## 5 Discussion
The range of subsidence A-DInSAR velocities, corresponding to each part of the dam section (eastern and the western sectors of the dam which show a similar pattern of vertical deformation), are also reported above the profile in Figure 8.  
According to [PERSON] et al. [30], the material type that forms the starter dam is sand mechanically condensed (2 and 2' in Figure 8). In this basal part of the dam ring, the subsidence rate can range from 5 to 10 mm/yr. Zone I consists of sandy tailings, not mechanically condensed (3 and 3' in Figure 8) and Zone II, which is the most inner part of the dam structure (4 and 4' in Figure 8) is made of uncondensed dusty tailings. The parts of the embankment affected by more intense subsidence (20-30 mm/yr) are founded on greater thicknesses of sandy tailings not mechanically condensed or on uncondensed dusty tailings (note the thickness of the Zone II in Figure 8). Therefore, the more pronounced subsidence rates in the highest sectors of the dam could be linked both to the most recent age of construction and to the larger thickness of compressible materials in the inner part of the dam body. As regards the horizontal deformation rates, the eastern sector of the dam moves towards the east and the western one moves towards the west, with vector intensity of more than 30 mm/yr in both cases. Northern and Southern sectors of the dam do not show evident horizontal deformations in the east-west direction. It could be possible that these sectors undergo horizontal deformations in the north-south direction along which the classical AD-InSAR technique cannot provide measurements [48; 49]. Figure 8: Scheme of tailings zoning in the site’s embankment and selected geotechnical parameters of the soil form [30]; 2’, 3’ and 4’ are for fully saturated material; \(\gamma\): unit weight; \(\varphi\)’: internal friction angle; \(c\)’: effective cohesion; AD-InSAR subsidence velocities measured in this work are also reported above the profile. By the analysis of the horizontal displacements that occur along the eastern and western sectors, the dam structure appears to deform towards the outside, with velocity vectors approximately radial to the center of the basin, and about the same intensity in both sectors of the basin. The velocities recorded with A-DInSAR along the dam were compared with those acquired by [PERSON] [1], who analyzed inclinometer measurements located along the dam. Along the eastern side of the dam, 26 deep inclinometers were placed and aligned along two cross sections, the XVIE and XVIIIIE (located in Figure 6a,b). These measurements highlight the occurrence of horizontal displacements localized at precise depths, where horizontal deformation takes place along sliding planes [1]. According to the author, the movement in the dam foundation is indeed concentrated along sub-horizontal shear planes, formed through glacio-tectonic phenomena in the Pliocene clays. Such planes can be described as 'horizontal thrusts' where the shear strength is close to residual, thus controlling the stability of the dam [1; 30; 51]. Therefore, the inclinometers measured the horizontal displacements of the top layers of Pliocene high plasticity clays, which are induced by tailings load. The inclinometer installed at the benchmark-208 (BM-208 located at the cross-section XVIE in Figure 6a), shows two active shear planes, at elevations 75 m and 45 m asl [1]. In Table 2 are listed the horizontal velocities of displacement at BM208, from 1999 to 2013, using data from [PERSON] [1], while from 2014 to 2018, the horizontal velocities of displacement have been retrieved by the A-DInSAR analysis, averaging the velocity for each analyzed year. Such east-west velocities are well comparable with the horizontal rates measured by [PERSON] [1] for previous years, i.e., 2011-2013. According to [PERSON] [1], towards the end of 1995, when the dam crest had reached the height of 40 m, the rate of horizontal surface displacement increased, so that during 2001-2009 displacement rates were between 40 and 50 mm/year.