PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
AWARDED THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR
NAMED AFTER S. M. KIROV

Volume 254
1975

METHODOLOGICAL ISSUES IN ENGINEERING-GEOLOGICAL INVESTIGATIONS OF BROWN COAL DEPOSITS IN SIBERIA

F. P. NIFANTOV, V. N. PULYAEV, E. A. PISAREV

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

The open-pit method of developing deposits of solid minerals is the most economical, but it requires comprehensive preliminary investigations and forecasting of the engineering-geological conditions that have a major influence on the choice of mining method and on the cost of development. This problem—and, in particular, the stability of quarry slopes—acquires especially great importance as the size and depth of quarries increase. Exceptionally large quarries, each with an output of up to 40–50 million tons of coal per year and a depth of up to 250 m, are planned at a number of brown-coal deposits of the Kansk-Achinsk Basin. In this connection it became necessary to determine the engineering-geological conditions of the deposits and to obtain preliminary data on the stability of the rocks in the slopes of the proposed quarries. These investigations were carried out during the exploration period.

As is known, many questions of the problem under consideration have been covered to a considerable extent in the works of P. N. Panyukov (1962, 1968), Yu. N. Malyushitsky (1957), V. D. Lomtadze (1952, 1956), G. L. Fisenko (1965), G. G. Skvortsov, L. I. Romanovskaya (1966), S. P. Prokhorov and E. G. Kachugin (1955), S. D. Babushkin (1969), and other investigators.

As applied to the brown-coal deposits of Siberia, the methodology of engineering-geological investigations and of forecasting the stability of rocks in quarry slopes has in recent years been developed at the Department of Engineering Geology of the Tomsk Polytechnic Institute. The main provisions of this method, applied during the exploration of the Itatsky, Nazarovsky, Berezovsky, and other deposits, are as follows.

At all stages of exploration work, engineering-geological investigations are carried out of the entire complex of conditions that will influence changes in the properties and stability of rocks in the walls, dumps, and floors of the planned open pits. The number of factors affecting the conditions of rock excavation always remains considerable. Therefore, the principal factors are first identified; their comprehensive investigation and proper accounting make it possible to choose the simplest and most effective methods for opening and operating deposits. At a number of brown-coal deposits in Siberia, the greatest influence during open-pit stripping is exerted by the geological structure, the water content of the enclosing rocks and coal, the strength and stability of all types of rocks, and, to a lesser extent, the relief, climate, and surface waters.

It turned out that all the deposits studied are brachysynclinal structures, filled within the overburden by various sedimentary, semirocky water-bearing rocks of the Jurassic and Cretaceous, overlain by Quaternary deposits. The strength and stability of the rocks vary over wide ranges. In this connection, the complex and varied factors that will influence the operation of future large open pits require their thorough and staged study. On our recommendation, engineering-geological investigations begin at the preliminary exploration stage and continue through the period of open-pit operation.

At the preliminary exploration stage, simultaneously with geological investigations, reserve calculation, and assessment of the quality of the mineral, the general conditions for opening the deposit are investigated mainly. In doing so, lithology, jointing, heterogeneity, water content, water resistance, properties, and stability of all types of rocks and coal are studied. At the same time, the characteristics, development, and regime of groundwater and surface water are determined. Based on the results of field and laboratory investigations, the following questions are resolved at this stage:

1. An engineering-geological classification is developed, and the stability and possible changeability of the properties of each engineering-geological type of rock are preliminarily assessed.

2. Hydrogeological and engineering-geological maps and sections are compiled, and engineering-geological zoning of the deposit is carried out. The most favorable areas for priority development are outlined.

3. Areas for detailed exploration for priority open pits are determined. At the same time, construction sites for a town, plants, thermal power stations, and other structures are selected.

At this stage, the strength and stability of rocks are assessed mainly in the field, according to data on drillability, core recovery, absorption of drilling fluid, swelling, and collapses of rocks in borehole walls, logging, and micropenetration of the core.

In our case, field micropenetration is carried out with a manual micropenetrometer designed at the department for application to investigations of semirocky rocks. It was found that the specific resistance to micropenetration of rocks that do not have visible jointing is directly proportional to the temporary resistance to crushing and is related to cohesion by the following formula:

\[ C = 9.8 \lg R_m - 0.262, \]

where

\[ C \]

is cohesion, t/m\(^2\);

\[ R_m \]

is the specific resistance to micropenetration, t/m\(^2\).

Density, moisture content, granulometric composition, and other necessary classification characteristics of the rocks are determined from log-

…diagrams and in the field or a stationary laboratory on monoliths. The value of the unit weight, according to density logging data, is estimated by the formula

\[ \Delta = 2.52 - 1.3 \cdot \frac{I_{\gamma\gamma}}{I_{0\gamma\gamma}}, \]

where

\[ I_{\gamma\gamma} \]

is the intensity of scattered γ-radiation,

\[ I_{0\gamma\gamma} \]

is the magnitude of the natural background with the working standard.

The purpose of engineering-geological zoning is to divide the area of each deposit according to the complexity of its geological structure, water-bearing capacity, and the properties and stability of the rocks. It is based on a point-rating assessment of engineering-geological conditions. The best areas of a deposit are considered to be those with the smaller number of points.

At the stage of detailed exploration, the engineering-geological conditions for the construction of the first open pits are clarified, and the stability of the rocks that will occur in the walls, floors, and dumps of the proposed cuts is assessed. To this end, the geological structure, water-bearing capacity, and rock properties of each site selected for the placement of an open pit are studied in detail. Investigations are carried out to a depth of 15–20 m below the pit floor, simultaneously with exploration work. In order to solve these tasks, the character of occurrence, solidity, homogeneity, composition, structure, cementation, weathering, and microfracturing of the rocks are determined in especially great detail. The engineering-geological classification is finalized and, where necessary, detailed. On the basis of generalized indicators, with allowance for probable changes in rock properties, design characteristics are determined for each engineering-geological type of rock. In the process of selecting design indicators we use methods of probability theory and information theory. As is known, by the present time a number of methods have been proposed for obtaining design indicators. Works by N. N. Maslov, N. V. Kolomenskii, G. M. Shakhunyants, R. I. Balli, Il. Iliev, E. A. Pisarev, and others are devoted to this question. The methods differ in the rigor and objectivity of the results obtained. We take as the design value of the indicator \(x_p\), at the selected confidence probability and a normal distribution of the partial indicators of rock properties, its mathematical expectation \(M_x\), into which coefficients are introduced that take into account unfavorable conditions of rock behavior in the walls of the planned open pits.

\[ x_p = \frac{M_x}{k_1 \cdot k_2 \cdot k_3}, \]

where

\(k_1, k_2, k_3,\ldots\) are factorial coefficients determined by analyzing the stability of slopes in operating open pits laid out under analogous engineering-geological conditions.

For the Kansk-Achinsk basin, the product \(k_1, k_2, k_3\) is taken as 1.1. It should be noted that, up to the present time, there is no sufficiently rigorous justification for choosing the values of the confidence probability; therefore it becomes necessary to search for new methods of estimating \(M_x\) on the basis of information theory, with the introduction of the concept of the entropy value of the scatter of the estimate \(M_x\), equal to

\[ 2.066 \cdot \frac{S}{\sqrt{N-1}}. \]

The method for determining this coefficient has recently been described in the work of P. V. Novitskii, where one may also find the justification of the coefficient

2.066. In this case, for each identified engineering-geological type of rock, the design parameters can be determined by the following formulas:

1. Angle of internal friction

\[ \varphi_p = 0.9 \cdot \left(\bar{\varphi} - 2.066 \frac{S_\varphi}{\sqrt{N-1}}\right), \]

\[ \varphi'_p = 0.9(\bar{\varphi} - 2.066 \cdot S_\varphi). \]

1. Cohesion

\[ C_p = 0.9 \cdot \left(\bar{C} - 2.066 \frac{S_c}{\sqrt{N-1}}\right), \]

\[ C'_p = 0.9(\bar{C} - 2.066 \cdot S_c), \]

where

\(\varphi'_p, C'_p\) are design values along planes of weakness;

\(\bar{\varphi}, \bar{C}\) are the mean values of cohesion and the angle of internal friction, obtained from a sufficient number of individual characteristics;

\(S_\varphi, S_c\) are standard deviations;

\(N\) is the number of individual characteristics determined in the laboratory.

The design value of the unit weight of each type of rock is taken from the arithmetic mean values. In the process of preliminary calculations, an accelerated method may be used for determining standard deviations from the ranges of values in the sample,

\[ S_x = \frac{x_1 - x_2}{\alpha_N}, \]

where

\(x_1, x_2\) are the maximum and minimum values of the indices in the sample,

\(\alpha_N\) is the sample-size coefficient, whose value is given in Table 1.

Table 1

Values of the coefficients \(\alpha_N\)

Taking into account the geological-structural features of deposits, the direction of advance of the working walls of open pits along gently dipping seams, and the mandatory preliminary dewatering of the rocks, calculations of the overall stability of pit walls were carried out by us using the multilayer-slope method. In this method it is assumed that the sliding plane in the upper part of the slope will pass at a steep angle to the bedding, and in the lower part along a weaker layer or fractured zone. To date, calculations of this character have been performed for a number of large open pits planned at the Itatskoye, Berezovskoye, and other brown-coal deposits. In all cases, the overall angles of inclination of the walls at open-pit depths of 150–250 m, provided that the rocks are preliminarily dewatered, proved to be 26–33° (Table 2).

Thus, a comprehensive study of the engineering-geological conditions of deposits and the justified application of sufficiently reliable calculation methods make it possible to outline the general slope angles of the walls of large open pits before they are laid out and, to a considerable extent, to predetermine the conditions for mining coal seams.

Table 2

Results of calculations of slope angles of the working walls of planned open pits

It is quite natural that engineering-geological investigations should not end at the stage of detailed exploration. Subsequently, comprehensive investigations and stationary observations are necessary, which will make it possible to verify the adopted preliminary forecast decisions regarding rock stability and to take into account all changes arising during the period of construction and operation of the open pits.

In this brief article only the general scheme of the sequence of investigations and of resolving questions in the complex problem of forecasting the engineering-geological conditions for developing the brown-coal deposits of Siberia by large open pits has been considered. More detailed questions of investigation methodology are, moreover, covered in the works of F. P. Nifantov, V. E. Olkhovatenko, V. N. Pulyaev, and E. A. Pisarev, partly noted in the bibliography [6—9, 14—18].

REFERENCES

1. S. D. Babushkin, D. I. Presetsko, S. P. Prokhorov, G. G. Skvortsov. Study of hydrogeological and engineering-geological conditions during exploration and development of deposits of solid minerals. “Nedra,” Moscow, 1969.

2. N. V. Kolomenskii. General methodology of engineering-geological investigations. “Nedra,” 1968.

3. V. D. Lomtadze. Methods of laboratory investigation of soil properties. Gosgeoltekhizdat, 1952.

4. V. D. Lomtadze. General principles and classification of rocks in engineering geology. Proceedings of “Gidroproekt,” collection 2, 1965.

5. Yu. N. Malyushitskii. Conditions of stability of open-pit walls. Publishing House of the Academy of Sciences of the Ukrainian SSR, 1957.

6. F. P. Nifantov. Main directions of engineering-geological investigations of coal deposits of Siberia. — In: Questions of the methodology of engineering-geological investigations, vol. 2, Tomsk, 1967.

7. F. P. Nifantov. Some questions of engineering-geological investigations of brown-coal deposits of the Kansk-Achinsk basin. — In: Materials on geology and mineral resources of Western Siberia. Tomsk University Publishing House, Tomsk, 1964.

8. F. P. Nifantov, V. N. Pulyaev, E. A. Pisarev. Engineering-geological conditions for the development of certain brown-coal deposits of Transbaikalia. — Proceedings of TISI, Tomsk University Publishing House, 1969.

9. F. P. Nifantov, V. E. Olkhovatenko, E. A. Pisarev. Engineering-geological characterization of the Itatskoe brown-coal deposit. — Collection of scientific works of TISI, vol. XII, Tomsk University Publishing House, 1967.

10. P. V. Novitskii et al. On the use of the entropy value of error as a criterion of the accuracy of instruments and measurements. — Izmeritel’naya tekhnika, 1966, No. 11.

1. S. P. Prokhorov, E. G. Kachugin. Methodological guide to hydrogeological and engineering-geological investigations in the exploration of deposits of solid minerals. Gosgeoltekhizdat, 1955.

2. P. N. Panyukov. Fundamentals of physical mineralogy and petrography, MGI, Part 1, 1966 and Part 2, 1968.

3. P. N. Panyukov. Engineering Geology. Gosgortekhizdat, 1962.

4. E. A. Pisarev. On the methodology of statistical processing of indices of the engineering-geological properties of rocks.—Proceedings of the Tomsk Civil Engineering Institute, 1967.

5. V. N. Pulyaev, E. A. Pisarev. On the question of the spatial variability of rock properties.—Izv. TPI, vol. 196, 1969.

6. V. N. Pulyaev, V. V. Kravchenko, Yu. S. Kazazaev. Method and results of investigations of semirocky rocks with a micropenetrometer.—Izv. TPI, vol. 201, 1971.

7. V. N. Pulyaev. Investigations of the engineering-geological conditions of the Berezovsky brown-coal deposit in connection with the construction of large open pits. Dissertation, Tomsk, 1969.

8. V. E. Olkhovatenko, V. N. Pulyasv. Investigation of the shear resistance of certain types of rocks of the Itat brown-coal deposit.—Izv. TPI, vol. 151, 1966.

9. G. G. Skvortsov, L. I. Romanovskaya. Engineering-geological investigations and forecasts in the exploration of mineral deposits. Nedra, 1966.

10. G. L. Fisenko. Stability of quarry and dump slopes. Nedra, 1965.