Some Results of Studying Seasonal Ground Freezing in the Area of the Tugan Deposit
Abstract Generated abstract
This study examines seasonal ground freezing at the Tugan bedded sand deposit in Tomsk Oblast, where severe winter conditions affect the feasibility and methods of open-pit mining. Using Danilin frost tubes during the 1958 to 1959 winter, the authors measured freezing depths on experimental plots and quarry benches under differing snow cover, vegetation, exposure, and soil moisture conditions. The observations indicate that natural snow cover of about 85 to 100 cm, especially over grass-covered ground, can almost completely prevent freezing, while exposed quarry surfaces froze to about 0.65 m and snowless or wind-swept surfaces to about 1.5 to 1.8 m. The paper concludes that winter stripping may be feasible in naturally insulated areas, but exposed mining surfaces will require insulation, loosening, or other measures to manage frozen ground.
Full Text
Some Results of Studying Seasonal Soil Freezing in the Area of the Tugan Deposit
B. L. Surnachev, Yu. A. Ryzhkov
(Presented by the Scientific and Technical Conference on the Utilization of Mineral Resources of the Tomsk Economic Region)
Introduction
The Tugan bedded sand deposit, from the standpoint of its industrial development, occupies a favorable position. The slightly hilly plain relief of the area is favorable for developing the deposit and for constructing industrial facilities and housing. The shallow occurrence of the productive bed from the surface and the relatively small stripping ratio over a considerable area of the deposit lead to the only possible and economically advantageous open-pit method of development.
The severe winter conditions of Tomsk Oblast will considerably complicate the conduct of mining operations. Seasonal freezing of the soil will require the use of special machines and mechanisms with increased cutting or chipping forces, drilling-and-blasting operations, or special measures to prevent or reduce the depth of soil freezing. To resolve these questions, it is necessary to have detailed information on the character of the seasonal frost in the area of the deposit: the depth of freezing as a function of the magnitude of the snow cover, duration, and so forth.
The purpose of the investigation carried out was to establish certain quantitative values of seasonal soil freezing under various conditions at the Tugan deposit.
Geological Structure of the Deposit and Selection of Sites for Studying Soil Freezing
The Tugan deposit is represented by separate bedded bodies lying shallowly below the ground surface. The overburden rocks, the productive bed, and the floor of the deposit consist predominantly of loose sedimentary rocks. An exception is formed by siliceous sandstones, which in some parts of the deposit overlie the productive bed.
The productive sequence is composed mainly of fine- and very fine-grained quartz sands and kaolin (15–20%). The average granulo-
The granulometric composition of the sands, according to data from the Tomsk Integrated Expedition, is presented in Table 1.
Table 1
Granulometric composition of productive sand
| Size class, mm | Yield in % | Cumulative minus yield in % |
|---|---|---|
| +1 | 3.50 | 100 |
| −1 + 0.5 | 10.75 | 96.50 |
| −0.5 + 0.25 | 8.10 | 85.75 |
| −0.25 + 0.01 | 58.55 | 77.65 |
| −0.01 + 0 | 19.10 | 19.10 |
| Initial | 100 |
Thus, the principal mass of sand particles (77.65%) has dimensions smaller than 0.25 mm.
The average thickness of the productive bed varies by sector from 5.4 to 8.7 m. The immediate roof and the soil of the bed are represented by poor sands, which in appearance do not differ from the commercial ones.
The overburden rocks consist of interbedded rocks: clays, loams, sandy loams, sands of various grain sizes, interlayers of brown coals and lignites, as well as siliceous cemented sandstones with thicknesses from 0.5 to 5 m. The total thickness of the overburden ranges from 0.4 to 80.3 m. The smallest thickness of deposits is noted in sectors No. 1 and No. 2 of the deposit, and the greatest—in sector No. 4.
The floor of the placer consists mainly of clays and is a water-resistant horizon.
The hydrogeological conditions of the deposit are quite diverse. More favorable conditions occur in the first and partly in the second sectors. The other sectors are strongly waterlogged and are for the most part represented by quicksands.
For observations of seasonal soil freezing, sector No. 2 was selected, since it to a greater extent combines features characteristic of the entire deposit as a whole and can be prepared for observations at the lowest cost.
Climatic Conditions of the Tugan Region
The climate of the region is continental, with a short hot summer and a cold, prolonged winter. The average duration of the frost period is 250 days; early autumn and late spring frosts are common.
The mean annual maximum air temperature is +2.7°, the minimum −0.4°; the absolute minimum air temperature is recorded in January at −45°, with a mean monthly temperature of −19.1°. The average date of the last frost is April 25, and of the first is September 8. The thickness of the snow cover at the end of winter reaches 85–100 cm.
Winds of southern and southwestern directions predominate. Wind speed is up to 5 m/sec in winter and 3.1–3.6 m/sec in summer.
Factors Affecting Seasonal Freezing of the Ground
Seasonal freezing of soils is a very complex phenomenon. The processes of freezing and thawing of soils depend on a number of climatic, soil-ground, and water factors. Although the influence of each individual factor, and of the various combinations of these factors, on the freezing process has not yet been sufficiently studied either theoretically or experimentally, it may nevertheless be considered proven that the principal ones are the following:
- The temperature regime of the air.
- Thermal insulation of the ground surface.
- The thermophysical properties of the soil.
The main factor affecting the freezing of the ground is air temperature, since only with the onset of a stable negative mean daily temperature is freezing of the ground possible. The lower the temperature and the longer the duration of the frosty period, the deeper the ground freezes.
The thermal insulation of the ground surface under natural conditions depends on the vegetation and snow covers. Both of these act as thermal insulators and reduce the depth of ground freezing.
The thermophysical properties of the soil (thermal conductivity coefficient and specific heat capacity) depend mainly on bulk density and moisture content. The denser the soil, the more thermally conductive it is; consequently, freezing proceeds to a greater depth. Moisture has a varying effect on freezing. When moisture increases to 30–40%, the depth of freezing increases; with a further increase in moisture, it decreases. This is connected with an increase in the heat capacity of the soil and, chiefly, with an increase in the latent heat of ice formation. Changes in the thermophysical properties of soils in the deposit area are caused mainly by moisture, whereas other indices, such as bulk density, granulometric composition, etc., remain constant.
The factor of air temperature in the winter period was not studied, since observations were carried out during only one winter, 1958–1959. Observations in subsequent years will make it possible to establish the dependence of freezing depth on this factor.
The experimental sites for observations were selected in such a way as to study the influence of various factors on the depth of freezing.
To study the dependence of freezing depth on the character of the vegetation cover, sites with natural grass cover and bare sites (without grass cover) were adopted. For the second case, a special pit was opened (Fig. 1).
The influence of the depth of the snow cover on the depth of freezing was studied on sites with natural snow cover under sheltered conditions and under conditions open to the wind, on sites without snow cover, and with increased snow cover.
The dependence of freezing depth on the thermophysical properties of the soil was studied on sites with normal soil (moisture content 20–30%) and with quicksand soil (moisture content over 40%).
Methodology for Studying Ground Freezing
During the winter period of 1958–1959, observations were carried out at the deposit to determine the magnitude of ground freezing as a function of the depth of the snow cover.
The depth of ground freezing was determined with the aid of Danilin’s frost tube. The operation of the frost tube is based on the property of dis-
distilled water freezing at \(0^\circ\) and below. The depth of soil freezing is determined by probing a rubber tube filled with distilled water and immersed in the soil. The lower boundary of the frozen column of water is taken as the depth of soil freezing, and the upper boundary as the depth of its thawing.
The wells for the frost-depth gauges are lined with wooden tubes of square cross-section. The frost-depth gauges were placed in the middle of each experimental plot. Measurements of soil freezing were carried out once per ten-day period.
The depth of the snow cover was measured on a scale marked on the casing pipe of the frost-depth gauge.
To study the freezing of the overburden rocks, five frost-depth-gauge points were equipped on experimental plots with natural grass cover and with varying snow-cover depth.
Plot No. 1 (frost-depth gauge 8), measuring \(20 \times 20\) m, is located in the eastern part of site No. 2 of the deposit, in an open area. The plot has a slight slope to the southwest. Throughout the entire winter period it is cleared of snow.
Plot No. 2 (frost-depth gauge 9), measuring \(20 \times 20\) m, adjoins plot No. 1. On this plot, with the aid of snow banks and wooden shields, an increased snow cover (up to 1.0 m and more) is created.
Plot No. 3 (frost-depth gauge 10) is located 20 m from plots No. 1 and No. 2 and is characterized by natural snow cover.
Fig. 1. General view of experimental quarry No. 2 of the Tugan deposit.
Plot No. 4 (frost-depth gauge 7) is situated in an open area of the terrain, subject to the constant action of winds, as a result of which the snow cover has an insignificant height.
Plot No. 5 (frost-depth gauge 6) is located in a wooded area and has a natural snow cover of considerable depth.
To study the nature of freezing of the productive sands during the period of their mining, a commercial sand deposit was opened up on the deposit site. A general view of the quarry is shown in Fig. 1. The quarry is located on the slope of a steep ravine. The total length of the quarry is 100 m, width 60 m, depth 9 m. The quarry has three benches: an overburden bench and two within the thickness of the commercial sands.
The height of the upper bench is 4 m, the width of the working platform 10 m; the height of the lower bench is 2 m, the width 5 m. The height of the overburden bench is 1.5 m. The plan of the experimental quarry is shown in Fig. 2.
Legend:
hatched area — area cleared of snow
○ — frost meter
Fig. 2. Plan of the experimental quarry.
Five frost-measuring points were equipped on the benches of the experimental quarry.
Frost meter 1 is located on the lower platform of the lower bench, which has a considerable natural snow cover.
Frost meter 2 is located on the upper platform of the lower bench at a distance of 5 m from the brow. The snow cover is natural.
Frost meters 3 and 4 are located on the same platform at distances of 3 and 6 m from the brow. The area is systematically cleared of snow; its dimensions are \(10 \times 10\) m.
Frost meter 5 is located on the upper platform of the upper bench. The snow cover is natural.
Some results of the study of soil freezing
The dependence of the depth of soil freezing on the height of the snow cover at the experimental fields and in the quarry is presented by the graphs in Figs. 3 and 4.
Analyzing the graphs, one may conclude the following.
In areas with natural snow cover, the maximum height of the snow cover and the depth of soil freezing were, for the frost meters: 1—110 cm and 23 cm; 2—95 cm and 65 cm; 5—110 cm and 66 cm; 6—95 cm and 38 cm; 10—65 cm and 4 cm. The minimum freezing depth (4 cm, frost meter 10) was recorded in an area with natural grass cover. The exposed surface (frost meters 2 and 5), despite the large amount of snow cover, froze to a depth of 65–66 cm, i.e., 16 times greater. The smaller freezing depth recorded by frost meter 1 (23 cm), in comparison with frost meters 2 and 5, is explained by the high moisture content of the soil (relative moisture above 40%).
The readings from frost meter 6 are not characteristic, since the snow cover in December and January was disturbed by drilling a borehole 3 m from it.
In areas with a small snow cover, the height of the snow cover and the freezing depth were, for the frost meters: 3—25 cm
and 151 cm; 4—20 cm and 161 cm; 7—15 cm and 177 cm; 8—25 cm and 164 cm. Comparing these figures with those given above for areas with an exposed surface (frost-depth gauges 3, 4 and 2, 5), it may be noted that, with a decrease in the snow-cover thickness from 95–110 cm to 20–25 cm (by a factor of 4–5), all other conditions being equal, the depth of ground freezing increased from 65–66 cm to 151–163 cm, i.e., by a factor of 2.3–2.5.
Fig. 3. Graph of the dependence of the depth of ground freezing on the height of the snow cover in the experimental quarry.
1, 2, etc.—numbers of frost-depth gauges.
Fig. 4. Graph of the dependence of the depth of ground freezing on the height of the snow cover on the experimental plots.
6, 7, etc.—numbers of frost-depth gauges.
In the area with an increased artificial snow cover (frost-depth gauge 9), its thickness was 105 cm, while the depth of ground freezing was 4 cm.
Conclusions
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The natural snow cover (on average 85–100 cm) provides practically complete protection of the ground from freezing in areas with a natural grass cover. Consequently, stripping operations may be carried out in winter.
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The quarry surface exposed after stripping, under a natural snow cover, freezes to a depth of 0.65 m; therefore, when excavating the ground, special measures will have to be taken to break up the frozen brow or to thermally insulate the ground.
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A snowless ground surface freezes to a depth of 1.5–1.8 m. For excavation, it will be necessary to apply a number of measures to reduce the depth of freezing or to loosen the ground.
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Observations of ground freezing must be continued in the future, with special attention paid to the dynamics of freezing.