## 2 Pneumatic Flow Mixing Method and Construction of Airport Island  
### Overview of Pneumatic Flow Mixing Method  
The pneumatic flow mixing method is a solidification method in which soil is solidified by chemical treatment to increase its shear strength and improve its compressibility, liquefaction resistance, and impermeability. Solidification methods can be divided into those that solidify the ground in situ and those that mix a chemical stabiliser (solidifier) with the soil as a pretreatment for backfill or reclamation materials; the pneumatic flow mixing method is classified as the latter. In this method, the soil and solidifier are stirred and mixed in a pumping pipe. This allows the equipment to be simplified, with a solidifier-adding device connected to the existing pneumatic pumping equipment. There are two methods for adding solidifiers to the pumping pipe: the 'pumping system', in which solidifiers are added before compressed air is injected, and the 'line system', in which solidifiers are added after compressed air is injected (Figure 1). Depending on the properties of the solidifier, the addition method can be classified as the'slurry method' or the 'powder method'.  
When attempting to pump only soil in a pumping pipe, friction with the pipe wall causes a large pressure reduction and the pressure in the pipe is high, making pumping difficult. Pneumatic pumping is a technology in which compressed air is continuously injected into a pumping pipe to generate a multiphase flow of dredged soil and air to transport the dredged soil to the outlet. Figure 2 shows an image of pneumatic pumping and kneading. In the multiphase flow generated by the compressed air injection, the dredged soil is broken up and forms a plug. During pumping, this plug of dredged soil constantly changes shape owing to friction with the pipe wall and intermittently collapses and reforms. The pneumatic flow mixing method utilises this change in plug shape to mix the dredged soil and solidifier. The pneumatic flow mixing method has been applied to cohesive soils with sand and gravel contents of 50% or less. This method uses pneumatic pumping; thus, clayey soils with sand and gravel contents of less than 30% and water contents of 90-110% (approximately 1.3-1.5 times the liquid limit) are the most suitable.  
Soil is not mechanically stirred in the pneumatic flow mixing method; thus, there is concern regarding variations in the strength of the treated soil. It is a common practice to allow for a certain degree of heterogeneity in treated soil based on experience, including the results of field trials. The strength of the soil treated by the pneumatic flow mixing method is set by assuming that the strength in the field follows a normal distribution and considering the variability and failure rate. The failure rate is defined as the percentage of the unconfined compressive strength of the in situ-treated soil that is below the design strength. The failure rate should be set according to the intended use of the treated soil; however, it is generally set at 25%. The actual amount of solidifier added is determined based on the results of laboratory mixing tests, considering the quality required by the design and the need to ensure fluidity during pumping. Variations in the strength of the in situ-treated soil and its ratio to the laboratory mix strength (strength ratio) are also considered. If the strength variation and strength ratio of the in situ-treated soil are not determined by field trails, a coefficient of variation of 0.35 and a strength ratio of 0.5 are used as guides when the amount of solidifier added is approximately 50-100 kg/m\({}^{3}\), based on past performance data. When treated soil is used as a landfill material, the amount of solidifier added is usually approximately 50-70 kg/m\({}^{3}\) and the design strength is often set at 100-200 kN/m\({}^{2}\). For a more detailed description of the pneumatic flow mixing method, please refer to [2].