Use of 3D geothermal modeling in complex thermosyphon design for decelerating creep-related rock slope deformation, Swiss Alps

ABSTRACT: The Piz Nair summit cable car station (elevation 3,030 m) is located in the canton of Graubünden, Switzerland and is a popular vantage point. The station was originally built in 1955 and upgraded in the early 2000’s to accommodate the current, 80-person gondola. The station is comprised of the cable car system (anchored into the bedrock) and a multi-use building on a shallow foundation that hosts a mountain restaurant. Ground temperature measurements show that warm permafrost (approximately 0 to -0.5°C for 20-30 m in thickness) is currently present in the subsurface. During the 2000’s construction, ice-rich conditions were encountered, confirming the presence of permafrost at this location, at that time. Deformation of the south-western rock slope has been measured via survey points (since 2005) and recently via slope inclinometers (Figure 1), extensometers and topographic change detection (structure from motion using unmanned aerial vehicle), as well as ground-based InSAR. Deformation accelerations seem to correlate with warming air temperatures and by penetrating surface water. Slope inclinometer deformation patterns are indicative of two distinct layers which are interpreted as zones of differing weathering and ground ice content. The subsurface deformation is impacting the integrity of the building and potentially the cable car operations. Therefore, a mitigation is needed to stabilize the foundation and allow safe operation for another 30 years.The project team completed multiple rounds of geothermal modeling to demonstrate the effectiveness of a thermosyphon option for stabilization. Conceptual models were completed using typical rock thermal properties, local wind speeds and air temperatures, and rough topography. Results demonstrated sufficient thermosyphon heat extraction in an Alpine context. A site-specific 2D thermal model, that included climate change projections, was advanced to support a site-specific feasibility study for the suitability of deformation stabilization using thermosyphons. After initial studies confirmed the feasibility of thermosyphons for heat extraction under alpine climate conditions, the project team was requested to complete detailed thermal design for a complex, thermosyphon-based passive cooling system to retard the thermal degradation and possibly restore the permafrost conditions in the rock slope and thereby increase its stability, hence arrest ongoing deformations. Note: the goal of the project was to slow the deformation of the rock via cooling for approximately 30-years.