| Knowledge of temporal changes in the shear strength of new snow is required in order to evaluate the stability of snow accumulating on slopes and determine avalanche risk during periods of snowfall. To clarify the effects of temperature and normal loading on temporal changes in the shear strength of new snow、 we carried out an experiment using artificial snow at the laboratory of the National Research Institute for Earth Science and Disaster Prevention located in Shinjo、 Japan.[*] In this experiment、 artificial snow consisting of dendrites fallen from a snow-making machine was allowed to accumulate to a depth of 60 cm. The shear strength、 which is referred to as the shear frame index (SFI)、 was then measured at three positions of accumulated snow using a shear frame with an area of 250 cm2. The temperature of the snow and the normal load from weight of snow above the measured layer were also measured. The environmental temperature in the laboratory was -10 °C during the snowfall period. After the snowfall stopped、 the environmental temperatures were controlled in two patterns to -3 °C (Pattern A) and -7 °C (Pattern B)、 and measurements with these patterns were continued for 76 and 24 hours、 respectively. To determine the factors affecting the temporal variation of shear strength、 SFI were measured while increasing the normal load by placing different weights on the snow within the shear frame. The measured SFI were treated with the Coulomb expression Σ = C + σ tanφ、 and cohesion factor C and internal friction factor tanφ were calculated using regression analysis of the SFI (Σ) measured with the different normal loads (σ) provided by the weights.[*] The results revealed that the SFI measured in Pattern A with a relatively high snow temperature increased more quickly than those in Pattern B with a lower temperature. The values of C exhibited the same tendency as those of SFI. According to multiple regression analysis on the rate of temporal variation for C using the snow temperature and the normal load due to snow above measured layer as dependent variables、 the rate of increase in C increased with large normal loads and high temperatures. Cohesion factor C is increased by the growth of bonds between snow crystals、 that contributes to increased SFI. However、 the value of tanφ decreased with high snow temperatures and increased with low snow temperatures. The results of multiple regression analysis show that the rate of temporal variation for tanφ mainly depends on the temperature. In particular、 a snow temperature of approximately -6 °C separates the positive and negative trends of tanφ within the range of normal loading measured in this experiment. In low-temperature conditions、 compaction of the snowpack makes progresses while the shape of dendrites is maintained、 so the degree of penetration with which the branches of each crystal interlock increases. This suggests that the normal load due to removed snow above the measured layer should be considered in SFI measurement of new snow in conditions of low temperature. |
