Impact of sample quality on CPTU correlations in clay – Example from the Rakkestad clay

Abstract

As part of the zoning plan for the new 30 km long highway, E16, from Nybakk to Slomarka, an extensive laboratory and field testing campaign was conducted by NGI. The deposit along the highway is a normally to slightly overconsolidated clay with a water content in the range of 30-45% and a plasticity index ranging between 7-25%. 72 mm diameter piston samples at 70 localities were taken and CPTUs were carried out at over 120 locations. About 180 CRS oedometer tests and 360 triaxial tests were performed. These results generally show good to excellent sample quality. However, due to the interpreted lower values of undrained shear strength from local CPTU correlations, 9 block samples were retrieved at 3 locations and additional laboratory testing was performed. The active undrained shear strength obtained from the block samples was up to 53% higher. Based on this data set, correlations were established to optimize the engineering solutions for the new road and to address the impact of sample disturbance on geotechnical engineering parameters. The results lead to important economical saving for the project and highlight the need for local and high quality samples for CPTU correlations in soft and sensitive clays.

Impact of sample quality on CPTU correlations in clay – Example from the Rakkestad clay