Mechanical properties of snow

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Abstract
The investigation of the mechanical properties of seasonal snow cover aims mostly at applications in avalanche release and avalanche control but also at no less important problems such as vehicle mobility in snow, snow removal, or construction on snow. Primary needs are (1) constitutive equations, that is, relations between the stress tensor and the motion, and (2) fracture criteria which limit the region of validity of constitutive equations. Both can be tackled from the aspect of continuum theories and structure theories. With modern continuum theories the characteristic nonlinear behavior of snow can be taken into account and also the strong dependence on stress and strain history. When thermodynamics is introduced, more insight into the deformation and fracture processes can be gained. High initial deformation rates cause low dissipation, elastic behavior, and brittle fracture, whereas when dissipative mechanisms can develop, ductile fracture occurs. The advantage of structural theories lies in the immediate physical insight into deformation mechanisms, but the disadvantage is that only simple states of stresses acting macroscopically on a snow sample can be considered. Different approaches have been elaborated: for low‐density snow the concept of chains (a series of stress‐bearing grains) or the neck growth model (consideration of stress concentrations in bonds between grains) and for high‐density snow the pore collapse model (snow idealized as a material containing air voids). Structural constitutive equations were applied to the calculation of stress waves in snow. Recorded acoustic emissions, indicating intergranular bond fractures, can also be used for the construction of constitutive equations. Structural failure theories model brittle fracture with series elements, where the weakest link causes fracture of the entire body, and ductile fracture by parallel elements, where fracture of one element leads merely to a redistribution of stresses and only after a sufficiently high increase of the load to a total failure. In this method the statistical distribution of link strength plays an important role. The mechanics of wet snow (snow containing liquid water) is considerably different from dry snow mechanics. While deformation of dry snow is dominated by (slow) creep and glide of ice grains and bonds, the densification of wet snow is mainly due to the (fast) process of pressure melting at stressed contacts of a grain.

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