## 2 Materials and Methods
### Test Procedure
#### 2.3.2 Filter Paper Test  
The contact filter paper method, initially introduced by [PERSON] et al. and [PERSON] [26; 27], has undergone improvements by [PERSON] et al., [PERSON] et al., and [PERSON] et al. [28; 29; 30; 31], who compared their proposed suction estimation procedures with the method outlined in American Society for Testing and Materials (ASTM) D5298-10. The enhanced test procedure had several advantages, including minimizing the exposure of soil samples and filter papers to the prelaboratory soil solvent, thereby changing the content caused by moisture loss or absorption from the air. As a result, their findings were considered to be reliable to determine soil-water characteristic curve (SWCC). According to [PERSON] et al., the compaction property of the material exhibited a significant dependence on the manner in which it was prepared [32]. Therefore, the scholars developed a specific procedure in which samples were prepared from a basic material with a specific water content instead of wetting the dry soil powder directly to reach the desired value. It was assumed that by completing this process, the homogeneity of the aggregated soil particles could be preserved [32]. This procedure was applied to the material used in this study (Figure 3). First, a specific weight of water was added to the soil powder, and the resulting combination was mixed continuously and thoroughly and stored in an airtight container. The water content was determined to be 6.13% after homogenization over two days. Then, the basic material was wetted to the target water content, and the rehomogenized phase was observed after two days. Each soil sample was prepared in a standard mold by using sample preparation equipment, and they all had smooth surfaces. A secure contact surface between the filter paper and the bottom surface of the soil sample was guaranteed. The nominal height and diameter of the mold were 20 mm and 61.8 mm, respectively. At each density level, a total of approximately 50 samples were prepared from the basic material and divided into 25 groups. Samples compacted during drying and at the optimal water content were saturated (water-cured) prior to performing filter paper suction measurements. Each sample took different durations to reach various estimated water contents, and the drying curve was obtained from these data. Whatman ashless Grade 42 quantitative filter papers (Cytiva, Shanghai, China) were placed in direct contact between two soil samples for each group to prevent contamination (Figure 3a,b) [33; 34]. Waterproof tape was used to adhere the two soil specimens in each group together to prevent the evaporation of water (Figure 3c) [26; 31; 34]. Specifically, to prevent evaporation during and after equilibration, the samples were wrapped in cling film and tinfoil sequentially, covered with wax, and placed in a sealed container (Figure 3d-f). After all of the above steps were completed, the samples were stored at 20 \({}^{\circ}\)C in a humidity-controlled dark room (Figure 3g). Following the recommendations of [PERSON] and [PERSON], a 2-week period was allocated to allow equalization [35]. Subsequently, the filter paper was meticulously extracted from the soil samples (Figure 3g) and weighted using a scale with an accuracy of 0.0001 g. The wet and dry soil samples were also weighed with a scale with an accuracy of 0.01 g to calculate the GWC. Estimations of matric suction were derived utilizing correlations that relied on the water content of the filter paper, as established by [PERSON] and [PERSON] [36].