Consolidation analysis of soft soil improved with short deep mixed columns and long prefabricated vertical drains (PVDs)

ABSTRACT: Soft soils may pose many challenges to construction of roadway embankments, such as low bearing capacity, excessive settlement, and slope instability. A combined method involving the use of deep mixed (DM) columns and prefabricated vertical drains (PVDs) was proposed to reinforce the soft soils. In the application of the combined method, the DM column is usually shorter than the PVD. To obtain an analytical solution of the composite foundation with short DM columns and long PVDs, the drainage system was converted into a cylindrical drain wall with equivalent properties, and only the radial drainage was considered. An axi-symmetric unit cell was established and the solution of average degree of consolidation was derived. The analytical solution was verified by comparisons with the results computed by three-dimensional finite element analyses. A parametric study was conducted to investigate the influence of diameter of DM column, moduli of DM column and soft soil, permeability of DM column, soft soil and smear zone, and improvement depth of DM column on the soil consolidation and some recommendations were proposed for its practical application. Based on the proposed solution a design chart was developed which can be used to easily obtain the degree of consolidation for soft soil improved by the combined method. The proposed design method was applied to a case history of an embankment on soft soil with short DM columns and long PVDs. Good agreement was found between the predicted average degree of consolidation and the field data.