# A Case Study of a Large Unstable Mass Stabilization:
## 2 Materials and Methods  
### Geographical and Geological Situation  
The case study is located at \"Sallent de Gallego\" (Huesca, Spain). That area belongs to the high basin of the Gallego River Valley (Figure 1), in the Central Spanish Pyrenees.  
The studied hillside corresponds with the southwestern slope of the spurs located between Petrusos Peak and the Old Pass of Sallent (latitude 42\({}^{\circ}\)48\({}^{\prime}\)4.83\({}^{\prime\prime}\) N; longitude 0\({}^{\circ}\)24\({}^{\prime}\)48.19\({}^{\prime\prime}\) O), with heights between 2128 m and 1848 m. The road next to which the parking lot was built, road A-136, is situated at the hillside foot, and links \"El Portalet\" Pass and \"Sallent de Gallego\" village. The Gallego River flows through lower elevations, parallel to the road. The excavation was performed at altitudes between 1735 and 1775 m. Figure 1 shows a parking excavation view, where road A-136 can also be seen.  
From the geological point of view, the area under study is located in the Pyrenean Axial Zone, primarily composed of Devonian and Carboniferous materials, amongst which slates (sometimes with sandstone interlayers) are common, with some calcareous zones, all of which are affected by an intense Hercynian folding, accompanied by low-grade metamorphism. From a tectonic point of view, all of these materials are part of the Gavarnie thrust, which contains part of the southern configuration of the Pyrenean chain. Most of the hillside is covered by recent colluvial and quaternary deposits [25; 26; 27].  
The area contains low strength rocks with intense active slope processes and with portions with low structural control. This is observed between the Inner Chains and \"El Portalet\" on the Gallego River basin, where slates, schists and clays have caused several slope evolution phenomena, associated with mass movements. Slumps are the most spectacular shapes, which leave a scar or tension crack on the head of the slope. Those slumps are deep and affect the substrata, also presenting deep weathering due to both their plasticity and tectonization. In some cases, Devonian slates have acted as a lubricant, their structure and faults located at the top of the slope sometimes causing an additional instability issue. The length of tension cracks can exceed 1 km, and the landslides portray the typical internal corrugation, a consequence of a mass movement.  
One of these slumps was mapped by [PERSON] [16; 17]. Two parallel tension cracks, which form successive steps (see Figure 1), match the southern slope of the Petrusos spurs and they also overlap with the convex surface presented by the rocky hillside. At its toe, the slumps can be found with two coalescent lobes that reach the fracture northwest of the sunken block of \"El Portalet\". The high density of such morphologies can be found on the Gallego head of the slope, especially between \"El Portalet\" and Formigal ski station in addition to the Pena Foratata southwestern slope, where superposed landslides tend to form great slumps. Most of them do not look very active, but other slumps are still moving or show a local reactivation.  
This area is also located in a seismic zone, presenting a basic seismic acceleration of 0.10 g (with g being the gravitational acceleration), according to the Spanish NCSR-02 standard [28].  
Figure 1: Geographical and geological setting of the area under study in the Central Spanish Pyrenees and view of the parking excavation works (note: bottom left image modified from [22]).