Full-flow CPT tests in a nearshore organic clay

Abstract

The T-bar is a full-flow CPT with a larger probe surface area compared with the conventional CPT probe. The tip has the shape either of a ball or as an upside-down T, a so-called T-bar. The term ‘full-flow’ comes from the assumption that the earth ‘flows’ around the tip, which is a realistic assumption for soils with extremely low undrained shear strength. This paper presents a case study where a large number of T-bar tests have been performed in a nearshore organic clay with high water content and very low undrained shear strength. The test site is located in Stockholm, Sweden, where planned land reclamation and capping of contaminated top soils are challenging from a stability perspective due to the low strength of the soil. T-bar tests were thus performed to characterise the shear strength profile of the soil in detail, especially at shallow depths where sampling was difficult and the shear strength values were under 5 kPa. A N-factor relating the net cone resistance (qnet) and the undrained shear strength of the soil (cu) was evaluated based on T-bar measured parameters and direct simple shear tests and undrained triaxial tests on samples taken at greater depths. This allowed to estimate shear strength profiles with depth. For all tests, both the penetration and extraction cone resistance were measured, and a good correlation was obtained between this ratio and the soil sensitivity measured in the laboratory. In addition, the sensitivity was correlated to the organic content of the clay. These correlations were found to be OCR-dependent. Further, cyclic tests were performed, and their results were correlated with the remoulded shear strength values. This facilitated mapping of the soil conditions across the site.