PREDICTION OF ROCK STABILITY AT THE ABAKAN BROWN COAL DEPOSIT

V. N. Pulyaev, N. S. Rogova

(Presented by the scientific seminar of the Department of Hydrogeology)

In 1969–71, investigations were carried out at the Abakan brown coal deposit, the purpose of which was to clarify the engineering-geological conditions for developing the deposit and to predict the stability of rocks in the areas where the first open pits were to be established.

The investigations included a comprehensive engineering-geological survey, engineering-geological testing of rocks and aquifer horizons, laboratory work, stationary observations of the regime of aquifer horizons and of the development of modern physical-geological phenomena on the slopes of ravines, in river valleys, and on the walls of operating open pits (Nazarovsky and Irsha-Borodinsky).

The engineering-geological survey was conducted in order to study the external conditions for the placement and construction of open pits, auxiliary industrial and civil structures. Particular attention was given to studying the geological structure, the water-bearing capacity of rocks, landform types, and physical-geological phenomena. Operating open pits working under similar conditions, such as the Irsha-Borodinsky and Nazarovsky pits, were examined in detail; there, the method of mining, the behavior of rocks in working and nonworking walls, the influence of water on rock stability, etc., were clarified.

Rock strength during the survey was assessed chiefly by micropenetration. In areas of the most unstable rocks and on the walls of open pits, field tests were conducted to investigate the shear resistance of rocks.

In the course of the investigations, data from radioactive and electrical logging were also used. Penetration and geophysical observation data were compared with the results of studies of rock properties in field and stationary engineering-geological laboratories. In the field engineering-geological laboratory, determinations were made of unit weight, moisture content, plasticity for clayey rocks, slaking, micropenetration, and, in small amounts (mainly for weak varieties), rock shear resistance.

In the stationary laboratory, in addition to the listed characteristics, the grain-size composition, specific gravity, temporary resistance to crushing and rupture, and chemical and mineralogical composition were studied, using an electron microscope, thermal analysis, and X-ray structural analysis. Much attention was devoted to the investigation of rock shear resistance. Taking into account the conditions in which—

... rock types will be in the walls of the planned open pits, the shear tests were carried out under loads below the in-situ loads, under conditions of full water saturation. The shears were directed mainly along bedding planes, normal to bedding, and along weakened microzones. The temporary resistance to crushing was determined using a method developed at the A. A. Skochinsky Institute of Mining.

Much attention was devoted to studying the physicomechanical properties of the rocks, the results of which were reflected in the engineering-geological classification. The classification included weak Quaternary and Middle Mesozoic semi-rocky rocks and brown coals, in places deeply altered in the process of hypergenesis, especially in zones of weathering and burning.

Each lithological type was subdivided by strength into the following varieties: very weak (silts), weak, medium-strength, strong, and very strong rocks.

Weak rocks include rocks loosened and degraded in the process of weathering and by other factors, having high porosity and moisture content, low density and strength. The specific resistance of the rocks to micropenetration is up to \(70\ \text{t}/\text{m}^2\), and cohesion along bedding is up to \(5\ \text{t}/\text{m}^2\). Clayey rocks are plastic.

Medium-strength rocks include the majority of rocks partially altered in the process of weathering. Their strength properties are higher: the specific resistance to micropenetration is up to \(250\ \text{t}/\text{m}^2\), and cohesion along bedding is \(5\)—\(11\ \text{t}/\text{m}^2\). In their natural state, as a rule, they do not slake; when dried, they break down into separate aggregates.

Strong rocks are rather dense and strong varieties of rocks on clayey-carbonate and carbonate cement, with low values of porosity and moisture content. The rocks practically do not swell and do not slake in water. Cohesion along bedding exceeds \(11\ \text{t}/\text{m}^2\), and the specific resistance to micropenetration is more than \(250\ \text{t}/\text{m}^2\).

Very strong rocks constitute no more than 0.5% of the volume of overburden rocks and are characterized by very low moisture content (0.44—1.64%) and porosity (6.55—10.37%). They occur as isolated, pinching-out lenses 0.1—0.6 m thick, rarely 1.0 m. As a rule, these are fine-grained sandstones with sideritic or siliceous cement.

The study of physicomechanical properties ultimately came down to assessing the strength of the rocks and their stability in the walls of open pits.

The engineering-geological assessment of rock stability is a rather complex task and includes the selection of design parameters of the rocks, the selection of methods, and the direct performance of calculations according to the adopted design schemes.

The correct selection of design parameters creates the prerequisite for the most objective calculation of the stability of the walls of planned open pits. Failure of slopes and open-pit walls most often occurs along surfaces or zones of weakening caused by places of stress concentration, the weakest rocks, fracturing, groundwater outlets, the degree of weathering of rocks, and other causes.

In this case, the use of arithmetic mean values of the property indices of each rock layer does not correspond to the actual engineering-geological conditions; their variability must be taken into account. Therefore, recently the mathematical expectation has been taken as the design value, with corrections introduced into it to account for errors in field and laboratory investigations, corrections for un-

sufficient reliability of the generalized indicators, possible changes in the properties of a given rock in space, etc.

\[ X_p=\frac{M_x}{K_\phi}, \tag{1} \]

where \(M_x\) is the mathematical expectation. In our case, the generalized value of the indicator is taken as the mathematical expectation;

\(K_\phi=k_1\cdot k_2\ldots k_n\) is a factorial coefficient reflecting all unfavorable conditions of rock performance in the pit walls;

\(k_1\) is a correction allowing for the variability of particular indicators;

\(k_2\) is a correction allowing for the reduction in rock strength in the walls and benches of the open pit; according to the recommendation of VNIMI, it is 1.1 for weak rocks;

\(k_3\) is a correction allowing for the imperfection of the calculation scheme; it is taken equal to 1.1;

\(k_4\) is a correction allowing for the influence of hydrostatic and hydrodynamic forces.

Taking into account the variability of rock properties, the lower confidence limits of the angle of internal friction and cohesion are adopted as the design values. The arithmetic mean value is taken as the design value of the unit weight.

In those cases when the slip line is predetermined by engineering-geological conditions (bedding, the presence of interlayers of weak rocks, etc.), the lower tolerance limits of the shear resistance indicators along the weakened surface, determined by the formula, are adopted as the design values:

\[ X_0=M_x-t_\alpha\cdot\sigma, \tag{2} \]

where \(t_\alpha\) is the standardized deviation corresponding, under the normal law, to the selected significance level;

\(\sigma\) is the standard.

Proceeding from the above formula, the design values of cohesion and the angle of internal friction along the weakened surface are determined as:

\[ C'_p=\frac{1}{\kappa_2}\left(c-t_\alpha\cdot\sigma_c\right); \tag{3} \]

\[ \psi'_p=\frac{1}{\kappa_2}\left(\phi-t_\alpha\cdot\sigma_\phi\right), \tag{4} \]

where \(c,\ \phi\) are the mean values of cohesion and the angle of internal friction, \(\sigma_c,\ \sigma_\phi\) are the standards.

The results of determining the design indicators for all engineering-geological types of rocks and coal are used as the basis for calculations of the stability of slopes of the walls of future open pits, depending on the adopted calculation methods.

At present there are several dozen calculation schemes used to determine the steepness of slopes and cuts. For the conditions of the Abakan deposit, the multilayer-slope method (the method of V. V. Sokolovskii) and the IX scheme of VNIMI are of greatest interest.

The calculation according to the method of V. V. Sokolovskii is based on constructing the slope contour by numerical integration of the differential equations of limiting equilibrium, the solution of which was accelerated by the use of an electronic computer and presented in the form of a graph constructed by I. S. Mukhin and A. I. Sragovich for a conventional rock. The results of calculations of the slope angles of the walls of the projected

Table 1

Results of calculations of slope angles

for open pits with the introduction of a correction factor of 1.1 are given in Table 1. In individual lithological types and engineering-geological varieties, under conditions of preliminary drainage, they were: for loams 24–82°, weak sandstones 28–59°, medium-strength sandstones 30–44°, weak siltstones 24–52°, medium-strength siltstones 33–70°, weak argillites 19–26°, and brown coal 32–57°. Significant slope angles in weak rocks occurred in the upper parts of the section, which does not contradict the theory of stress distribution in slopes.

When calculated according to VNIMI scheme IX, which takes into account the presence of weakened zones in the section, the overall slope angles of the pit walls were obtained. They are given in Table 1.

These angles may decrease significantly under the action of groundwater in filtering slopes. In forecasting the overall stability of slopes, hydrostatic buoyancy and hydrodynamic pressure, which adversely affect rock stability, should be taken into account. For the conditions of the Abakan deposit, in this respect the greatest danger is posed by weakly cemented rocks above the Moshchnyi seam, especially in the weathering zone, to depths of 40–50 m. In them, under conditions of hydrostatic buoyancy, the angles of internal friction are substantially reduced: in sandy loams from 26 to 14.5°, in sands from 32 to 20°, and in weak sandstones from 30 to 16°. The stability coefficients under the possible action of filtration forces amounted in the calculations to 0.42–0.61.

Thus, the calculations showed that the angles of filtering slopes decrease almost twofold in comparison with drained rocks, which once again emphasizes the need for preliminary drainage of deposit areas before exposing the coal seams.

Simultaneously with the engineering-geological investigations at the Abakan deposit, the stability of rocks in the walls of operating open-pit coal mines of the Kansk-Achinsk basin and of a number of large exposures was studied. These include, above all, the Nazarovo, Irsha-Borodino-

sky and Bolshesyrsky open pits. Since 1966 these studies have been carried out most fully at Open Pit No. 2 of the Nazarovo deposit, whose engineering-geological conditions are in many respects analogous to those of the Abakan deposit.

The Nazarovo coal open pit was laid out in the late 1940s in the marginal part of the basin of the same name, composed of Jurassic semi-rocky rocks and coals overlain by Quaternary alluvial deposits. The mining system is combined: transport on the lower overburden benches and hydromechanized on the upper bench in the southeastern part of the open pit. Bench heights range from 5.7 to 12.6 m. In all, the pit has 4 rock benches (35.5 m) and 3 coal benches.

The annual advance of the overburden benches ranges from 94 to 100 m. The rocks differ in stability. The most stable in working slopes after preliminary drainage are sandstones, which withstand slope angles of 50–70° in benches up to 10–12 m high. In clayey varieties they decrease to 45–50°. The overall slope angle of the working wall, because of the wide berms, is 12–14°.

Under the action of weathering processes the rocks lose their strength and stability. According to the results of investigations, the generalized stability coefficient according to P. N. Panyukov proved to be 0.63 for sandstones, 0.78 for siltstones, and 0.87 for argillites, i.e., argillites are more unstable. The most unstable in the pit walls are loams and silts, which in 1968 began to slide in the southeastern corner of the pit. The landslide-flow that formed halted the advance of mining operations for 1.5 months. Subsidence and landslides often occurred on the external and internal dumps. These phenomena are often associated with insufficient strength and watering of the rocks in the foundation, with the dumping of thawed rocks onto a frozen crust, with excessive slope angles and heights, and with other causes.

The Borodino open pit is located south of the Abakan deposit. The depth of the pit is 83 m. Coal is mined in 10 benches with wide berms of 20–40 m. Although coals and rocks in drained benches have, for the most part, angles of 60–75°, the overall angle of the working wall of the pit is no more than 15–18°. The slope angles of the inactive walls are 33–35°. Traces of landslides are visible on them, especially on the second inactive bench.

The Bolshesyrsky open pit is located in the Balakhta district and mines coal for local needs. The depth of the pit is 15–17 m. Productivity is up to 500 thousand tons per year. There are 2 benches. Mining is carried out without preliminary drainage; therefore Quaternary deposits often slide.

Along the banks of the Chulym and Uryup rivers there is a whole series of large exposures 35–42 m high. The angles of drained slopes are 45–50°, decreasing to 27–30° in the wet state.

Thus, the majority of operating open pits in the Kansk-Achinsk basin work under conditions of preliminary drainage. The calculated angles of individual low slopes (up to 10–12 m) under conditions of preliminary drainage of future Abakan open pits approximately coincide with the bench angles of operating open pits.

The research materials make it possible to draw the following conclusions:

1. In the process of uncovering the Moshchny seam, rocks differing in composition and strength will be encountered—from weak to very strong. Weak rocks, in places intensively weathered, are most widespread in the overburden strata. Very strong varieties of rocks occur rarely. Taking into account the nature of their distribution,

they will not have a substantial effect on the stability of the rocks in the pit walls.

1. Taking into account the fairly high water abundance of individual aquifer horizons, as well as the low strength and stability of the rocks, coal mining should be carried out with preliminary drainage.

2. Provided preliminary drainage is carried out, the overall slope angle of the pit wall at depths of 50–70 m may be taken as up to 35°; at greater depths (up to 100–120 m), as no more than 29–32°.