Analytical Approach for Determination of Soil Shear Strength Parameters from CPT and CPTu Data

Abstract

Shear strength parameters of soils C and ${\phi}$ can be determined by means of laboratory tests and in situ testing data. Common approaches for determination of soil strength parameters from CPT data are on the basis of bearing capacity and cavity expansion theories. A new method is proposed for C, ${\phi}$ prediction using all quantities, q _c, u ₂ and f _s from CPTu considering bearing capacity mechanism of failure at cone tip and direct shear failure along the penetrometer sleeve. One advantage of this method is the improvement of accuracy in the case of erroneous data using all three outputs of CPTu. Laboratory test results, two sets of nonlinear equations by the proposed approach, and existing correlations of C and ${\phi}$ angle parameters have been compared by applying them on a data base compiled from six sources. In current methods, cohesive parameters are not considered in bearing capacity equation and they cause the failure loads to be transferred to the second part of the basic bearing capacity equation which is a function of the internal friction angel. It has been considered that the internal friction angle obtained by current methods is relatively higher than the measured values. Also, the comparison of predicted and measured C and ${\phi}$ angle values indicates good consistency and low scatter for the proposed method. Accordingly, this can lead to optimize design for geotechnical practice.