Deep Mixing Method (DMM), a deep in-situ admixture stabilization, has been developed in Japan to reinforce soft alluvial clays. In practice, a special equipment
having some mixing shafts and blades is used to manufacture a treated soil column by one penetration of the shaft. Many patterns of the improvement which is formed by overlapping the soil colums have been used for construction of port facilities. Recently, a group colum type DMM with high replacement is expected to be applied for a breakwater. This is a new-pattern of improvement where treated soil columns are adjacent to each other without overlapping.
　Centrifuge model test and finite element analysis were performed to investigate the behavior of the new-pattern improvement. A series of model tests were carried out by changing the width of the improved region. In the tests, the model grounds were brought to failure by increasing the horizontal force acting on the super structure. Test results were compared with those of the block-type improvement and reliability of the new-pattern improvement was investigated.