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.
The isolated deep mixing columns, particularly the columns near the slope of the embankment, were reported to experience a minor horizontal resistance while supporting an embankment. In order to reinforce these columns and increase the stability of the improved area, which includes the group of columns, a shallow layer produced by shallow mixing method was proposed as a countermeasure. While many studies reported that the isolated columns may fail either by the external failure or the internal failure, the failure of this combined structure including the deep mixing columns and the shallow layer has not well studied yet. In the previous study, overturning and sliding failures were observed as the leading patterns in terms of the external stability, when the combined structure to supports embankment slope. In addition to the study of the external stability, the internal failure pattern of the columns reinforced by the shallow layer was concentrated in this study. It is worth to note that the lowstrength columns are used here. Four centrifuge model tests were performed by varying the column strengths in models with and without the shallow layer of reinforcement. The results showed a significant influence of the shallow layer on the failure pattern of the deep mixing columns. The study further discusses the effect of the shallow layer and the column strength on the stability of the supported embankment.
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