Since the coastal area of Japan, is mainly composed of soft alluvial clay, it is important to know how to improve the soft clay deposits. In order to achieve rapid construction of structures with heavy super structures and to meet environmental restrictions which are strengthened year after year, new improvement techniques are needed. The deep cement continuous mixing method DECOM was developed for this purpose. This method is based on cementation rather than the usual concepts of consolidation, drainage, compaction, and replacement. Since the DECOM method enables rapid stabilization without pollution, many soil improvements using this method have been conducted. In this paper, the problem of how to achieve the quality between previously improved and newly improved zones, called the overlapping problem, is discussed as well as the effect of DECOM on environment conditions using field measurements from Yokohama Harbour.