Changes in the physical and mechanical properties and structure of rocks depending on the depth of mining operations in iron ore quarries

The trend of the activity of open-quarry mining enterprises of iron ore raw materials is an increase in depth quarries. It is investigated in the work, changes in physical and mechanical properties and rock structure depending on the depth of mining operations in quarries. Natural geophysical and laboratory methods of research were applied. It was established that the angle of internal friction, Poisson’s ratio and rock density practically do not depend on depth of mining operations. But with an increase in the depth of iron ore quarries, there is an increase in the compressive and tensile strength of rocks. Indicator of specific adhesion has significant dynamics of fluctuations depending on the mineral composition of rocks and can either increase or decrease in deep horizons, which must be taken into account in each specific calculation. For example, by determining marginal angles of slopes of sides and ledges. According to the data of geophysical studies, it is established a characteristic increase in the intensity of creep shear processes and surface deformations rocks when the quarry depth increases. The results of research are recommended to be taken into account when organizing the mining and technological process in deep iron ore quarries.


Introduction
Huge industrial reserves of iron ore of the Kryvyi Rih iron ore basin (more than 18 billion tons) include both iron-rich and relatively iron-poor minerals.The rich iron ores (martite, hematite-martite, iron content 51-67%) are mined underground at a depth of 800-1400 m.Poor iron ores (magnetite ferrous quartzites, iron content less than 38%) are mined in open quarries and need beneficiation, therefore all quarries are part of mining and beneficiation plants (MBP) [1].As of 2023, there are 7 main quarries in Kryvyi Rih: Hannivskyi and Pershotravnevyi as part of the Northern MBP, Hlijevatskyi as part of the Central MBP, careers No. 3 and No. 2-bis PJSC "ArcelorMittal Kryvyi Rih", Skelevatskyi quarry of the Southern MBP, Inhuletskyi quarry of the Inhuletskyi MBP.All these quarries have been operated for more than 50 years, therefore, the depth of mining operations reaches 250-400 m and will continue to grow.It is known that with an increase in the depth of conducting open mining operations becomes much more difficult mining technical conditions, the technical and economic indicators of production deteriorate [2].
An important reserve for improving the technical and economic indicators of careers is scientific justified choice of mining and geometric parameters of slopes and ledges of quarries.Development methods of managing the state of the rock massif during iron ore mining is based on knowledge of mechanical processes occurring in rocks.In turn, the emergence and the nature of the development of IOP Publishing doi:10.1088/1755-1315/1348/1/012049 2 mechanical processes in rocks are determined by physical and mechanical factors properties and structural features of the massif of rocks [3].
The technological processes of ore extraction by drilling -explosive method are characterized as volume and surface deformations of rocks, which greatly changes the natural ones physical and mechanical properties and structural features of the rock massif [1].Besides real values of physical and mechanical properties and structural features of the massif of rocks depend on a number of natural factors, namely: mineralogically petrographic composition, the layerage of layers, the state of the external environment, etc.
The purpose of the conducted research is the analysis of changes in physical and mechanical properties and structure of rocks depending on the depth of mining operations in the quarry.
As the objects of the study there were chosen the quarries No. 2-bis and No. 3 of the Public Joint-Stock Company "ArcelorMittal Kryvyi Rih" (PJSC "AMKR"), mining accordingly by Novokryvorizke and Valiavkynske deposits of ferruginous quartzites, which are composed of metamorphic rocks and are covered by a weak cover of sedimentary rocks [3].Against the background of the complex folded structure of deposits significant fracturing has developed.The fracturing is characterized by chipping cracks and stratification cracks.A common feature of these deposits is the presence of oxidation zones, in which magnetite is replaced by martite, and the synclines pass into a mixture of hydromica, kaolinite with discrete hematite and goethite [4].
In quarry No. 2-bis (surface area 200 ha), mainly magnetite iron ores are mined quartzites of the I and II iron horizons, mining operations are carried out without expansion of earlier formed career boundaries on the surface.Excavation of the quarry below the +30 m horizon is carried out by 15-meter ledges, with their subsequent doubling upon reaching the final contour career.The current depth of the quarry is 285 m, according to the project is 415 m.
In quarry No. 3 of PJSC "AMKR" (surface area 365 hectares), mining operations are related to the development magnetite quartzites of the IV iron horizon of the Saksahanskyi site, which occupies the central part of the quarry.This horizon is in contact with VI along the Valiavkynske fault an iron horizon exposed on the eastern side of the quarry.Magnetite quartzites both stratigraphic horizons underwent weathering, as a result of which they were replaced hematite "oxidized" quartzites.Hypergenic changes are manifested up to a depth of -300 ÷ -500 m in section VI of the iron horizon and to a depth of 150 -210 m in section IV of iron horizon [4].Thus, in parallel with the magnetite quartzites in quarry No. 3, oxidized ferruginous quartzites, concentrated on the lower ones, are also developed horizons and east, as well as in the south and north of the quarry.
From the day surface to the depth of the horizon +30 m, the exploitation of quarry No. 3 was mining by ledges 12 m high.Below this mark to the bottom of the quarry, the height of the ledges is 15 m.As of 2023 year, the main mining operations are carried out in the central part of the deposit at a depth of 390 m, according to project -500 m.
Preliminary analysis of data on the structure of discontinuous faults in the rock structure of the edge of quarries shows that these violations are caused not only by spatial location, morphology, internal structure and evolution of the rock massif itself, as well as consequences explosive works, which are carried out in quarries and significantly affect engineering and geological deposit conditions and lead to geodynamic and structural transformations in rock massifs [3].

Methods
In order to fulfill the set goal, physical and mechanical studies of properties and structure of rocks in 8 working horizons of each quarry were carried out.In work two types of methods were used: -geophysical methods of instrumental non-destructive testing at the level of the massif of rocks; -laboratory methods of destructive control using rock samples.With the help of geophysical methods, directly at the level of the ledges, it was determined as physical -mechanical properties of rocks and the structure of the rock massif.The physical and mechanical properties of the rocks were studied using an ultrasonic device UK-39 by measuring the speed of propagation of ultrasonic waves (UH) through a section of the massif in the natural conditions of the occurrence of rocks by surface method by a fixed base of measurements or by an end-to-end method.When determining the speed ultrasonic transducers were installed in the surface method of spreading UH directly on the surface of the area of at least 200x200 mm of bare rock massif.Measurements were made along and across the layering of rocks.For measurement of the speed of propagation of UH by a through method, ultrasonic transducers were installed on both sides of the rock block at a distance of at least 120 mm.A total of 689 measurements were carried out by two methods.
Based on the database of ultrasonic measurements, it was determined the strength, elasticity, viscosity, acoustic, rheological, density properties of rocks and cracking.According to speed measurement data the coefficient and angle of the passage of longitudinal and transverse elastic UH were calculated internal friction and Poisson's ratio [6,7].
During the study of the physical and mechanical properties of the rock massif, it was also forecasting of the degree of weakening of the rock massif during development was carried out deposits by quarries.Due to the presence of fissuring of the massif of rocks, its total strength is usually less than the individual components (structural blocks).With the decrease in the degree of cracking, the strength of the rock mass decreases and the deformation characteristics increase.To determine the actual strength rock mass due to the strength limit in the sample, a coefficient was entered into the calculation structural weakening (ks), which was determined experimentally -justified formula: where Lmin -the minimum size of the structural block in the massif of rocks, cm; La -the average distance between individual cracks, cm; σt -tensile strength limit of massif rocks, Pa; σc -compressive strength of massif rocks, Pa.Since the rocks in quarry No. 3 were practically undisturbed, when determining the specific adhesion on all horizons (ks) it was taken at the level of 0.9.For quarry No. 2-bis (ks) on all horizons was also taken at the level of 0.9, with the exception of the -165 m horizon, where violations were recorded in the mountain massif, and (ks) was taken at the level of 0.8.
The structural state of the rock massif was also investigated by the geophysical method.As is known, the structural components of the rock massif are the result of deformation processes and selforganization of the geological environment [2].It is known that within the sedimentary cover, before the opening of the rock massif, changes in the structure of this massif occur in strict dependence on the depth and density of the rocks lying above [3].In iron ore quarries, the technology of using explosives to break up the rock mass has a significant impact on the structure and stress of the rock mass, which leads to the development of cracking, landslides and collapses.
A significant impact on the state of the structure of the rock massif is created by the waterlogging of rocks by underground water, the depth of which is determined by the geological structure, tectonics of the territory, the depth of the spread of tectonic disturbances, the composition and condition of the rock massif, the mode of recharge, transit and discharge of groundwater.Obstacles that block the path of groundwater flow lead to a rise in their level [8].
Studying the structural features of rocks on models in laboratory conditions does not allow for an objective assessment of the actual state of the rock massif, therefore, the most adequate method is to carry out such determinations in natural conditions, directly on the working ledges of quarries using geophysical methods.It is known [3] that geodynamic processes in the rock mass are manifested by different frequencies of pulses of natural electromagnetic emission.Thus, in the zones of the formation of cracks (including in the zones of tectonic faults) there is electromagnetic radiation in the frequency range of 0-1 kHz; electromagnetic radiation in the frequency range of 1-2 kHz occurs in zones where there are creep (slow sliding) processes; in the frequency range of 2-7 kHz, electromagnetic radiation reflects the presence of surface deformations that occur due to the pressure of the rock above the rocks, or from additional man-made load; registration of electric pulses in the frequency range of 7-50 kHz reflects the presence and migration of underground water in the rock massif.
In order to register the Earth's naturally pulsed field, in our research we used a microprocessorbased indicator MIEMP-4/1 of the "Simeiz" series, which has 4 independent measuring channels capable of sensing pulses of the Earth's electromagnetic field in the range from 0 to 50 kHz.The device allows to determine the number of pulses of the natural pulsed electromagnetic field of the Earth, their amplitude and the ratio of the total duration of the signal to the measurement time.As external sensors for the MIEMP-4/1 indicator, active electromagnetic antennas of mutually perpendicular placement were used, which made it possible to determine the direction of movement of the Earth's natural pulsed electromagnetic field in the controlled point of the territory.Measurements were made along linear profiles with a distance between control points of at least 5 m.At each control point, the measurement was carried out by three sensors (X, Y and Z) for 0.01 s to 1 minute.The total route of research (measurements) was 4,440 m, of which 2,670 m were covered along profile lines in 10-m increments and 1,770 m in 25-m increments.Measurement data were recorded in the device's memory, and then analyzed using the "Geoimpulse" computer program" with differentiation of quantity, frequency and amplitude of pulses.The results of data processing were presented in the form of text files and graphs using color gradation (a certain shade of color corresponds to a certain amplitude of the signal, the density of which is proportional to the intensity of electromagnetic pulses).
54 samples representing various mineralogical species were selected for conducting research on the physical and mechanical properties of rocks in laboratory conditions.In the process of laboratory research, the density, strength, porosity, rheological and deformation properties of rocks were determined by the method of destructive control.
In order to control the results of instrumental measurements of physical and mechanical properties of rocks by non-destructive ultrasonic method, relevant indicators were studied on samples of rock control samples in laboratory conditions.According to the "Instructions ..." [7], the volume of control samples was 15 samples.Control samples were made from the cores of drilled column wells, which contained types of ores similar to those that made up rock samples selected in quarries for laboratory research, namely: biotite-chlorite schist, hematite-magnetite quartzites, magnetite-silicate quartzites, magnetite quartzites, hematite-magnetite quartzites, silicate-magnetite quartzites, silicate-carbonatemagnetite quartzites.

Results and discussion
The results of the conducted research are presented in tables 1-2.For an objective comparison of the results, data analysis in all tables was carried out for rocks of the same mineralogical nature composition of different horizons.
The analysis of the data in table 1 shows that for the main productive rocks of quarry No.2-bis (magnetite -carbonate -silicate and silicate -magnetite quartzites) with increased quarry depth, the average values of compressive and tensile strength limits increase relatively.The angle of internal friction, Poisson's ratio and rock density practically do not change with increasing quarry depth.Cohesion indicators for magnetite -carbonate -of silicate rocks to a depth of -165 m increase, and with a further increase in depth significantly decreases, while for silicates -magnetite quartzites, they practically do not change.
For ore-free rocks (biotite-chlorite slates), the investigated indicators of physical and mechanical properties practically do not depend on the depth of work in the quarry.
According to the quarry No. 3, it was established that for all investigated rocks, the limit of compressive and tensile strength with increasing depth of development (especially up to -200 m) tends to increase, but for hematite-magnetite quartzites with further deepening of the quarry (up to -300 ÷ -330 m) this indicator decreases.The angle of internal friction, Poisson's ratio and the density of the examined rocks depend little on the depth of mining operations.For magnetite-carbonatesilicate and silicate -magnetite quartzites, the rate of rock cohesion increases significantly with increasing quarry depth; for magnetite quartzites, this indicator practically does not change, and for 5 hematite-magnetite quartzites, it decreases.It should be noted that the strength of rocks significantly depends on the lithologic -petrographic and mineralogical features of the rocks (the coefficient of variation is 12-30% on average).The strongest rocks are martite hornblende and jespelite.Then come magnetite, hematitemagnetite and silicate-magnetite hornblende, diabase, amphibolite, arkose, phyllite and biotite hornblende.Due to the great diversity of the mineral composition of rocks in the section of the vertical composition of the sides of iron ore quarries, it is practically impossible to establish a clear gradient of changes in rock strength with changes in quarry depth.
The results of geophysical studies of the rock structure of different horizons are shown in table 2. The table indicates: "-" the absence of the phenomenon; "+" the presence of the phenomenon; "++"; average process intensity; "+++" is high intensity of the process.
The obtained data indicate that in quarry No.2-bis the processes of crack formation with high intensity occur at a depth of -180 ÷ -200 m.The intensity of rock creep (slow shift) processes increases with increasing quarry depth.The presence of surface deformations of rocks is manifested with the same intensity in all studied horizons, and the migration of groundwater has a high intensity in the horizons from -70 ÷ -165 m.
In quarry No. 3, the zones of crack formation on all horizons are not very intense, the processes of creep and surface deformations of rocks increase with increasing quarry depth and reach a high intensity at a depth of -330 m.Underground water migration occurs most intensively in the range from -60 m to -240 m, and with further deepening of the quarry, this indicator decreases significantly.

Conclusions
The mechanical properties and state of the rock structure are the determining parameters for selection of technology and techniques of conducting open quarry mining operations.These properties are also important for substantiation of deformation characteristics, calculations of rock strengthening parameters massif, for substantiation of the limit angles of slopes of sides and ledges of iron ore quarries.Therefore, establishing the nature of changes in these properties when the mining depth increases is important information for the effective and safe organization of the mining process.The research results allow us to draw the following conclusions: 1.When the depth of iron ore quarries increases, there is a certain change in the physical and mechanical properties of rocks, the most characteristic of which is an increase in compressive and tensile strength.
2. The angle of internal friction, Poisson's ratio and rock density practically do not depend on the depth of mining operations in the quarry.
3. Changes in the specific adhesion index have significant fluctuations depending on the mineralogical-petrographic composition of the rocks and can either increase or decrease in deep horizons, which must be taken into account in each specific calculation, for example, when determining the limit angles of slopes of sides and ledges.
4. According to the data of geophysical studies, a characteristic increase in the intensity of creep shear processes and surface deformations of rocks was established with increasing quarry depth.
5. The migration of groundwater has a high intensity at the initial horizons of quarries and up to a depth of -250 m, which is due to the flow rate of surface waters of Quaternary deposits, which are mainly formed by precipitation, as well as the level of the horizon of fractured waters of crystalline rocks.
6.With the help of geophysical observation, it is possible to establish the actual position of the stressed areas of rock massif, the zone of formation of rupture cracks and landslide zones.It is also possible to predict the development of cracks and landslide zones phenomena earlier than visually.This is important for safe organization of mining operations in the quarry.
7. The results of the research are recommended to be taken into account when developing project documentation and organizing the mining and technological process for deep iron ore quarries.

Table 1 .
Dynamics of physical and mechanical properties of rocks depending on the depth of quarry development (results of determinations by ultrasonic and laboratory methods).

Table 2 .
Changes in geophysical indicators of the structural state of the rock massif at different depths of mining operations in the quarry (according to the results of the registration of the natural impulse field of the Earth).