## 2 Materials and Methods
### Testing Method
After assembling, the retractable ring, which contained the loading piston, porous stone, specimen, and lateral miniature pressure transducer component, was positioned above the HAEV disk in the pressure chamber, as shown in Figure 1.  
Figure 1: A (a) schematic view and (b) photo of the modified UC oedometer. Figure 2: Schematic view of the retractable ring. Extensive trials were conducted to calibrate the transducer readings and establish an optimal ring closure protocol. As depicted in Figure 3, the lateral pressure generated during the expansion of the specimen can be simulated by a controlled vertical load. Therefore, the calibration process started with a customized \"T\"-shaped loading frame, as shown in Figure 3a, which can be connected to the threaded loading head of the force gauge through a threaded hole in its upper part. The pressure would be gradually applied to the loading frame by the force gauge and transmitted through the curved plate and thin acrylic spacer to the miniature pressure transducer. The deformation is then monitored by two displacement gauges placed on both sides of the loading frame. Finally, the real-time readings of deformations and pressures are entered into the software interface (Ver 1.0) via displacement and pressure data loggers, respectively. It was observed that maintaining lateral stresses 2-3 kPa higher than the bulk stress state ensured reliable contact while minimizing boundary effects. In addition, as shown in Figure 3b, the maximum deformation obtained was 0.021 mm when the miniature pressure transducer reached the full scale of 600 kPa for the force gauge. Based on a specimen diameter of 61.8 mm, it can be calculated that the maximum radial strain is 0.032%, which is less than the requirement of the ASTM standard [31], which states that the maximum radial change in the ring should not be greater than the original diameter of 0.04% to achieve the \(K_{0}\) condition. In addition, the influence of air pressure on the miniature pressure transducer was also considered. In the subsequent laboratory tests, the maximum value of the applied mattic suction was set to 200 kPa; therefore, in the calibration process, the water pressure applied by the pump was always kept constant at 40 kPa, and the air pressure applied by the air pump ranged from 40 to 240 kPa to ensure that the formation of mattic suction was in the range of 0-200 kPa. The results show that the readings of the miniature pressure transducer remained unchanged whether the air pressure was reduced from 240 kPa to 0 or increased from 0 to 240 kPa, which indicates that the effect of air pressure on the miniature pressure transducer in this study could be negligible. Considering the disturbance and loss of moisture content during the transportation of the soil samples retrieved from the field, the specimens prepared in this way can no longer reflect the actual situation in the field. Therefore, remodeled soil samples were used in this study, and the pre-specimens were prepared in the manner of layered static compaction to approximate the layering phenomenon produced via the deposition of the original samples in the field. Specimen preparation involved drying, grinding, and sieving before compacting the samples under predetermined conditions. The expansive soil samples from the field were first dried in the oven at 105 \({}^{\circ}\)C, then ground to powder and passed through a sieve with a diameter of 2 mm. Subsequently, the samples were stored in a sealed plastic bag in the shade for at least 24 h. In addition, the expansive soils should be passed through  
Figure 3: Calibration of the retractable ring for the (**a**) process and (**b**) result. the 2 mm sieve once more before specimen preparation since expansive soil easily clots after being mixed with distilled water. The specimens were all prepared at an initial moisture content of 18.0% and a dry unit weight of 16.0 kN/m\({}^{3}\). Specimen homogenization entailed the static compaction of the prepared wet soil within a custom cube mold at each 10 mm layer.