Treated soil by the deep mixing method (DMM) is characterized by the extremely high shear strength and Youndg's modulus in comparison with those of untreated soil.
　In the present study, the mode of failure of the treated soil mass is examined by centrifuge modeling technique. The scope of the study is limited to the treated soil block which is seated on a reliable sandy layer and bears the combination of vertical force due to the self weight of upright breakwater and horizontal force due to waves. In this condition, the failure model of the treated soil block is by the development of cracks. At the peak load level, crack appears on the vertical line passing through the toe of the caisson. In the cases where the unconfined compressive strength of the treated soils are lower than 2 kgf/cm2, relatively small reduction of the load is observed after the peak load is attained. Whereas, large reduction of the load is observed for the cases in which unconfined compressive strengths are approximately 5 kgf/cm2. Also found is that the current design procedure for the internal stability gives a reasonable index of the peak load if the boundary stresses are appropriately selected.