PROTECTION OF SOIL FROM SEASONAL FREEZING UNDER THE CONDITIONS OF THE TUGAN DEPOSIT

Yu. A. RYZHKOV, B. A. SURNACHEV

(Presented by the Scientific and Technical Conference on the Use of Mineral Resources of the Tomsk Economic Region)

Introduction

The development of deposits in the winter period in regions with seasonal freezing of soils can be ensured: first, through the use of earth-moving machines with high cutting forces, which make it possible to excavate frozen soil without preliminary preparation; and, second, through the application of measures that ensure excavation of the soil after preliminary preparation.

Preparation of soils for excavation in wintertime may be carried out by one of the following three methods: protecting soils from freezing, loosening frozen soils, and artificially thawing the soil. Of these methods, the most effective and economical measure is protection of the soil from freezing.

Preventive measures may be reduced to the following groups: preliminary mechanical treatment of the soil surface, covering the soil surface with heat-insulating materials, excavation under shelters with artificial heating and without it, and special preventive measures. In turn, each of these groups may be subdivided into a larger number of individual methods.

In general, the corresponding methods of preparing soils for excavation in wintertime may be classified as follows.

Protection of soils from freezing

I. By mechanical treatment of the soil: 1) plowing and harrowing; 2) deep loosening by excavators.

II. By covering with heat-insulating materials: 1) snow; 2) dry insulating materials.

III. By shelters with artificial heating.

IV. By special methods: 1) ice shelters; 2) artificial snow; 3) flooding with water; 4) chemical treatment of the soil.

Loosening of frozen soils

I. By blasting. II. By special mechanisms and devices.

Artificial thawing of soils

I. With the aid of electricity. II. With the aid of water. III. With the aid of steam.

The choice of a method for protecting soil from freezing depends on the purpose of the measure being applied, the duration of use of the thermal protection, the nature of the relief of the surface to be insulated, the type of soil, the nature and quantity of local insulating materials available in the district, the maximum permissible depth of freezing of the soil being insulated, and the meteorological conditions.

The purpose of the protective measures is to insulate the surface and the slopes of benches over a considerable area. The duration of use of the thermal protection corresponds to the duration of the frosty period and, for the conditions of the Tugan deposit, is 5–6 months (from October through March).

In the area of the deposit, the overburden rocks are represented by interbedded rocks: clays, loams, and vari-grained sands; the productive deposits consist mainly of fine-grained sands with an admixture of kaolin (15–20%).

The moisture content of the loose overburden and underlying beds of sand and loam ranges from 7.72 to 23% and reaches a maximum in clays—27.4–37.3%. The moisture content of the productive sands varies from 1.4 to 16.8%.

Methods of protecting soil from freezing

Plowing and harrowing reduces the thermal conductivity of the surface layer of soil. Plowing should be carried out to a depth of not less than 35–50 cm, followed by harrowing to a depth of 10–15 cm. These works must be completed before the onset of frosts. The use of this method is advisable when soils are being worked during the first third of winter. More often this method is used in combination with snow retention.

Among other methods of mechanical treatment of the soil surface, it is necessary to note deep loosening, carried out by excavators. This method has found application in large sandy quarries with a capacity from 150 thousand m³ to 1 million m³. To avoid prolonged exposure of the loosened part of the sand to autumn precipitation, it is recommended that loosening be carried out in late autumn and at a rapid pace.

Insulation with snow. Snow retention may be achieved by constructing snow ridges, installing snow-retaining shields and wattles, and making use of shrubs and trees present on the area being protected from freezing, the removal of which should be carried out before earthworks begin. The method of insulation with snow is recommended for large areas of soil being worked. It can also be used to cover bench slopes of small height. Snow-retaining shields, wattles, and ridges are placed perpendicular to the direction of the prevailing winds, at mutual distances of the order of 10–15 times the height of the shield, wattle, or snow ridge. Snow-retaining shields are usually made 1.5 × 2 m in size, with an open-area ratio of 30–50%.

Experimental work carried out by the base laboratory of the Tomsk Council of National Economy established the high effectiveness of snow retention as a means of combating soil freezing.

Insulation of the quarry with heat-insulating materials is used mainly when working small areas.

soil or for covering the slopes of benches. The most suitable materials for insulating the slopes of benches are straw mats (straw), snow, and also matting sheaves or felt mats. Sawdust, peat, and leaves cannot be used because they cake together and lose their thermal-insulating properties, and their removal requires a great deal of manual labor.

The thickness of the layer of insulating materials depends on the number of days with negative temperature and on the temperature on those days. The depth of soil freezing also depends on these same factors. Therefore, when determining the required thickness of the layer of insulating materials, the initial value adopted is the depth of soil freezing at the given geographic location, determined from handbooks, tables, or formulas. The required thickness of the insulating layer and the freezing depth of uninsulated soil over the design period can be determined from formulas (1) and (2).

The thickness of the insulating layer

\[ H_y = \frac{H_{\Gamma}\cdot \lambda_y}{\lambda_{\Gamma}}\cdot K,\ \text{m}, \tag{1} \]

where \(H_{\Gamma}\) is the depth of soil freezing over the design period, m;
\(\lambda_y\) is the coefficient of thermal conductivity of the insulation, kcal/m·h·deg.;
\(\lambda_{\Gamma}\) is the coefficient of thermal conductivity of the soil, kcal/m·h·deg.;
\(K\) is a correction coefficient.

The freezing depth of uninsulated soil

\[ H_{\Gamma} = H_M \sqrt{\frac{T_P}{T_M}},\ \text{m}, \tag{2} \]

where \(H_M\) is the freezing depth over the entire winter period, m;
\(T_P\) is the sum of degree-days over the design period;
\(T_M\) is the same over the entire winter period.

The design value of the insulating layer, depending on the insulation and the freezing depth, is given in Table 1. The thickness of the insulating layer was calculated for sandy soil with a thermal-conductivity coefficient of 1.8 and a moisture content of 20%. The correction coefficient \(K\) was taken as 1.2 to allow for nonuniform distribution of the insulation over the area and for the case of an increase in moisture.

Table 1

The data obtained by calculation are confirmed by observations carried out directly at the deposit during snow surveys. Where the height of the snow cover exceeded 1 m, the soil was thawed.

Insulation of the soil surface by various greenhouses without artificial heating and with artificial heating is used on small areas, most often in the development of clay—

open-pit mines with low productivity. For insulating large areas this method is not applicable.

Of the special methods, methods of insulation with artificial snow, flooding of the site and its development by earth-suction dredges, as well as chemical treatment of the soil, merit attention. These methods have not been sufficiently tested under production conditions; therefore, it is not yet possible to recommend them for use under the conditions of the Tugan deposit.

Methods of loosening and artificial thawing are widely used in the development of frozen soil both in areas of seasonal frost and in permafrost areas. They are not considered in the present work.

Methods recommended for protecting soil from freezing at the Tugan deposit

Of the heat-insulating materials, in Tugan and directly at the deposit there are snow, peat, and also straw (agriculture is developed in the area), leaves, coniferous branches, sawdust, shavings, and other materials.

In order to establish the distribution of the thickness of the snow cover in the deposit area depending on the terrain relief and vegetation, a snow-measurement survey was carried out on the most characteristic natural sites of the deposit, covering all relief elements encountered in the area. The area of a site ranged from 5 to 8 ha. Measurements were made during the period of maximum snow cover—on March 10, 1959—with a ruler graduated in 1 cm. At each site from 150 to 300 measurements were made, and from them the average snow-cover value was derived. On average, one measurement corresponded to an area of 250 m². Data for five characteristic sites are given in Table 2.

Table 2

Height of the snow cover in the area of the Tugan deposit
(as of March 10, 1959)

From the results of the snow-measurement survey, the following conclusions may be drawn.

1. On open sites subjected to the constant action of winds, the snow cover is distributed extremely unevenly, with fluctuations from 18 to 130 cm, and on average amounts to 50–85 cm.

1. In areas with sparse forest, shrubs, and young dense forest, the snow cover is distributed more uniformly, averaging 95–100 cm.

2. In areas with deep excavations (river valleys, ravines, etc.), snow accumulates to a considerable height, reaching 130 cm.

Among other insulating materials, preference should be given to straw and straw mats. Peat, foliage, coniferous branches, sawdust, and shavings are unsuitable, since they have higher thermal conductivity than snow and therefore require a larger volume of materials to cover 1 m² of the ground surface, making them uneconomical.

The maximum permissible depth of soil freezing when working with mechanisms not adapted for excavating frozen ground should not exceed 20–30 cm [1].

Economic Section

The cost of sand extracted in winter by the open-pit method from an insulated quarry depends mainly on the length of the working face, the thickness of the bed (bench height), and the cost of the insulating materials.

The total cost of insulating the slopes and the bench crest during winter sand extraction, depending on the length of the working face, may be calculated by the formula

\[ S = q_{1}\frac{L \cdot h}{\sin \alpha} + Q_{\text{win}}\left(\frac{q_{2}}{b \cdot \sin \alpha} + \frac{q_{3}}{h}\right),\ \text{rub.}, \tag{3} \]

where \(L\) is the length of the working face, m;
\(h\) is the bench height, m;
\(\alpha\) is the angle of the bench slope, degrees;
\(b\) is the width of the cut, m;
\(Q_{\text{win}}\) is the quantity of sand extracted during the winter period, m³;
\(q_{1}\) is the cost of the initial insulation of 1 m² of the bench slope, rub.;
\(q_{2}\) is the cost of uncovering before excavation and subsequent covering of 1 m² of the bench slope, rub.;
\(q_{3}\) is the cost of insulating 1 m² of the bench crest, rub.

It follows from formula (3) that:

1) the costs of insulating the bench crest and the operating costs for insulating the bench slope do not depend on the length of the working face;

2) the initial costs of insulating the bench slope are directly proportional to the length of the working face;

3) with an increase in the bench height (bed thickness), the costs of preparing and insulating 1 m³ of soil decrease.

The cost of insulating the slope and surface of the bench for the conditions of quarry No. 2 of the Tugan deposit, calculated by formula (3), is: a) when the working face is located along the bench slope, 14–15% of the total cost of extracting 1 m³ of sand; b) when the working face is located at the end of the bench, 8–10%.

The method of insulation for slopes is covering with straw matting; for the surface of benches, snow retention with the aid of shields.

Conclusions

For the conditions of the Tugan deposit, located in a region of seasonal freezing, the most effective method protecting

...protecting the soil from freezing is to cover it with thermal-insulating materials. It is advisable to insulate the slopes of benches with straw mats, felt mats, or snow (when the bench height is small), and to protect the surface of the benches from freezing by retaining snow with the aid of shields.

REFERENCES

1. Cherkashin V. A., Development of Sand and Clay Quarries in Winter. Moscow, 1956.