Geotechnical and geo-acoustic characterization 0 f sediments from Eckernförde Bay, Germany, which are organic in nature, very soft, and contain variable amounts of dissolved and undissolved methane gas, are combined with a theoretical model to predict the influence of gas bubble concentration on key engineering parameters. At the typically low gas volume fractions measured at the study site, 0 n average less than 1%, gas pressure and shear strength are predicted to fall within small to moderate ranges. The instantaneous, undrained, elastic shear modulus i s expected to decrease very little, whereas the bulk modulus and the compressional wave velocity show dramatic decreases.
Offshore organic sediments, particularly those found in shallow water depths, are often unsaturated due to the presence of gas bubbles. Methane is the most common type of gas present in surface deposits and is derived from bacterial breakdown of organic matter. Organic-rich sediments are found in many coastal areas such as in the North Sea, the Black Sea, and near the deltas of several major rivers. The presence of undissolved gas can have important consequences for the mechanical and acoustic behavior of these sediments, which constitutes the focus of this article. A recently completed major research initiative, the Coastal Benthic Boundary Layer program (CBBL; Richardson, 1994), included an exhaustive investigation of the soft, organic, methane-rich sediments from Eckernförde Bay, Germany (Figure 1). The fine- grained sediments near the central basin are relatively uniform in composition in the upper 5 meters and consist of a black, silty clay with an average organic content of 11.2% by weight. Near the surface the sediments are characterized by unusually high water contents, plastic limits, compressibility (C, between 2.7 and 6.8), as well as low permeabilities (average of 4×10−6 cm/s in the upper 40 cm), and very low shear strengths (Figure 2; Silva and Brandes, 1998; Brandes et aI., 1996).
