Dynamic magnification factors for snow avalanche impact (with pile-up) on walls and pylons

Abstract

Snow avalanches are a significant natural hazard in many mountainous regions throughout the world. Although long recognized that avalanche impact can easily destroy buildings and other infrastructure, design codes rarely, if at all, consider dynamic magnification factors. The underlying problem is that flowing snow at impact behaves both as a solid and as a fluid. The combination of solid and fluid behaviour makes it difficult to characterize the avalanche impact loading. The solid behaviour of flowing snow leads to pile-up regions of highly compacted snow in front of the structure. This dead-zone often adopts the form of a prismatic wedge with two distinct shear planes. The wedge serves to deflect avalanche snow around the impacted object. Because avalanche snow likewise exhibits fluid behaviour, the deflected snow can easily bypass the structure without stopping. In this paper we model the compaction, pile-up and deflection of avalanche snow around walls, pylons and trees. The work energy theorem is used to model the forces assocated with the compaction process and therefore pile-up phase. We show why the pile-up produces intensive, short duration loadings in the form of a triangular impulse. We quantify the duration time as a function of the pile-up geometry and compactive properties of the flowing snow. This allows us to both quantify the impact duration as well as select the appropriate form of the impulsive loading. If the eigenfrequency of the structure is known, dynamic magnification factors can be calculated for avalanche impact on a wide range of structure geometries, especially ski-lift masts, walls and power transmission towers.