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.