FINITE ELEMENT STUDY OF THE BEHAVIOR OF STONE COLUMNS

Abstract

Stone columns have been successfully used as a ground improvement technique for cohesive soils to increase the soil bearing capacity, to reduce the resulting settlement, and to speed up the consolidation effects in the ground. The behavior of this composite ground is investigated through studyin"g three topics. The first is the installation process where an investigation and quantification of the effect of .the vibro-installation process was studied. The second is the effect of the foundation size on settlement improvement. Finally, the third is the stress concentration factor; an important design parameter defined as the ratio between the stress carried by the column and that carried by the surrounding soil. The contradiction found in literature concerning the behavior• of this parameter with different design parameters was resolved, then design charts were presented and a parametric study was carried out. A numerical model has been developed usmg. a finite element code to study the subject topics. To ensure the reliability of the analysis, the model was calibrated using records of a field, load test executed on weak layered soil improved with stone columns. The results of the study showed that the vibro-installation process significantly alters the soil state of stress. A practical range for the quantification of this effect was obtained. The study also revealed that the gain in settlement improvement increases as the foundation size gets larger. The findings of the current study proved that the conventional modeling idealizations so far used may lead to under estimation of the settlements of limited size foundations. Finally, the conflicting statements on the values and trend of stress concentration factor, n, were analyzed. Extensive numerical work was undertaken to determine the reasons behind such controversial statements. The findings of this study could finally resolve and illustrate the controversy. It could also furnish a systematic way to •estimate a reasonable value of n for a given replacement ratio and ground characteristics. The values of the stress concentration factor, n, presented in the systematic design charts showed good agreement with several field measurements.