Engineering-Geological Conditions of the Uryup Area of the Berezovsky Brown-Coal Deposit

F. P. Nifantov, V. N. Pulyaev

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

Coal production by open-pit methods in the Soviet Union is increasing from year to year. Great prospects for the development of this method are opening up at a number of deposits of the Kansk-Achinsk Basin, where enormous reserves of brown coal are concentrated in thick seams at shallow depths. In the coming decades, coal extraction by open pits at many explored deposits can be brought up to several hundred million tons at a cost of 30–40 kopecks per ton. Such conditions also exist in the Uryup area, located in the northwestern part of the Berezovsky deposit, which was investigated from an engineering-geological standpoint in 1966–1967. Below, the results are briefly presented of studies carried out at the stage of detailed exploration, essentially with the use of methods developed at the Department of Hydrogeology and Engineering Geology of Tomsk Polytechnic Institute [2, 3, 4, 5]. At the same time, the method of combining exploration and engineering-geological boreholes drilled by the Itatka Party of the “Kuzbassuglegeologiya” Trust ensured a reduction in engineering-geological workings and in the duration of investigations and, thus, savings totaling more than 20 thousand rubles.

General Engineering-Geological Conditions of the Area Investigated

The main task of engineering-geological study of mineral deposits being prepared for open-pit mining is to determine all engineering-geological conditions and to assess the stability of rocks in the walls, dumps, and floor of open pits, taking into account changes in the conditions, properties, and stability of the rocks. With such a comprehensive approach, the number of factors affecting mining conditions is always considerable. Therefore, the principal factors are usually first determined and taken into account; their thorough investigation and proper consideration make it possible to choose the simplest and most effective methods for stripping and exploiting the deposit. In our case, among the engineering-geological factors, the greatest influence on coal extraction by open pits will be exerted by the geological structure, the water-bearing capacity of the enclosing rocks and coal, the strength and stability of all types of rocks, and, to a lesser degree, the relief, swampiness, and periodic flooding of the southern part of the deposit.

Geological Structure

Within the study area, which comprises about 100 sq. km, exploration has revealed a second-order brachysyncline, somewhat elongated along the meridian and separated from the main Berezovsky syncline by a structural swell of the same strike, complicated by disjunctive disturbances (Fig. 1). The dip of the limbs of the fold is 3–6°, and only in the northeast is it up to 20°.

Throughout the area of the brachysyncline, Jurassic and Cretaceous sedimentary rocks are developed, overlain by Quaternary deposits. The Jurassic deposits, composing the productive sequence of the deposit, have been divided by geologists into three suites: the Makarovskaya, Itatskaya, and Tyazhinskaya. The latter is not clearly separated from the overlying rocks of the Lower Cretaceous.

Fig. 1. General geological map and section of the western part of the Berezovsky brown-coal deposit. 1—Quaternary deposits; 2—Upper Jurassic and Lower Cretaceous deposits; 3—Middle Jurassic deposits of the coal-bearing subsuite of the Itatskaya suite; 4—Middle Jurassic deposits of the barren subsuite of the Itatskaya suite; 5—Lower Jurassic deposits of the Makarovskaya suite; 6—Carboniferous deposits; 7—Devonian deposits; 8—outcrops of the coal seam; 9—burned rocks; 10—collapsed rocks; 11—tectonic disturbances; 12—geological boundaries; 13—eastern boundary of the Uryup area; 14—line of geological section A—B.

The Makarovskaya suite occurs in the central part of the brachysyncline at depths down to 500 m. It overlies the uneven surface of the Paleozoic and is represented by sandstones, siltstones, and argillites with several coal interbeds. Owing to their deep occurrence, the rocks of this suite will not exert any significant influence on the conditions of open-pit mining of the Berezovsky seam.

The Itatskaya suite overlies the deposits of the Makarovskaya suite without visible unconformity. It is subdivided into two subsuites: barren and coal-bearing (Fig. 1). The barren subsuite is represented by sandstones (up to 70%), siltstones, and argillites. The total thickness of the subsuite is up to 130 m. The sandstones in places reach 50 m and contain confined waters. The coal-bearing subsuite includes a number of coal seams, including the 50-meter Berezovsky seam, which most often occurs at depths from 15 to 220 m. The maximum thickness of the subsuite reaches 200 m. Rhythmicity is observed in the section; the rocks below the Berezovsky seam are represented predominantly by sandstones, while in its roof they are represented by weak coaly argillites, siltstones, and, rarely, sandstones.

The Tyazhin Formation is usually composed of fine-grained sandstones, siltstones, and argillites. In the composition of the psammite-pelites, an increased content of carbonates, pyrite, muscovite, and chlorite is sometimes observed. The total thickness of the formation is about 150 m. The upper member, 30–60 m thick, is variegated and possibly belongs to the Lower Cretaceous. Here the rocks are more often weak, weathered, fractured, and water-saturated.

Quaternary deposits, developed over the entire area, are represented by loams, clays, and, less commonly, sands and pebble beds. In the pit walls there are also weak silty loams. The total thickness of the sequence is up to 36 m.

Thus, the geological conditions of the deposit—shallow occurrence of the seam and not very competent overburden rocks—may be considered favorable for open-pit mining. The entry trenches of the pits should be laid out along the seam outcrops, excluding the water-bearing burnt rocks (Fig. 1). Subsequent extraction should proceed down the dip, which will ensure a low stripping ratio in the first years of operation and higher stability of the pit walls.

Groundwater of the Investigated Area

The geological structure described and the alternation of rocks create favorable conditions for the accumulation of confined and unconfined waters. Within the area, the most clearly distinguished aquifers are those of Quaternary deposits, burnt rocks, sandstones, and coals.

Groundwater in Quaternary deposits is distributed unevenly. More often it occurs as low-yield horizons beneath flat watersheds, swamps, and at the contact of loams with the underlying Cretaceous and Jurassic deposits. In the cover loams, a perched water zone is developed in places at depths from zero to 10 m, reducing the stability of the rocks. More water-abundant are the alluvial sands and pebble beds in the valley of the Uryup River, where stable aquifers 7–12 m thick have been recorded, with spring discharges along the channel of up to 1 l/sec. This aquifer, together with surface waters, causes waterlogging of the floodplain surface and the formation of small landslides along the left bank of the Uryup River. If coal is mined beneath the floodplain, special measures for draining the pebble beds will be required, in addition to preventing collapse.

The aquifer of the burnt rocks, with a thickness from several meters to 44 m, is especially water-abundant, but limited in area (Fig. 1). Specific yields from pumping tests in boreholes are 1.26–10 l/sec, and filtration coefficients are 7.32–84.6 m/day. Owing to its high water abundance and static reserves, this aquifer may provide large inflows of water during the initial periods of coal mining and in spring, which should be taken into account in designing and driving the pits.

In the Mesozoic deposits, several aquifers are clearly distinguished, confined to sandstones, coal seams, and sometimes siltstones. In the sandstones, confined waters are developed with heads of up to 250 m and uneven water abundance, increasing in the weathering zone. According to data from the Itat geological party, specific yields during pumping from boreholes were up to 1.0 l/sec, and the filtration coefficients of the rocks were 0.02–8.24 m/day. The fractured brown coals of the Berezovsky seam proved to be more water-abundant, where specific yields reached 6.59 l/sec and the average filtration coefficients were 2.88 m/day.

Thus, the water-permeable rocks and coals of the investigated area are sufficiently water-bearing; therefore, taking climatic conditions into account, the seam should be mined with preliminary drainage, while bearing in mind the possibility of spontaneous combustion of the coal in overdried areas during the dry summer period. If quarries are developed on the floodplain of the Uryup River, in addition to drainage it will be necessary to straighten the channel and construct protective embankments.

Classification and engineering-geological characterization of rocks

In developing the engineering-geological classification, the composition, degree of diagenesis, properties, and strength of each lithological rock type were taken into account. The classification included rocks developed to a depth of 250 m. Within this sequence, three complexes are clearly distinguished by lithology and properties. The first comprises Quaternary deposits with weak bonds between particles; the second, burned and collapsed rocks; the third, Upper Mesozoic semi-rocks, ranging from very strong to weak. Within each complex, groups and lithological rock types differing in age and lithology were distinguished. In turn, each lithological type was subdivided by strength into engineering-geological varieties: very weak, weak, medium-strength, strong, and very strong rocks. Generalized indicators for the rock varieties are given in Table 1.

Very weak rocks include silty loams and strongly weathered, collapsed, and kneaded argillites, having cohesion up to 1.5 t/m² and low angles of internal friction. Weak rocks include Quaternary loams, clays, sands, sandy loams, and Mesozoic rocks weakened in the process of weathering and the action of other factors, having bedding-plane cohesion of 2–5 t/m² and temporary compressive strength normal to bedding of up to 50 t/m². Medium-strength rocks include sandstones, siltstones, and argillites, partly altered in the process of hypergenesis, with temporary crushing strength of 50–250 t/m² and cohesion of 5–10 t/m². Strong rocks include dense and sufficiently strong rocks with clayey-carbonate cement, which slake slowly in water and possess a temporary crushing strength of 250–1500 t/m². These rocks will be stable in quarry walls and can be excavated by heavy excavators. Sandstones and siltstones with sideritic and siliceous cement, having low moisture content and porosity, proved to be very strong. Their temporary crushing strength is 1500–9500 t/m², and tensile strength is up to 1180 t/m². These rocks occur as thin lenses and constitute no more than one percent of the volume of the overburden sequence; they will be excavated only with the use of blasting.

Thus, in the overburden sequence of the future quarries there will be rocks heterogeneous in lithology, properties, and strength, ranging from very weak to very strong. Weak and medium-strength, often water-bearing, rocks predominate.

Results of calculating the overall stability of working quarry walls

Determining the overall stability of the working walls of large quarries is an extremely necessary and complex task. At the stage of detailed exploration, when quarry designs have not yet been developed, this task is solved approximately, with the aim of obtaining slope angles for stable walls that are needed for calculating stripping ratios and estimating coal reserves in higher categories. Calculations usually include a comprehensive analysis of engineering-geological conditions, selection of calculation parameters, selection or development of methods, and perfor—

Table 1

Generalized indicators of the principal physicomechanical properties of the identified soil types of the Uryup area

Note. With a small number of determinations, the coefficients of variation are not substantiated.

…of the calculations. In our case, the design characteristics were selected for each engineering-geological type of rock according to generalized indices (Table 1). As the design values of the angles of internal friction and cohesion coefficients, the lower confidence limits obtained at a significance level of 0.05 were adopted, with inclusion of a stability safety factor of 1.1. As the design values of the unit weight of each rock, the arithmetic means were adopted.

Taking into account the substantially gentle monoclinal occurrence of the rocks and of the coal seam on the limbs of the brachysyncline, the high facies variability, and the recommendations on stripping and preliminary drainage of the rocks, calculations of the overall stability of working pit walls were carried out by the multilayer-slope method (1) and according to the ninth scheme of VNIMI (6).

As a result, the slope angles of the working pit walls obtained by calculation proved to be: for depths up to 50 m, 38°; at depths of 90–110 m, 31–32°; and for depths of 220–250 m, 27°. The values given have been adopted for practical decisions in the process of estimating coal reserves in the higher categories and will be taken into account during the design of the open pits.

REFERENCES

1. I. S. Mukhin, A. I. Sragovich. Construction of Limiting Contours of Equally Stable Slopes. Akademizdat, 1957.

2. F. P. Nifantov, V. N. Pulyaev. Engineering-Geological Conditions of the Uryup Brown-Coal Deposit. Scientific report, TPI archives, 1967.

3. F. P. Nifantov. Some questions of engineering-geological investigations of brown-coal deposits of the Kansk-Achinsk Basin. Materials on Geology and Minerals of Siberia. Tomsk, 1964.

4. V. E. Olkhovatenko, V. N. Pulyaev. Investigations of the shear resistance of certain types of rocks of the Itatka brown-coal deposit. Izv. TPI, Tomsk, 1966.

5. E. A. Pisarev. On the methodology of statistical processing of indices of engineering-geological properties of rocks. Transactions of TISI, XII, 1967.

6. G. L. Fisenko. Stability of Open-Pit Walls and Dumps. Nedra, 1965.