Scientific and technical problems of transition from open pit to combined technologies for raw materials mining

In Kryvyi Rih iron ore basin, long-term and intensive mining of deposits applying open pit methods have resulted in significant areas disturbed by open pits, dumps and tailings storage facilities. Hundreds of thousands of hectares of fertile arable land are already unsuitable for agriculture and sometimes for living. Thousands of tonnes of dust from dumps and tailing ponds deteriorate the surrounding nature every year and pollute the atmosphere in mining basins. Industrial ore breaking in open pits using bulk blasting causes emissions of a significant amount of harmful dust and carcinogenic substances. The environmental situation in open pit mining areas is often close to critical. In addition, bulk blasting forms a seismic wave, which in some cases destruct civil buildings and industrial facilities. The article proposes ways of gradual transition from mining mineral deposits by open pit methods to environmentally friendly technologies of open pit-underground and underground mining. It is established that such transition is a forced and at the same time choiceless way of developing mining areas, in particular Kryvyi Rih iron ore basin. The paper notes that the main problems of this transition include geomechanical stabilization of the rock massif during construction of underground mines in areas of possible impacts of open pit fields. The paper develops the fundamental study of the problems of controlling the stress-strain state of the massif during transition from the open pit to combined technology of deposit mining. Technologies of combined mining that involve formation of a transition belt with backfilling the worked-out space with compound mixtures are studied as well. It is proved that development of theoretical foundations for controlling the stress-strain state of the massif during transition from the open-pit to combined technology of deposit mining, study and design of technological solutions that contribute to transition to open pit-underground and underground mining, is an urgent scientific, technical and practical problem of great importance.


Introduction
In terms of explored iron ore reserves Ukraine ranks first-second among the most powerful mining countries.The main reserves of iron ores are located on the Ukrainian crystalline shield in Kryvyi Rih iron ore basin.This basin along with Kremenchuk and Bilozersk iron ore districts forms the region of Greater Kryvyi Rih, or Kryvyi Rih iron ore basin -Kryvbas.
Oxidized quartzites and brown iron ores are difficult to process and require roasting-magnetic or gravitational-flotation processing methods [2].
The thickness of magnetite and oxidized ferruginous quartzites is up to 100-200 m, occasionally up to 500 m, the content of iron oxides in ore is from -15 to 46%, phosphorusfrom -0.03 to 0.16%, sulfur -from -0.02 to 0.24% [3].
Of the total volume of iron ore raw materials (IORM) mined in Ukraine, Kryvbas accounts for about 80%, at that the share of the open pit mining method is about 70-80% [4].
Ferruginous quartzites are mined mainly by open pit and partly underground methods.
The open pit method of mining causes alienation of much more land.In Kryvyi Rih basin, which accounts for up to 85% of marketable IORM production in Ukraine, mining has disturbed thousands of hectares of fertile arable land.
According to the State Enterprise "State Institute for Design of mining enterprises "Kryvbasproekt", in Kryvyi Rih basin land expenditure per unit of marketable products is 13 times larger when applying open pit mining methods than in underground mining.Air pollution by dust and gas emissions is also much greater as compared to underground works.
During open pit mining, the damage caused to the environment per 1 t of marketable products is 3.5 times greater than in underground mining, and considering the quality of marketable products it is 3 times larger [6].
The above-mentioned conditions of iron ore deposits operation and their impact on the environment deterioration when applying the open pit mining method are summarized (table 1).As compared to the open pit methods, underground mining of deposits demonstrates the following significant environmental advantages: • reduction of area losses caused by formation of open pits and waste dumps; • possible separate extraction of rich iron ores of different varieties; • use of overburden rocks as raw materials for by-product production; • lower losses of ore raw materials in the subsoil; • possible creation of nature reserve areas, recreational areas after completion of the deposit operation.
Thus, one of the main ways to reduce the negative impact of open pit mining on the environment of the basin consists in gradual transition from the open pit mining of mineral deposits to environmentally friendly technologies of open pit-underground and underground mining.Such a transition is found to be a forced and at the same time choiceless way of developing mining areas, in particular Kryvyi Rih iron ore basin.The main problems of such transition will include those of geomechanical stabilization of the rock massif during construction of underground mines in areas of possible impact of open pit fields.

Purpose
The paper develops the fundamental study of the problems of controlling the stress-strain state (SSS) of the massif during transition from the open pit to combined technology of deposit mining.Technologies of combined mining that involve formation of a transition belt with backfilling the worked-out space with compound mixtures are studied as well.
So, the paper aims to develop theoretical foundations for controlling the stress-strain state of the massif during transition from the open-pit to combined technology of deposit mining, study and design technological solutions that contribute to transition to open pit-underground and underground mining.

Analysis of researches and publications
Deterioration of mining conditions (increased depth of mining, reduced thickness of ore deposits) with a simultaneous increase in energy consumption leads to reduction in parameters of working sites, an increase in angles of the working wall and conservation of a significant part of the open pit [1].Deconservation of the open pit reserves after completion of every stage of mining results in a sharp increase in the volume of stripping, and given limited investments and a long period of their return, technical and economic indicators deteriorate significantly.The environmental problems of the open pit method of mining add to the already difficult condition of modern mining and processing plants (GZKs).
One of the directions of solving this theoretical and technical problem is the wider applicationt of combined mining with the integrated use of open pit-underground and underground methods of deposit mining.
Combined open pit-underground mining allows for maintaining the production capacity of mining enterprises for a long time.Along with that, introduction of scientific recommendations and technological solutions for combined mining of reserves has enabled a number of foreign companies that traditionally applied the open pit method to involve areas of deposits considered unpromising into intensive operation [7].
The sequence of application of open pit and underground mining methods is determined considering the required productivity of the enterprise and specifics of the deposit [8].
Depending on the location of underground mine and open pit fields within the deposit, three characteristic schemes can be distinguished [7]: • with a combination of work in one vertical plane (underground mining operations (UMO) are carried out under the open pit); • with a combination of works in the horizontal plane (UMO are carried out in the pit wall); • with a partial combination in both the vertical and horizontal planes.
The Kidd Creek Mine in Ontario (Canada) is an example of a systematic transition from open pit to underground mining, [9].
Construction of the underground mine started when the depth of the open pit was actually 150 m (the design depth is 250 m), i.e. 7-10 years before completion of open pit mining operations (OPMO).
The Australian copper-gold deposit Osborne located in Queensland has been mined by underground methods in the pit wall since 1996.The pit was decommissioned the same year.The underground work site is opened out through an inclined adit with a portal at hor. 80 m of the depleted pit, and vertical shafts [10].
The Australian Northparkes deposit operated by a group of mines that conduct UMO not after completion of OPMO but simultaneously with them is of particular interest.The company produces high-quality copper-gold concentrate [11].
Considerable experience in the simultaneous conduct of open pit and underground operations has been accumulated at the largest copper-gold deposit Grasberg which was explored in 1988.
Grasberg is the largest gold mine and the third largest copper open pit, as well as the world's highest located open pit.It is located in the province of Central Papua, Indonesia [12].
The mine consists of an open pit and an underground mine.The pit provides high production at low cost.The underground mine operates the under-pit ore massif and several individual deposits near the pit.
The Pyhäsalmi mine operates a copper polymetallic deposit with the 3-4% copper content in the ore and also extracts lead, zinc, sulfur and rare earth elements.The upper part of the deposit is worked out by a pit to the depth of 120 m, the lower part -by the underground method, using room systems with backfilling, as well as a system of horizontal layers with consolidating backfill [11].
The Virtasalmi copper mine operates a polymetallic ore deposit and is being worked out by open pit and underground methods.To the depth of 175 m, the deposit was worked out by both open pit and underground methods with taking the rock mass from the underground part to the daylight surface along the ramp.With transition to underground mining, ventilation raises were created from the non-operating wall of the pit to air underground mining operations [13].
"Sherrit Gordon" (Canada) has transited from the open pit method to the underground one.The Ruttan mine operates the copper ore deposit by the open pit to the depth of 120-160 m.The orebody thickness is 36 m, its length is 820 m, the dip is 70 degrees.The explored reserves for UMO are 27 Mt of ore with the copper content of 1.74%.The development system is room mining with subsequent backfilling of the worked-out space with processing tailings [14].
The copper-cobalt deposit Kamoto in Katanga is also mined by the open pit-underground method.The steeply dipping part of the deposit directly under the pit is worked out by a room system with backfilling, and the flat part is worked out by a room-and-pillar method [15].
To the depth of 170 m the deposit was mined during 15 years, then 10 years later they started underground mining operations.The period of simultaneous OPMO and UMO made 8 years [16].
Initially, the deposit was supposed to be worked out in combination, by an open pit to the depth of 380 m and an underground mine with a 60 m high safety ore pillar left in the bottom of the pit, which is equal to the height of a level.At the underground mine, room systems are applied with the backfilling of the worked-out space and the use of self-propelled equipment.
The Gaiske copper-pyrite deposit is represented by a series of orebodies with the thickness of 35-40 m and over.It is mined by the combined open pit and underground method in one vertical plane [18].
Finsch deposit is another good example of combined mining.It was developed by the open pit method until the 1990s [19].Since 1991, mining has been carried out by the underground method under the shell of the depleted open pit.
The Kiruna Mine is the world's largest and most modern mine with open pit-underground iron ore mining (figure 2).The mine is located in Kiruna, Sweden, and owned by the Swedish mining company Luossavaara-Kiirunavaara AB.In 2018, the mine produced 26.9 Mt of iron ore.
Until the 1960s, the mine applied the open pit ore mining method.Now iron ores are mined by the underground method [20].
Based on the analysis performed, it is established that the choice of combined mining technology is to a great extent impacted by the stress-strain state of the rock massif.Mathematical methods enable assessing current conditions and obtaining predicted data for enhancement and development of new mining methods, improvement of the flowsheet, selection of optimal parameters for stoping.

An important element of transition from technologies of open pit mining of magnetite quartzites
to those of open pit-underground and underground mining is the control of rock landslides in areas of pit fields undermined by underground workings.Such areas may be potential sources of man-made disasters.The stress-strain state of the massif is known to significantly depend on both natural and artificial conditions of the mining environment.
Among natural properties, geological heterogeneities, in particular the availability of structural, tectonic and other disturbances, are of special significance.
Among artificial disturbances of the rock massif, those occuring during the preparatorydevelopment and especially stoping operations have the greatest impact [21].In combined open pit-underground mining, the nature of the massif SSS change is impacted by both underground and open pit mining operations.Therefore, information about such disturbances is extremely necessary both at the design stage and in the process of deposit mining [22].
Consequently, a comprehensive assessment of values of current stresses in the massif, prediction of their nature and causes of their change during combined open pit-underground and underground stoping are necessary.This information will enable assessing current conditions and obtaining initial data for improvement of the applied and development of new combined flowsheets of open pit-underground mining, selecting optimal parameters for stoping operations and determining their rational sequence [23].
Consider application of mathematical methods on the example of the impact of the Pershotravnevyi open pit of the PrJSC "PivnGZK" on geomechanical properties of the rock massif at the bottom and in walls of the open pit.
Consider the geomechanical conditions for transition to underground operations during mining the reserves of the Pershotravneve deposit by the open pit-underground method.Minerals are mined by the H k deep open pit with the angle of the walls φ k .Underground mining operations are supposed to be carried out under the pit at the depth h from the daylight surface.
Assess the deposit area adjacent to the open pit on the basis of solving the plane problem of the theory of elasticity.The components of the stress fields are determined in stages.First, determine the amount of load removed from the pit bottom.It is a uniformly distributed load with an intensity γ p H k .
The values of the stress components can be written as , where a k is half the width of the pit bottom; γ p is the volumetric weight of ore in the massif; x 1 , z 1 are the coordinates along the corresponding axes.Analysis of the results obtained shows that the value of the horizontal component grows with an increase in the pit bottom depth and width.The value of the vertical component increases only with an increase in the width of the pit bottom, and the change in the pit depth does not impact the value of this parameter.
The values of the lateral earth pressure coefficient λ f are also maximum in the area that is directly adjacent to the contour of the open pit and decreases as the depth and width of the pit bottom increase [24].
In this paper, SolidWorks 2015 software is used to determine the SSS of a rock massif related to the determination of the stress field [25].
Table 2 presents initial physical and mechanical properties of rocks and the backfill material for calculating stresses and strains applying SolidWorks 2015.To calculate the stress-strain state of the combined massif, options of the classical scheme of open pit-underground mining of magnetite quartzites are accepted for the average statistical conditions of transition from open pit to underground mining.
The current average depth of open pits at the level of 350 m is accepted as the final pit depth (or the boundary of transition from open pit to underground mining).
The second stage of the deposit opening during transition from the open pit to underground mining method is carried out by a vertical shaft located outside the open pit in the footwall of the deposit.
The scheme of this kind is expedient for opening at the deposit depths of over 800 -1000 m, which is characteristic of all Kryvbas deposits.In addition, Kryvbas mining enterprises have accumulated a huge experience of applying this particular scheme for underground opening of deposits.
With less than 800 m depths of deposits, or in case of no funds for construction of mine shafts and absence of prospects for long-term mining by the open pit-underground or underground method, other opening schemes are applied.
Creation of ramps or spiral ramps from technological platforms in the open pit is the main scheme.It is expedient if self-propelled drilling, loading, and transport equipment is used.But in case of low costs of magnetite quartzites, such a scheme may not always be economically viable.
A grid of finite elements of the initial design model is given in figure 3. The model is adequate in size to the site where the technology under study is applied and is used to calculate stresses during open pit-underground mining.
Below are given the results of calculating stress fields in the rock massif when stoping under the open pit bottom, as well as when backfilling stopes with consolidating backfill mixtures.
Isolines of the main stress field around the rooms with an open stoping space and rooms backfilled with a consolidating mixture and collapsed waste rocks are presented in figure 4.
To visually determine the stresses, all isolines have a certain stress value in Pa and correspond to a certain color scale.
All the options proposed by the authors have the following general characteristics.The vertical shaft of the mine is located in the footwall of the deposit.The height of the underground level is 90 m.
The main crosscuts, drifts of the foot-and hanging walls are driven from the shaft to the orebody at levels -440 m and -530 m.
The level-room system is used for stoping.Three fundamental schemes of work development are investigated.
The idea behind the first flowsheet of transition from the open pit to underground method of mining an ore deposit is illustrated in figure 4.
After the complete drawing of the broken ore, the stoping space of the worked-out room is filled with waste rocks resulted from driving underground and open workings.The calculated isolines of the main stresses σ 1 of the rock massif when applying open pitunderground mining of the magnetite quartzites deposit with ore breaking and drawing and subsequent backfilling the stoping space of the worked-out room with waste rocks are presented in figure 4.
As is seen in figure 4, the general picture of the distribution of the field of stresses is classic: the greatest absolute magnitude of stress occurs near the angles of the formed stope from the side of the ore massif.Small concentration of stresses is observed in the corners on the bottom of the room.
Significant maximum stresses σ 1 in the corners of the room appear due to the action of compressive stresses.The stresses σ 1 decrease depthward into the ore massif, and their distribution becomes more uniform.
Concentration of maximum stresses σ 1 is observed in the upper and lower corners of the rooms that are not backfilled, and only in the lower part of the backfilled ones.
In some cases, lateral exsposures of the unfilled stope may be distinguished by the fact that tensile stresses σ 3 appear in the central part of the lateral generatrix.In this case stresses σ 1 are reduced from the boundary of the room depthward into the ore massif.

Results
Today, development of deposits of ferruginous quartzites of Kryvbas by underground mining has equaled the cost of open pit operations.At the same time, the cost of open pit operations will steadily increase, but in underground mining it will remain rather stable.In order to improve the technologies of transition from open pit to underground mining of magnetite quartzites, on the example of Kryvyi Rih iron ore basin, three fundamental technological schemes are proposed.
The idea behind the first flowsheet of transition from the open pit to underground method of mining an ore deposit is illustrated in figure 5.
In addition, the proposed solution to store waste rocks in the worked-out space of underground mines has a significant environmental effect, improving the environment of the mining basin.
In the future, in order to develop the environmental safety of the region, it is proposed to store all available waste rock, including overburden at possible parallel ore mining by the open pit method, in the worked-out space of underground mines.
Further underground extraction of magnetite quartzites will be carried out by systems of level caving of ore under overlying waste rocks.
This technology is the cheapest due to no consolidating backfill used that is provided for in the following flowsheets.
The idea behind the second flowsheet of the transition from the open pit to underground method of mining an ore deposit consists in backfilling the stoping space of the room with consolidating backfill after the complete drawing of broken ore.After the consolidating backfill Further underground extraction of magnetite quartzites will be carried out by level-room mining under protection of an artificial pillar made of consolidating backfill.
To reduce costs for consolidating backfill in case of its necessary application in the initial version of the combined open pit-underground mining of magnetite quartzites, the third flowsheet with a partial artificial pillar made of consolidating backfill is proposed.
The idea behind the third flowsheet consists in backfilling the bottom of the worked-out room with extra strong consolidating backfill after the complete drawing of broken ore.After the consolidating backfill gains its standard strength, it can be overlaid with waste rocks as an inert backfill.
Formation of an artificial pillar of consolidating backfill on the bottom of the stope requires a much smaller amount of the backfill material.At the transitional stage, this volume can be obtained by using a mobile backfilling complex.Consequently, no need for construction of a stationary backfilling complex significantly reduces the cost of the proposed technology, the cost of magnetite quartzites production by the proposed option of the technology of combined open pit-underground mining.

Conclusions
It is determined that nowadays development of deposits of ferruginous quartzites of Kryvbas by underground mining has equaled the cost of open pit operations.At the same time, the cost

Figure 1 .
Figure 1.Dynamics of iron ore raw material production by mining enterprises of Ukraine in 2011-2021.

9 Figure 3 .
Figure 3. Grid of finite elements of the initial design model adequate in size to the site where the technology under study is applied to mine magnetite quartzites.

Figure 4 .
Figure 4. Results of calculation of the main stresses σ 1 when applying open-underground mining of the magnetite quartzites deposit with ore breaking and drawing and subsequent backfilling the stoping space of the worked-out room with waste rocks: 1 -the contour of the open pit; 2 -waste rock backfill of the upper worked-out room; 3 -broken ore of the lower underground level; 4 -the orebody.

Figure 5 .
Figure 5.The proposed technology of open pit-underground mining of the deposit of magnetite quartzites with backfilling the worked-out room with waste rocks: 1 -the contour of the open pit, 2 -the vertical shaft, 3 -workings of level -440m, 4 -workings of level -530 m, 5 -waste rock backfill of the upper worked-out room, 6 -broken ore of the lower underground level, 7the orebody.

Table 1 .
Conditions of iron ore deposits operation and the impact of production processes on the environment in open pit mining.

Table 2 .
Physical and mechanical properties of rocks and backfill material.