Hi Fred
Interesting question. On the one hand, when I think of snow, there is that fluffy moldable white stuff of our childhood where we built snow forts with vertical walls, on the other hand with additional moisture or pressure does this condition change? Here is what Bernhard Muessen, an expert in snow thermo-mechanics has to say on the subject. Glean what you will.
"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."
So....it appears that poisson's ratio may be in evidence for polar snow regions such as these, but for practical purposes I'd guess that lateral creep may not be present or not sufficiently large under normal seasonal conditions to warrant building design consideration in the lower US.
Hope this helps.
PE in NE
