Abstract: The variable diameter deep mixing method is proposed to economically and efficiently treat a layered soft ground, where the soft layer is at the middle depth. A relatively high column replacement is used for the high compressive soil layer and a relatively low column replacement is used for the medium compressive soil layer, resulting in a layered composite foundation with the column replacement varying with the soil layer. 3D numerical analysis is carried out to investigate the performance of the layered soft ground treated by variable diameter shaped deep mixed columns with different design parameters. The different numerical cases of variable diameter shaped deep mixed columns-treated layered soft ground have the same equivalent area replacements (volume replacements), but different column diameter ratios (D/d). The results indicate that the settlement at the ground surface is remarkably affected by D/dand there is an optimum D/dfor the minimum settlement. However, the maximum lateral movement under the embankment toe is only slightly affected by D/d.