Internal Stability of Group Column Type Deep Mixing Improved Ground Under Embankment Loading

The Deep Mixing Method (DMM), an in-situ soil stabilization technique using cement and/or lime as a binder, is often applied to improve soft soils. The group column type pattern is extensively applied to stabilize foundations of embankment or lightweight structures. An improved-ground design procedure in Japan assumes two failure patterns related to external and internal stabilities. For the external stability, a collapse failure pattern where the DM columns tilt like dominos could take place instead of a sliding failure pattern. For the internal stability, the DM columns show shear, bending and tensile failure mode, depending not only on the ground and loading conditions but also on the location of each column. However, the current design does not incorporate the effect of these failure modes, but only the shear failure mode. In this study, a series of centrifuge model tests was carried out to investigate the internal stability of group column type improved ground under embankment loading. This paper describes the failure modes of DM columns and a proposed simple calculation that takes into account the bending failure mode.