Summary
Little is known regarding the engineering properties of deepwater sediments. Bathymetric data from the Gulf of Mexico (GOM) show unusually steep slopes which are inconsistent with the anticipated low strength sediments whereas limited high resolution subbottom data indicate gassy sediments and evidence of creep and slope instabilities on low angle slopes. The goal of this research is to investigate the behavior of deepwater sediments, examine potential geohazards, determine the relative importance of these features and propose analysis techniques if appropriate.
Part of the OTRC strategic plan is to develop improved understanding of the critical engineering properties of deep ocean sediments as they interact with foundations under realistic long term loading conditions. Three theme sites have been identified to initiate this study. This project will focus on obtaining information from the theme sites and analyzing the data.
The first step in the program was a preliminary survey in May 1994 at one of the theme sites – the Pigmy Basin. Sediment cores were obtained at several locations including the rim, slope and floor of the basin. Samples from these cores are being subjected to extensive consolidation, strength and stress path controlled tests to develop their engineering behavior and sedimentation history. A deep tow survey system will be used to obtain higher quality and more detailed subbottom high resolution and side scan sonar records of the Pigmy Basin and the Aliminos Canyon in February and March 1996. More cores will be obtained for additional testing and analysis with emphasis placed on determining the in situ state of stress on the floors and sidewalls of the theme site basins and canyons. This project interacts with four other OTRC projects regarding characterization of the seafloor and slope stability analysis.
There is scant research being conducted elsewhere on the characterization of deepwater sediments, other than for purely scientific purposes. We are in regular contact with the Ocean Drilling Program, but most of their work is not applicable to locations of interest to us. Some of the deepwater geotechnical data obtained by oil companies and service companies have been made available, but analyses of these largely proprietary data are not forthcoming.
Examining three sites in the GOM will not expose each type of sediment condition and geohazard that will exist in the deep water environment. However, each site has distinctive characteristics that are worthy of study. This project should provide the following: create an awareness of the potential geotechnical problems in the deepwater environment; help to optimize techniques for conducting deepwater site investigations; develop analytical approaches for analysis of some of the critical geotechnical problems in the deepwater environment.
