Deep mixing columns have been widely used to support embankments constructed on soft ground condition. A layer of shallow mixing is proposed to fix and reinforce these columns to increase their stability and the loading capacity of columned improved area. This study focuses on the external failure pattern of deep mixing columns reinforced by a stabilized shallow layer where the high strength columns fail due to large deformation without any failure inside the columns. Centrifuge model tests were carried out to investigate the effect of shallow layer on reducing embankment displacement and on the failure mechanism of deep mixing columns. High and low strength bottom layers where the columns deposit on were also considered. Acrylic pipes were used as deep mixing columns in the centrifuge tests to avoid unexpected internal failure. As a result, the tilting and sliding failures were observed as two main failure patterns of the improved ground in term of external stability. The tilting failure was found as the main failure pattern of the isolated columns. The stabilized shallow layer can significantly reduce the tilting displacement of improved area while the sliding displacement is considerably influenced by the bottom layer's strength. The effect of the shallow layer on the embankment displacement and the stability of the columned improved ground are also discussed in this study.
2016 Japan Society of Civil Engineers