# A Case Study of a Large Unstable Mass Stabilization:
## 1 Introduction  
Slope stabilization is probably one of the most typical, ancient and challenging issues of civil engineering. Commonly, problematic landslides affecting buildings and infrastructures are solved by installing a series of measures that lead to completely and immediately stopping the ground movement. Those measures normally include rigid retaining walls, anchors or a substantial variation of the ground profile's geometry, and their implementation can give rise to a compromise between cost and safety [1, 2, 3]. However, when dealing with a great amount of ground material, stopping the movement absolutely and instantaneously may not be the optimal solution. The literature shows the possibility of using different techniques for reducing the mobility of landslides, including small excavations and adjustments of the slope profile, the implementation of flexible walls or piles and the installation of drainage [4, 5, 6, 7, 8, 9]. The latter is especially important since landslides are often triggered by precipitations and the resulting change in the groundwater level. Water reduces ground strength and increases pore pressures, contributing to the instability of a slope [7, 10, 11, 12, 13]. Controlling the water table is essential when dealing with shallow and slow movements, as any rainfall may accelerate or even reactivate a landslide. The previous issues are particularly relevant in areas characterized by rainy seasons followed by dry periods, such as the Mediterranean region and, more precisely, the Iberian Peninsula. The \"El Portaler\" Pass is located at the municipality of \"Sallent de Gallego\", Huesca province, in Spain, and belongs to the Central Spanish Pyrenees. In 2004, an excavation tocreate a parking lot next to a local road was performed in the area. Parking excavation was conducted and resulted in the activation of a large landslide. That landslide occurred on a hillside in the surroundings of Petrusos Peak, in an area of about 0.35 km\({}^{2}\), where superficial cracking and diverse instabilities at different movement rates had been identified in the past [14]. A carboniferous substrate characterizes the zone, affected by faults of slate nature, with gravel colluvium and sandy clay materials on foothills as a result of previous paleo slides. Natural slopes (average angles of 12\({}^{\circ}\)) were also identified to be on the verge of instability [15]. The parking excavation reactivated small slides previously identified in the area [16; 17] and produced new ones. The first instability signals were displayed at the end of the year, when large cracks appeared in the head of the slope and propagated afterwards along the hillside, showing cracks and longitudinal and transversal deformations, even breaking the slope toe. This type of deformation characterizes an paleo-slide failure mechanism involving an important volume of material [15; 18; 19; 20]. Due to the great deformations and large unstable masses involved in the landslide, the typical solutions aimed at absolutely and instantaneously stopping the ground movement would have represented a higher cost than the total elimination of the unstable hillside. Instead, the approach was based on controlling the hillside movement evolution and implementing a series of measures to slow down the ground movements, achieving the stabilization of the slope after some years, but also ensuring no damage to any infrastructure during that time. This paper shows and analyses the stabilization measures projected and implemented on \"El Portalet\" Pass. The area has been intensively investigated since 2004 by diverse authors. Classical geotechnical investigations, including boreholes and the installation of inclinometers [15], were conducted after landslide activation. Several Differential GPS campaigns as well as the use of different satellite surface monitoring techniques, including the ground-based SAR (GB-SAR) technique and the Differential Synthetic Aperture Radar Interferometry (DinSAR) technique [14; 21; 22; 23; 24], were performed from 2006 to 2010.