Deep mixing (DM) is one of the most commonly used techniques to improve soft soils under embankments. When Bishop's modified method and Spencer's three-part wedge method are used to analyze slope stability of column-supported embankments, they assume that the embankment fails along a slip plane through soils and DM columns that mobilize their shear strengths. However, some numerical studies and centrifugal model tests showed that DM columns might fail in a form of bending or rotation under an embankment loading. The three-dimensional (3-D) finite element method incorporated in the ABAQUS software was used in this study to investigate the failure modes of DM column-supported embankments over soft soils. The subsoil consisted of a 10 m thick soft soil layer, underlain by a 2 m thick dense sand layer. Two conditions were considered in the numerical models: (a) the DM columns were embedded into the dense sand layer and (b) the DM column bases were seated on the top of the sand layer. The shear strength reduction technique was implemented to evaluate the factors of safety of the DM column-supported embankments over soft soils. The factors of safety computed by the numerical analyses were compared with those using Bishop's modified method and Spencer's three-part wedge method. Numerical results indicated that the failure modes of bending and rotation might happen to the DM columns. Bishop's modified method yielded a higher factor of safety than those from the numerical method and Spencer's three-part wedge method.
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