Construction projects have often encountered very soft soil deposits, which can pose headache problems of stability and/or excessive settlement. To solve these problems, many varieties of ground improvement techniques have been developed and put into practice so far. Deep Mixing Method (DMM), a deep in-situ soil
stabilization technique using cement and/or lime as a binder, was developed and put into practice in Japan and in the Nordic countries independently in the 1970s. Numerous research efforts have been paid in these areas to investigate the properties of treated soil, behavior of DM improved ground under static and dynamic conditions, design method, and execution techniques.
　Research and development of DMM in Japan was initiated by Soil Stabilization Laboratory at Port and Harbour Research Institute(PHRI) of Ministry of Transport, Japan. During the 1970s and 80s, many research efforts have been done to study the engineering properties of lime and cement improved soils and to investigate the
behavior of improved ground. The Design Procedure includes the block type and wall type improved ground applications for foundation stability, settlement reduction purpose. This Procedure is based on the allowable stress concept, in which the induced stresses within the improved ground are confirmed to be lower than the allowable strengths in the internal stability calculation. This Design Procedure was revised twice in 1989 and 1999.
　Ministry of Land, Infrastructure and Transport (former Ministry of Transport) has a plan to modify the current design procedure and to introduce a reliability based design for marine construction works in 2007, in which the Design Procedure on DM improved ground will also be modified to the reliability based. In the new Design
Procedure, the average and variation of soil parameters and external loads are incorporated by partial safety factors to investigate the stability of improved ground. The authors have been studied the design procedure and magnitude of partial safety factors. In this article, the studies on reliability based design of the method are
described to provide detail information on the Design Procedure for practical and research engineers as well as academina.