CPTU-detection of thin clay layers in sand: Results from calibration chamber tests
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The detection of thin clay layers (i.e. ≲ 20 cm) is challenging for all conventional geotechnical field investigations techniques, including high quality CPTU tests. During the last two years, a research program has been carried out in the geotechnical laboratory at NTNU in Trondheim. The work aims to identify possibilities and limitations in detecting thin clay layers and assess their properties using the CPTU tool. Tests were run in a pressurized chamber where thin horizontal clay layers of both pottery clay and natural, sensitive clay were embedded in a homogenous, medium dense sand. Both a standard piezocone (10 cm2) and a mini-piezocone (5 cm2) have been utilized. The results show to what degree the CPTU response in thin layers is influenced by the surrounding sand, and how this influence in practice may lead to serious overestimation of shear strength in thin layers. In addition, the effect of depth-offset of measurements in connection with soil type characterization was evaluated.