Evaluation of Soil Strength from Variable Rate CPTU Tests in Silt

The interpretation of cone penetration test (CPTU), at the standard rate of 20 mm/s, results in clays assuming undrained penetration tests, whereas the interpretation in sands is carried out using a drained approach. However, for silts, the drainage conditions during a standard rate test are less well understood, and often partial drainage occurs. The main objectives of this work are to show the limitations of applying existing correlations for sand and clay to a silt material and to give guidance to practicing engineers on how to establish strength parameters for silts that allow a safe design. This paper presents the results of a comprehensive laboratory and field test study that was carried out on a silt site in Halsen-Stjørdal, Norway. Varied rate CPTU tests were conducted, and various soil samples were taken. Using a conventional Nktapproach to determine undrained shear strength, standard rate CPTU tests provided 20 % higher values compared with high-rate tests. Thus, the undrained strength may be overestimated from standard rate tests. Care needs to be taken when using undrained shear strength in silts. Nevertheless, this value represents a limiting strength value, which is often of interest when investigating the response as if it were undrained. The friction angle could be successfully determined from CPTU results by applying the modified Norwegian Institute of Technology (NTH) method to the measured data set. The results reveal a rate dependency of the friction angle, resulting in an underestimation of 20 % if a standard rate test is used compared with using low-rate CPTU tests, giving drained results that match the reference values from the laboratory. To overcome uncertainties in picking design strength values, it is highly recommended to carry out at least one fast and one slow CPTU test to evaluate the drainage situation and to adjust the design values accordingly.