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Two intrusive in-situ measurement systems, the C-CORE Shear Wave Profiling System and the Oxford University Densimeter, were used to carry out in-situ profiles of bulk density and shear wave velocity in contaminated dredge spoils. The spoils have high water content and, at the time of the survey, exhibited densities in the range of 1160 kg/m3 to 1302 kg/m3 and undrained shear strengths in the range of zero to 2.7 kPa, in the uppermost 3.85 metres of the deposit. The objective of using the above technologies was to provide a profile of in-situ small strain shear stiffness of the deposit through the combination of the bulk density and shear wave velocity measurements.
Small strain shear stiffness values were calculated to produce a profile 3.85 metres deep within the deposit. These data revealed much information about the in-situ conditions of the deposit. The upper 1.00 metre was unable to support the transmission of shear waves and therefore was assumed to have zero shear stiffness. This upper region represented the most recent deposit in this area of the repository and it is likely that this material had had insufficient time to develop a continuous, soil matrix. These zero stiffness measurements were coincident with the zero undrained shear strength measured using a shear vane apparatus. Beneath this surface layer, three other distinct layers were found based on the small strain shear stiffness measurements. The maximum value of stiffness, which was measured at the bottom of the profile, was 129 kPa ± 3 kPa. There was some evidence to suggest that shear wave amplitude and velocity were affected by gas contained within the sediment. Close correlation between shear velocity and stiffness and that of undrained shear strength was found in the upper region of the deposit.
in-situ measurement, small strain shear stiffness, bulk density, undrained shear strength, dredge spoils, soft soil
Director, Geophysics at C-CORE, Memorial University of Newfoundland, St. John's, Newfoundland
University Lecturer, University of Oxford, Oxford,