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A site investigation in deep water often relies solely on laboratory tests to evaluate the geotechnical properties of the sediments. This imposes two fundamental limitations on the investigation: (1) the maximum depth to which the properties can be profiled and (2) the uncertainty of sample disturbance and de-pressurization effects on the measured data. This paper uses results from investigations performed in a water depth of 5.4 km at an abyssal plain site, Great Meteor East (GME), to illustrate how ambiguities can arise in laboratory measurements of strength, and discusses how a simple in-situ test, the expendable “penetrator,” can be used to corroborate the laboratory data.
Despite the layered nature of the sediments at GME, both laboratory and penetrator data indicate the overall trend to be a linear increase of strength with depth. However, triaxial tests where the sample was consolidated back to the estimated in-situ stress σ'ν indicate a strength gradient 70% greater than that determined from quick undrained (UU) tests; normalized data from tests performed at higher consolidation pressures show better agreement with the UU data. Odometer test results show little evidence of over-consolidation or cementation in the sediments, suggesting sample disturbance to be the most likely explanation of the discrepant triaxial strengths.
The results of some 14 penetrator tests allow the sediment strength profile to be extrapolated to a depth of 58 m. Tests performed up to 60 km apart yielded similar resistance profiles, and the inferred strength gradient agrees broadly with that based on the UU data. It is concluded that the sediments at GME are normally consolidated, unindurated, and not fundamentally different to other soft cohesive deposits found on land or in shallow water.
sediments, deep sea, marine geology, soil mechanics, geotechnology, shear strength, laboratory testing, in-situ testing, penetrators (or penetration testing), radioactive waste disposal
Head of Foundation Section, Building Research Establishment, Watford, Herts
Head of marine geology, Rijks Geologische Dienst (Geological Survey of the Netherlands), Haarlem,