MINING

Purpose. To establish the feasibility of refining deep open­pit mines below the boundary of the use of combined motor­con­ veyor transport with an increased slope angles of the pit walls using the developed transport unit for reloading rocks to overlying horizons during the reactivation of pillars under transport berms. Methodology. Preparation of a digital block model of the deposit, the elaboration of 3D geomechanical models for the dynam­ ics of mining, 2D and 3D numerical simulation of the rock stress­strain state of the outcrops of opencast workings, mathematical modeling of stepwise ore reserves and mining schedule, patent research and feasibility study. Findings. It is advisable to carry out mining in terms of the marginal rock state with an increase in the slope of the pit sides below the limit of application of the cyclic and continuous method in ultra­deep open pits. Such design of pit sides is achieved when benches are mined from top to bottom within the boundaries of steeply inclined layers with the use of inter­bench loaders of the developed designed in the completion zone. Provisions for the selection and feasibility of using the loader in the deep zone are formulated based on demarcation of application zones of cyclic (road transport) and cyclic­flow (combined road­conveyor trans­ port) technologies. Originality. Schematization of the mining operation was performed based on the calculated values of safety factor of sides, which allows increasing the slope angles of the pit walls of even ultra­deep open pits in the completion zone. It was found that with deepening of mining, the zones of potential sliding move away from the loose overburden to lower ore benches closer to the final depth of the Kacharsky open pit (760 m), but the safety factor corresponds to the required value according to the design standards. Practical value. An increase in the slope of the pit walls in the completion zone can be achieved using the developed loading installation, the main difference of which is that it can be moved without dismantling under conditions of reactivation of transport pillars (with an increase in lifting height by 1.5–4.5 times compared to the known equipment).


Introduction.
Ensuring effective and safe mining of steeply dipping deposits through deep and ultradeep (more than 600 m) open pits, especially circular open pits, remains a pressing problem in mining science and practice. Studies by leading scientific centers demonstrate the costeffectiveness of switching to the progressive cyclic and continuous method (CCM) for mining. However, the optimal depth of emplace ment of CCM complexes on rounded open pit fields is limited to a depth of 330-350 m even when steep conveyors are used [1,2]. Reducing the flattening of open pit sides while finding technological solutions to decrease transportation costs should be comprehensively solved and connected with the develop ment of the whole mining transportation system in the final part of deep and ultradeep open pits [3,4].
Therefore, the development of technological solutions for the effective implementation of mining operations below the limit of reasonable use of the cyclic and continuous method on roundshaped open pit remains an urgent scientific and tech nical problem for the safe extraction of ore reserves with the lowest possible side flattening.
Literature review. Modeling of geomechanical processes is a necessary element in planning mining operations in modern conditions. Any technological solution to improve the effi ciency of mineral extraction should be optimal from the point of view of the safety of mining, which is primarily related to the stability of rock openings. Ensuring the stability of the open pit walls remains the most topical problem for mining companies, as it is associated with a large number of influencing factors, whose variability results in changes in the stressstrain state (SSS) of the rock mass [5,6].
In the paper [7], it was shown that due to physical weather ing, unpredictable rock displacements often occur, leading to the reduction in rock strength and failure of slopes in open pits. The deformations of the pit walls are analyzed based on 3D numerical simulations. As a result, the potential mode of destruction for the described type of slopes is predicted and the role of crack development along the weak layer is found.
The authors of the article [8] point out that in the last decade many cases of catastrophic destruction of the open pit walls have been recorded worldwide. The importance of detailed modeling of the geomechanical processes is underlined.
The adequacy of slope stability modeling is ensured only if the initial data are reliable, especially with respect to the me chanical properties of the lithological differences. The authors of paper [9] state that the evaluation of shear strength is crucial for stability analysis, but that few data are available to estimate the actual properties of soils. The authors of paper [10] em phasize that backward analysis combined with monitoring of soil displacements can fill the gap in determining the mechan ical properties of soils. They use the FLAC3D software for nu merical simulation with the finite difference method.
The mechanism of destruction of the weak layer was deter mined, and the angle of internal friction of developing weak layers was determined, which compensated for the lack of laboratory tests and allowed the transition from qualitative to quantitative analysis. This can provide a reliable basis for the safe operation of openpit mines, which was clearly demon strated in the paper [11].
The modeling of critical conditions of the pit walls is simi lar in many respects to the analysis of landslide processes. The authors of the paper [12] point out the problem of modeling a landslide in weak soils. A decrease in the strength of the clay layer during plastic deformation leads to irreversible strains, so that soil displacements on a small slope can lead to a series of regressive failures and thus to an unexpected catastrophic landslide. It is noted that successful landslide prediction re quires reliable numerical modeling capable of reproducing ex treme soil deformations. The positive experience with finite element modeling using the RS3 code is presented.
Thus, when it comes to justifying the stability of open pit walls in mining ore deposits with a thick layer of overlying sandclay deposits and the presence of areas with a gradient of lithological differences toward the mining area, there is no al ternative to 3D numerical simulation of rock stressstrain state.
Ensuring the stability of open pit walls before and after the transition of the surface contour bounding the quarry is an im portant technological task in the mining of steeply inclined ore deposits. The deformation processes affect a significant part of the mined massif and are caused by the influence of a number of factors, among which the rock physical and mechanical properties dominate, taking into account watering, loading by mining equipment and seismic effects. The authors of [13] point out the need to analyze in detail the stability of the pit walls and individual benches in connection with technological operations in order to substantiate the technology of high rhythm ore mining. Based on the study on the geological structure and physicalmechanical properties of different lith ological varieties, the parameters of the slopes and pit walls in clay rock of the overburden are concretized.
The upper slopes are often at risk of collapse under the in fluence of natural and manmade factors, which affects the intensity of mining, especially during the rainy season [14]. In this context, the application of laboratory tests and numerical simulations is appropriate for studying the influence of rock watering on the stability of slopes and pit walls. In the article [15], the results of studies on the stability of slopes in Fushun open pit mine (China) subjected to extreme precipitation over a long period of time are presented. Earthquakes are a trigger ing factor for the stability of slopes, which requires their tech nical protection. Digital photogrammetry, depth telemetry and infrared scanning technologies are used to evaluate the behavior of overburden [16]. They are effectively used to pre dict geomechanical deformations of slopes in open pit mines under difficult geological conditions.
The effectiveness of mathematical modeling of rock and soil state depends on the correct choice of failure criteria. Nonlinear plasticity behavior and fracture mechanisms can be considered using the HoekBrown criterion and developing stability diagrams that take into account loading conditions and rock mass quality [17].
Estimating the stability of fractured rock slopes is a com plex problem of nonlinear and uncertain systems. In the article [18], an interactive method for analyzing the dynamic stability of steep rock slopes with internal cracks is proposed. The ap plication of special interface elements in FEManalyzes allows one to determine the state of jointed rock mass and, accord ingly, the opening stability.
Probabilistic approaches to geotechnical calculations have advantages over traditional deterministic methods because they account for various degrees of variability and uncertainty that are common in rock properties. In the article [19], an evaluation of slope stability using the probabilistic approach in combination with a kinematic analysis based on stereographic projection methods is presented. It is followed by a kinetic analysis using the limit equilibrium method.
The above analysis of methodological approaches to eval uating the stability of mine walls demonstrates that the use of multifactorial geomechanical analyzes in the development of mine plans is feasible.
Unsolved aspects of the problem. There is still no single ap proach to the safe reactivation of the pillars under the trans port berms and increasing the inclination angles of the sides of deep and ultradeep open pits in completion zone. Such tasks require 3D numerical modeling of the mined rock mass when mining the benches with transverse panels in steeply inclined layers from top to bottom. This implies the problem of expedi ency of pit walls flattening at the open pit depths of more than 600 m. Existing interbench loaders have a limited scope of application due to the need for their dismantling, transfer and installation at a new location. At the same time, the height of loading the rock mass on the overlying horizons does not ex ceed 30 m.
The purpose of research is to establish the expediency of cleaningup below the boundary of the use of combined mo torconveyor transport in the lower zone of deep open pits with minimal pit side flattening and increase in their inclina tion angle. The goal is achieved due to the developed device for transporting rocks to the overlying horizons during the reacti vation of the pillars under the transport berms.
Results. A general pattern has emerged whereby the stabil ity of the rock mass decreases as overburden and ore bodies are mined with steeply inclined layers. The schematization of the mining model within the boundaries of the steeply inclined layers was performed based on the calculated values of the re sultant slope angle and safety factor (SF) (Fig. 1). Stages Nos. 17, 19, 21 and 25 of the ore body mining on the 19 th profile were selected for modeling. According to the regulations, SF should not exceed 1.3 to provide the safe panel mining. This should be taken into account when mining deepening and reaching the maximum depth of the open pit. To not exceed the limit value of SF, the overlying overburden should be mined simultane ously with the ore body using the transverse panels.
It was found that SF drops from 1.63 to 1.38 in the comple tion zone of the Kacharsky open pit mine (Fig. 2). FEManal ysis with PHASE2 Rocscience software shows that the layers of the loose overburden are subject to the most intense defor mations. However, as the ore deposit is mined and mining op erations become lower, the potential sliding zones shift to the lower ore benches of the open pit side.
The 3D numerical simulation was carried out using RS3 code (Rocksciense). The authors of the paper [2] point that an "intermediate" zone should be allocated in addition to the "deep" zone in deep and ultradeep open pit mines. Mining of this "interme diate" zone is expedient using the cyclical and continuous method with combined motorconveyor transport.
It has been proven in [2] that the use of steeply inclined conveyors is effective while mining the inclined ore body. At the same time, it was found that, under conditions of the Kacharsky ultradeep open pit, the economically justified maximum depth of the horizon serving for rock mass reloading from motor transport to the conveyor is 344 m [2]. If the final depth is 760 m, the rock mass should be transported to this horizon by road to a height of about 416 m. Therefore, several problems will have to be solved at once: ensuring the com pleteness of the deep reserve with minimal flattening of the pit walls and mining the deep reserves at minimal cost.
Studying the maximum strains and displacements in the rock massif nearby the pit contour showed that the decrease in the SF when using the technology of mining with transverse panels in steeply inclined layers occurs due to the complicated deformation processes. That is why the crucial task is the cal culation of SF, at which the maximum shear strains in rock mass near the pit wall contour can lead to largescale sliding.
It has already been mentioned that, according to the de sign standards, the safety factor should not be less than 1.3 if the slope of the lithological differences in the mined space is smaller than the average angle of the rock internal friction. The 3D numerical simulation with RS3 showed that the criti cal SF at the final stage of mining is not less then 1.38. At the same time, the inclination angles of the pit sides are the largest in the completion zone, which indicates the possibility of min ing with a minimum number of narrow transport berms. This can be achieved with the use of loading devices in the comple tion zone.
The stability analysis was carried out taking into account a possible deterioration of the rock quality, especially consider ing the occurrence of regular joint systems. The 2D numerical simulation was performed to consider the rock mass fracturing with use of PHASE2 special options. In this case a greater de crease in the factor of safety and an increase in rock displace ments have been obtained as a function of the depth of mining. For the profile No. 19 (Fig. 5), it was found that the maximum displacements increase from 1.4 to 8.5 m when the depth of the open pit is increased from 385 to 760 m, which corresponds to a decrease in SF from 1.72 to 1.1. Such an increase in maxi mum displacements implies the risk of a sliding surface forma tion (Fig. 6) in the study area.
A feature of mining operations within the boundaries of steeply inclined layers is that benches are mined with trans verse panels from top to bottom. Therefore, the extraction of ore in the mined steeply inclined layer can be started only after the end of the excavation of overburden. Consequently, the time of mining the overburden of the next steeply inclined layer should not exceed the duration of ore extraction in the previous layer. Therefore, the width of the steeply inclined layer should be minimally sufficient to ensure a safe loop turn of the dump trucks. It is important to start mining the rock overburden in a timely manner. This provision allows justify ing the depth of the open pit, from which it is possible to pro ceed to mining the rocks with steeply inclined layers and to ensure a reduction in peak calendar volumes of overburden We analyzed the period of 10 years before the end of open pit operation, corresponding to the beginning of the decline in ore production relative to the design production capacity. This gives us the following picture. Ore production volumes will de crease from 23 to 1 million tons, and stripping operations will decrease from 15.6 to 0.4 million tons. On average, over the past 10 years, annual ore production will amount to 10.7 mil lion tons, and the excavation of the remaining volumes of rock overburden will be 6.9 million tons.
Rock overburden volumes will decrease by an average of 9 times compared with the beginning of mining the rock over burden with transverse panels in steeply inclined layers. There fore, it is impractical to flatten the pit sides at great depths of mining. Under such conditions, a more effective solution is the elimination of transport berms with an increase in the re sulting slope of the pit side in the completion zone and the use of interbench loaders as well as cyclical and continuous method for transporting rock masses.
Existing interbench loaders, as a rule, are equipped with steeply inclined belt conveyors, such as, the SIC30 or MPU 5000K loaders manufactured by the Ukrainian company PJSC Azovmash. A device is also known which consists of sequentially arranged elevators in a tracked base, placed on adjacent benches to load the rock mass from the excavator face to the surface or into a vehicle on a higherlevel elevator through a system of lifts.
Such transport system includes plate loaders, as well as sev eral boom lifts on a tracked base, equipped with buckets con nected by endless bushingroller chains. The lifts are located on the platforms of the upper benches one after the other, forming a transport system for interbench loading of the rock masses, and the last lift carries out direct loading to another transport vehicle. The system of boom lifts on a tracked base moves following the face advance. The installation is intended for the reactivation of nonworking sides of the open pit mine.
The disadvantage of this system is high metal capacity, as well as the need to keep a constant height of the benches and the width of the platforms on which the switch lifts are in stalled. This is difficult to implement under conditions of open pit ore mining. The refinement of the end caps under the trans port pillars for opening the deep horizons of the deposit with out additional flattening of the pit sides provides for an increase in the slope angle of the pit side due to the elimination of trans port berms, which leads to a decrease in the width of platforms.
The introduction of new elements made it possible to de velop a transport installation for shipment of rocks to the over lying horizons together with JSC SSGPO (KZ 34721) [20]. Its main purpose is to depreserve transport pillars and increase the inclination angle of the pit sides in the completion zone. The supports of the transport installation are connected to the transport gallery by hydraulic legs with a hinged or bearing joint at the end of the hydraulic cylinder, while the hydraulic leg is attached by a hinge to the skip rails, and the hydraulic leg foundation is rigidly fixed to the crawler support. Fig. 8 shows a transport installation for loading of endto end stocks under the pillars of transport berms, respectively, in the crosssection and plan, on which 1 is a dump truck; 2 is a lifting bridge; 3 is a crawler reloading device; 4 is a skip under loading; 5 is a crawler support; 6 is an impact machine on a track; 7 is skip under unloading; 8 is unloading guides; 9 is a vehicle body; 10 is a drive station; 11 is a skip rails; 12 is a skip cable; 13 is articulated (bearing) joint; 14 is a hydraulic leg; 15 is a rigid connection; 16 is a crawler; 17 is a well for the reload ing device; 18 is a skip cover; 19 is a skip block. It may also be recommended to load the rock mass through the feeder hopper directly into the power plant, without the involving the dump trucks. The support on the transport hori zon can be equipped with a hopper loader and overload the rock masses into the vehicle body through a plate feeder. There is no need to construct the well under the loader if the loader is installed on the lower horizon of the zone being cleanedup and equipped with a selfpropelled plate feeder. The rock mass is stacked and loaded onto the plate feeder by a wheel loader or excavator. A belt conveyor can be used in the transport gallery instead of a skip lift.
The reduction of the pit side flattening is achieved by using dump trucks with a lifting capacity of 90 tons in the deep zone. At the same time, a reduction in the number of transport berms can be obtained using loading devices, described above. In this regard, the main provisions on the selection and justifi cation of the feasibility of using the reloading device for opera tion in the deep zone were formulated: 1. The total costs of transporting rock mass under the new scheme of combined inpit transport with the use of the load ing device for operating in the deep zone should be less than the costs of transportation under existing (traditional) schemes.
2. The amount of capital costs for the construction of load ing devices, which constitute a new scheme of combined inpit transport, should not exceed the difference in the cost between the existing and new schemes of combined inpit transport.
3. The volume of mining and capital works when changing the scheme of combined inpit transport and structures of the reloading device while deepening of mining should be minimal. 4. The new scheme of combined inpit transport should make the most effective use of existing transport communications. The construction of new loading devices for operation in the deep zone is permissible only if the total costs of mining are reduced.
5. The introduction of a new scheme of combined inpit transport using a reloading device for working in the deep zone should not lead to an increase in mineral losses. 6. A smaller number of pillars should be left in order to increase the completeness of the ore body extraction under the transport berms with the use of the loading device for opera tion in the deep zone.
7. The total distance of rock mass transportation according to the new scheme of combined inpit transport with the use of developed loading device should not exceed the distance of transportation according to the existing scheme.
8. The cost of mining counted the extraction of overburden when introducing a new scheme of combined inpit transport with the use of the loading device should not be more than the cost of using the existing scheme and not more than the eco nomically feasible cost. 9. The life of the loading device for operation in the deep zone should be such as to amortize the investment cost of its construction.
10. The downtime of the pit transport during the construc tion of the loading device for operation in the deep zone should be minimal.
The economic effect of using the transport system to trans fer rock masses to horizons above when finishing pillars under transport berms is calculated according to the formula, mln 1 Here Q is the annual productivity of the transport chain, mln tons; H is the height of the rock mass lifting, m; C 1 , C 2 are costs of lifting with the use of vehicles and the proposed trans port unit, respectively, $/t•km; і 1 , і 2 are the slopes of the motor transport route and the skip hoist, respectively, ‰.
The use of the transport installation for reloading the rock mass to the overlying horizons during the cleaningup of the pillars under the transport berms allows for the reactivation of the nonworking pit side below the zone of application of the CCM complex under conditions of an increase in the resulting slope angle of the pit side. Due to hydraulic legs that regulate the height of the supports 5, it is possible to move the complex within the zone of completion of the pillars. Conclusions.
1. The possibility of increasing the slope angle of the pit walls in completion zone up to the limit value when reaching the final depth of 760 m is proved even for ultradeep ore pits, using the example of the Kacharsky iron ore deposit with a thick layer of loose overburden (on average 160 m). A general pattern of decreasing the pit wall stability was revealed as a function of the slope angle. Numerical simulation showed that with the resultant slope angle of the working pit side of 22-30°, the slope angles in lower part in the completion zone can be increased from 36-38° degrees up to 42-47°. There is a real opportunity to refine ultradeep open pits without sig nificant flattening of their sides with the use of highperfor mance technology of mining the pit benches with transverse panels in steeply inclined layers until the completion of min ing operations.
2. An increase in the slope of the lower part of the pit sides in the completion zone below the boundary of the application of cyclical and continuous method is provided by the elimina tion of part of the pillars under the transport berms using the developed design of the transport device for overloading the rock mass to the above horizons. It is advisable to use skips as a loadbearing body of the interbench loader. The design of the loader supports allows it to be built with a lifting height of more than 30 m with the possibility of moving along the pit side with variable berm elevations.
The construction of the bridge provides for unloading the trucks directly into the skip on the lower support, and the con veyor belt eliminates the need to use the drive station on a separate support (it reduces the width of the transport horizon up to 18-24 m). The presence or absence of a hopper loader on the transport horizon also affects its width, and, as a conse quence, the inclination angle of the transport gallery. The lo cation of the well on the lower horizon affects the mobility of the installation and the angle of its inclination.
3. It was found that at the wall inclination angle of 22-30°, the safety factor (1.38) exceeds the limit value (1.3) by 6.1 %. At the same time, the inclination angles of the pit sides are the largest in the completion zone, which indicates the possibility of mining with a minimum number of narrow transport berms. This can be achieved with the use of loading devices in the completion zone. Thus, it becomes possible to increase the inclination angles of the pit sides in the comple tion zone of the ultra deep iron ore open pit mine from 36-38 to 42-47° in the direction of the main development of mining operations.
The reduction of lateral flattening in the completion zone of the ultradeep open pit below the zone of application of the cyclic and continuous method, as well as the annual transpor tation costs up to 4.33 million USD is achieved by the use of skip transportation with an increase in the transport gallery slope on deep horizons.