Design method for calculating settlement of stiffened deep mixed column-supported embankment over soft clay

As a composite column, stiffened deep mixed (SDM) column is formed by inserting a precast concrete core pile into the center of a deep mixed (DM) column. The SDM columns have been successfully used to support highway and railway embankments and buildings over soft soil. However, there has been still a lack of feasible method to calculate the settlement SDM column-reinforced soft soil under an embankment load. This paper developed a theoretical solution to calculate the settlement of SDM column-supported embankment over soft soil. Based on the unit cell concept, the total settlement of the SDM column-reinforced soft soil consisted of three components, i.e., the compression of soil within the length of stiffened core pile, the compression of soil from the core pile base to the SDM column base, and the compression of soil below the SDM column base. The upward and downward penetrations of stiffened core pile were considered in the analysis. The analytical solution was verified by a comparison with the results computed by three-dimensional finite element analyses. A parametric study based on the derived solution was conducted to investigate the influence factors of modulus, length and diameter of DM column, length and diameter of core pile, and interface friction angle between DM column and core pile on the settlement of SDM column-reinforced soil, and some recommendations were proposed for its application in practice. The design charts for settlement calculation were developed for the ease of use in design. The design method was applied to two case histories of SDM column-supported embankments, and good agreements were found between the predicted settlements and the field measurements.