## 2 Materials and Methods
### Experimental Setup  
Six soil samples, each with a volume of approximately 250 cm\({}^{3}\), were prepared by adjusting their initial water contents through the addition of predefined amounts of distilled water to achieve six different soil moisture conditions. Before testing, the water contents were estimated based on approximately 20 g of fragments taken from each sample, using the gravimetric method (ASTM D2216-19 [44]). Based on the representative water content recognized during the in-situ investigation [39], the determined water contents were 22.1%, 26.7%, 28.8%, 34.4%, 37.4%, and 41%. The unit weight of the samples was determined to stay within the range of 15 and 17 kN/m\({}^{3}\), considering the volume, mass, and water contents of the soil samples. As environmental conditions impact soil suction [16], the tests were carried out in a geotechnical laboratory with thermo-hygrometrically controlled conditions, with a temperature of 20 \(\pm\) 1.5 \({}^{\circ}\)C and a relative humidity of around 50%. To measure the matric suction, the hydraulic contact between the soil and the sensor is a prerequisite for efficient operation of the device. Therefore, part of the sample was used first to surround the sensor's ceramic disc. Then, the sensor was inserted into the soil specimen and subsequently sealed in a plastic bag (Figure 2a). The plastic bag containing the sensor and the soil specimen were all placed inside a thermocol box. A type E thermocouple was also adopted to measure the temperature variation in the soil sample. A CR1000X (Campbell Scientific, Inc., Logan, UT, USA) series data logger was used to record and store the values of temperature and suction, with a data acquisition interval of 10 min. In the first test, the 6 soil samples were divided to obtain 12 different samples based on initial water content. The measurement was concurrently conducted in two modes using the TEROS 21: under initially wet and initially dry conditions of the ceramic discs (i.e., six sensors for each condition). The initial states were achieved through two methods: for the series of measurements with initially saturated discs, prolonged immersion in distilled water was used; for the series of measurements with initially dry discs, natural drying was  
Figure 2: (**a**) Sensor inserts in the soil sample (test 1); (**b**) soil samples at the same initial water content put together at the end of the equilibrium time with (**c**) four filter papers attached to soil samples (test 2). employed by exposing the sensors to the laboratory air. The suction readings of initially wet and dry sensors corresponded to about to 1 kPa and 25,800 kPa, respectively. An equilibrium condition was assumed to be reached when the variation in the measured suction remained within about \(\pm\)2.0 kPa over a period of 6 h [7]. Some tests were run for a longer time interval, even after the suction equilibrium was attained. The long-term tests may provide information on the stability of the electrical signal of the sensors, corresponding to stability in terms of thermal and hydraulic equilibrium conditions [7]. At the end of the equilibrium time, part of the soil was taken from the 12 samples to re-measure the water content in order to evaluate the effect of the initial condition of the sensors (wet or dry). Then, a second test was carried out. The six soil samples, together with the sensors, starting with the same initial water content, were put together (Figure 2b). In addition, Whatman No. 42 filter paper was used to measure matric suction (Figure 2c). At the end of the equilibrium time, part of the soil was taken from the six samples to measure the final water content. In addition, a third test was carried out. Three undisturbed samples from the same material were subjected to drying paths, during which suction measurements were conducted with either the FP technique or the HTC. All suction measurements were performed on specimens of 56 mm diameter and 20-40 mm height.