Assessment of Undrained Shear Strength based on Cone Penetration Test (CPT) for Clayey Soils

This paper presents a rational way of developing the correlation of undrained shear strength based on the cone resistance from the Cone Penetration Test (CPT) for clayey soils. The field cone penetration test program included CPT, the index test, the one dimensional test and the triaxial test. The cone factor, which is most important for reliable estimation of undrained shear strength from cone resistance, has been evaluated considering the plasticity index of soils. The cone factor is influenced by the test methods that are used for obtaining the undrained shear strength. The isotropic Consolidated Undrained Compression Test (CIUC) for shear strength assessment has been used to effectively reflect in-situ strength. Following the field cone penetration test program, clayey soils from nine sites in Indiana have been investigated. Based on the results from the test program, the cone factor ranges from 8.0 at plasticity index of 7.9 to 12.1 at plasticity of 20.0 for Over Consolidated (OC) clays. This result parallels the increasing trend of the cone factor as the plasticity index increases, which was reported by some researchers, while other researchers showed the decreasing trends. An equation for estimating the cone factor is suggested for better geotechnical design.