Geotechnical and Geological Factors Affecting Offshore Engineering and Seabed Utilization on a Carbonate Margin: St. Croix, V.I.

The northern margin of St. Croix, V.I. is characterized by three morphological features: a narrow shallow water carbonate shelf; a steep slope; and a deep-sea basin. Shallow water reef material and carbonate debris occurs along the shelf. A thin carbonate ooze overlies the steep slope where occasionally rock outcrops occur. Trough sediments are clayey silts and turbidite deposits. Average shear strengths range between 3.1–10.0 kPa for the slope and 5.8–28.3 kPa for the trough (1-m cores). Values range as high as 69.1 kPa at sediment depths greater than one meter. Average sensitivities range between 3.2–6.1 for the slope and 4.1–12.1 for the trough (upper 1 m). Higher sensitivities [8–12] indicate the presence of metastable deposits and considerable strength loss upon disturbance. Variation in the sand, silt, and clay sized fractions, the nature of the carbonate particles, and bioturbation strongly affect the variability in the geotechnical properties. Quantitative studies of local morphology, processes and sediment properties reveal the need for detailed data in specific areas of engineering interest.