*2.1. Construction Methods*

Ground improvement has various sustainable effects on soft grounds, such as the prevention of liquefaction, increase in bearing capacity, and promotion of consolidation. Currently, there are many ground-improvement methods, such as the replacement method, accelerated consolidation method, compaction method, solidifying method, reinforcement method, and injection method. They are selected for use according to the characteristics of the targeted soft ground and the purpose of the improvement. Deep mixing methods (DMMs) comprise one of the solidifying methods among ground-improvement methods. In DMMs, while the stirring wing penetrates the targeted soft ground, slurry-like or powder-like solidifying material and the soft ground are forcibly stirred and mixed to construct a columnar improved body in the targeted soft ground. One of the DMMs, the relative stirred deep mixing method (RS-DMM), is a method of rotating the inner and outer wings of the stirring wing in opposite directions at different speeds while discharging a slurry-like solidifying material from the tip of the stirring wing [9,12,13]. By rotating the inner and outer wings in opposite directions, it is possible to add the effects of "kneading" and "mixing" to the mixing of the targeted soft ground and the solidifying material. One of the issues that can occur when a DMM is applied to a soft ground is "co-rotation". Co-rotation is the phenomenon in which the unimproved parts remain due to their rotating together with the cohesive soil that becomes attached to the stirring wing during stirring and mixing. It occurs especially in cohesive soil and causes variations in the quality of the improved bodies. Cohesive soil adheres to the stirring wing due to the cohesion force of the cohesive soil, further cohesive soil adheres due to the cohesion force of the adhered cohesive soil, and cohesive soil also adheres to the outside. In this way, the mass of cohesive soil becomes larger and larger and results in the stirring capacity of the stirring wing becoming significantly reduced. The RS-DMM prevents such co-rotation by "kneading" and "mixing" and makes it possible to construct a high-quality, high-strength improved body without variation.

DMMs are used not only as liquefaction countermeasures but also as measures against the subsidence of embankments, the prevention of the slip destruction of grounds including embankments, the stabilization of foundation structures, and so on, and they can be applied under a wide range of ground conditions. The construction flow of DMMs is shown in Figure 1 and is described in the following:

**Figure 1.** Outline of construction process of DMMs.


#### *2.2. Occurrence of Displacement and Displacement Reduction Type of Stirring Wing*

One of the situations that may arise when performing ground improvement using the relative stirred deep mixing method (RS-DMM) is the effect of "displacement". When applying the RS-DMM, it is necessary to penetrate the stirring wing into the targeted soft ground and inject a slurry-like solidifying material. When this solidifying material is injected, the material pushes the surrounding soft ground out, increasing the pressure inside the soft ground and causing "displacement" [14,15].

If there are railroads or buildings around the construction site, the construction must be executed while giving due consideration to the impact of displacement. Especially in construction works near railways, there must be strict control of track displacement, and the construction must be handled with the utmost of care.

The stirring wing of the displacement reduction type (DRT) is used in the above situations. The stirring wing of the DRT uses the stirring wing rod as a spiral rod. In order to suppress the displacement, it is necessary to prevent the solidifying material from pushing out the ground, but the effect of the spiral rod promotes the discharge of excess soil equivalent to the solidifying material injection, as shown in Figure 2, and the displacement can be suppressed by releasing the internal pressure in the soft ground. By using the stirring wing of the DRT, it is possible to reduce the displacement as well as its influence on the surrounding structures.

**Figure 2.** Stirring wing of NT and stirring wing of DRT: (**a**) Stirring wing of NT; (**b**) Stirring wing of DRT.
