Deepwater Soil Properties and Their Impact on the Geotechnical Program

Abstract

Abstract The sometimes unpredictable and highly variable nature of soil conditions in deepwater regions must be recognized in planning geotechnical site investigations. Examples of four different soil stratigraphies in deepwater are presented to demonstrate the variability and peculiar nature of some sediments and how important it is to plan for comprehensive programs both in the field and in the laboratory phases of a geotechnical site investigation. The authors will show how reliance on correlations based on index properties and failing to understand their limitations can lead to misinterpretation of soil properties. Introduction In contrast to the continental shelf, shallow geologic and soil conditions on the continental slope, in water depths of 600 ft to 10,000 ft, can be complex. The geologic conditions can include:steep slopes of 15 degrees or more;irregular and sometimes rocky topography with relief ranging from several to tens of feet;active faults with seafloor relief of up to more than 200 ft;modern and relic landslides covering large areas;gas hydrates; and seafloor erosion of tens to hundreds of feet of sediment. The soil conditions can range from underconsolidated sediments to rock.1, 2 In addition, the engineering properties of soil deposits may vary horizontally across different geologic provinces as anticipated, but can change unexpectedly with depth as well. Such complex geological and soil conditions could have adverse effect on structural siting and design. In the initial geological/geophysical phase of an integrated site investigation, most of the site condition and geologic processes should be well defined. The results of this phase of the site investigation are typically used to develop a geologic model of the anticipated soil conditions, which in turn can be used to plan the geotechnical phase. However, in the geological/geophysical phase, soil engineering properties can only be estimated, and certain localized features or changes in soil properties that can impact foundation design are unlikely to be identified. Such features or changes in soil properties include indurated layers of about 5-ft thickness or less, unusual changes in porewater or in soil stress history, and dissolved gas concentrations that can affect the quality of retrieved soil samples. The best way to investigate the site-specific engineering properties of a proposed offshore structure location is by means of a comprehensive geotechnical program both in the field and laboratory testing phases. Sometimes the field program is kept to a minimum with the idea that a comprehensive laboratory testing program will compensate for the deficiency of the field phase; in other instances, the converse is true. However, such approaches may or may not work, depending on the site geology. In the following sections of this paper, the authors provide four different examples of soil stratigraphies that have been encountered in various deepwater sites. The water depths at these sites range from over 600 ft in the upper continental slope to 4,300 ft farther down the slope. Three of the sites are from the north-central Gulf of Mexico (GOM) while the fourth is from a deepwater site outside the Gulf of Mexico.