STUDY OF THE SHEAR RESISTANCE OF CERTAIN ROCK TYPES FROM THE ITATSKY BROWN COAL DEPOSIT

V. E. OLKHOVATENKO, V. N. PULYAEV

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

In studying the engineering-geological conditions of mineral deposits mined by the open-pit method, it is important to determine the strength of rocks not only in a monolithic specimen, but also in the massif [1, 2]. For this purpose, field tests of the shear resistance of rocks were carried out on the territory of the Itatsky brown coal deposit. In view of the absence of operating open pits at the deposit, the investigations were conducted in outcrops of natural slopes occurring along the right bank of the Barandatka River. At the same time, monoliths were taken from boreholes and studied in the engineering-geological laboratory of the Tomsk Polytechnic Institute. This made it possible to determine the coefficient of structural weakening of rock cohesion in the massif, which is the ratio of the cohesion of rocks in the massif to the cohesion in a monolithic specimen.

Among the existing methods for determining the shear resistance of rocks in a massif, the method of the All-Union Scientific Research Mine Surveying Institute (VNIMI), based on the use of hydraulic cushions or hydraulic jacks [1, 2], is the most widely used, as are also the methods of extrusion, collapse, and horizontal shear proposed by V. B. Shvets and P. P. Kazakov. According to the VNIMI method, to calculate the magnitude of cohesion it is necessary to know the angle of internal friction, which is determined in the laboratory on samples taken from the sites of in situ tests. The method of V. B. Shvets and P. P. Kazakov makes it possible to determine directly the angle of internal friction and cohesion without additional laboratory investigations. The essence of this method is as follows. In the rock massif, a prism of specified dimensions is cut out, separated from the rest of the massif by slots filled with compacted soil of disturbed structure. A force is applied to one of the walls of the prism; as a result, the rock is extruded by the vertical wall with the formation of a sliding surface. The installation scheme and the experimental procedure are described in detail in the works of V. B. Shvets and P. P. Kazakov [3, 4].

In our case, extrusion was carried out with a screw jack. During the tests, a dynamometer recorded the maximum force \((Q_{\max})\) at the moment of shear and the minimum force \((Q_{\min})\) during displacement. After the experiment, the coordinates of the sliding surface were measured in several vertical sections, and their average values were determined. To derive the calculation formulas, the authors of this method consider the case of limiting equilibrium of the entire extruded part of the rock, isolating in it a wedge of unit width (Fig. 1). In this wedge there acts a ho—

horizontal force \(Q\), equal to \(Q_{\max}\), divided by the wall width \(L\). This horizontal force is distributed uniformly throughout the entire volume of the wedge. The selected wedge is plotted to scale on millimeter paper and divided into a number of blocks. For each block its weight \((g)\), the length \(l\) of the sliding line, and the angle \(\alpha\), equal to the angle between the direction of the force of gravity and its component normal to the shear plane, are determined (Fig. 1).

Fig. 1.

The shearing force \((P)\) falling on each block is determined by the formula:

\[ P=\frac{Q}{G}\cdot g, \]

where \(G\) is the weight of the selected wedge.

The equilibrium condition of block I is expressed by the equation

\[ T_1=N_1\cdot \operatorname{tg}\varphi+T_1^1, \]

where \(T_1\) and \(N_1\) are the tangential and normal components of the weight of the block and of the shearing force \(P\):

\[ T_1=P_1\cdot \cos\alpha_1+g_1\cdot \cos(90+\alpha_1); \]

\[ N_1=P_1\cdot \cos(90-\alpha_1)+g_1\cdot \cos\alpha_1; \]

\[ T_1^1=cl_1; \]

\(\operatorname{tg}\varphi\) is the coefficient of internal friction of the rocks;
\(c\) is the cohesion of the rocks.

Transforming the above formulas, one can obtain the equation of limiting equilibrium of the entire wedge, divided into \(n\) blocks:

\[ \frac{Q}{G}\sum_{1}^{n} g_n\cdot \cos\alpha_n-\sum_{1}^{n} g_n\cdot \sin\alpha_n = \]

\[ = \operatorname{tg}\varphi\left(\frac{Q}{G}\sum_{1}^{n} g_n\cdot \sin\alpha_n+\sum_{1}^{n} g_n\cdot \cos\alpha_n\right)+C\sum_{1}^{n} l_n, \]

where

\[ \sum_{1}^{n} g_n \cdot \cos \alpha_n = g_1 \cdot \cos \alpha_1 + g_2 \cdot \cos \alpha_2 + \ldots g_n \cdot \cos \alpha_n; \]

\[ \sum_{1}^{n} g_n \cdot \sin \alpha_n = g_1 \cdot \sin \alpha_1 + g_2 \cdot \sin \alpha_2 + \ldots g_n \cdot \sin \alpha_n; \]

\[ \sum_{1}^{n} l_n = l_1 + l_2 + \ldots + l_n. \]

The magnitude of cohesion over the entire sliding surface is determined according to the formula

\[ C \sum_{1}^{n} l_n = Q - Q_1, \]

where

\[ Q_1 = \frac{Q_{\min}}{L}. \]

The cohesion of the rocks determined in this way is substituted into the limiting-equilibrium equation, and by solving it we obtain \(\operatorname{tg}\varphi\), and then \(\varphi\). According to the procedure described above, we determined \(\varphi\) and \(C\) for argillites, siltstones, and sandstones occurring in exposures of natural slopes along the valley of the Barandatka River. The results of the investigations are given in Table 1. In the same table, for comparison, the results of investigations of \(\varphi\) and \(C\) on monoliths are given. The latter were determined in “Gidroproekt” apparatuses (new model), with preliminary compaction and water saturation of the rocks. Other characteristics of the rock properties (Table 1) were determined according to the generally accepted standard method. From Table 1 it is evident that the specific gravity of the rocks tested in shear under field and laboratory conditions differs only slightly. Its average value for argillites is \(2.69\)—\(2.73\ \text{t/m}^3\), for siltstones \(2.69\)—\(2.71\ \text{t/m}^3\), and for sandstones \(2.67\)—\(2.68\ \text{t/m}^3\). The volumetric weight of weathered rocks from exposures proved to be lower than that of the same rocks taken from boreholes. Thus, argillites from exposures have an average volumetric weight of \(1.96\ \text{t/m}^3\), siltstones \(1.90\ \text{t/m}^3\), and sandstones \(1.80\ \text{t/m}^3\), whereas for monoliths from boreholes it proved to be, respectively, \(2.05\), \(2.06\), and \(2.0\ \text{t/m}^3\) (Table 1). The latter is explained to a considerable degree by the greater weathering of rocks in exposures. This is also indicated by the porosity of these rocks. As a result of the drainage of groundwater by the Barandatka River, the rocks in exposures during the dry season have low moisture content. The degree of moisture of siltstones and argillites does not exceed \(0.45\)—\(0.64\), and that of sandstones \(0.4\). These same rocks, occurring away from the Barandatka River, are in a state of almost complete saturation and have, as a rule, a degree of moisture above \(0.8\)—\(0.9\). Only for some types of sandstones does the degree of moisture not exceed \(0.45\) (Table 1). The angles of internal friction of monolithic specimens proved to be close to the angles of internal friction of the rocks in the massif (in the slope). The exception is argillites, whose angles of internal friction in monolithic specimens are somewhat lower than in the massif.

The cohesion in the massif of weathered rocks, determined by field tests, proved to be significantly lower than in rocks according to monoliths. The reduction in cohesion is explained by the weakening of the rocks in the process of their weathering.

From the data presented it is evident that the coefficient of structural weakening of weathered rocks proved to be \(0.34\) for argillites, for silt-

Table 1

Results of studying the engineering-geological properties of certain rock types of the Itatsky deposit

Note: In the upper part of each cell, the range of values is given; in the lower part, the arithmetic mean.

0.24 for siltstones, and 0.22 for sandstones. A similar picture may partly be observed when the deposit is worked by open pits, since all the overburden rocks have, for the most part, a clayey cement.

Conclusions

1. The decrease in the strength of rocks in outcrops is due chiefly to a reduction in cohesion, whereas the angle of internal friction changes only slightly.

2. The main reason for the significant decrease in the cohesion of rocks in natural and artificial slopes should be considered the general weakening of the structure of the rocks studied during the process of weathering.

3. Simultaneous investigations of the shear resistance of rocks, carried out in natural slopes and on monoliths from boreholes, make it possible to determine the coefficient of general weakening of rock cohesion, which should be taken into account when assessing the stability of open-pit walls.

References

1. P. L. Rudakov, I. I. Popov, A. P. Li. Prevention of Landslides in Open Pits. Gosgortekhizdat, 1960.

2. Guidelines for Determining the Optimal Slope Angles of Open-Pit Walls and Waste-Dump Slopes. VNIMI Publishing House, 1962.

3. V. B. Shvets, P. P. Kazakov. Testing soils for shear by the punch method under field conditions. Transport Construction, No. 5, 1962.

4. B. V. Shvets, P. P. Kazakov. Determination by field methods of the shear resistance of soils. Foundations, Footings, and Soil Mechanics, No. 1, 1963.