Abstract: The working and failure mechanism of soft ground improved by the cement deep mixing method (CDM) is studied with centrifuge model test for the foundation under retain wall of pile supported pier and the foundation under caisson. The main factors influencing the complete stability of improved ground are clarified. The failure type of ground improved by CDM in different geological conditions and with different width of improved ground are obtained. It is verified that the active and passive earth pressures on two sides of improved ground can be calculated with Rankine theory.