# Recent Case Histories of Carbon-Neutral Activity Using Ground Improvement Technology in Japan
## 3 Recent Developments in and Application of Environmentally Friendly Ground Improvement Techniques
### Soil Admixture Stabilization Techniques  
Many soil admixture stabilization techniques have been developed and applied in many infrastructures since the 1970s due to their many benefits, such as rapid strength gain, wide applicability to many types of soil material, etc. In these techniques, cement is one of the materials most frequently used as a binder. However, about 800 kg of CO\({}_{2}\) is emitted during the manufacturing 1 ton of cement, and the energy consumption related to cement material and cement production is about 4% of the total CO\({}_{2}\) consumption in Japan. In cases where 200 kg cement per m\({}^{3}\) soil is used for stabilization, 160 kg of CO\({}_{2}\) is emitted. As the soil volumes to be stabilized are large in general, with the result that CO\({}_{2}\) emission should also be large, it is desirable to use new types of binder that can reduce CO\({}_{2}\) emission. (1) Use of materials with a low carbon footprint  
One of the new binders is high-furnace-slag cement [12; 13], which replaces the ordinary Portland cement (OPC) content to 30% with CaO and to 10% with SiO\({}_{2}\) to compensate for the hydration reaction induced by the OPC. This new cement can reduce CO\({}_{2}\) emission by 30% compared to OPC and 50% compared to furnace-slag cement type B. This cement has been applied to the shallow mixing method and the deep mixing method, in which the amount of CO\({}_{2}\) emission can be reduced by about 51%, about 78 kg/m\({}^{3}\) to 40 kg/m\({}^{3}\), respectively. The other new material is calcia, one of the by-products of the steel slag convertor. The calcia material is composed of calcia as the mother material, has a controlled chemical composition and is graduated. The calcia material can achieve a strength gain to about 200 kN/m\({}^{2}\) through the dewatering effect and the calcium hydration effect. It has been applied to stabilize dredged soft soil for backfilling, shallow stabilization and land reclamation [14; 15]. (2) Mixing of biochar in the ground together with chemical binder  
Recently, a new ground improvement method has been developed in which molten slag and biochar, as well as cement, are mixed with soil in situ [16; 17; 18]. The molten slag is produced by incinerating ash melted at a high temperature and cooled down, which is expected to improve the soil properties of clayey soil and can then reduce the required amount of cement for stabilization. The Japanese Industrial Standards (JIS) specified the quality and environmental safety requirements of the molten slag in 2006 to promote its utilization in civil engineering work and to prevent adverse impacts on the environment. Biochar is a material carbonized by the incomplete combustion of biomass raw materials, and functions to aid in the storage of carbon dioxide in the ground. By increasing the amount of biochar mixed, it is expected that carbon negativity will be achieved, in which the sum of CO\({}_{2}\) emission reductions and fixed amounts exceeds the emissions, which can bring about a sort of decarbonized ground improvement method. In construction, molten slag, biochar and cement are spread on the ground and mixed and compacted to build a stiff, shallow layer (Figure 6[17]). The amounts of molten slag and biochar are 550 to 1750 kg/m\({}^{3}\) and 10 to 30 kg/m\({}^{3}\), which can reduce the amount of cement that needs to be used to about 60% of the cement used in the existing methods, as shown in Figure 7.  
(3) Beneficial use of spoil  
Another method is to reduce and beneficially use spoil, which can reduce the required amount of binder and reduce the construction energy used. A large amount of cement slurry is usually injected into the ground using soil admixture stabilization techniques. Cement slurry with a low water-to-cement ratio is preferable for strength increase. However, this leads to less fluidity of the slurry, which requires high-capacity pumping machinery to inject it into the ground.