An improved technique for evaluating the CPT friction ratio

Abstract

This report examines a statistical technique known as the cross-correlation function (CCF) for determining the shift distance associated with the cone penetration test (CPT). When evaluating the friction ratio, FR (= fs/qc), for soil classification purposes, it is essential that the measured values of qc and fs are shifted relative to one another because of the depth discrepancy between the cone and the friction sleeve. Generally, the shift distance is estimated by means of empirical and subjective methods. Using a series of case studies, this report demonstrates that the CCF is a very useful and objective technique for estimating the shift distance. In addition, a phenomenon, associated with sleeve friction measurements which is related to elastic rebound of clay soils, is discussed. Finally, in iterative technique is proposed for determining the layering of a soil profile based on the CPT.