Influence of Microstructure on the Consolidation of Compressed Snow

Snow falling on roads is rapidly compacted into a hard crust which is difficult to remove, and which may become slippery at temperatures near melting. To avoid this, chemicals are often spread in a measure called anticompaction to keep the snow plowable until it can be removed by highway maintenance personnel. The use of chemicals is, however, harmful both to the environment and to structures and vehicles, meaning that more efficient anticompaction measures are needed. Testing the efficiency of an anticompaction measure in the field is an extensive job, and as an alternative different lab tests have been developed. Unfortunately these correlate poorly with field tests and one of the causes for this has been hypothesised to be the type of snow which is used. In this paper the effect that different degrees of metamorphism had on the hardness of snow immediately after compaction was studied. Three different snow types were used, as follows: (1) dendritic snow, (2) granular snow, and (3) snow consisting of large round grains. The snow types were characterized by specific surface area, snow type classification, and micrographs. The dendritic snow was allowed to coarsen between 0 and 168 h to increase the number of different snow morphologies. The different snow types were compressed at $-6°\mathrm{C}$ to a density of $500\mathrm{kg}/{\mathrm{m}}^3$, after which their hardness was measured with a micropenetration test. The results show that the compact hardness decreases rapidly with the degree of metamorphism; after 24 h the hardness was 80% of that of fresh snow and snow left to coarsen for 1 week before compacting only had 40% of the fresh snow hardness. The granular snow was still weaker and the snow consisting of large rounded grains did not consolidate at all, but remained as a collection of loose particles. This shows that when comparing the effect of different chemicals on anticompaction it is important that the experiments are performed on a snow similar to what can be expected to fall on a road. Old natural snow or granular artificial snow are likely to give misleading results.