Deep ocean clay crusts: behaviour and biological origin

Abstract

In water depths of 500m to greater than 2,000m, off the West coast of Africa, sediments comprise very soft clays with extremely high water contents and plasticity. In situ CPT and T-bar testing in these areas have identified 'crusts' with undrained shear strengths of up to 15kPa at 0.5m depth, before the strength reduces by an order of magnitude to normally consolidated strengths by 2m depth. This thesis presents an investigation into the behaviour and origin of these crusts. Mini ball-penetrometer tests on natural cores confirm the crustal strength, and indicate a sensitivity of 3 within the crust. However, pipeline interface tests using the Cam-shear device demonstrate a significant variability in the measured interface friction coefficient. Particularly low strengths are observed when shearing in an undrained manner on a rough interface. These results are attributed to the heterogeneity of natural samples, and demonstrate the need to better understand the origin of the crust material in relation to interface micro-mechanics. A microbiological investigation of crust material by extracting bacteria DNA from clay samples is described, and identifies the presence of the bacterium, Marinobacter aquaeolei. This bacterium is then used to inoculate sterile samples to determine its ability to produce crustal strength. Through this work, it is concluded that M. aquaeolei is unable to create crustal strength, although extracelluar polysaccharides produced by this bacterium will influence the permeability of sediments through the clogging of voids. It is therefore also concluded that future geotechnical investigation into marine sediments should consider the presence of bacteria and their ability to influence the soil properties. Wet sieving of crust material shows that the crust comprises a mixture of burrowing invertebrate faecal pellets and clay. Pellets are found to represent 20% to over 55% of the crust material by dry mass. Individual pellets are shown to exhibit unconfined compressive strengths of between 5kPa and 50kPa, thus demonstrating their strength and robustness. Consolidation behaviour is governed by the percentage of pellets in natural samples. Based on their location, abundance and strength, it is concluded that the origin of crustal strength lies with the presence of burrowing invertebrate faecal pellets. When sheared on rough pipeline interfaces, however, pellets are observed to crush, expelling void-filling fragments that may generate positive excess pore pressures. Smeared clay produced when shearing natural samples obstructs the dissipation of pore pressures, which may encourage hydroplaning, and explain the observation of very low interface friction coefficients. It is therefore suggested that smooth pipelines offer more sliding resistance by minimising the risk of pellet crushing. This thesis proposes that wet sieving of core samples should be undertaken during the site investigations for future deep-water, hot-oil pipeline installations to provide design information on both the consolidation and strength behaviour of natural sediments.