Sample Disturbance Effects in Deep Water Soil Investigations

This paper presents the first phase of a joint industry project. The objectives of the project have been to examine the problem of sample disturbance in deep water sampling practice. The problems with dissolved gases are not addressed in this first phase. The Norwegian Geotechnical Institute (NGI) has carried out a comprehensive laboratory test programme on two clays. The low plastic Lierstranda clay (I{sub p} = 10-17%) and the medium to high plastic Bothkennar clay (I{sub p} = 45%). Benchmark reference soil parameters have been obtained from CAUC triaxial and CRS odometer tests on high quality block samples. The same laboratory tests have been repeated on samples obtained with piston tube sampling and samples subjected to extra disturbance to simulate thick wall gravity core sampling. A series of special tests have also been carried out where sampling in 1500 m water depth have been simulated in the laboratory. Based on these test series it has been possible to quantify the amount of disturbance caused by thick wall tube sampling, and also by large stress relief which occurs when sampling in 1500 m of water. The results have been used to develop a new criteria for sample disturbance and will also be used to establish procedures for correcting results of CAUC and CRSC tests to arrive at soil parameters that are as close as possible to in situ conditions. (author)