Engineering-Geological Conditions of the Development of the Nazarovsky Open-Pit Mine

F. P. Nifantov, N. S. Rogova, V. N. Pulyaev

The state plan for the coal industry envisages a considerable expansion of the Nazarovsky open-pit mine in the coming years. In this connection, in recent years the Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute has carried out investigations aimed at clarifying the engineering-geological conditions for the development of the open-pit mine. The work was performed simultaneously with supplementary exploration and included detailed engineering-geological mapping, engineering-geological testing of rocks in exploration boreholes and on the pit walls, laboratory studies of 130 monoliths, and an assessment of the conditions for stripping the Moshchny seam. The results of the investigations are presented more fully in a scientific report and briefly in the present article.

Geological Structure and Water-Bearing Capacity of the Rocks

Structurally, the Nazarovsky deposit is an asymmetric brachysyncline composed of Jurassic rocks overlain by Quaternary sediments. The coal-bearing strata at the deposit and within the open-pit area are Jurassic deposits lying on an eroded Paleozoic surface. According to the unified scheme (1960), the entire sequence of Jurassic formations is divided into the Makarovskaya and Itatskaya suites. The deposits of the Makarovskaya suite, distributed over the uneven surface of Paleozoic rocks, are represented by gray and dark-gray argillites, locally carbonaceous, as well as siltstones and sandstones. Beds and lenses of brown coal are sometimes encountered within the sequence.

The rocks of the Itatskaya suite lie without visible unconformity on the Lower Jurassic deposits. In terms of lithological composition and coal content, two subsuites are distinguished here: the lower, barren one and the upper, coal-bearing one. The deposits of the lower subsuite are represented by sandstones with interbed-

siltstone and argillite members; beds of conglomerate-like sandstones are sometimes encountered.

In the upper subsuite, sandstones, siltstones, argillites, and beds of brown coal are predominantly developed. In all, 10 coal beds with thicknesses from 0.1 to 6 m have been exposed. The structure of the beds is simple, and they occur at depths of 2–83 m. The most persistent of them is the Moshchnyi bed, sometimes reaching 20 m, which is being mined by the open pit. In the roof of the bed, sandstones are more often developed; in the floor, argillites, and more rarely clayey sandstones and siltstones. Among the enclosing rocks of this bed, the most widespread are quartz–feldspar sandstones, reaching in places 25–30 m, with a contact-pore, substantially clayey cement. In places in the section, coarse-grained, very strong quartzite-like sandstones occur.

In geomorphological terms, the area under study is located on the high fourth terrace of the Chulym River, bounded on the south by a 30–40-meter bedrock slope. The surface of the terrace is even, slightly inclined to the north. The alluvial deposits of the terrace, more often developed on eroded Jurassic sandstones, reach a thickness of 3–4 m. The pebbles are usually poorly rounded and strongly silted. They are overlain by loams and clays up to 10–40 m thick. Along the Bezymyannyi brook, on the terrace of the Adadymka River, especially in the area of the former Chertovo Lake, silts and silty loams of fluid consistency are developed, reaching in places 23 m in thickness.

Groundwater in the area studied occurs in Quaternary and Jurassic deposits. In Quaternary sandy-loamy, and in places gravel-pebble rocks, weak aquifer horizons occur beneath the thalwegs of gullies, at the foot of the bedrock slope, and in silts beneath swamps, in particular in the area of the former Chertovo Lake. On watersheds, on the slopes of gullies, on the fourth terrace of the Chulym River, and around the open pit, the groundwater of these rocks is to a considerable extent drained. Only locally, in loams and sandy loams of the aeration zone, after rains and snowmelt, temporary low-yield perched waters appear.

In the upper coal-bearing subsuite of the Itat Formation, the most water-abundant are two aquifer horizons that exert a substantial influence on the mining of the Moshchnyi bed.

The first of them is distributed in the over-coal sandstones and coals of the Moshchnyi bed, reaching in places a thickness of 40 m. Beneath rivers and the thalwegs of gullies, they are locally recharged by surface and alluvial waters. On the other hand, in the floor of the coal bed this horizon is often replenished by the confined waters of the second horizon, developed in a thick layer (30–50 m) of gray subcoal sandstones.

Physico-mechanical properties of the rocks of the overburden sequence of the open pit

Over the entire area investigated, above the Moshchnyi bed, sedimentary semirocky rocks of the coal-bearing subsuite of the Itat Formation are distributed, overlain by Quaternary deposits. The principal properties of these rocks, required for assessing stability, are described below and in Table 1.

Quaternary deposits are represented chiefly by loams, clays, silts, sandy loams, sands, and, locally, pebble beds, varying in genesis and lithological composition.

Loams, more often loess-like, are widely developed in the upper part of the Quaternary sequence and average 10–12 m in thickness. This

Table 1

Engineering-geological properties of the principal rock types

usually silty, very porous and macroporous, weak rocks that readily soften in water, with a total porosity of 37–51% and relatively low shear-resistance characteristics (Table 1). When excavated by shovels and placed at the bases of external and internal dumps, they readily soften under the action of moisture and overlying rocks. In a dry state they are more stable, but on slopes they become covered with deep cracks, most often of vertical orientation; when steep slopes and benches are undercut, this leads to the formation of collapses.

Clays are often silty and in their properties differ little from the loams that overlie them (Table 1).

The most unstable within the thickness of Quaternary deposits are silts and silty loams, distributed in the southwestern part of the area studied, on the territory of the former Chertovo Lake. When they were opened by the eastern wall of the pit in the summer of 1968, they turned into a flowing mass and slowly flowed through a cut into the mined-out space of the pit. Over the course of a week about 100,000 m³ of silty mass flowed out, and stripping operations were halted. At the recommendation of the department, a bypass alternative for the silty deposits and the resulting landslide-flow was developed. Subsequently, stripping operations were carried out according to this alternative. The data on the properties of these rocks given in Table 1 emphasize their high water saturation and low stability.

The semi-rocky rocks of the coal-bearing subformation, represented by sandstones, aleurolites, and argillites, are more stable, but heterogeneous in their properties. Therefore, some lithological varieties are subdivided into types according to their strength (Table 1).

Weak sandstones occur frequently. They are characterized by a silty composition and low cohesion. In a disturbed, moist state they exhibit plastic properties, which is explained by the high content of silty and clayey particles; by degree of moisture they are classified as water-saturated rocks. The angle of internal friction varies from 14° to 38°, and cohesion is up to 5 t/m².

Medium-strength sandstones occur as lenses up to 5 m thick. In granulometric composition these are also silty, but stronger rocks, with a cohesion coefficient of 5–10 t/m²; they soften only slightly in water and are stable in the walls of the pit.

Strong sandstones are very limited in distribution and occur as small lenses up to 2.0 m thick. They are distinguished by high strength, increased density, and in places, during excavation, require the use of blasting. They are very stable in the walls of the pit.

Weak aleurolites occur as small lenses up to 4.0–5.0 m thick. The principal engineering-geological properties are given in Table 1, from which it is evident that the rock is weakly compacted (average porosity coefficient 0.80) and saturated with water. The angle of internal friction varies from 11° to 26°, and cohesion is 0.6–5.0 t/m².

Medium-strength aleurolites are of limited distribution, mainly in the floor of the Moshchny seam. In composition they are silty and heterogeneous. They are distinguished by fairly high density and strength. They do not soften in water. The angle of internal friction is 17°–33°, cohesion 5.0–10.0 t/m². In the walls of the pit they remain stable for a long time.

Strong aleurolites occur as small lenses; their share in the overburden thickness is no more than 0.5%.

Weak argillites are of limited distribution and more often occur in the floor of the Moshchny seam. The rock is heterogeneous

in composition; it is distinguished by low density and high moisture content, and it softens in water. In the sides of the open pit, in a wetted state, it is insufficiently stable.

Brown coal is predominantly dull and brittle. In dry summer weather, in air, it rapidly loses moisture and cracks into pieces, subsequently turning into dust. Lenses of sooty coal occur. Specific gravity is \(1.44—1.71\ \text{t}/\text{m}^3\), volumetric weight \(1.20—1.31\ \text{t}/\text{m}^3\). Under conditions of natural occurrence beyond the drainage zones, the coals are in a state of complete saturation.

The angle of internal friction for crushed coal ranges from \(23^\circ\) to \(32^\circ\), and cohesion is \(3.75\ \text{t}/\text{m}^2\). In strongly drained zones the coal undergoes spontaneous combustion (eastern side, 1968).

Engineering-Geological Assessment of the Area Investigated

The entire investigated territory of the deposit, in terms of engineering-geological conditions for open-pit development, is clearly divided into the following sectors: eastern (the fourth terrace of the Chulym River), western (the slope of the same terrace), and southwestern (the area of the former Chertovo Lake). The engineering-geological conditions of the first two sectors are favorable for the development of the open pit. In particular, within the limits of the fourth terrace of the Chulym River, which encompasses \(2/3\) of the investigated area, a surface gently inclined to the north is observed, together with favorable conditions for the runoff of surface waters. The overburden rocks are dry or only slightly water-bearing and will prove most stable in the sides of the open pit. Along the slope of the fourth terrace of the Chulym River, rocks similar in composition are distributed, but they are more weathered. At high slope angles (exceeding the design values) and with improper conduct of mining operations, small landslides and collapses are possible here in places, especially in the southern part of the open-pit field.

In the area of the former Chertovo Lake, as noted above, the least stable will be the weak silty rocks, which will flow out at the foot of the open-pit side. Therefore, mining operations in this sector should be carried out according to a specially developed design, taking into account all the engineering-geological features of this sector, which are described in detail in the report.