## 4 Results
### Back Analysis  
Once the pre-failure topography has been recreated, its stability was studied using a 3D model with the software fast Lagrangian analyses of continuous (FLAC) 3D, a software developed for the geotechnical calculation by ITASCA group. It is based on the method of finite differences, which allows the use of non-linear behavior laws, with an explicit resolution scheme of the differential equations of the geotechnical problem. Although the medium is divided into elements formed by quadrilaterals, FLAC internally divides the quadrilateral into two superimposed groups of triangles of constant deformation of triangular elements. The values of the geomechanical parameters of the sandstones and limestones substratum do not control the failure but must be reasonably estimated. All materials are modeled with a Mohr-Coulomb failure criterion, with parameters assigned according to Table 2, where the average values proposed by ([PERSON], 2018) for the sandstones and limestones have been used. The lowest strength corresponds to lutites, where instability is predictable and the geotechnical characteristics that have been considered correspond to the study interval of the geotechnical research referenced ([PERSON], 2018). Therefore, it is advisable to conduct a sensitivity analysis to study global stability with different geotechnical parameters that define the level of luitites. For the boundary conditions, horizontal constraints for literals contours and both horizontal and vertical constraints for the lower boundary of the model are considered. First of all, initial stresses under static conditions (only under vertical gravity loading) are calculated, considering the water pressure caused by the situation of the water table at the head as is indicated in Section 4.3. The estimation of horizontal effective stresses has been performed with [PERSON]'s formula because, even if this is not a horizontal layered soil configuration, it can be considered accurate enough for this estimation. For the back analysis, a sensitivity analysis by varying the strength parameters of the geotechnical unit of lutites is considered. It has been necessary to reduce cohesion to values of 20 \(\mathrm{kN/m^{2}}\) and friction angles of 24\({}^{\circ}\) to induce breakage. A lower value of cohesion for this material is hardly realistic as previously mentioned. As can be analyzed, these values are consistent with those studied by [PERSON] el al. (2018). Figure 7 shows the 3D failure surface obtained where the extent of the landslide can be clearly seen, which entirely affects the lutites unit and which acquires a spoon shape inserted in this stratigraphic unit. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Stratigraphic** & **Apparent** & **Effective** & **Effective** & **Young’s** & **Poisson’s** \\
**Unit** & **Density** & **Cohesion** & **Friction Angle** & **Modulus** & **Modulus** \\
**V** (kN/m3) & **c (kN/m2) & **\(\varphi\) (\({}^{\circ}\))** & **E (kN/m2)** & \(\varphi\) \\ \hline Sandstone * & 26.5 & 200 & 40 & 250,000 & 0.30 \\ Limestones * & 26 & 50 & 38 & 200,000 & 0.30 \\ Luitites & 24 & 20–100 & 17–33 & 70,000 & 0.25 \\ \hline \end{tabular}  
* The geomechanical values of these strata can be consulted in Table 1 of the chapter IX and Table 3 of the chapter XIII of ([PERSON], 2018).  
\end{table} 
Table 2: Geomechanical parameters of the different lithologies. A main section (marked in Figure 7) allows the failure surface to be analyzed (Figure 8). In this way, the geomechanic situation is obtained using the geotechnical parameters indicated and researched for the level of lutites. A failure surface with a unit safety coefficient that indicates the strict balance of the mass considered in this calculation is obtained. As can be seen from Figure 8, the breakage occurs in the body of the lutites, being clearly circular and being delimited at the head by the highest area (where the traction cracks were probably located at the change in level, and a main scrap as developed) and on the foot by the glacier.