SOME RESULTS OF ENGINEERING-GEOLOGICAL INVESTIGATIONS OF THE WESTERN PART OF THE BEREZOVSKY DEPOSIT

Yu. A. Usynin

(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology)

In the development of the productive forces of Eastern Siberia, one of the most important tasks is the exploitation of the Berezovsky brown-coal deposit of the Kansk-Achinsk Basin. On its area, the largest open pits, with a capacity of 40–50 million tons per year, will be established. Exploration over the area of the deposit has been carried out since 1959 by the Krasnoyarsk Geological Administration; by the present time the western part of the deposit has been explored in detail. The Berezovsky deposit is located in Krasnoyarsk Krai and partly in Kemerovo Oblast. The principal geological structure of the area of the Berezovsky deposit is the Berezovsky trough, composed down to a depth of 400–500 m of gently dipping Jurassic and Cretaceous rocks overlain by Quaternary deposits, and having a length of approximately 100 km and a width of 20 km. Of industrial significance is the coal-bearing subformation of the Itat Formation of the Middle Jurassic, enclosing the thick, 40–70 m Berezovsky seam of brown coal and represented by an alternation of beds of aleurolites, argillites, sandstones, and brown coal. The trough-like structure with monoclinal occurrence and the alternation of sandy-clayey, weakly cemented rocks on the flanks of the structure contribute to the formation of confined groundwaters. The principal aquifer horizons are the first, confined to the overburden sandstones; the second, confined to the Berezovsky seam; and the third, confined to the sandstones occurring below the coal seam.

The Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute, under the direction of Associate Professor F. P. Nifantov, carried out engineering-geological investigations over the area of the western part of the deposit. The purpose of the engineering-geological investigations was to study the physico-mechanical properties of the rocks and to determine the stability of the slopes of the working walls of the planned first-priority open pit. The work was carried out according to the method developed by the department [3]. Twenty-four engineering-geological boreholes, with a total footage of 2720 m, were drilled for the purpose of taking monoliths for the study of the physico-mechanical properties of the rocks. The engineering-geological boreholes were drilled along five exploration lines, uniformly distributed over the area of the western part of the deposit. From these boreholes 260 monoliths were selected.

In the laboratory a complete set of investigations of the physico-mechanical properties of the rocks was carried out, with primary attention being paid

investigations of the shear strength of rocks. In addition, the temporary crushing strength of the main rock types was determined. At the same time, studies were carried out on the petrographic and mineralogical composition of all rock types by methods of petrography, electron microscopy, and thermal methods. On the basis of the investigations carried out, a large body of factual material on physical-mechanical properties was obtained. For the western part of the Berezovsky deposit, an engineering-geological classification was compiled (Table 1).

Table 1

Engineering-geological classification of rocks of the western part of the Berezovsky deposit

According to the classification, within the thickness of deposits of the western part of the deposit, down to a depth of 150–200 m, seven geologo-genetic complexes are distinguished, differing in conditions of formation, composition, and physico-

Table 2

Basic physico-engineering-geological properties of rocks in the western part of the Berezovsky deposit

NOTE: $\gamma$ — unit weight, t/m³; $\Delta$ — volumetric weight of the solid rock, t/m³; $w$ — moisture content, %; $n$ — porosity coefficient, %; $\varepsilon$ — internal-friction angle coefficient; $\varphi$ — internal-friction angle; $c$ — cohesion, t/m²; above the line is the interval of fluctuations; below the line is the arithmetic mean value of the characteristic.

mechanical properties. Each geologic-genetic complex is subdivided into engineering-geological types according to lithological composition, structure, texture, and physico-mechanical properties. Within the engineering-geological types, engineering-geological varieties are distinguished, differing in strength properties.

The most thoroughly studied as a result of the engineering-geological investigations proved to be Quaternary clays and loams; Jurassic and Cretaceous argillites, siltstones, sandstones; brown coal (the Berezovsky seam); and Jurassic siltstones, argillites, and sandstones occurring below the seam, which are most widespread in the western part of the deposit and make up 90–95% of the volume of the lithological section. The principal engineering-geological properties of these rocks are summarized in Table 2. Less well studied are alluvial sand-and-pebble deposits, silts, eluvium of the weathering crust, as well as burned and collapsed bedrock, owing to the absence of monoliths from these rock types. The rocks listed are poorly developed in the western part of the deposit and occur mainly in the area where the coal seam crops out beneath the Quaternary deposits, in the southernmost part of the deposit. In the future, during construction and operation of the open pit, it will be necessary to undertake further investigations of these poorly studied rocks.

For each rock type, individual values of indices of physico-mechanical properties were processed by methods of mathematical statistics, and design indices were selected for carrying out preliminary calculations of the stability of the slopes of working benches. As design indices for the unit weights of the rocks, arithmetic mean values were adopted; for the coefficients of internal friction and cohesion, the lower confidence limits were adopted at confidence probability $\Phi(t)=0.95$. In this case the design indices were as follows: for loams—the angle of internal friction 19°, cohesion 2.2 t/m²; for clays—the angle of internal friction 19°, cohesion 3.2 t/m²; for overburden siltstones—the angle of internal friction 25°, cohesion 5 t/m²; for overburden argillites—the angle of internal friction 18°40′, cohesion 4.0 t/m²; for overburden sandstones—the angle of internal friction 27°, cohesion 2.9 t/m²; for brown coal—the angle of internal friction 29°40′, cohesion 3.2 t/m². Values of unit weights are given in Table 2. For the slopes of working benches, preliminary calculations of overall stability were carried out for depths of 115, 159, and 185 m by the method of N. I. Golovanov for a multilayered mass and according to the VNIMI schemes [1, 2, 4]. These calculations yielded the following stable slope angles for the main rock types, with a safety factor of 1.1 and design indices at confidence probability $\Phi(t)=0.95$: for brown coal—28°; for overlying argillites, 18–25°; for overlying siltstones—27–28°; for sandstones—27–29°. For shallow-lying (down to 30–40 m) coal seams, the angles of a stable slope calculated by the method of N. I. Golovanov increase considerably, which is explained by the specific nature of the calculation method. In this connection, for shallow-lying (30–40 m) Quaternary and weathered Jurassic rocks, the angle of a stable slope should be adopted, according to calculations by the VNIMI scheme, as no more than 28°.

REFERENCES

1. N. I. Golovanov. Stability of slopes of earth structures according to the theory of limiting equilibrium. Collection of Works of the Institute of Mining of the Academy of Sciences of the Ukrainian SSR, No. 4 (13), 1956.

2. Yu. N. Malyushitsky. Conditions of Stability of Open-Pit Walls. Kiev: Publishing House of the Academy of Sciences of the Ukrainian SSR, 1957.

3. F. P. Nifantov. Some questions of the methodology of engineering-geological investigations of the Kansk-Achinsk brown-coal basin. In: Materials on the Geology and Mineral Resources of Western Siberia. Publishing House of Tomsk State University, 1964.

4. G. L. Fisenko. Stability of Walls of Coal Open Pits. Moscow: Ugletekhizdat, 1956.