Abstract
A weak layer consisting of non-rimed plate-type snow crystals that formed during the night of February 4-5, 2010, was observed on February 8, 2010, at the Shinjo Cryospheric Environment Laboratory. It was confirmed that the weak layer included not only crystals with broad branches, but also large dendrites, and the density of the weak layer was lower than the densities of those of other studies. The shear strength of the weak layer was nearly identical to that of normal snow with the same density. A micrograph of a thin section of the layer showed that a majority of the snow crystals were deposited individually and horizontally. Thus, it was assumed that the weak layer could possibly collapse while under shear stress, since the unique microstructures depend on the mechanical properties of single ice crystals. However, in the case of brittle failure, the shear strength of the basal plane of the single ice crystals was similar to that of the perpendicular plane. In the case of ductile failure, the shear strain rate under plastic deformation calculated for a typical overburden load showed susceptibility to collapse, but in nature, this was not observed. Therefore, another mechanism is needed to control the deformation.
