## 3 Ground Improvement Methods
### Forced Replacement Method  
Firstly, the forced replacement method has advantages such as the low cost and the simple method. On the other hand, if this method applied for the very soft soil such as organic soil, the shear failure, the significant heave and settlement may occur. The concept of this method is that the very soft subsoil is replaced by the materials of good quality, which have the high strength and unit weight, such as sand and broken rock, leading to improve the ground strength. Figure 4 illustrates the schematic diagram of the forced replacement method.  
Figure 3: Properties of organic soil: (**a**) Water content; (**b**) soil cohesion; (**c**) organic content.  
It is basically assumed that the embankment unit load \(p\) is equal to the unit ultimate bearing stress of soft soil \(q_{ult}\). The embankment unit load \(p\) can be estimated as follows:  
\[p=H\gamma_{t}+D_{z}\gamma_{sub} \tag{1}\]  
where \(H\) = height of embankment; \(\gamma_{t}\) = unit weight of embankment; \(D_{z}\) = replacement depth; and, \(\gamma_{sub}\) = effective unit weight of embankment.  
Many researchers recommended the replacement depth \(D_{z}\) based on the limit equilibrium method. According to [PERSON] [22], the replacement depth \(D_{z}\) using \(N_{c}\) value of 5.7 can be written as  
\[D_{z}\ =\ \frac{5.7c-H\gamma_{t}}{\gamma_{sub}\ -\ \gamma_{s}} \tag{2}\]  
where \(c\) = soil cohesion; \(\gamma_{s}\) = unit weight of soft soil (original soil).  
[PERSON] and [PERSON] [23] suggests the replacement depth \(D_{z}\) using \(N_{c}\) value of 5.3 can be estimated as  
\[D_{z}\ =\ \frac{5.3c-H\gamma_{t}}{\gamma_{sub}\ -\ \frac{7}{6}\gamma_{s}} \tag{3}\]  
[PERSON] [24] reported the equation to estimate the replacement depth \(D_{z}\) using \(N_{c}\) value of 5.14 as follows:  
\[D_{z}\ =\ \frac{5.14c-H\gamma_{t}}{\gamma_{sub}\ -\ \frac{7}{6}\gamma_{s}} \tag{4}\]  
These existing methods can predict the replacement depth simply and indirectly, however, those can not predict the height and influence range of heave soil that would be required in design and construction stage.  
For this site, the field test was conducted to review the constructability of the forced replacement method. The replacement depth, settlement, and soil heave were investigated. The width and height of embankment for field test were 6.0 m and 2.0 m, respectively. The settlement of embankment was measured by using the survey and visual observation. In this site, we confirmed that the entire embankment was settled to the subsurface in a day, thus, we regarded it as a 'punch failure' or 'forced replacement'. Based on the investigation, the settlement was about from 2.0 to 3.0 m, hence, we could predict the replacement depth of 4.0-5.0 m because of the sum of the settlement and the height of embankment. Also, the soil heave near the embankment occurred. The range and height of soil heave were measured by using the visual observation. The range and height of soil heave were 20 and 1.5 m,  
Figure 4: Schematic diagram of the forced replacement method.  
respectively, as shown in Figure 5. Especially, the soil heave influenced the nearby agricultural land, thus the damage occurred.  
The replacement depths in this case are estimated using the theoretical existing methods, and are compared with the results of field test. As mentioned above, the entire embankment in this site was settled to the subsurface, thus, the height of embankment is zero when the replacement depth is estimated using existing methods. The replacement depths estimated by [PERSON] [22], [PERSON], [PERSON] [23], and [PERSON] [24] represents 1.95, 1.91 and 1.85 m, respectively. When compared with the field investigations, it is shown that estimated replacement depths from existing methods are underestimated. Additionally, the influence range and height of heave soil cannot be estimated using the existing methods. The existing methods have the limitations because those are based on the limit equilibrium method.