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Stone columns have been widely used as a cost and environmental friendly method for soft soil treatment. For situations when the undrained shear strength (c u ) of soil is too weak, stone columns may lose their effectiven ess as the surrounding weak soils may not provide enough confinement to the columns. In that case, geosynthetic (i.e. geotextile or geogrid) encased stone columns (GECs) overcome the shortcomings and provide lateral confinement to the stone materials to im prove the bearing capacity of the soils. GECs have been successfully used for road embankment or dike foundations. However, the stability of GECs in soft soil under embankment is still far from clear. In this study, centrifuge model tests were performed on embankments supported by GECs with different reinforcement stiffness and encasement lengths. The test results show that, for the ordinary stone columns (OSCs) composite foundation, columns under embankment are apt to bulge with no shear slip trend to incu r significant settlement because the stones are squeezed into the soft soil. For the GECs with half length encasement composite foundation, columns at the centerline of the embankment are mainly compressed and incur obvious bulging deformation at the junct ion of the encased and un encased portions. Meanwhile columns under embankment slope and near slope shoulder can tilt and bend largely due to insufficient bending stiffness of columns. The above two factors lead to the greatest settlement of the GECs with half length encasement composite foundation. For the GECs with full length encasement composite foundation, columns at the centerline of the embankment suffer vertical compression deformation while those under the embankment slope bend outwards. The bendin g deformation and settlement decrease with an increase of encasement stiffness. Full length encasement with high stiffness is required for the GECs composite foundation embankment to reduce the settlement and to ensure stability in practical application.