Monitoring and reconstruction of a chairlift midway station in creeping permafrost terrain, Grنchen, Swiss Alps

The midway station of a chairlift located in the ski resort Grächen (Swiss Alps) was originally built in 1997 at 2453 m ASL in alpine permafrost terrain. The chairlift conveys 300,000–330,000 passengers every winter season and constitutes an important link between two cable cars in the ski area. In winter 1997–1998, it became evident that the terrain at the mid-way station was unstable: one of the two concrete foundations started to creep and settle rapidly and cracks formed in the structure. To investigate the properties of the ground, two 25 m boreholes were drilled near the foundations and equipped with inclinometer casings and thermistors. The presence of permafrost with an exceptionally thick active layer and a 20-m talik containing water was confirmed. The horizontal and vertical deformation rates of the ground attained very high values between 2002 and 2003. As a consequence, and in the interest of the safety of the passengers, the original midway station had to be destroyed and a specially developed new station was built in 2003. The excavation trench was lined with insulating material in order to avoid thermal disturbance of the underlying permafrost by hydration heat. The new foundation consists of a concrete T-girder with three point bearings. Repositioning of the entire structure in response to creep is possible, due to the unique character of the structural bearings which can be raised or lowered using hydraulic cylinders and steel plates. The thermal regime of the entire structure, ground temperatures and slope movements continue to be monitored to determine the long-term evolution of the mid-way station.