Methods for controlling the strength properties of snow cover

. The paper and the work are devoted to laboratory experiments on the controlling methods of compacting and obtaining strength properties of snow in the refrigerating room for snow cover engineering needs. The relation regulation of equivalent cohesion of obtained snow on its temperature and density was established.


Introduction
The strength properties of snow can be measured by various methods and technical tests, depending on the level of accuracy and reliability of the expected results, as well as on the available resources.
The Snow micro pen (SMP) is a device that allows for rapid, 4 μm resolution, carrying out and recording data of measurements of the ice matrix resistance force to vertical penetration (at a constant speed) of the probe into the snowpack (snow strength) and the amplitude of this force, i.e., providing information on the condition, stratigraphy and stability of the snowpack [1,2].
Due to the relevance of using the device in various spheres of economic activity, developers and scientists in different countries around the world continue to investigate its possibilities. The most important technical issue today is to determine the regularities of the interaction between the probe tip and snow crystals and the mechanics of breaking bonds between crystals under external influence.
Through the use of a high-resolution snow penetrometer, it is possible to investigate the resistance to snow penetration and its meso-and microstructure. The variation of the signal is characteristic of different types of snow. The penetrometer can be used both in the field and in the laboratory. The density of snow measured with the penetrometer can vary from freshly fallen light snow (0.05 g/cm3) to very dense snow on ski slopes (0.5 g/cm3) (GOST 30416-96/).

Strength measurement experiment
As part of the practical part of the work, a number of experiments were conducted in the research laboratory of the Avalanche and Mudflow Laboratory of Lomonosov Moscow State University. The essence of the experiment was to consider the dependence of equivalent cohesion of snow on various factors (Fig. 1). Equivalent cohesion is a term derived from permafrost engineering. It is a complex characteristic of the strength of frozen ground, taking into account both the cohesion itself and the presence of internal friction. [3][4][5]. In general, the equivalent cohesion reflects the strength of the bonds between the structural elements of the soil. The value of equivalent cohesion is most useful for setting the ultimate bearing capacity of snow with different characteristics for the construction of various snow and ice roads and runways.
The method of ball indentation consists in pressing a rigid ball-shaped die into the snow under a given load p and measuring the depth of its immersion in the process of indentation (Fig. 2). Readings of deformation measuring devices are used to determine the depth of immersion of the ball die into the ground at the end of the test (when the deformation has conditionally stabilised).   The essence of the experiment I conducted was to observe and determine the factors on which the value of equivalent cohesion depends. For this purpose, four samples of snow and one sample of ice were prepared. Snow was used for the experiment.
Each sample was immersed in a special steel casing and weighed exactly 50 grams (except ice). Using a UNITRAUM UNY10003 hydraulic press (Fig. 3, 4), the samples were brought to different density values. The humidity of the snow used was measured using a special Moisture Meter type DM4A. It is worth noting that the snow samples showed moisture values in the region of 30%. This value is only possible in freezer conditions, as in the natural environment the humidity is much higher. Table 1 shows the characteristics of the samples.  The main characteristics I considered in the experiment are: temperature (and humidity), density and vertical load of the ball die. In each sample, I varied one of the characteristics to clearly show on which parameters the equivalent cohesion was most dependent.
-Sample No. 1. This is the reference. The temperature of the snowpack at -9…-10 C, density 0.7 g/cm3, a vertical load of 0.7 MPa on the ball die. The measurement gave a value of equivalent cohesion equal 0.0107 MPa.
-Sample No. 2. The target density is also 0.7 g/cm3, but the temperature of the snowpack is increased to -6 С, and the vertical load is reduced to 0.5 MPa. The example produces low values of equivalent cohesion.
-Sample No. 3. The temperature of the snowpack has dropped again to a value of -10 С, the vertical load is increased to 0.7 MPa, but the density is reduced to 0/.584 g/cm3. In spite of the reduced density, the equivalent cohesion of the sample is virtually the same as that of the reference.
-Sample No. 4. Temperature of the snowpack again -10 C, the vertical load is also 0.7 MPa, but the density is brought up to 0.896 g/cm3. This sample showed a high equivalent cohesion value of 0.0290 MPa.
-Sample No. 5 (ice). To compare and demonstrate the differences with the fourth sample, a piece of ice with similar density parameters was chosen as the fifth sample. When the same vertical load was applied, the ice showed similar, but still slightly lower values of equivalent cohesion.
Below is a graph of the distribution of the samples as a function of the value of the equivalent cohesion as a function of temperature (Fig. 5). There are no differences between samples No. 1 and No. 3, and although the density differs by almost 150 g/cm3, at the same temperature, the density values are evened out and give similar values of equivalent cohesion (0.0107 and 0.0106 MPa). However, when the density value is increased by a further 150 g/cm3 (Sample No. 4), the highest value of equivalent cohesion is achieved. Moreover, the difference in density between Sample No. 4 and Sample No. 5 (ice) is small (0.896 g/cm3 and 0.901 g/cm3), but the equivalent cohesion of sample No. 4 is higher. This is most likely due to the fact that the fourth sample is composed of compacted snowflake particles, while ice represents frozen water molecules and forms a rigid crystalline lattice.   Figure 8 shows that the ball die was able to push through the ice, but cracked. This may also be due to the lower value of the equivalent cohesion value than in sample no.4, shown in Fig. 7 on the left.

Conclusion
In summary of the results of the experiment, it can be concluded that the temperature of the snowpack has a direct influence on the value of the equivalent cohesion. Furthermore, with similar density values, snow appears more advantageous than ice, since it does not crack when reaching a conditionally stable deformation, which reduces the further strength of the sample. The work has been performed in accordance with the state budget theme "Evolution of cryosphere under climate change and anthropogenic impact" (121051100164-0), "Hazard and risk of natural processes and phenomena" (121051300175-4).