Glacier- and permafrost-related slope instabilities

Glacier retreat and permafrost degradation are considered to be major hazards in alpine regions, as both induce slow rock slope deformation and rockfall activity that can endanger infrastructure and cause casualties (Figure 9.1). While the scientific observation of glaciers and changes in their extent dates back more than 150 years (e.g. Louis Agassiz in 1841, James D. Forbes in 1842), the observation of permafrost as a hidden subsurface phenomenon in alpine soils, debris, and rocks has only received serious attention since the late 1970s [1,2]. The time span of scientific observation of permafrost in mountains and of the respective instabilities is relatively short and focused on a handful of well-observed study sites. This explains why the retrospective correlation of instability and changing climate conditions is a difficult task. This chapter rather exploits our physical understanding of changes in stress and the physical strength of slopes over time in order to understand systemic patterns of glacier and permafrost-related slope stabilities. Permafrost is a thermally defined phenomenon referring to ground that remains below 0 °C for at least two consecutive years, irrespective of the presence of water or ice in the system [3]. Rock permafrost is not synonymous with perennially frozen rock, as rock often only freezes significantly below the datum freezing point T₀(0 °C), due to the effects of solutes, pressure, pore diameter, and pore material [4]. Ice develops in pores, cavities, and joints (hereafter used as general term) such as fissures and (macro-) fractures (>0.1 mm aperture) [5]. The systemic difference between non-permafrost and permafrost rock walls is, thus, the potential perennial presence of ice (i.e. cryospheric), and its serious implications for the thermal, hydraulic, and mechanical properties of the rock wall system. In cryospheric systems, glaciers and permafrost display a complex interplay and a high level of interconnectivity [6–8]. Principles of the thermal and mechanical interconnectivity of glaciers and permafrost are displayed in Figure 9.2, assuming that (1) active layer thickness decreases with altitude [9]; (2) cold glaciers and onfrozen glacierets are based on and favour the development of permafrost bedrock [10]; (3) warm glaciers conduct massive advective heat transfer with adjacent rocks [11,12]; and (4) the active layer is ‘semiconductive’ as thermal conduction in the frozen rock mass performs better than in unfrozen rock and across air-filled rock discontinuities [13].