## 3 Survey Locations and Methods
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Test** & **Part** & **2002** & **2006** & **2012** & **2017** & **2022** \\ \hline Water content test & Interior & x & x & x & x & x \\ Test for bulk density of soil & Interior & x & x & x & x & x \\ Unconfined compression test & Interior & x & x & x & x & x \\ Interior & & & x & x & & \\ Needle penetration test & Surface & & & & x & x \\ Near surface & & & & & x \\ Chemical analysis of suspension & Near surface & & & & x & x \\ EPMA & Near surface & & & & & x \\ Analysis of groundwater & Boring hole & & & x & & \\ \hline \end{tabular}  
\begin{tabular}{c c c c c} \hline
**x**: test implemented. & & & & & & \\ \hline
1. Physical property tests & & & & & \\
2. Unconfined compression test & & & & & \\
3. Needle penetration test & & & & & \\ \hline
3.4 & & & & & \\
1, 2 & 1C2 & 2C2 & 1, 2 & 1C2 & 2S2 & 1, 2 \\
1, 2 & 1C3 & 1, 2 & 1C4 & 2C4 & 1, 2 \\
1, 2 & 1C4 & 2C4 & 1, 2 & 1C5 & 2C5 & 1, 2 \\
4 & & & & & 3-5 & 3-5 \\
2017-1 & 2017-2 & & & & & \\
2022-1 & 2022-2 & & & & & \\
2017 survey & & & & & & \\
2022 survey & & & & & \\ \hline \end{tabular}  
\end{table} 
Table 1: List of tests conducted on samples. Figure 5: Boring and sampling: (**a**) boring operation; (**b**) core sample. Figure 6: Tested parts of cores. diameter for compaction tests; thus, the diameter was taken as sampled (approximately 66 mm), and the height was twice that. In surveys up to 2012, no signs of strength reductions were observed in the treated soil, whereas in the 2017 and 2022 surveys, needle penetration tests were conducted against the surface exposed to the untreated soil. It was difficult to determine the exact exposure surface for the treated soil sampled in the field; however, after lightly removing the surface sand and gravel with a finger, the surface that seemed to be solidified was considered the exposure surface. In some samples, gravel was embedded in the exposed surface of the treated soil and needle penetration tests were conducted on the surface after removing this gravel. Two types of needle penetration tests were conducted near an exposure surface: one in which the needle penetrated the exposure surface perpendicularly and the other in which the needle penetrated the sample from the side at several depths. In the former method, the penetration resistance can be continuously determined from the exposed surface; however, the measurable depth is limited. With the latter method, there is no limit to the depth at which the penetration resistance can be measured; however, the data are discrete in the depth direction. In the 2022 survey, the latter method was also utilised because the former method did not provide a clear degradation depth. For chemical analyses, soil suspensions were prepared and analysed according to the Japanese Geotechnical Society Standards [22; 23]. The four measurement parameters were the pH, calcium ion concentration, sodium ion concentration, and electrical conductivity, which were determined using Horiba compact instruments. The pH and calcium ion concentration decreases with the progressive degradation of the treated soil, and the electrical conductivity decreases accordingly. According to a previous study [13], there is a correlation between these parameters, and chemical analyses were performed to determine the signs of deterioration. The test locations are shown in Figure 7. In the 2017 survey, only one measurement was made near each end of the borehole, whereas in the 2022 survey, the specimen was divided into smaller sections from the exposure surface and analysed. However, the upper and lower ends of core 2022-2 (especially the upper end) contained large amounts of gravel, were brittle, and collapsed during cutting. Therefore, the soil was analysed in batches of a few centimetres. Figure 7: Testing near exposed surfaces. In the 2022 survey, an EPMA of the surface (elemental mapping) was implemented on samples near the exposed surface.