Deep mixing technology has been increasingly used to support embankments over soft soil. Deep mixed (DM) columns and soft soil form a composite foundation. Similar to other foundation systems, the DM foundation has a limit to support a certain height of the embankment. After this critical height, the columns and/or the soil between the columns start to yield and result in significantly increased settlement, which is often intolerable. In this study, a numerical study was conducted to investigate the influence of four key factors including strength, size, and spacing of DM columns and strength of soft soil on the critical height of the embankment under an undrained condition. The soft soil, the embankment fill, and DM columns were modeled as linearly elastic perfectly plastic materials with Mohr-Coulomb failure criteria. The embankment was constructed by lifts with equal thickness. The critical height was determined corresponding to a sudden nonlinear increase on the maximum settlement versus the embankment height plot. Empirical correlation was developed to estimate the critical height of the embankment built over DM foundations. In addition, a formula was proposed to estimate the bearing capacity of a DM foundation under an embankment.
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