Avalanche defence strategies and monitoring of two sites in mountain permafrost terrain, Pontresina, Eastern Swiss Alps

Snow-supporting avalanche defence structures are increasingly being built at high altitudes in potential permafrost areas. Special construction methods and guidelines have been developed to ensure a minimal stability of the structures, which have a vital role in the protection of underlying settlements and transport infrastructure against snow avalanches. If the avalanche slopes are located on ice-rich permafrost terrain, as is the case in a steep avalanche gully above Pontresina (Eastern Swiss Alps), other means of protection must be used - such as deflection or retention dams - as construction on ice-rich sediments can be very problematic. Experimental snow-supporting structures were built in 1997 in order to test different types of structures and their foundations, to develop specially adapted construction methods and to monitor the long-term behaviour of the structures in moderately creeping frozen ground with volumetric ice contents under 20%. Snow-nets were found to be the most suitable type of protection against avalanches in this type of permafrost terrain due to their deformability and because they are well adapted to rock fall. The structures do not improve slope stability but contribute towards maintaining permafrost as they delay snow melt by modifying the spatial and temporal distribution of the snow cover. The results of the project described have led to a better understanding of permafrost-related avalanche defence problems.