## 5 Discussion
Therefore, in this sector there are no localized areas of weakness, as is evident for the eastern sector, and the horizontal movement is uniformly distributed along the perimeter of this part of the dam. According to the above-mentioned hypothesis that the whole embankment structure is moving outwards, the horizontal displacement of the northern and southern sectors of the dam should be, respectively, northward and southward. As aforementioned, the typical limitation of the InSAR technique in measuring displacements in the North-South direction  
Figure 10: Longitudinal section of cumulative horizontal displacement along the eastern dam (Section 1, location in Figure 6a); curves from 2004 to 2013 are from [1]; curves from 2014 to 2018 are from AD-InSAR measurements. could therefore lead to an underestimation of north and south displacement of the dam. In support of this hypothesis, the north dam started exhibiting horizontal displacements in 2006/2007, and the rates increased with time, according to [PERSON], [1]. These displacements were localized along two sections of the dam: section VIIN and section XVIN (localized in Figure 6a,b), with a total cumulated displacement since 1987 to 2013 of 283 mm and 408 mm, respectively. Even in the northern sector the horizontal deformation did not occur uniformly but affected mainly these two specific areas. As regards the southern sector of the dam basin, on the other hand, we do not have any precise information on the possible occurrence of southward displacements at the embankment. The displacement of the dam towards the outside of the dam basin can be produced by two combined factors: (1) the load of the tailings pushing on the containment banks, (2) the sliding along sub-horizontal shear planes at depth, in the high plasticity, over-consolidated Pliocene clays, where the shear strength is close to the residual (details on shear strength parameters can be found in [PERSON] [1], and reference therein). The high-plasticity clays exhibit a strong spatial variability, discontinuous pattern, and variable lateral extension [1]. The occurrence of some localized areas of weakness, with greater horizontal deformations, observed in the eastern and in the norther parts of the dam embankment (sections XVIE, XVIN and VIIN in Figure 6a,b), could be linked to peculiar settings of the Pliocene clays at depth; otherwise, it could be linked to structural issues of the dam structure. Horizontal displacements potentially could cause the dam failure, as the development of progressive failure in a weak soil layer in the dam foundation is indeed considered an important mechanism of failure for tailings dam build up with the upstream method [21]. A failure simulation at a Zelazny Most dam crest elevation of 205 m asl. (or dam height of 91 m), has been developed by [21]. This height is expected to be reached in 2023 if the present filling rate is continued. The failure mode would result in an active rupture mechanism that continues in depth into the Pliocene clays down to the deepest measured shear zones. The authors state that this failure mode is pushing the dam forward along the pre-sheared zones in the clays at the dam foundation, ending with a passive failure mechanism in front of the dam. A further destabilizing factor for the Zelazny Most TSF stability is represented by the effects of the mining-induced seismicity. The intense seismicity documented at LGCD (e.g., [52; 53]) counts thousands of small tremors every year; more than 2000 events with magnitudes between 0.9 and 3.5 are recorded yearly in this district [53]. The earthquakes with magnitude >4 could have an influence on the susceptibility of tailings to failure and can represent a triggering factor for such a system, affected by active and continuous deformation processes.