On the Use of a Water Potential Probe for Suction and Temperature Measurements in Unsaturated Natural Clayey Soil  
[PERSON]  
1 Department of Civil, Environmental, Land, Building Engineering and Chemistry, Polytechnic University of Bari, 70126 Bari, Italy; [EMAIL_ADDRESS] (V.T. ); [EMAIL_ADDRESS] (O.B. ); [EMAIL_ADDRESS] (F.C. )1 Department of Civil, Environmental, Land, Building Engineering and Chemistry, Polytechnic University of Bari, 70126 Bari, Italy; vito. in slope stability analyses, especially when both the energy and the hydraulic gradients due to climatic action exhibit their maximum intensity in the topsoil. Continuous monitoring data on these parameters supports a range of stability models, including limit equilibrium-based closed-form equations [6; 10], physically based models [11; 12], and finite element models [1; 13]. Such models usually require the knowledge of two constitutive relationships, i.e., the soil water retention curve (SWRC) and the soil hydraulic conductivity function, which relates the matric/total suction to the water content and the hydraulic conductivity, respectively. These relationships characterize the HM behavior in unsaturated soils [6]. However, the SWRC is characterized by a hysteretic nature, which occurs with cycles of wetting and drying processes [6; 14; 15], and that could be modeled by a major loop that extends between the two extreme soil water contents (\(\theta_{\text{s}}\); \(\vartheta_{\text{r}}\)), bounded by two main curves: the main drying curve and a main wetting curve [16]. The hysteretic behavior of the soil has further implications for the water movement, soil deformation, and shear strength in unsaturated soils [17]. Thus, the retention curve is more accurate, the more precise and reliable measurements of suction and water content are, which evolve during drying or wetting processes. The choice of a specific device for measurements of suction depends on several factors, such as the suction range, resolution, accuracy, equalization time, operating temperature, and stability/continuity in measurement [7; 8; 18; 19; 20; 21; 22; 23]. Traditional methods for measuring matric suction, such as tensiometers [24; 25] and high-suction probes, may have limitations, particularly in fine-grained soils, in terms of the equalization time and stability measurement, the latter being due to a greater sensitivity to the phenomenon of cavitation. Thus, in a complex geotechnical context, an approach based on combined measurement techniques should always be pursued to increase the reliability of the measured data. In this respect, to allow a fair comparison between different measurement techniques, the choice of the sample volume becomes important On the one hand, the sample must have a sufficiently large volume to ensure that the measurements are representative. On the other hand, an excessively large volume could introduce too many material heterogeneities, thereby compromising the reliability of the measurements, especially in the context of the Southern Apennines [26; 27; 28], in which structurally complex formations [29] and heterogeneity are present. Among many, an example of the reliability of the filter paper (FP) technique is that reported by [PERSON] et al. [30], who compared the matric suction measured by means of 13 filter papers placed \"in-contact\" on a Blue Clay sample (of about 7000 cm\({}^{3}\)) with the value obtained using a high-capacity tensiometer (HCT) [25]. Consistency was found between the latter and the mean suction value of 1692 kPa (with a standard deviation of 190 kPa) as calculated by averaging the 13 values obtained using the filter papers. From this perspective, this study investigates the consistency of suction measurements using indirect measurement methods such as those obtainable through commercial sensors. The TEROS 21 is a water potential sensor widely used in agriculture as a cost-effective tool for in-situ monitoring of matric suction in unsaturated soils [22; 31; 32; 33; 34; 35].