Natural ventilation through open bedrock fracture systems and its influence on rock slope stability

Open bedrock fracture networks are characteristic structural features in unstable rock slopes. They affect the subsurface bedrock temperature field due to fracture ventilation and temporary water infiltration. The ground thermal conditions are a key factor influencing slope stability. Ascending air circulating through fracture networks during winter (the so-called chimney effect) facilitates the cooling of the ground and leads in some cases to the development of extra-zonal permafrost. In addition, fracture networks exposed to the atmosphere have an impact on gas exchange processes at the Earth-atmosphere interface. Natural ventilation of the underground compartments can thus lead to increased gas exhalation to the surface. Especially the radioactive gas radon (²²²Rn) has been used in Earth science and environmental studies of natural ventilation systems and has due to its relatively long half-life great potential to characterize the subsurface bedrock fracture systems.

We present a case study of a natural ventilation system from the Stampa rock slope instability (Aurland, Norway). The area above the slide scar is characterized by a relatively low slope angle and bedrock lineaments, which correspond to morphological depressions and open subsurface fractures. Natural ventilation through these fractures has been observed at several locations at Stampa, where air flows out or in. Such chimney ventilation depends upon outside air temperature compared to subsurface temperature but also on locality and other atmospheric conditions such as wind and air pressure. Rock-surface and air temperature loggers in open fracture systems can provide information about both subaerial temperature and the subsurface temperature field, which can be use to model the chinmey ventilation. Instruments continuously monitoring air flow and radon concentration at selected vents, in addition to sporadic alpha track radon surveys are used to identify the extent and connectivity beween individual ventilation systems and verify ventilation patterns. In situ measurements are combined with UAV surveys using both optical and thermal imaging. We found that air ventilation through several individual systems of open bedrock fractures leads to cooling of the ground and to the development of sporadic extrazonal permafrost far below the regional permafrost limit. Radon concentration of outflowing air is depending on the air flow rate and the rock-atmosphere contact area which, in turn, depends on ground water level and the extent of ice in subsurface fractures. The subsurface bedrock reaches its highest temperatures in late autumn/early winter which coincides with enhanced slope deformation.