Stability problems under a chairlift midway station in creeping permafrost terrain, Grächen, Swiss Alps

The mid-way station of a chairlift located in the ski resort Grachen (Swiss Alps) was originally built in 1997 at 2453 m ASL in alpine permafrost terrain. A few months after construction it became evident that the terrain at the mid-way station was unstable: settlement and creep occurred and cracks formed in the structure. Two 25 m boreholes were drilled near the foundations and equipped with inclinometer tubes and thermistors. The presence of permafrost with exceptionally large active layer depths and a 20 m thick talik containing water was confirmed. The deformation rates of the ground attained high values between 2002 and 2003. As a consequence, a specially developed new mid-way station had to be built in 2003. The excavation trench was lined with insulating material in order to avoid thermal disturbance of the underlying permafrost during the setting of the concrete. 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. Ground temperatures and slope deformation continue to be monitored to determine the long term evolution of the mid-way station.