## Materials and Methods
### Suction Measurement Methods: Filter Paper Technique and High-Capacity Tensiometers  
The water retention behaviour of the Paola Dece fissured clay has been investigated by measuring the degree of saturation, S\({}_{r}\), void ratio, e, and matrix suction, s, on undisturbed samples taken at depths ranging between 0.7 and 1.6 m. These values were measured just after sampling the material in different periods of the year to identify the in situ conditions and see how they change as a result of the interactions with vegetation and atmosphere. Moreover, S\({}_{r}\), e, and s were also measured while subjecting to drying paths 10 of the 16 samples analysed in the present study, to have a more comprehensive idea of the water retention behaviour of the Paola Dece clay over a larger suction range. Matrix sections were measured by means of both the Imperial College London (ICL) high-capacity tensiometers [31] and the filter paper technique [32], according to the procedures discussed in the following. All suction measurements were performed after preparing, from the undisturbed samples, specimens of 50-to-56 mm diameter and 20-to-40 mm height (Figure 3).  
Whatman No. 42 filter paper, neither pre-treated [32] nor pre-dried in the oven [33], was used to measure matrix sections roughly ranging between 30 kPa and 30 MPa [33; 34]. For each measurement, a couple of filter paper disks was put in contact with each of the two specimen bases (hence, four filter paper disks in total were used in every measurement). For each couple of filter paper disks, only one disk was in direct contact with the base of the specimen (referred to as \"in-direct-contact\" filter paper) and had to act as protection for the other disk (referred to as \"protected-in-contact\" filter paper) [35]. An equilibration time of two weeks was waited for each suction measurement [36], the latter derived by means of the calibration curve deduced by [37] using the pressure plate apparatus. From the two \"in-direct-contact\" filter paper disks, one average suction value was calculated. Similarly, one average suction value was derived from the \"protected-in-contact\" filter paper disks. Hence, every measurement was associated with two average suction values. Nevertheless, only the suction value obtained from the \"protected-in-contact\" filter paper disks has been considered in the present study, because the \"protected\" filter paper disks were certainly not contaminated with any soil \"left-overs\" (hence, they were likely to provide more accurate suction estimates).  
Figure 3: Specim prepared for suction measurements (50 mm diameter, 40 mm height).  
ICL high-capacity tensiometers were also employed in the experiments, as previously mentioned. Before performing each measurement, the tensiometers were saturated according to the procedure proposed by [38] and then pre-pressurised according to the procedure reported in [39]. As shown in Figure 4, all the measurements were conducted by: (1) fixing the tip of the tensiometer in the centre of a Perspex disk laid on one base of the tested specimen; (2) spreading a kaolin paste on the ceramic tip of the tensiometer to guarantee full hydraulic connection with the soil; (3) wrapping in cling film all the sides of the specimen, except the one where the measurement was taking place, in order to avoid water evaporation from the soil. Most of the times, tensiometer measurements were repeated by performing them on both the two bases of the specimen, also adopting different tensiometers. In these cases, the matrix suction value reported corresponds to the average value obtained from the different measurements.