Vol 49 Iss. 1

At the Zapolyarny mine of the Norilsk Combine, the complex conditions of the disseminated ore deposit make it difficult to develop mining systems and select their structural elements. The lack of similar examples in worldwide mining practice made it necessary to search for new ways of solving mining engineering problems, to check and correct the decisions made. Rapid growth rates of ore production meant that the experimental application of new variants of systems and methods of rock pressure control took place simultaneously with production operations. Sometimes this helped to find prompt solutions to emerging problems, and in other cases, on the contrary, it made it difficult to reach the right conclusions.

The disseminated copper-nickel ore deposit developed by the Zapolyarny Mine is represented by a tabular gabbro-diabase body up to 30 meters thick, dipping at an angle of 12-20°. Above the ore-bearing intrusion lie effusive rocks (porphyrites, gabbro-diabases) with a total thickness of 180-250 m.

It is known that mass blasting of ore by boreholes or mine charges under the system of forced level caving can be carried out on pre‑formed compensation chambers or without them. The latter method – single‑stage ore breaking in a confined environment – is currently used at the S. M. Kirov apatite mine, at the Zapolyarny mine of the Norilsk Combine, at the Salair mine, at the mine named after the XXII Congress of the CPSU of the Zyryanovsky Combine, and others. The breaking method affects the physical and mechanical properties of the broken ore: if in variants with compensation chambers the coefficient of initial loosening of the ore immediately after blasting the block varies from 1.3 to 1.6, then with single‑stage breaking this coefficient ranges from 1.1 to 1.3.

The correct choice of the underground development system and its main structural elements is especially important for enterprises developing complex vein forms. At such enterprises, the production process usually ends with the production of concentrates, so an efficient development system should ensure the minimum cost of the final product. In this regard, the choice of development system includes the following stages: selection of effective development systems based on mining and technical and technical-economic factors; comparative economic evaluation of selected development systems; industrial research of selected development systems and clarification of their basic design elements; technical and economic analysis of systems and recommendation of the most effective options.

The Rasvumchorr‑Tsirk and Tsentralny apatite open pits are located in the central part of the Khibiny massif and are worked on part of the arc‑shaped apatite deposit. The deposit area is characterized by harsh climatic conditions: low negative temperatures in winter (up to ‑36°C), snow blizzards and storms. The number of days per year with negative temperatures on the high‑altitude plateau of Rasvumchorr reaches 300, on the Rasvumchorr‑Tsirk open pit – 270. The duration of the winter period is 9 months; the thickness of snow cover on the plateau exceeds 3 m. Frequent fogs and avalanche‑prone slopes significantly complicate the work of open pits in winter. According to the project of the institute "Giproruda" the productivity of open pits: Rasvumchorr‑Tsirk 1.8 million tons, Tsentralny 12 million tons of ore per year. However, the actual productivity of the Rasvumchorr‑Tsirk open pit significantly exceeded the planned capacity.

One of the polymetallic deposits of the Union, where vein thickness varies from a few centimeters to 2‑3 m and dip angles from 25 to 90°, is represented by a variety of vein forms, among which branching ore bodies are widespread. The development of veins branching up‑dip is particularly difficult, as the acceptability of a productive stope‑and‑fill system on thin veins is determined by the possibility of partial ore drawing from the branched stopes to the main level drift. This determines the choice of the development system option, the volume of development and stoping preparation works, labor productivity and ore cost. The possibility of partial drawing during simultaneous breaking in all branches of the vein (main vein and apophyses) is determined by the rate of ore flow from the stopes along the vein and apophyses.

In the underground development of non-ferrous metal deposits quite often ore inter-chamber pillars are replaced by artificial usually concrete “pillars” - supports to achieve the most complete extraction of rich ores, to get the opportunity to use high-productivity methods of ore breaking both in the chambers and in the mining of pillars. From the point of view of reducing the cost of ore, the thickness of supports and the strength of concrete should be as small as possible. At the same time, the concrete supports should be strong and stable enough until the end of ore extraction operations, and sometimes for a longer period of time. Therefore, choosing the optimum dimensions of concrete supports is very important and responsible. However, there is still no definite methodology for calculating the parameters of concrete pillars. Their sizes and concrete strength grades are usually determined on the basis of practice data.

At two-stage development of powerful steeply dipping deposits with replacement of inter-chamber ore pillars with concrete wall supports it is very important to choose the right wall thickness and concrete grade. The methodology of such calculations is insufficiently covered in mining engineering literature. Laboratory studies on models at the Leningrad Mining Institute have shown that inter-chamber concrete supports in mining systems with ore storage or sublevel breaking (with subsequent filling of chambers with backfill) resist lateral loads like thin slabs with different restraint conditions along the contour. The final destruction of the wall under the action of lateral loading occurs from the loss of stability by overturning or shear (wall subsidence).

In the mines of the Baltic oil shale basin, extensive industrial tests of different variants of the chamber mining system have been carried out for several years. In order to correctly determine the parameters of the system, the behavior of rock pressure was studied at the mine No. 3 of the Slantsy Combine, in particular, the stability of the pillars was investigated. The commercial oil shale bed with a total thickness of 2–3 m is almost horizontal at a depth of 30–100 m under the conditions of the basin and consists of four layers of oil shale and three rock partings. In the roof and floor of the seam, limestones are interbedded with clay and oil shale.

Usually rocks are fractured, so to judge the deformation of the rock during impact or explosion, it is necessary to be able to determine the wave impedance of fractured rocks. As a first approximation, let us assume that parallel cracks in the rock are combined into systems, in each of which the crack opening d and the distance between them a are constant. Let us assume that the rock is penetrated by three mutually perpendicular systems of cracks and that the cracks are filled with a substance with wave impedance. Let us denote the ratio of crack opening in each system (i = 1, 2, 3) through ei. Then the wave impedance of fractured rock can be represented as a Taylor series.

In the development of any deposit, numerous variants of development systems are possible according to the order of development of the working face in the open pit. The final evaluation of the variants with the choice of the most economical one is carried out according to the standard methodology.

The development of calculation methods for rock movement beyond the final pit limit is associated with the design of the development of a group of bauxite deposits and further work on the definition and implementation of rational parameters of mining operations for rock movement by walking draglines. Standard design calculations are carried out on many cross‑sections with plotting of each strip. The labor intensity of this method is high, and the accuracy, depending on the distance between adjacent cross‑sections, is low, because the number of sections to speed up calculations is limited.

Tikhvin bauxite deposits, on the example of which the relationships are considered in the development of the working face at open pits prone to landslides, are characterized by an irregular shape, a small width (an average of 200‑300 m) and a significant length (0.5‑3 km). Due to the small thickness of deposits, mining operations are carried out by one bench, sometimes with division into sub‑benches, using upper and lower loading by excavators with buckets of 1‑4 m³. The overburden sequence is characterized by a layered structure, change of lithological composition in transverse and longitudinal directions, dip of clay layers in some areas towards the mined‑out space of the quarries, as well as water saturation of gravel‑pebble deposits, sands, fractured limestones and bauxite rocks.

At quarries in the Leningrad region, granite breaking is conducted, placing explosive charges in vertical wells with a diameter of 105 mm. Usually after explosions the floor of a bench turns out extremely uneven, protrusions (sills) sometimes reach one third of the height of the bench. Due to the uneven floor of the bench, the productivity of excavators EKG-4 does not exceed 250 thousand m³/year, i.e. about half as much as at leading mining enterprises

As is known, the hydraulic method of development provides a comprehensive mechanization of labor and high technical and economic performance. As applied to the development of alluvial deposits, one of the urgent tasks is the determination of design parameters and improvement of hydraulic mining systems. Specific water consumption in short‑season hydraulics with natural head depends both on the natural conditions of each particular placer (physical and mechanical properties of the soil, the thickness and width of the placer, the slope and nature of the bedrock, etc.) and on factors related to the organization of placer mining (head and water quantity, mining system, adopted equipment, the length of the extraction block or the greatest distance from the face to the sluice, and the design of the sluices).