Permafrost Changes in Rock Walls

The present thesis discusses for the first time systematic investigations of permafrost in rock walls of high Alpine mountain regions. However, the focus of this thesis are the physical processes that are taking place within these permafrost occurences rather than their spacial distributions.

Field investigations were carried out in the region Jungfraujoch-Aletsch. At Jungfraujoch (3500 m asl) Europe's highest railway station, tourist facilities and a scientific research station are located. Contracted by the Jungfrau Railway Company the consulting firm Geotest is monitoring the geological and geotechnical properties in the vicinity of the various installations. In conection with new construction activities at Jungfraujoch boreholes in rock were equipped with thermocouples and deformation sensors. The present thesis is in part based on the resulting data. However, because these measurements are thermally and mechanically affected by the presence of the buildings we performed comparative measurements at the east ridge of Jungfrau. On both sides of the ridge twenty metres deep boreholes were drilled and equipped with sixpoint-extensiometres and eight thermistors. Data are acquired automatically with a datalogger station which can be controlled via telefon link from ETH. The measurement and data aquisiton system was mainly constructed, calibrated and programmed at VAW-Glaciology and has been in operation since its installation in summer 1995. As boundary conditions for model calculations, surface temperatures in rock walls at different altitudes and with different geographical orientations were required. For this reason a number of miniature temperature dataloggers were installed in several rock walls around Konkordiaplatz.

Freeze/thaw processes in rock can be identified in the measurements. The annual variations in rock temperatures are frequently out-of-face with the rock deformations such that extensions are measured during cooling and contractions during warming periods. Furthermore, there is a relation between the extent of the strain and the location of the measurement with respect to the mechanically active joint system. Clear annual strain variations were also observed in regions were the bedrock never warms above -4°C. Phase changes in rock are therefore not isothermal at 0°C. After excavating a 100 m long elevator shaft through the Sphinx ridge, the vertical joint system opened by 3.5 mm. To inhibit further deformation the building foundations had to be reinforced to withstand horizontal tensile forces. Boreholes equipped with several thermistors yielded spatial and temporal temperature information. Based on the pure heat conduction theory, this information could be used to calculate the latent heat production during freezing/thawing of water/ice in the joint system. The calculated ice/water content within the active layer of the gneisses at Jungfrau east ridge is between two to three volume per cent. The measured strain is about ten times smaller than expected from the freeze/thaw processes. A parameterisation of the current mean annual surface temperature in dependency of altitude and aspect was carried out for the granitoidic rock walls in the region of investigation.

The results of the field and the laboratory investigations were used in a variety of model calculations. With these transient model calculations the thermal and thermo-mechanical behaviour of simple two-dimensional geometries was analysed. The thermal disturbance during excavation of the elevator shaft in Sphinx ridge (autumn 1994) and the expected future steady state situation were analysed. For comparison, the thermal and thermo-mechanical behaviour of the undisturbed Jungfrau east ridge were modelled. The influence of the glacier retreat on the temperature field in a north facing rockwall at Konkordiaplatz was investigated. In transient heat transfer calculations a 1000 years long climate history with changing surface temperatures and glacier fluctuations was considered. The model calculations demonstrate that changes in the permafrost distribution are strongly attenuated by the latent heat production. For the same reason the freeze/thaw activity is considerable.