Shear tests on deep-ocean clay crust from the Gulf of Guinea

Natural cores from the crust that is found over deep-ocean sediments off the west coast of Africa have been tested in direct shear following one-dimensional consolidation under normal stresses relevant to pipelines. Shearing tests were conducted at different speeds of 0·5, 0·05, 0·005 and 0·0005 mm/s, both within the soil and against interfaces of different roughness. The behaviour in these tests is observed to be strongly influenced by the structure of the natural soil, which contains abundant faecal pellets. The data of shear tests, especially when conducted at faster speeds, display significant randomness, which is attributed to the random location of pellets and their associated macro-voids. Nevertheless, certain trends are evident. Two key observations are made. The first is that the interface friction factor when shearing against a smooth interface can be greater than that of a rough interface. The second is that the rate of shearing has a strong influence on the residual interface-friction value when shearing against a rough interface. When shearing at 0·5 mm/s, the apparent friction factor on a rough interface was observed to drop to zero, whereas the apparent friction factor of a smooth interface tended to be larger. This is attributed to excess pore pressures created by damage to the natural structure of the soil. Slow shearing at 0·005 mm/s produced higher apparent friction factors in the range 0·15–0·30, although excess pore pressures may still be present. It is concluded that the testing of natural cores is essential for a complete understanding of the behaviour of hot-oil pipelines that are subject to pipe walking and buckling. If designers rely instead on tests that have been conducted on reconstituted soils, pipelines may misbehave.