An experimental programme has been performed to investigate the cause of the faults that have been detected geophysically at study areas on Atlantic abyssal plains The effects of differential settlement over an uneven basement have been studied in the Cambridge geotechnical centrifuge by deforming a block of clay using hinged flaps in the base of the model The tests have been performed on normally consolidated kaolin, although in some cases slight over consolidation of the upper layer was unavoidable Laboratory tests at one gravity and numerical analyses have been used to supplement the centrifuge tests and to study a problem with boundary conditions closer to those found in the abyssal plains.
The development of shear planes was observed in all the centrifuge models, even when no over consolidated clay was present. In the single gravity tests, the low stress levels appeared to encourage the formation of tension cracks in preference to shear planes. Finite element modelling of both the centrifuge model and the ‘abyssal plain problem’ did not indicate the formation of shear planes, however, this is probably due to the limitations of the soil model used It is concluded therefore that differential settlement can cause faulting in normally consolidated sediments and is a likely explanation of the phenomenon observed m the abyssal plains.
Seismic reflection profiles of the seabed sediments of the Nares and Madeira abyssal plans have shown the apparent presence of offsets in some layers which appear to indicate that some faulting has occurred in these soft surface sediments (Buckley and Grant, 1985, Dun, Mesdag and Kok, 1984; Dun, 1985; Williams, 1987) These ‘faults’ do not appear to be associated with any movements in the basement rocks but appear to be associated with the physical topography of these underlying rocks Some of the sediments in the abyssal plain have been deposited as pelagic sediments in a layer of uniform thickness, but the majority of sediments are deposited as turbidites which, when they fall out of suspension, tend to be deposited m greater thicknesses in the valleys than on the hills It has been suggested that the faults are the result of the different amounts of consolidation settlement that occur in the soft sediments between high points and low points of the basement There is uncertainty concerning the origin and nature of these faults and there is concern that if such faults exist or develop near to potential radioactive waste disposal sites; they may provide preferential pathways through which radioactive substances may find their way out of the seabed.
Studies of the significance of the faulted sediment as preferential pathways for radionuclider migration have formed the basis of other research contracts performed at Cambridge (Gronow, 1987) The work reported here relates to the investigation of possible mechanisms of formation of such faults The work described falls into three areas centrifuge model tests, single gravity model tests, and numerical finite element analyses.