The abyssal plains of the Great Meteor East (GME) at 30–33 °N, 23–26 °W and the Southern Nares Abyssal Plain (SNAP) at 22–24 °N, 62–65 °W have distinctly different sediment stratigraphies sand stress states The GME site, with laterally continuous, thick interbedded turbidites, has higher strengths than at the SNAP site, and is normally consolidated to the cored depth of 34 m The sediment lithology and properties in the SNAP site are vertically and laterally more variable, with interbedded fine-grain thin turbidites and pelasgic clays Consolidation tests on large-diameter high-quality core samples from the SNAP reveals an 11 m zone of significantly underconsolidated sediment from 9 m to 20 m subbottom. The results of stress strain tests are consistent with the hypothesis of underconsolidation in the SNAP site. Possible reasons for the underconsolidation are discussed.
The isolation of nuclear wastes within the seabed would rely primarily on the sediment to act as a barrier between the waste materials and the overlying waters Knowledge of the engineering properties and behaviour of the sediments is necessary at virtually all stages of the subseabed programme. Many natural processes, such as slumping, erosion, compression, and water migration are controlled by geotechnical properties The interactions between the delivery system and the sediment, such as penetration and closure of the cavity, are closely related to engineering properties. Long-term processes due to thermal effects can have important implications on the suitability of the sediment strata for containment of radioactive waste.
Some of the more important processes and associated geotechnical properties for an idealized homogeneous clayey sediment column (i e, no stratification, faulting, etc) are illustrated in Fig 1. The upper part of the diagram shows some of the possible stress states due to natural processes and overburden conditions Of particular importance is the possibility of having an underconsolidated condition at depth leading to vertical advection of pore water. Obviously any preexisting condition, such as excess pore water pressures, would have to be factored into ion migration models The lower part of the diagram illustrates the effects of a localized heat source Elevated temperatures can result in important mineralogical alterations which could lead to changes in engineering and geochemical properties. Temperature gradients will cause flow of pore water, cause localized densification due to consolidation, and result in other changes in sediment properties that must be considered in near-field modelling (Percival et al, 1987, Technical Report by Ove Arup and Partners, 1987)
In this chapter we focus our attention on two different abyssal plan areas m the Atlantic that have been considered as possible sites for disposal of high-level nuclear wastes. The samples used in these laboratory studies were collected in 1985 during the ESOPE cruise on the RN Marion-Dufresne (ESOPE Cruise Data Report, 1986)