| The ratio of shear strength to shear stress for snow cover on a slope (referred to as the stability index SI) is often used as a metric to gauge avalanche risk. The shear strength of snow begins to gradually increase with the passage of time immediately after it falls. However、 the quantitative relationship of temporal changes in the shear strength of snow[*]cover with grain type、 temperature、 snowfall conditions and other factors has not yet been clarified. In this study、 field measurements were conducted to clarify the characteristics of temporal changes in the shear strength of snow cover after heavy snowfall、 and their effects on the stability of snow cover on slopes were examined.[*] Snow pit observation was conducted for snow cover with a thickness of approximately 80 cm accumulating over a period of 24 hours between January 7 and 8、 2007 in the Nakayama Pass near Sapporo. Shear strength、 density and temperature of snow were measured、 and snow grain types were identified nine times at intervals of three to nine hours two to four days after the end of snowfall.[*] The snow cover in this study was new or compacted、 and that the snow temperature was -3 to -7 degrees Celsius. The existing relational equations and the measurement results of this study were compared in terms of the relationship between shear strength and density. The results revealed that the relationship corresponded with the relational equation[*]for cases with weak layers in a past study up to a density of around 150 kg m-3. However、 as the density increased、 the shear strength increased at a higher rate、 and the relationship became closer to that indicated by the relational equation for cases without weak layers. [*] Next、 the SI was calculated based on viscous compression theory to clarify the effects of such temporal changes on the SI. As a condition for calculation、 the observation results of this study were applied to the relationship between density and shear strength. The result revealed that the SI during snowfall was lower than in the case of the existing[*]relational equation for snow cover without weak layers、 and that it increased gradually after snowfall stopped. Since snow that accumulated in large amounts during a short period of time has less shear strength than normal snow、 it is considered necessary to pay attention to the stability of snow cover on slopes for a longer period of time. |
