Deep Mixing Method (DMM), a deep in-situ admixture stabilization, has been developed in Japan to reinforce soft alluvial clays. In practice, a huge treated soil
mass whose shear strength exceeds 1 MPa is formed by the method to support a superstructure. The treated soil mass is assumed to behave as a rigid structure buried in the soft ground because of the large difference of the engineering characteristics between treated soil and untreated soil. Therefore, stability of the improved ground is examined in two stages, external and internal stability analyses. In these analyses in both stages, earth pressures acting on the vertical faces of
treated soil mass are assumed to be a combination of active and passive earth pressures. However there was no case histories which verify these assumptions and the current design procedure. A series of model tests has been carried out by means of geotechnical centrifuge to investigate the prototype behavior of the improved ground.
　It is known that external forces are carried solely by the treated soils, that the magnitude and distribution of contact pressures at the surface of treated soil mass are dependent on the magnitude of factor of safety against external stability, and that the pressures change with time due to consolidation process of soft soil.