Geotechnical Properties of Sediments from the Continental Slope Northwest of the British Isles

To the west of the Shetland Islands the continental margin is constructed from a thick sequence of sediments of Pliocene to Holocene age. In this paper the sedimentology and geotechnical properties of five vibrocores are described. The cores selected for study can be divided into two groups on the basis of their sedimentology, which relate to the acoustic signatures of the horizons from which they were collected. The first group consists of clay-silts of probable mass flow origin. These sediments are acoustically transparent and the cores are lithologically very uniform throughout their depth. The second group is drawn from sediments which are acoustically well-layered at the sample sites, and the cores show major facies variations between muds, muddy silts and sandy silts. Consistent differences are seen between the two groups over a wide range of engineering properties, including water content, grading, plasticity limits, undrained shear strength and compressibility. These differences arise from the sedimentological character of the cores, in particular the mud content and the presence of depositional facies variations, which exert a strong control on the engineering behaviour. Those facies which are rich in mud have high plasticity, high water contents, low strength and high compressibility, whereas those facies which are low in mud content are of correspondingly low plasticity, high density and low compressibility.