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Theses

Physical and mechanical characterization of deep water sediments

Manh Hai Le
Abstract : Offshore oil exploration and production has been enlarged to the continental slope and some fields are now being developed in Gulf of Mexico and Gulf of Guinea in water depths from 1500 to 2000m. The sediments encountered by great water depths on the continental slope of the Gulf of Guinea are composed or very fine particles and are characterized by very high water contents (w >100%), very high plasticity (IP>80), moderate sensitivity (3< St <5) and strong potential in shear strength regain due to thixotropy. Moreover, a crust zone of anomalous high strength immediately below the seabed has been reported in several deepwater oilfields. These characteristics are different from sediments of Gulf of Mexico. The aim of the experimental studies presented in this thesis is to improve the understanding of the soil behaviour and to investigate the effect of their nature (mineralogy, physico-chemistry) and microstructure. Bioturbation and geochemical changes in the sediment have been thought as possible causes for the crust zone. The effect of sedimentation rate's change in this zone during the last ice age is also discussed. Bioturbation has been shown in previous studies to affect geotechnical engineering properties. Mechanical and physicochemical measurements have been performed on samples coming from sites with and without crust zone. At this stage, the origin of the crust zone remains partially unknown. Several improvements in testing procedures for the identification of soils have been proposed (measurement of water content, organic matter content, mineralogy, grain size distribution). In order to understand the thixotropy characterization of soft deep marine soils, some rheological tests have been also carried out on rheometer following an existing approach for colloidal suspensions like drilling mud. The mechanical behaviour of marine soils has been investigated through a large number of triaxial and oedometric tests. The mechanical behaviour of Gulf of Guinea soils is characterized by very high compressibility weakly correlated with the index properties, contrary to what usually observed in clayey soils. Progressive breakdown of the sediment initial structure under compression and shearing has also been observed. Once this occurs the soil tends to behave as its parent remoulded soil. The effect of structure on the mechanical behaviour of Gulf of Guinea sediments seems well related with their sensitivity as observed in many continental types of clay. The soil microstructure has also been examined by using Scanning Electron Microscopy and Mercury Intrusion Porosimetry. Observation carried out on intact and remoulded soils and on soils consolidated at various levels in one dimensional compression allowed an insight into soils destructuration features.
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Submitted on : Tuesday, June 17, 2008 - 8:00:00 AM
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Manh Hai Le. Physical and mechanical characterization of deep water sediments. Sciences of the Universe [physics]. Ecole des Ponts ParisTech, 2008. English. ⟨pastel-00003828⟩

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