Analyse multi-méthodes de la déstabilisation d’un pylône de remontée mécanique implanté sur un glacier rocheux des Alpes françaises

Abstract

La station de Val horens (Savoie, France) concentre plus de 79 composantes de remontées mécaniques en contexte de permafrost probable. Au cours de la dernière décennie, trois d' entre elles (deux gares amont et un pylône), situées vers 3 000 m d'altitude, ont subi des déstabilisations partielles nécessitant d'importants travaux de confortement et de stabilisation. Cet article documente la déstabilisation du pylône n° 2 du funitel de horens au cours de l' été 2016. Ses fondations ont été afectées par un déplacement horizontal et un afaissement à l' origine d'un décalage métrique de la tête de pylône par rapport au câble. Le pylône en question est implanté sur un glacier rocheux terrassé dans le cadre de l'aménagement d'une piste de ski. Une analyse multi-méthode a été conduite ain d' évaluer le rôle du permafrost dans la déstabilisation et de caractériser l' évolution des processus ainsi que l'impact des modiications anthropiques sur la dynamique du glacier rocheux à travers (i) l'analyse stratigraphique de données de forages, (ii) une analyse du terrain porteur fondée sur des proils géophysiques de résistivité des matériaux acquis par tomographie électrique, et (iii) la reconstitution des modiications géomorphologiques de surface par comparaison de modèles numériques de terrain, obtenus par photogrammétrie numérique (images aériennes IGN et drone) avant et après terrassement. Cette étude met en évidence le rôle préparatoire du terrassement et des écoulements d' eau de surface mal contrôlés dans un milieu riche en glace tel qu'un glacier rocheux. ABSTRACTIn the Val horens ski resort (Savoie, France), more than 79 infrastructure elements related to ropeway transport systems are located on permafrost. During the last decade, three of them (two top stations and a pylon located around 3,000 m a.s.l.) experienced destabilization which required important adjustment and stabilization work. his study investigates the destabilization of the pylon #2 of the horens funitel that occurred during the Summer 2016. Its foundations were afected by horizontal displacement and subsidence, causing a metric shit of the pylon top with respect to the cable. he pylon was built on a rock glacier which morphology was severely altered by earth work aiming to develop a ski slope. A multidisciplinary analysis was conducted in order to understand the processes that led to the pylon destabilization with a focus on the role of the morphological alterations of anthropogenic origin on the rock glacier dynamic. We used (i) a stratigraphic data analysis from boreholes and (ii) a subsurface analysis based on geophysics (Electrical Resistivity Tomography proiles) to understand the rock glacier internal structure, while (iii) geomorphological surface modiications were investigated by comparing digital terrain models obtained by aerial photogrammetry before and ater the earthwork. his study highlights the preparatory role of the earthwork and a non-control of the surface water low in an ice-rich environment such as a rock glacier.

Qualitative risk assessment and strategies for infrastructure on permafrost in the French Alps

“…Despite the cost of building and adapting some infrastructure, only 5 infrastructures on the 17 had used geotechnical conception adapt on permafrost (long term solution) to prevent or to cope a destabilization event. For example, the Thorens cable car (Val Thorens) built during summers 2010 and 2011 for 10 M€ required additional spending of 700 k€ in 2017 in order to consolidate the foundations of one of the pylons, representing 7 % of the original investment only 5 years after its building (Duvillard et al 2019b). In 5 cases, the reconstruction or consolidation solution was adapted in order to prevent new damage due to further degradation of the permafrost.…”

“…(2) Conversely, the rebuilding or maintenance (adaptation or consolidation) could locally accelerate the permafrost degradation due to heat input from the work, and induce a negative feedback on the permafrost. The excavating and drilling of 38 piles during the summer 2017 to stabilize pylon 2 of the Funitel de Thorens probably induced heat flows in the rock glacier and increased the permafrost degradation (Figure 7b; Duvillard et al 2019b). In order to take into account these positive and negative feedbacks in the risk index calculation, a more in-depth vulnerability (s.s.) assessment and possible assessment of permafrost state would have to be considered.…”