Calibration Chamber Studies of Piezocone Test in Cohesive Soils

Abstract

The authors have presented a series of piezocone dissipation tests in reconstituted kaolin specimens. From a comparison of the measured radial coefficient of consolidation and the values interpreted from dissipation data, it was concluded that interpretation of the dissipation results to evaluate the radial coefficient of consolidation should be based on the initial dissipation values of excess pore pressure, and not the penetration pore pressure. The discusser will show that the dissipation data presented may be interpreted rationally using the analysis by Teh and Houlsby (1991), which was based on the penetration excess pore-water pressure. Firstly, some features of the measured dissipation data will be examined. It was noted that the tests performed on soil specimen 1 showed a distinctly different behavior from the other tests. The penetration pore pressure measured at the cone tip in soil I was higher than that measured at the base. The opposite was true in other tests, with higher pore pressure at the cone base. This difference was also reflected in the dissipation data presented in Figs. 5-8. A significant drop in tip pore pressure was observed at the early stages, with reduction of 12%, 26(/(., and 16% in specimens 2, 3, and 4, respectively. The initial pore-pressure reduction in specimen I was comparatively smaiL The authors attributed the initial drop in pore pressures to the reduction of normal stresses on stopping penetration. However, any reduction in normal stresses would be reflected in a decrease in cone-tip resistance, and this could be verified by monitoring the tip resistance during dissipation. Teh and Houlsby (1991) have studied cone penetration into an isotropic material using the strain-path method. Although the analysis cannot properly account for the different stress histories of the soil, some features of the analytical results may provide better insights to the interpretation of piezocone dissipation test. The analytical results show that penetration pore-water pressure, is highest at the cone tip and cone face region. This is consistent with the data for soil specimens 2-4 but at variance with data in specimen I. The dissipation rates monitored at the cone tip and the cone base are governed by the spatial distribution of excess pore pressure around the cone. Cavity expansion theories assumed a one-dimensional variation of flu, which decreases logarithmically with distance. However, the pore-pressure distribution around the cone is more complicated and one-dimensional analyses such as cavity expansion theories may not provide an adequate basis for interpreting piezocone data. It should be noted that the theoretical dissipation curves presented by Teh and Houlsby (1991) were based on the penetration excess pore pressure, flu. Hence, it was incorrect to use it to interpret dissipation data normalized by initial