An improved statistically based technique for evaluating the CPT friction ratio

This paper 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 (=ƒs/qc), for soil classification purposes, it is essential that the measured values of qc and ƒs are shifted relative to one another because of the physical offset between the cone and the friction sleeve. Generally, the shift distance is estimated by means of empirical and subjective methods, a value of 75 to 100 mm is adopted, or it is ignored all together. Using a series of case studies, this paper demonstrates that the CCF is a useful and objective technique for estimating the shift distance. In addition, a phenomenon associated with sleeve friction measurements related to elastic rebound of clay soils is discussed.M B Jaksa, W S Kaggwa, P I Brooke