A new composite column technique referred to as Stiffened Deep Mixed (SDM) column has been adopted to support embankment over soft soil to mitigate the post-construction settlement, increase bearing capacity and enhance stability. The SDM column was formed by inserting a precast concrete core pile into the center of a Deep Mixed (DM) column immediately after the construction of DM column. Due to the complicate interaction between concrete core pile, DM column and surrounding soil, the load transfer mechanism between SDM column and surrounding soil has not been well understood yet. This paper presents a series of three- Dimensional (3-D) finite-element analyses to investigate the load transfer effect of SDM column-reinforced ground under embankment. First, the numerical method was validated by a comparison with field data in terms of settlements and vertical stress on the head of SDM column. Then, based on the validated simulation method, the 3-D numerical model of the SDM column-supported embankment over soft soil was established, and the influence factors of length of DM column and concrete core pile, cross section area of concrete core pile, column spacing and embankment height on the load transfer effect were investigated, respectively. The results showed that the column spacing and height of embankment fill had significant effects on the load transfer between SDM column and surrounding soil, while the influences of length of DM column and concrete core pile, and cross section area of concrete core pile on the load transfer effect were minor. Finally, four current commonly-used design methods were assessed in the prediction of the efficacy of SDM-reinforced soil under embankment loading. The current commonly-used design methods showed great differences in the load transfer predictions of the SDM column-supported embankment over soft soil due to the existence of outer DM column socket. A modified method was proposed for the use of EBGEO (2011) in the design of SDM column-supported embankment over soft soil.

A combined ground-improvement method has recently been adopted for embankments on soft soil that involves the use of both deep mixed (DM) columns and prefabricated vertical drains (PVDs) to increase the bearing capacity and accelerate the consolidation of the soft soil. A well-instrumented embankment was constructed in a field using this ground-improvement method to verify its performance. Three- (3D) and two-dimensional (2D) finite-element analyses were conducted to evaluate the performance of an embankment constructed over soft soil improved by the DM columns and PVDs. In the 2D analysis, a plane-strain conversion method was used to simulate the 3D condition. The finite-element software ABAQUS was used for these analyses. The computed settlements, excess pore-water pressure, load transfer from the surrounding soil to the columns, and lateral displacements in both analyses are compared with the field data, and good agreement was found between the computed and measured data. The field and numerical results show that the DM columns enhanced the stability of the embankment over soft soil, whereas the PVDs accelerated consolidation of the soft soil.