Soft soils treated by the deep mixing method (DMM) are not always homogeneous and often consist of heterogeneous soils such as clay, loose sandy soil, and organic soil. In such cases, it is common to set the amount of binder on the safe side to avoid insufficient strength. However, this method may cause variations in the strength of the improved soil. It is also uneconomical and has a large environmental impact. In order to optimize the strength and cement content of improved bodies obtained in the improvement of heterogeneous soft ground, it is first necessary to quantitatively analyze the effects of ground characteristics and DMM application conditions on the difference between the in-situ strength of improved bodies and the design strength. Therefore, this study first identified trends in DMM construction conditions employed in road construction projects over the past 10 years. In addition, construction data for 8 representative cases were analyzed. As a result, it was found that the mechanical mixing by wet method with a construction depth of 5 to 20 m and an improvement diameter of 0.6 to 2.0 m was frequently adopted. Furthermore, it was confirmed that the higher the fine-grain content of the soft ground, the greater the in-situ strength of the improved material tended to exceed the design strength. On the other hand, there was no clear relationship between the number of improvement diameters or mixing shafts and the strength of the improved material.
