On creep flow of snow and firn

Abstract

The exploration and exploitation of polar regions requires the ability to work in and with the firn layer, which is the topmost layer of the continental ice sheet. A firn layer consists of meteoric snow which changes by metamorphosis from original snow flakes to a stable firn body. As the firn is compressed by subsequent layers of snow, a porous granular material evolves with density that is increasing with depth. This leads to polycrystalline ice in the bottom layers. The porous structure of the firn gives rise to volumetric viscous behaviour. Its granular build-up leads to an increase of volume under shearing conditions (dilatation). Furthermore, firn exists at high homologous temperatures, thus it shows creep behaviour.

A material law for snow and firn is proposed, which accounts for these characteristic features of the material. Following the approach by von Mises, it is based on the assumption of an elastic and a viscous potential. The dilatation is described by the third invariant of the stress tensor to model a strain orthogonal to the shearing direction. Creep experiments are used to evaluate the material parameters.