Consolidation and Permeability Behavior of High Porosity Baltic Seabed Sediments

Abstract

The consolidation and permeability characteristics of high porosity surficial sediments from the Coastal Benthic Boundary Layer (Naval Research Laboratory) Special Research Program test site in the EckemfOrde Bay Baltic Sea were studied using a backpressured, constant rate of deformation (CRD) consolidation and permeability testing system driven by flow pumps The silty clay sediments from the Baltic test site are characterized by high void ratios (above 6), large organic content and dissolved or free methane gas bubbles. The high organic content (up to 17%) influences several of the unique features of these sediments including high water contents (up to 500%; porosities up to 93%), high consistency limits and high compressibility. The sediments are dominated by clay and silt size fractions (42% and 55% respectively) with a small percentage (3%) of sands. Organic content determinations performed on selected samples indicated a variation from 9% to 17% in the organic fraction (dry weight basis). Atterberg Limits were determined for several samples from different locations within the site and appear to be high (wL between 190% and 280%, Ip from 130% to 195%) in comparison with available data on other marine sediments. The Atterberg Limits were used in conjunction with the grain size and organic content data to classify the sediment as an organic clayey silt, OH in accordance with Unified Soil Classification System (USCS). Results from CRD consolidation tests show apparent overconsolidation of sediments at the surface layers with a gradual transition to normally consolidated state at about 300 cm. The highly compressible nature of the material is reflected in the large values of compression index (3.2 to 6.8 for box core samples; 2.7 to 4.3 for gravity core samples). These high values are consistent with the large values of Plastic and Liquid Limits. Recompression indices range from 0.3 to 0.5. The coefficient of consolidation for the samples tested range from 10-3 cm2fs to 10-5 cm2fs which are in the typical range for marine sediments. In situ permeability ranges from 9.8 x 10-7 emfs to 3.2 x 10-4 emfs with the larger values corresponding to deeper samples. The permeability values plot into two fairly distinct zones depending on whether the samples were from box cores upper 40 cm) or gravity cores (deeper zones). The permeability of the box core samples ranges from 2.5 x 10-7 emfs to 3.0 x 10-6 emfs for void ratios ranging from 4.5 to 7 .0 and the permeability