Cone penetration testing in the clay tills of Skåne - An investigation of the cone factor

Abstract: The undrained shear strength is a key parameter in the design process of geotechnical structures. One of the most frequently used methods to investigate this parameter is the cone penetration test (CPT). The CPT does not measure the undrained shear strength directly, but instead the resistance that a conic probe encounters as it is pushed into the ground. A calibration factor, the cone factor (Nkt), relating the two quantities is, therefore, needed. The Swedish Geotechnical Institute suggests to use a cone factor of 11 when using CPT in the clay tills of Skane. However, in Denmark, a value of 10 is recommended for similar soils. The magnitude of the calibration factor is critical when determining the undrained shear strength and, consequently, for further design. To investigate the cone factor, measurements of the undrained shear strength and the net cone resistance were compared for five types of clay till found in Skane at a total of three different sites. Depending on availability, other soil parameters were considered in the analysis in order to explain variations of the calibration factor Nkt. The comparison between the undrained shear strength measurements, from field vane tests, and the net cone resistance gave a mean value of Nkt in the range of 9.3 to 10.2. However, a significant standard deviation, ranging from 0.7 to 1.7, is connected to the evaluated cone factors. Due to the significant degree of scatter in the data, partially caused by the inhomogeneous nature of the soil and partially influenced by primarily using field vane tests to evaluate the undrained shear strength, it is hard to draw exhaustive conclusions. However, for the different soils studied in this project, it seems reasonable to use a cone factor lower than 11. It also appears that the cone factor is closely linked to the overconsolidation ratio, though more tests at various locations are needed to verify this hypothesis.