometer and complementary laboratory experiments to characterize the key physical properties of sandy seafloors in areas
dominated by quartzose (North Sea, Germany) and calcareous (Hawaii, USA) mineralogy. The carbonate sands have higher friction
angles (carbonate: 31–37°; quartz: 31–32°) and higher void ratios (carbonate: 1.10–1.40; quartz: 0.81–0.93) than their siliceous
counterparts, which have partly been attributed to the higher angularity of the coral-derived particles. During the in situ
tests, we consistently found higher sediment strength (expressed in deceleration as well as in estimated quasi-static bearing
capacity) in the carbonate sand (carbonate: 68–210 g; quartz: 25–85 g), which also showed a greater compressibility. Values
were additionally affected by seafloor inclination (e.g., along a sub-aqueous dune or a channel), or layering in areas of
sediment mobilization (by tides, shorebreak or currents). The study shows that the differences in in situ measured penetration
profiles between carbonate sands and quartz sands are supported by the laboratory results and provide crucial information
on mobile layers overlying sands of various physical properties. |
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