## 2 Unsaturated Subgrade Filler Triaxial Testing System
### Sample Preparation  
The soil used in this study was sourced from a high embankment along a heavy-haul railway. Its characteristics include a coefficient of uniformity, \(C_{u}=d_{60}/d_{10}\), of 10.894 and a coefficient of gradation, \(C_{c}=d_{30}^{2}/d_{60}\cdot d_{10}\), of 1.02, placing it within the classification of Group A fillers according to the guidelines outlined in the TB10001-2016 (2016) [35]. The detailed particle grading distribution is presented in Table 1. The embankment filler was subjected to sieving in accordance with the TB10102-2010 (2010) [36], and the particle grading curve for the unmodified embankment filler is illustrated in Figure 4. This filler exhibited a maximum dry density of 2.06 g/cm\({}^{3}\) and an optimum moisture content (OMC) of 8.5%, as determined through standard light compaction testing. A compaction level of 95% was used to prepare the samples. Figure 5 shows the compaction test curve for the embankment soil.  
Based on the fundamental properties of the embankment filler, the sample preparation process was carried out as follows. First, the soil is dried, and distilled water is added at a mass ratio of 16% relative to the moisture content. During the weighing process, an additional 2% of distilled water is added to compensate for moisture loss during mixing. The distilled water is divided into three portions, each of which is sprayed evenly onto the soil sample using a spray method, including mixing for 3 min per portion. This process is repeated until the soil sample is thoroughly mixed. After mixing, the sample is shaped, wrapped in plastic film, and placed in a moisture box for 24 h for curing before further use. The light compaction method is then applied to divide the soil sample into three layers, which are compacted into a soil column with a diameter of 100 mm and a height of 200 mm.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Particle Size/mm** & **\textgreater{}10** & **10–5** & **2–5** & **1–2** & **0.5–1** & **0.25–0.5** & **0.074–0.25** & \textless{}**0.074** \\ \hline Percent content/\% & 1.13 & 10.129 & 11.234 & 27.7 & 15.613 & 13.144 & 18.188 & 2.862 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Grading distribution of filler particles.  
Figure 3: Schematic and actual diagram of matric suction tensiometer components. Notes: (**a**) structural schematic, (**b**) sensor section schematic, (**c**) actual product diagram.  
After compaction, the sample is exposed to the atmosphere for approximately 15 min each time, with the exposure time precisely controlled to gradually adjust the moisture content and accurately control its evaporated moisture. Finally, the treated soil sample is sealed in a plastic bag and stored in a humid room for 30 days to ensure that the suction of each layer in the sample reaches equilibrium. The sample parameters under different initial conditions are presented in Table 2, where the suction values measured for three sample groups under varying net confining pressures are listed, with net confining pressures of 5 kPa, 200 kPa, and 600 kPa.  
Figure 4: Grain size distribution curve for the embankment soil.  
Figure 5: Compaction test curve for the embankment soil.