In-situ detection of quick clay using modified penetration test apparatus

The report on the Gjerdrum landslide highlights the need to detect quick clay without sampling. Although multiple dimensionless-number-based methods have been developed, in Norwegian industrial practice, decisive detection of quick clay relies on sampling followed by laboratory testing. This is both time-consuming and expensive. It would be a great advantage if accurate identification of quick clay could be efficiently made by some sort of geotechnical sounding in the field. Ideally such a new sounding method should be based on modifying some conventional sounding equipment where a rod with an instrumented tip is pushed or drilled into the ground. The current paper addresses work based on modifying a Conical Penetration Test device (CPTU) for this purpose. Three different, modified penetrometer tips were fabricated and tested at several sites in Norway with quick clay. In addition, cyclic CPTU tests (using a conventional CPTU tip) were performed to investigate how repetitive movements remould the clay. The results show some improved remoulding in monotonic tests by using modified tips and by cyclic testing. Still, the results seem to indicate that the fully remoulded layer around the rod in quick clay is surprisingly thin, which may be part of the reason why remoulded shear strength typical for quick clay (<0,33kPa from fall cone test according to current Norwegian standard) could not be measured in the monotonic tests. Even after several cycles the cyclic tests show higher resistance than one intuitively would expect from moving the tip in a fully remoulded clay. Another important conclusion is that the CPTU devices used in our study have insufficient accuracy on sleeve friction. The o-rings connecting the sleeve to the rod introduce friction forces that prohibit measuring remoulded shear strengths in the order of 0,33 kPa or less. The preliminary results of our study will be used to develop a sounding tool that can both properly remould the clay and accurately measure remoulded shear strength.