Analysis of combined open-pit and underground mining method for Кuzbass coal deposits

. Modern development of coal mining industry in Kuzbass is carried out using advanced mining techniques. Analysis of its geological and technological conditions is performed with respect to the development of advanced technologies in related coal mining methods, developing combined (open-pit and underground) geotechnologies for complex mining of coal deposits.


Introduction
According to the project aimed at coal-mining industry complex development in Russia up to 2035, both in new coal-mining sites and in the operating enterprises there will be developing coal-technological, coal-chemical, and coal-energy clusters, which will allow for the complex use of the coal deposit possibilities [1].
Geotechnological cluster of the complex physical and technical technology is a combined knowledge about the convergence of open-pit and underground mining of coal deposits in time and space, regularities in the behavior of system "open-pit mineunderground workings" in rock mass, technical, economic, ecological, and organizational interrelations of technological processes during coal mining [2].
In order to develop the technologies for coal open-pit mining in slightly dipping and angled sheet deposits using full advantage of technology-related resources of the open-pit coal mine gobs, a method for choosing a reasonable way of development for slightly dipping and angled sheet deposits by means of two lines with internal stacking for in-pit crushing and conveying system has been developed. The method is based on new principles of production space organizing and developing, which ensures the most favorable conditions for deposit development compared to the well-established principles used in the production methods with traditional longitudinal and lateral course of open-pit coal mine operating area development [3].
This makes it possible to adapt process engineering solutions relating to the autonomous mine sites with modular structure (in projects "minelongwall face") to the operating open-pit mines having combined (open-pit and underground) development of gently sloping seams following the concept "Smart Mine in the Coal Open-pit Mine" [4]. The method enables to determine required and sufficient mining-technological structure of mine site uncovering and preparation, which is adapted to the technological workflow of coal open-pit mine development for one high capacity mining face and simple repeatability of mining activities following the combined geotechnology on 3-5 and more meters thick coal seams.
Institute of Coal of the Federal Research Center for Coal and Coal Chemistry of the Siberian Branch of the Russian Academy of Sciences developed modular miningtechnological structures of mine sites, adapted to the combined coal-mining techniques, protected by 15 industrial patents of the Russian Federation. These intangible assets are an intellectual property in the form of industrial patents. They are of special value for business, focused on conducting scientific research and commercialization of intellectual activity results.
According to the program for coal-mining industry development in Russia up to 2035, Table 1 reveals the structure of coal mining in Siberia. This program for coal mining in Siberia up to 2035 considers two variantsconservative and optimized.
One of the main goals of this program is to ensuring technological development of coalmining industry. At present moment coal-mining industry in Russia is represented by 58 mines and 133 open-pit mines. Moreover, almost a half of them were launched after 2000. New enterprises are equipped with high-performance equipment. These companies use the most advanced coal-mining techniques. 42 mines (which accounts for 76% of the operating mines total number) are working using advanced technology "mine -longwall face" [1]. This technology was developed in Institute of Coal (Siberian Branch of the Russian Academy of Sciences) as a modular geotechnological structure of mine sites and has been patented in Russia [5]. Development of coal deposits in Siberia is based on new scientific theories and construction principles of facilities following structural-network theory of gently sloping coal seam uncovering, using modular geotechnological structure. In contrast to the traditional statistical analysis (mines, open-pit mines, total coal production) the authors have identified coal production on an annual basis in mines and open-pit mines using single production infrastructure based on the combined (open-pit and underground) method of coal deposits development in Kuzbass (Fig. 1). The authors found out that from 2010 to 2020 coal mining using combined method had increased by more than 2 times (from 31.1 million tons to 63.7 million tons). The authors also analyzed coal production in Kuzbass using combined mining method, identifying as open-pit mining, as underground mining on the grounds of the modular geotechnological structure of mine sites in the open-pit coal mines using single production infrastructure of the coal-mining complex (Fig. 2).

The application areas of the combined development in Kuzbass
The authors found out that coal extraction on modular mine sites of coal open-pit mines has increased more than two times for 10 years (from 12.9 up to 29.9 million tons). As for open-pit mining, used within the combined coal extraction technology, it has increased 1.8 times (from 18.2 up to 33.8 million tons). The authors give reasons for high efficiency of the combined method for coal extraction, as it helps to reach the profit 1.5 times higher than the average level of profit in Kuzbass.
Practical application of the combined method of extraction began in 2000s in Kuzbass as a result of scientific and methodical reasoning of using combined physical and technical geotechnology in Institute of Coal of the Siberian Branch of the Russian Academy of Sciences by PhD (Engineering) Yalevskiy V.D., PhD (Engineering) Fedorin V.A. [5] and feasibility report for investment project prepared for "Sibirginskaya" mine of the open-pit coal mine "Sibirginskiy" (Giprougol, Novosibirsk) and open-pit coal mine "Mokhovskiy" ("Baykaimskaya" coal mine) [6]. At present moment, 11 coal-mining companies are using open-pit and underground mining in Kuzbass. In 2019 coal extraction using combined method was 66,502 thousand tons, which is more than 26 % of the total value, amounting to 250,070 thousand tons in Kuzbass (Fig. 1). 58,837 thousand tons of coal was mined using underground mining method, and 124,731 thousand tons of coal was extracted using open-pit mining [6].   To increase the scope of mine sites usage, it is necessary to use open-pit opening method of the deposits with longitudinal initial cut made on the strike of the seams, which permits to consider it as a part of the mining-technological structure with underground mining operations as a single facility. Longitudinal initial cut of the coal open-pit mine is a distribution of the mine sites divided in space for extraction of the deposits using combined method (Fig. 3).