## 3 Survey Locations and Methods  
Five borehole investigations (with 25 boreholes drilled immediately after the reclamation and two drilled in each of the other investigations) were conducted on the improved ground over a period of 20 years (2002, 2006, 2012, 2017, and 2022). A planar survey location map and a schematic of the columnar sections are presented in Figure 4. In 2002, the airport was not in service, which made it possible to survey a wide area. After the airport was put into service, the survey locations were limited owing to the operational constraints of the airport, and the survey locations after 2006 were different from those immediately after the reclamation was completed. The location was an unpaved area adjacent to a taxiway. As  
Figure 3: Central Japan International Airport and areas where dredged soil was used. shown in the sampling columnar sections, the lower part of the treated soil produced by the pneumatic flow mixing method consisted of fine-to-medium-grained sands, whereas the upper part was a reclaimed soil layer consisting of sand and gravel mixed with cobbles. Although the elevation varied slightly depending on the borehole drilled, the treated soil lay at a depth of approximately 2-7 m, which was below the residual water level (at a depth of approximately 1.4 m). The residual water level is higher than that of the treated soil; thus, it can be said to be in a non-drying condition. Specimens were collected at each site using double-tube samplers with built-in sleeves and triple samplers at the boundary between the overburden (sand and gravel) layer and treated soil. Examples of the bores and treated soil samples are shown in Figure 5. As shown in the photograph, the sampling caused cracks at several positions, but the number of cracks was small enough to ensure that a 132 mm specimen was obtained for the unconfined compression test. The laboratory tests for the sampled soil are listed in Table 1. Continued tests included water content and wet density tests, as well as unconfined compression tests, with two-three test specimens per metre of sample being taken for compression testing. Needle penetration tests were also conducted up to 2012 on the cut surface inside the treated soil and in 2017 and 2022 on the nearby treated soil exposed to the untreated soil. In addition, chemical tests and areal analyses (elemental mapping) using EPMA were implemented. The location maps of the tests are shown in Figure 6. Groundwater  
Figure 4: Survey points and schematic columnar sections: (**a**) survey points (left: immediately after construction; right: second and subsequent surveys); (**b**) schematic of columnar section. samples were collected in 2012, and chloride ion concentrations were measured using ion chromatography. Unconfined compression and needle penetration tests were conducted according to the Japanese Geotechnical Society Standards [20,21]. The purpose of these tests was to investigate the continuous changes in strength (strength development and reduction). The specimens used for the unconfined compression tests were approximately 66 mm (diameter) \(\times\) 132 mm (height). The treated soil samples were hard and difficult to shape into the standard 50 mm