Interpretation of Cone Penetration tests in Cohesive Soils

Abstract

This report focuses on the evaluation of the factors affecting cone resistance measurement during cone penetration in saturated clayey soils and the application of the result to pile shaft capacity analysis. In particular, effects of drainage conditions around the cone tip were studied. Rate effects related to both drainage and shear strength dependence on loading rate were studied. In order to investigate the effects of drainage during penetration, penetration tests were performed with various velocities in the field and in a calibration chamber, and the obtained data were analyzed. For the field tests, two sites which have homogeneous clayey soil layers below the groundwater table were selected, and CPTs were performed with various penetration rates. Penetration tests in the calibration chamber were performed to investigate the transition points between undrained and partially drained, partially drained and fully drained conditions based on cone penetration rate and the coefficient of consolidation. A series of flexible-wall permeameter tests were conducted for various mixing ratios of clays and sands to obtain values of the coefficient of consolidation, which is a key variable in determining the drainage state during cone penetration. Nine piezocone penetration tests were conducted at different rates in calibration chamber specimen P1 (mixture of 25 % clay and 75 % sand) and eight penetration tests were carried out in calibration chamber specimen P2 (mixture of 18 % clay and 82 % sand). From the results of the penetration tests in the calibration chamber, a cone resistance backbone curve, with qc plotted versus normalized penetration rate, was established. Guidelines were proposed for when to interpret CPT tests, whether in estimating soil properties or in estimating pile resistances, in soils for which penetration takes place under conditions that cannot be established as either drained or undrained a priori.