Modelling the Portalet landslide mobility (Formigal, Spain)

The Portalet landslide is a large, active slope failure located on the upper part of theGállego River valley, in the Central Spanish Pyrenees (Sallent de Gállego, Huesca). Thelandslide is located on a southwest-facing hillside of Petrasos Peak, close to the ski resort ofFormigal. The dimensions of the landslide are 500 m long and up to 700 m wide at the toeextended over an area of 0.35 km2 that shows superficial cracking and distinct grounddisplacements. Displacements are quite important (several cm in a month) from summer2004 when the construction of a parking area was carried out by digging the foot of theslope. Field observations show a close relation between ground movement and rainfallintensity. Monitoring and predicting the landslide behaviour is necessary to guarantee thesafety of this ski resort area. Monitoring was performed with four differential GPS (DGPS)campaigns between May and November 2006 and installing a novel ground based SAR(GBSAR) monitoring system. GBSAR measurements were recorded continuously for 47days, from the 5th of October 2006 to the 21st of November 2006, providing up-to-datedisplacement maps of the landslide area at a rate of 1 per hour. Two numerical models havebeen proposed to forecast the recorded movements directly from rainfall intensity. A simple1D infinite slope model solving the momentum equation gives a first approximation. Themodel includes a viscoplastic behaviour and takes into account directly the known dailyrainfall intensity and the dissipation of the excess pore-fluid through a simple consolidationequation. A fully coupled hydro-mechanical finite element analysis in plane strainconditions using an elastoplastic constitutive model has also been performed. The resultsshow that prediction of slope behaviour is feasible if data parameters are well documented.