## 3 Results
### Typical Drying Paths of the Paola Docc Fissured Clay  
After evaluating the initial state of the Paola Docc clay, 10 specimens were subjected to staged drying paths, with suction measurements performed, at the end of each drying stage, by means of both filter paper technique and/or ICL high-capacity tensiometers. The drying stages were imposed by allowing the specimens to be in contact with the atmosphere, so that the pore water could evaporate. However, even during drying, the material was covered (but not wrapped) with cling film to allow for a very slow and homogeneous drying process to take place across the specimens. In order to better discuss the typical behaviour exhibited by the tested fissured clay during drying paths started from in situ conditions, representative water retention data are reported in Figure 6, with reference to a representative specimen, named \"Specimen 1\". The data are shown in terms of matrix suction, \(\mathrm{s}\), against: degree of saturation, \(\mathrm{S_{r}}\) (Figure 6a), void ratio, \(\mathrm{e}\) (Figure 6b), and volumetric water content, \(\mathrm{\theta_{w}}\) (Figure 6c). In Figure 6a, it can be observed that \(\mathrm{S_{r}}\) decreases slightly during the first two drying stages, even if the matrix suction increases of almost one order of magnitude. When \(\mathrm{s}\) values of around 1000-2000 kPa are exceeded, the slope of the water retention curve increases significantly, showing a roughly linear response. Normally, \(\mathrm{s}=1000\)-2000 kPa would be interpreted as the \"air-entry value\" (AEV) of the material [41]. However, as previously observed, the desaturation of the tested fissured clay is expected to take place at very low suctions as a consequence of the water drained out of the inter-element pores (i.e., the fissures). Hence, due to the (at least) double-porosity structure of the material, the AEV \"stricto sensu\" is likely to correspond to very low suction values, while \(\mathrm{s}=1000\)-2000 kPa is likely to correspond to the AEV of the clay elements (shown in Figure 2). This interpretation seems to be corroborated by all the other water retention data shown in the following. Figure 6b reports the water retention curve of Specim 1 in terms of matrix suctions against void ratios. In this figure, a significant reduction in void ratio is observed at suctions of around 1000-2000 kPa, i.e., in correspondence of what has been interpreted as the AEV of the clay elements. For \(\mathrm{s}>2000\) kPa, Specim 1 shows very small void ratio reductions and a stiffness (herein defined as the ratio between matrix suction and volumetric strain) comparable with the one exhibited at the start of the drying path. It is interesting to notice that a similar behaviour was observed, for both reconstituted and natural clays, by [42; 43], who referred to the abrupt reduction in void ratio as \"volumetric collapse\". Refs. [42; 43] observed that the volumetric collapse not only occurs at the air-entry value, but also in correspondence of the yield point in isotropic compression. Such a correspondence can be also identified for other materials, such as the reconstituted silty clay tested by [44]. Isotropic compression tests were not performed on the Paola Docc clay sampled at Pisciolo at depths ranging between 0.7 and 1.6 m, so a similar correspondence between volumetric collapse, air-entry value and yield stress was not verified. Figure 6: Representative water retention data of the Paola DOCe fissured clay reported in terms of matrix suction (measured with the filter paper technique, FP) against: (**a**) degree of saturation; (**b**) void ratio; (**c**) volumetric water content. The water retention data of Specimen 1 are also reported in Figure 6c in terms of matrix sutions against volumetric water contents. The data in Figure 6c clearly reflect the variation in void ratio shown in Figure 6b, highlighting the presence of a major water content reduction at s = 1000-2000 kPa.