# A Case Study of a Large Unstable Mass Stabilization:
## 4 Discussion  
The case study presented in this paper at \"El Portalet\" Pass in the Spanish Pyrenees is an example of dealing with a large landslide by slowing it and controlling its behaviour by different instrumentation and monitoring techniques. The proposed approach was fostered by the slow surface movements (about 0.3 m/year) observed during the previous geotechnical investigations and recorded in the inclinometers installed [15]. This value led to the conclusion that the hillside material had a slow type of movement [46] with both distensile and compressive areas. Therefore, permanent infrastructures (such as the parking lot) situated over the hillside were expected not to be seriously harmed by the natural movement of the slope. Nevertheless, stability of the landslide in the long term must be ensured due to both safety and economic efficiency of the parking lot and the surrounding area. The slow movement allowed for the projection of a series of hillside stabilization measures that tended towards the reduction of the movement rate velocity, which eventually attained the total stopping of the landslide evolution. These measures included the modification of the slope geometry, the installation of drainage and the construction of a flexible retaining toe wall. Data obtained from diverse geotechnical investigations, satellite monitoring [14, 15, 18, 19, 21, 22, 23, 24] and numerical models were used to assess the effectiveness of such measures. In addition, new geotechnical investigations, involving new boreholes and the installation of new inclinometers, were conducted, showing that the evolution of the landslide tends towards its total stabilization (no movement). The initial design of the parking lot included a basic stability analysis of the hillside and the implementation of a rigid retaining wall to contain the ground [15]. The landslides activated once the parking area was excavated however, which showed an inefficacy of the analysis and measures. As indicated in Section 3.2, the stability analysis of the area under study should have considered the entire slope and not have been restricted to the area just adjacent to the parking excavation to avoid erroneous results and misunderstand the measures needed. A stability analysis that leads to a safety factor greater than 1.0 does not guarantee the slope to be stable, especially when involving large areas, as is the case under study. Besides, it should be noted that limit equilibrium methods do not consider deformations, so a slope may be unstable sometimes even though the safety factor is greater than 1.0, as reported in some cases found in the literature [50]. An important aspect involving a large area and also shown in this case study is the possibility that a landslide triggers other landslides in the area. The cascade effect that a potential landslide may have in other points of the slope should never be neglected. These additional landslides are difficult to predict unless a geological exploration is conducted in the area potentially affected. All in all, the stability analysis of potential landslides involving large ground masses, even though adjusted to the corresponding design requirements, should be supported on thorough geological and geotechnical surveys that enable an assessment that the analysis conducted properly fits the natural processes. In the case under study, the influence of the water table plays a critical role in the stability of the landslide. In particular, the drainage time needed for lowering the water table was shown to be decisive in the instability of the system. Thus, establishing only a future situation and assuming this situation will be reached in a short term may give rise to errors. As shown in this work, stability takes time to be reached, an issue that should be considered in the design of slope stability measures and the subsequent monitoringprocess.