METHODS AND SOME RESULTS OF AN ENGINEERING-GEOLOGICAL ASSESSMENT OF THE ROCKS OF THE TATAUROVO BROWN-COAL DEPOSIT

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

(Submitted by the scientific seminar of the Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute)

In the current five-year plan there has been extensive development of industry and construction in Chita Oblast; in connection with this, the demand for energy fuel has increased substantially, and prospecting for coal is being widely carried out. As a result, a number of deposits have been discovered, including the Tataurovo brown-coal deposit, located 70 km from the city of Chita. In the priority area of this deposit, by 1966 detailed geological exploration and hydrogeological work had been completed; at the same time, the Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute carried out engineering-geological investigations, the results of which are partially presented below.

Geologically, the Tataurovo deposit is a fairly large brachysyncline situated in the valley of the Ingoda River, bounded from the northwest and southeast by high mountain ranges—the Yablonovy and Chersky ranges. The structure is composed mainly of undivided continental deposits of the Cretaceous and Jurassic, up to 2000 m thick. The Jurassic and Cretaceous rocks are everywhere overlain by Quaternary deposits. From the standpoint of assessing rock stability, the greatest interest is presented by the coal-bearing deposits of the Cretaceous and Jurassic down to depths of 200–250 m, and by the Quaternary formations.

The coal-bearing sequence lies conformably on the underlying coal-free sequence and in the central part of the deposit reaches up to 215 m. The lower member, up to 50 m thick, is represented by interbedded sandstones, siltstones, and argillites with thin interlayers (0.1–0.9 m) of coal. Above these beds are fairly thick coal seams suitable for open-pit mining. The average thickness of the upper seam is 8.95 m, of the middle seam 3.86 m, and of the lower seam 7.33 m. The enclosing rocks are represented to a significant extent by coarse- and medium-grained sandstones (57.9%), to a lesser extent by coal (20.4%), siltstones (10.7%), and argillites (9.6%). A gentle dip (3–5°) and considerable facies variability of the rocks are observed on the limbs of the brachysyncline.

The Quaternary deposits within the valley of the Ingoda River at the deposit are represented by pebble beds, sandy loams, loams, and sands, among which modern alluvial sand-and-pebble deposits of the Ingoda River floodplain are most often encountered; these everywhere cover the deposit with a layer 10–11 m thick, and in the overdeepened channels of the Praingoda up to 32 m. The listed rock types were investigated

in engineering-geological terms, on 203 monoliths taken from 15 exploratory boreholes.

The investigations included determination of the composition, structural features, and properties of all types of rocks of the overburden strata of the future open pits. The main attention was devoted to studying the shear resistance of rocks, taking into account their probable change in the walls of the open pits. In particular, considering the process of permafrost degradation in a number of open pits and mines of Transbaikalia, the investigations of properties were carried out mainly for thawing rocks. For each monolith, shear tests were performed under loads of 2, 4, and 6 kg/cm² under conditions of complete water saturation. Shearing was carried out according to the scheme of accelerated consolidated shear [1]. The shear planes were directed chiefly along bedding and weakened zones, which usually corresponds more closely to the development of probable slip surfaces in open-pit walls. The primary processing of the results of each test, in order to determine the coefficient of internal friction \(f\) and cohesion \(C\), was carried out by the method of least squares. For intact rocks the following formulas were used:

\[ f=\frac{\tau_6-\tau_2}{4}; \qquad C=\frac{4\tau_2+\tau_4-2\tau_6}{3}, \]

where \(\tau_2, \tau_4, \tau_6\) are the shearing forces under vertical loads of 2, 4, and 6 kg/cm². For disturbed rocks, the value of cohesion, as a rule, was close to zero, and \(f\) was calculated by the formula

\[ f=\sum_1^N P_i \tau_i \bigg/ \sum_1^N P_i, \]

where

\[ \begin{aligned} P_i &\text{— vertical load;}\\ N &\text{— number of vertical loads.} \end{aligned} \]

Determination of the mineralogical composition, microstructure, and composition of the cement was carried out with the use of ordinary and electron microscopes. Other indices of rock properties were determined by generally accepted and standard methods. Particular characteristics of rock properties were processed using methods of mathematical statistics in order to obtain generalized and design indices necessary for classifying the rocks and assessing the stability of the walls of the planned open pits.

Engineering-geological classification of rocks. In general terms, the purpose of such a classification is to divide rocks into separate classes in which the rocks are characterized by close values of the principal physico-mechanical properties. As the principal indices we adopted unit weight, the coefficient of internal friction, and the cohesion of intact rocks. The construction of the engineering-geological classification with respect to these indices was carried out on the basis of the modern theory of geological classifications [2, 3], taking into account the three principal situations listed in Table 1 that are encountered in constructing geological classifications of rocks. In the same table, cases typical for engineering geology are emphasized.

Using the data of Table 1, for the rocks of the Tataurovo deposit a diagnostic classification was constructed (Table 2), in which, as sets \(U^1\) and \(U^2\), known lithological concepts, defined in the manual [4], were adopted; the sets of concepts \(U^3\) and \(U^4\) refer to engineering-geological indices (\(U^3\) is moisture content, \(U^4\) is the ultimate compressive strength of rocks). From Table 2 it is also evident that \(U^4\) follows \(U^3\), and this is quite natural, since the change in strength of the rocks under consideration depends substantially on the change in their moisture content. On the basis

Table 1

2, 5, 7, and 9 are typical cases for engineering geology.

For the listed sets, Table 3 presents an engineering-geological classification of the list according to the system of attributes \(U\), with the author’s volume \(N(U)=10\). Subsequently, for each class \(A_i\), the corresponding statistical processing was carried out for the partial values of bulk weight, internal-friction coefficient, and cohesion of the rocks constituting the system of diagnosed attributes \(Q_i\), which is also illustrated by the aforementioned Table 2.

The statistical processing of partial indicators was performed using a procedure developed at the Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute [5, 6]. The method includes testing the normality of the distribution of partial characteristics and determining the parameters (the arithmetic mean and standard deviation) of the distribution, which, as is known, serve as the basis for obtaining calculated indicators of rock properties.

Calculated indicators and assessment of the overall stability of working quarry slopes. The construction of a computational model of quarry slopes or dumps is rather closely linked with the classification scheme prepared for the rocks under study. The rocks listed in the classification form the basis for the elementarization of the computational model. At the same time, each class of rocks is characterized by a definite set of calculated characteristics, which are brought into correspondence with the selected computational model and the scheme for calculating the stability of a quarry slope or dump. In engineering geology, the mathematical expectation is taken as the calculated indicator of rocks, into which a number of factor coefficients are introduced that take into account the structure of the massif, the conditions under which the rocks operate in the sphere of action of the quarry, etc.; this is expressed by the formula:

\[ x_p=\frac{1}{K_1\cdot K_2\cdot K_3\ldots}\cdot Mx, \]

where \(Mx\) is the mathematical expectation of the indicator; \(K_1, K_2\ldots\) are factor coefficients that are determined experimentally or by analogy with already operating quarries. As applied to the Tataurovo deposit, for the characteristics of rock resistance to displacement, the value \(K_1\cdot K_2\cdot K_3\ldots\) was taken equal to 1.3, taking into account all

Table

Diagnostic engineering-geological classification of rocks of the Tatarovsky brown-coal deposit

Note. $N$ is the number of determinations; $\overline{X}$ is the mean value; $S$ is the standard deviation. Rocks with $\sigma_{\mathrm{cж}} = 0$–$5\ \mathrm{kg/cm^2}$ are classified as weakly cemented, $5$–$25\ \mathrm{kg/cm^2}$ as medium-strength, and $25$–$150\ \mathrm{kg/cm^2}$ as strong.

Classification of the possible principal types of stability disturbance of working pit walls at the Tatarskii brown-coal deposit

Table 3

The schemes of slope deformations are shown in Fig. 1.

unfavorable conditions that may arise when frozen and partly thawed rocks (sandstones) of the overburden are stripped.

Another sufficiently important point in the process of obtaining the design characteristics is the estimation of their mathematical expectation from sample data [6]. The following situations may occur here:

1. For \(Mx\), one or another worst interval estimate is adopted (a pessimistic estimate).

2. The mean value of the parameter is adopted as the estimate of \(Mx\) (an optimistic estimate). The optimistic estimate of \(Mx\), as a rule, is used for parameters of rock properties that have insignificant variability and when the rock characteristic within the sphere of action of the structure will not decrease. In our case, these parameters include the unit weights of the rocks. On the other hand, for the characteristics of the shear resistance of rocks, owing to their significant variability in each rock and their decrease at the sides of open pits, one of the pessimistic estimates of \(Mx\) is recommended: the lower confidence limit or the lower tolerance limit, obtained at a specified significance level \(\gamma\) [5].

On the basis of a careful analysis of the geological structure and the physico-mechanical properties of the rocks of the Tataurovo brown-coal deposit, in order to obtain the design shear characteristics the following estimates of \(Mx\) were adopted. For weakened contacts, the lower tolerance limits of the friction and cohesion of the rocks were used, while in the remaining cases the lower confidence limits obtained by a grapho-analytical method at a significance level of 0.05 [6] were adopted.

Taking into account all the remarks made above, design characteristics were calculated; for each class of rocks they are given in an earlier work by the authors of the present article [5] and in Table 2.

For the conditions of the Tataurovo deposit with gently dipping strata, taking into account the adopted design model of slope deformation (Table 3, Fig. 1) and the physico-mechanical properties of the rocks, the overall angles of inclination of the drained sides of open pits were determined. They proved to be no greater than \(33^\circ\).

Fig. 1. Design model of slope deformation.
1 — coal; 2 — sandstone; 3 — siltstone; 4 — claystone.

In conclusion, we note that the described method for assessing the stability of rocks and quarry slopes may be adopted for similar deposits at the stage of detailed exploration and the early stages of design.

REFERENCES

1. Resolutions and instructional guidelines of the Gidroproekt conference on the unification of methods for soil investigation. Gidroproekt, Moscow, 1964.

2. Geology and Mathematics. Nauka, Novosibirsk, 1967.

3. Yu. A. Voronin. On the mathematical-logical assimilation of geological classifications. Geology and Geophysics, 1963, No. 9.

4. Reference Manual on the Petrography of Sedimentary Rocks. Gostoptekhizdat, Leningrad, 1958.

5. F. P. Nifantov, E. A. Pisarev, V. N. Pulyaev. Engineering-geological conditions of the Tataurovo brown-coal deposit. Scientific report. TPI Funds, Tomsk, 1966.

6. E. A. Pisarev. Some applications of statistical methods to the processing of results of investigations of soil properties. In: Problems of Methods of Engineering-Geological Investigations. Tomsk, 1966.

7. T. L. Fisenko. Stability of Quarry and Shaft Slopes. Nedra Publishing House, Moscow, 1965.

8. Moscow–Volga Canal. Geotechnics. Moscow–Leningrad, 1940.

9. Yu. N. Malyshev. Conditions of Stability of Quarry Slopes. Kiev, 1957.

10. K. Schubert. Die Verteilung der Schubspannung unter Böschungen. Bergbautechnik, No. 3, 1964.