Abstract

The full range of stress states; normal consolidation, "true" or mechanical overconsolidation, underconsolidation, and apparent overconsolidation; are reported for typical locations on the slope/rise of the northwestern Gulf of Mexico. On the flatter plateau regions, the upper 2-3 meters exhibits apparent overconsolidation (AOC), and the deeper sections are either normally consolidated or underconsolidated within the cored depths of 20 meters. The underconsolidated zones, with OCR/SSR values of about 0.5, seem to correlate with thick deposits of finely laminated sediments. The possible high rates of sedimentation of the laminates and presence of the AOC zone may be the cause of underconsolidation, but other hypotheses are considered. The in situ effective stresses would be reduced due to the excess pore water pressures that have implications for slope stability analyses and other geotechnical applications. Deep tow, bathymetric data, and core data from most of the slope regions within the deeper basins indicate the full range of mass wasting processes including translatory/slab slides, deep seated rotational slumps and debris flows. Consolidation test results from overconsolidated zones can be used to estimate the amount of sediment removed by mass wasting processes. The analysis of three basin slopes where cores were obtained indicates that 5, 14, and 15 meters have been removed from these sites.

Introduction

During the 1990's there was increased interest by engineers and scientists in the seabed of the deepwater areas of the northwestern Gulf of Mexico. In order to better understand the seabed processes in this region, a five-year joint project by the University of Rhode Island and Texas A&M University was initiated in 1996. This project is sponsored principally by the National Science Foundation, with supplemental funding from industrial sponsors that now include Chevron, Conoco, Marathon, Phillips, and Texaco (Amoco was a sponsor for one year). The general study area, which includes most of the typical seabed features on the slope and rise, is shown in Fig. 1. Several OTC papers resulting from the first few years of research have been published in proceedings of OTC 1999 and OTC 2000. This paper summarizes the results of laboratory experimental studies relating to the state of stress (stress history) of the sediments in the study area and the relation to slope processes of specific areas.

In this paper we are interested in the sediment stress profile, i.e. the vertical variation of effective stress and pore water stress, of the upper 20 meters of the seabed in typical geological settings. If excess pore water pressures (often termed overpressures) exist in the sediment column, there are obvious implications for engineering applications and for analyses of seabed processes such as stability of slopes. Knowledge of the stress state, especially overconsolidation, can also be very useful in determining where and how past slope failures have occurred and assist in interpretation of similar features.

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