## 2 Materials and Methods
### Filter Paper Method to Determine the Matric Suction  
To predict the unsaturated hydraulic conductivity of the compacted specimen, the water retention curve (WRC) was formulated. The WRC is the relationship of the moisture content with the matric suction. Hence, in this study, the matric suction at a specific moisture content level was quantified using ASTM D5298 [31] or the standard test method for the measurement of soil potential, or suction, using filter paper, commonly referred to as the filter paper method. The Whatman No. 42 filter paper and ashless quantitative filter paper were oven-dried for at least 16 h to ensure that water was not present or absorbed by the filter papers. The oven-dried filter papers were placed in a desiccant jar to prevent moisture accumulation before being used in the experiment. The two cylindrical compacted specimens were repeatedly weighed separately until the desired weights corresponding to the target moisture content level were achieved. Once the compacted specimens undergoing the drying process achieved the target moisture content, a set of filter papers was placed in between the two compacted specimens with the same target moisture content. The set of filter papers was initially weighed before being put in between said compacted specimens to ensure an accurate data collection of the filter paper moisture content after the period of experimentation. The set of filter papers consisted of a single 55-mm diameter Whatman No. 42 filter paper in between two 60-mm ashless quantitative filter papers, shown in Figure 4, to ensure that no debris adhered to the Whatman No. 42 filter paper. The two  
Figure 3: Wet–dry cycle of the compacted specimens. Figure 2: Saturation of the compacted specimens. compacted specimens with the set of filter papers in between them were wrapped in plastic to maintain their shape as they were placed into the glass container. The glass container was sealed with vinyl tape, as shown in Figure 5. The glass container containing the specimen was then placed inside an insulated chest for 7 days to regulate the temperature of the containers as the water within the void spaces of the desaturated compacted specimens equilibrated with the set of filter papers in between. According to ASTM D5298 [31], the 7-day period for the equilibration of the moisture content is already sufficient for normal conditions in soil mechanics, and in many conditions, the equilibration is completed earlier than 7 days. Moreover, instead of a prolonged testing, it was also opted to use the 7-day testing period to refrain from using formaldehyde to prevent the growth of organisms in the moist environment and the decomposition of the filter paper. After 7 days, the moisture content of the Whatman No. 42 filter paper in between the compacted specimens was determined, which was used to obtain the matric suction together with the calibration curve from ASTM D5298 [31]. Since the compacted specimens were allowed to equilibrate for 7 days inside the glass containers, a new set of compacted specimens that underwent the necessary number of wet-dry cycles was used for every experimental run. In the experiment, the matric suction of the compacted specimen was determined as the specimen dried after re-saturation; hence, the WRC obtained was for the drying cycle, which was different from the WRC for the wetting cycle. The matric suction of the soil specimens in the study was determined using Equation (1), as stipulated in ASTM D5298 [31]. ASTM D5298 [31] also provided the typical calibration curves for determining the suction using different types of filter paper, such as Whatman No. 42 and Schleicher and Schuell No. 589. The upper portion of the calibration curves, above the filter paper water content break point, represented the  
Figure 4: Set of filter papers: (**a**) Whatman No. 42 and (**b**) ashless quantitative filter paper. Figure 5: Experimental setup of the filter paper method. moisture retained as the water films were absorbed into the soil particle surfaces.