Geotechnical properties and behavior of high-porosity, organic-rich sediments in Eckernforde Bay, Germany

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The organically rich, fine-grained, very soft, high porosity sediments in the inner portion of Eckernforde Bay, Germany have varying amounts of methane gas, with the horizon of gas fluctuating vertically on a seasonal cycle. The sharp vertical gradient in water content, with values exceeding 500% at the sediment-water interface, and corresponding gradient in density can be expected to cause a significant subbottom acoustic impedance contrast in these surficial sediments. Equations are presented to characterize geotechnical property variations of the upper 5 m. The upper 1.5 m exhibits appreciable 'apparent' overconsolidation with a trend toward a normally consolidated stress state at 2.5 m depth. The coefficient of permeability of the upper 40 cm is low (4 x 10-6 cm s-1) and the sediment is highly compressible with compression indices of 2.7-6.8. Triaxial compression test results indicate that the sediment behaves as a normally consolidated clay with a low friction angle (22°). The rheological behavior of the upper 20-30 cm, determined with a small vane device, is indicative of a shear-thinning material, implying that the resistance to penetration decreases with increasing velocity.

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Continental Shelf Research