Stone columns with granular material are often used to support embankments over soft soil. In extremely soft soils, encasing the granular column with geosynthetic material improves their stiffness and strength. Based on the soil and column properties, settlements and stresses can be determined by means of analytical solutions or numerical analyses, and this paper compares the results of these two procedures by means of parametric analyses. Two-dimensional finite-element analysis (FEA) was used to simulate the behavior of a geosynthetic-encased stone column in soft clay loaded by an embankment. Parametric studies are then carried out by varying the thickness of the soft soil layer and the tensile stiffness of the geosynthetic encasement for a given set of soft clay and column parameters. The results of FEA have shown that the influence of the geosynthetic encasement on settlement improvement increases in shallow soft soil layers. It was also observed that the dependence of stress concentration on the thickness of the soft soil increases with geosynthetic stiffness. The critical embankment height where ground settlement is negligible was also investigated.

A full-scale test embankment was constructed on a soft deposit (S_u < 20 kPa) improved by geotextile-encased granular columns to assess its serviceability performance for a nearby stockyard foundation. The behaviour of the improved foundation was monitored by means of instrumentation with respect to surface settlements, excess pore pressures, total vertical stresses acting on the top of the encased column and surrounding soft soil as well as geotextile radial deformation. Two-dimensional finite-element analysis was performed using an axi-symmetric unit cell, and then the results were compared with field measurements. It was observed that the soil arching progressed during post-construction due to a decrease in apparent stiffness of soft clay from quasi undrained to drained stiffness. Furthermore, there was a difference in settlement between the top of the encased column and the surrounding soft soil, midpoint between the columns, which increased during both construction and consolidation stages. A measured relation was also found to correlate the geotextile expansion with the maximum settlement occurring below the embankment.

Compacted granular columns are commonly used to support embankments over soft soils. Using a reinforcement layer under the embankment causes the total stress to be further transferred to the column rather than soft soil, thus reducing total deformations of the subsoil. In this paper two dimensional (2D) numerical analysis was used to study the influence of stone columns and basal geosynthetic on deformations and stability of an embankment over soft deposit by means of Plaxis 2D finite element code. Unit cell to plane strain conversion approach was applied to transform columns into equivalent walls thus allowing to simulate a full embankment over a group of columns. Comprehensive parametric analysis was then performed to investigate the role of different critical parameters on embankment behavior. Results showed that the use of stone columns yielded the total deformations of the subsoil to significantly reduce, while its influence was less remarkable as a high stiffness geogrid was placed under the embankment. It was also found that the stone column length was the most influential parameter on the embankment total deformations, so that increasing columns length from 0.25Hs to 0.75Hs reduced the vertical and horizontal deformations by about two and five times, respectively. In addition, the use of a high stiffness basal geogrid caused the stability of the embankment to remarkably improve as the value of safety factor at the end of construction increased from 1.25 to about 1.9.

This paper compares the field response of two test embankments constructed on similar soft soil deposits. The un-strengthened foundation had vertical drains underneath the embankment and the strengthened foundation had geotextile-encased granular columns, both provided with basal geogrid reinforcement. The effectiveness of the encased granular columns was assessed by comparing the settlement below the embankments, the horizontal displacement of the soft foundation and the excess pore pressures in the middle of the soft clay layer. The results showed that the granular encased columns significantly reduced the maximum settlement occurring below the embankment as well as the maximum horizontal displacement in the clayey foundation. At the same settlement level, the load-carrying capacity of the strengthened foundation was found to increase by as much as around 2·5 times that of the un-strengthened one. It was also observed that the geotextile-encased granular columns caused a noticeable reduction in the maximum excess pore pressure following load application. Furthermore, the degree of consolidation achieved at the end of embankment construction was quite high when encased granular columns were used.