Effect of Column Stiffness on Consolidation Behavior of Stone Column-Treated Clay

Stone columns have been proved an effective ground improvement technique. The increase in land prices and the existence of soft clay deposits in many areas around the world has encouraged the use and development of this technique. Stone columns can be used to accelerate the consolidation of soft clay deposits through two mechanisms. First, the stone columns act as vertical drains that provide additional drainage path for excess pore water pressure to dissipate. Second, they reduce the excess pore water pressure by transferring more load from the soil to the columns because they are stiffer than the surrounding soil. Stone columns may not work well in very soft soil due to bulging of the columns. Woven geotextile has been used to minimize bulging of the columns and improve their performance (capacity and stiffness). For the geotextile encased stone columns, geotextile filtration adds another benefit by minimizing fine particles migrating from the surrounding soil into the stone columns and maintaining their high permeability in a long term. The increased stiffness of the stone columns by the geotextile is expected to have an effect on the consolidation rate. However, this effect has not been well investigated. This study investigated the effect of column stiffness on the consolidation behavior and the stress transfer mechanism of the stone column-reinforced foundation through several small-scale model tests. Both ordinary and geotextile encased stone columns were utilized. For comparison purposes, a special PVD “column” was formed, which had nearly zero stiffness. In each test, a soil bed was first formed from a slurry by preloading in a rigid steel chamber with a dimension of 280 mm in diameter and 450 mm in height. A model stone column of 100 mm in diameter was installed in the center of the soil bed to the bottom of the steel chamber. Piezometers were placed inside the chamber at different depths and distances to the center to measure the generation and dissipation of excess pore water pressure. The pressure was applied in increments on a steel plate seated on the soil. After the application of each pressure increment, the pressure was maintained until the plate displacement was smaller than 1 mm/day. The vertical stress transfer between the soft soil and the column was monitored by earth pressure cells on the soil and the column. The test results show that the ordinary stone columns effectively reduced the consolidation time by 25%, as compared with the PVD “column”, while the encased stone columns further reduced the time by 40%. The ordinary stone columns also reduced the settlement by 36% as compared with the PVD “column” while the encased columns further reduced the settlement by 56%. The test results show that the stone columns with and without geotextile encasement carried more load than the surrounding soil. The steady stress concentration ratio, defined as the stress on the column to that on the soil, was found in the range of 4 to 6 for ordinary stone columns while it ranged from 4 to 11 for the encased stone columns. The modulus improvement factor, defined as the modulus of the PVD “column” treated foundation to that of a stone column treated foundation, was found to be 1.7 and 2.4 for the ordinary and encased stone columns, respectively. Finally, the theoretical solutions developed by Han and Ye (2001, 2002) were used to compare with the test results. The comparisons show good agreement between the theoretical solutions and the test results