Developing a Technology for Driving Mine Workings with Combined Support and Friction Anchors in Ore Mines

Abstract

This study summarizes the experience of conducting and maintaining mining operations in unstable zones in mines in Kazakhstan, assessing the mining and geological technical conditions of their operation under difficult mining and geological conditions. A field study and analytical modeling with an assessment of the stability of the boundary mass using the Barton mining method enabled the development of a technology for sinking mining operations with combined fasteners and friction anchors in ore mines.

1. Introduction

Ensuring the stability of mine workings in weakened rock conditions is one of the most critical tasks in the mining industry. The collapse of rock masses and the instability of workings can lead to significant economic losses and pose serious risks to the lives and health of workers. According to international studies, incidents related to mine collapses result in millions of dollars in losses and numerous injuries each year [1]. These factors underscore the need for developing effective support technologies to enhance safety and improve the economic efficiency of mining operations. The stability of ore and rock outcrops in complex geological conditions is often assessed using the Barton empirical method (Q-rating), which evaluates factors such as the rock strength and quality, mining depth, cross-sectional dimensions of the workings, the stress state in the surrounding mass, and the presence and condition of cracks, including their discreteness and alteration [2,3]. This method has proven effective under deep mining conditions and weakened rock formations. In recent years, ground support technologies in mining have significantly advanced. Modern methods incorporate various types of anchors and combined supports, which help minimize the risk of collapse and the development of metasomatic processes in rocks during operations. Special attention has been paid to SZA friction anchors, which provide reliable fixation of rock masses in complex geological conditions. Friction anchors have proven effective in stabilizing weak rocks without significant additional costs [4]. Studies confirm that combined support systems, such as reinforced concrete structures and anchoring systems, substantially improve the stability of workings in challenging environments.

This study considers various materials and support systems, such as the cement mixture MasterRoc STS 10, fiber-reinforced concrete (FTB), and steel-reinforced shotcrete (ATB) produced by Elasto-Plastic Concrete (EPC), which have been tested under challenging geological conditions. These materials and systems have demonstrated high efficiency in industrial trials, significantly improving the stability of workings under high rock pressure [5]. However, unresolved challenges remain concerning the optimization of these technologies for weak and exceptionally weak rock formations. According to Barton’s classification, category V rock formations are particularly difficult to support due to their low resistance to external loads and tendency to collapse [6]. The current solutions often fail to provide adequate stabilization for such formations, which necessitates further research and improvements in support technologies. The preliminary results of pilot tests of the cement mixture MasterRoc STS 10 revealed a reduction in rebound from the sides and roof of the workings during shotcrete application, as well as an increase in the thickness of the applied layer in one pass. To enhance the deformability of the shotcrete support, it is recommended to add steel or synthetic fibers, such as FTB or ATB, which further improve the strength properties of the support.

One promising approach is the combined use of SZA friction anchors and shotcrete. While this method has shown positive results in weak rock conditions, further analysis and refinement of its parameters are required. Additionally, successful implementation of these technologies depends on the detailed development of geomechanical models to predict the interaction between the rock mass and support systems. Such models would allow for better forecasts of mine stability and help minimize the risk of collapse. Deep-lying rope anchors with a two-level support system have become the standard in deep mines. These systems use rope anchors in combination with frame supports and anchor bolts, increasing stability and safety under high stress at the junctions of mine workings [7].

This study aims to develop and improve the support technology for mine workings using combined supports and SZA friction anchors. The research focuses on the interaction between the rock mass and the support system, as well as optimizing support parameters based on the geomechanical characteristics of rocks with varying degrees of stability. Particular attention is given to rocks of stability categories II, III, IV, and V, which require specialized support methods. Categories II through V represent varying degrees of rock stability, with category I being the most stable and category V comprising the weakest, most failure-prone rocks. Consequently, category V rocks present the greatest challenge for stabilization during mining operations. In conditions of increased horizontal pressure, particularly in areas where stress is perpendicular to the strike of ore bodies, reinforced combined support systems are recommended. These systems, which integrate steel arches with rope anchors, have proven effective in deep mines, ensuring the stability of roofs and walls in difficult conditions [8,9]. For tunneling in unstable, non-water-saturated soils, protective screens made of hollow pipes filled with concrete or reinforced concrete are recommended. These screens significantly enhance the strength and stability of tunnels, forming thin-walled spatial structures [10,11]. The methodology of this study includes an analysis of existing support technologies, field experiments, and the development of geomechanical models of the stress–strain state of the “enclosing rock–support structure” system. Extensive geological data from Kazakhstan’s mining sites were collected and analyzed to evaluate their impact on support technology selection. Particular attention was paid to the effectiveness of SZA friction anchors in combination with shotcrete for the support of weak rock formations.

The scientific novelty of this study lies in identifying the significant impact of the rock category on changes in the stress–strain state of a rock mass. The study determined that the maximum compressive stresses in the roofs of the workings range from 40 to 42 MPa, while the stresses in the walls range from 27 to 30 MPa. It was also noted that weak and exceptionally weak rock formations exhibit a 5–10% increase in maximum compressive stresses compared to rock formations of categories I-IV. These findings enable more precise adjustments to support parameters in weakened rock conditions. An analysis of the existing studies, design solutions, and practical approaches to mine support reveals that the current regulatory framework offers a wide range of options for selecting materials and support methods. Modern technologies, such as fiber-reinforced concrete, shotcrete, and combined support systems, have been validated through industrial trials and are widely applied to enhance the safety and stability of mine workings in complex geological conditions [12,13,14,15,16].

The practical significance of this study lies in the development of recommendations for supporting rocks of stability categories II, III, IV, and V in mines. Based on the collected data and modeling results, it has been demonstrated that using a technology combining SZA friction anchors and shotcrete ensures stable mine workings. The results of this study can enhance both the safety and economic efficiency of mining operations, particularly in complex geological conditions. This study thus provides valuable recommendations for selecting and optimizing support technologies in weakened rock conditions, contributing to improved safety and efficiency in mining operations not only in Kazakhstan but also in other regions with similar geological challenges.

2. Analysis of Ground Support Methods in Unstable Rock Conditions

Unstable zones include rocks of stability categories IV–V within the mine mass, as well as areas of faults, tectonic disturbances, and metamorphosed rock formations. In category IV zones, combined support structures are predominantly used. These consist of anchors, metal mesh, and shotcrete or arched metal supports of a special SVP profile. The most commonly used anchors include reinforced concrete anchors, SPA (steel–polymer anchor) bolts, Swellex (hydraulic expansion anchors), and KRAs (wedge expansion anchors).

The use of reinforced concrete anchors in mines with similar mining and geological conditions, despite their low cost and ease of installation, has revealed certain disadvantages. These include the necessity of using cement- and polymer-based fastening compositions, which can increase installation costs, the risks associated with anchor installation in unsupported areas, and the insufficient flexibility of the support design. This makes these anchors unsuitable for rock-prone deposits and for supporting unstable rocks as part of reinforced combined supports.

The use of SPA and Swellex partially addresses the limitations of reinforced concrete in ground support, offering greater flexibility and efficiency under dynamic loading conditions. However, these anchors still require manual installation in unsupported areas, limiting their effectiveness in complex mining environments.

Due to increased lateral pressure (resulting in sidewall collapse and floor heaving), steel–polymer anchoring measures were found to be insufficient. Issues such as anchors breaking, support tiles tearing from anchors, and rockfalls between anchors occurred. Additionally, rockfalls and the destruction of shotcrete support on the sides of the workings led to emergency situations, necessitating the bolting of the roof with metal mesh. KRAs, particularly near stopping operations and fixed workings, experience weakening and eventual loss of the locking part, resulting in rockfalls.

Concrete supports are prone to cracking during production and operation for several reasons [17,18,19]. The primary factors include deformation in hardened concrete, which is mostly caused by tensile or bending loads, environmental exposure, and corrosion. Over time, the development of defects significantly affects the stress–strain state of structural elements. The aforementioned causes of concrete cracking can be mitigated, or their effects minimized, by using dispersed reinforced concrete.

Several types of dispersed reinforced concrete are available, being classified mainly by the physical nature of the fibers used. These include steel fiber concrete (with metal fibers made from cold-drawn wires or sheet steel bent into zigzag shapes), glass fiber-reinforced concrete (fiberglass), synthetic fiber-reinforced concrete (polyethylene and polyester), carbon fiber-reinforced concrete (using carbon, aramid, or Kevlar), and fiber-reinforced concrete containing mixed fibers [20,21,22].

Despite the variety of dispersed reinforced concrete types used in construction, mixed fiber types play a leading role in preventing crack formation and reducing the rate of crack development. There are two approaches to addressing this issue: the first involves using fibers of the same type but different sizes, while the second involves combining two or more types of fibers, particularly a mixture of steel and synthetic fibers.

The simultaneous use of fibers of varying lengths helps reduce the number of both micro- and macrocracks. Short fibers help minimize the formation of microcracks, preventing significant stress dislocations. Long fibers, which can reduce the workability of the concrete mixture, are needed to limit the formation of larger cracks under heavy loads. Importantly, the volume of long fibers should be smaller than that of short fibers. The presence of fibers in the range of 1 to 2% increases the tensile strength, resistance to crack development, and impact resistance, making this composite suitable for shotcreting applications. Concrete reinforced with steel fibers of various lengths is characterized by enhanced fire resistance, low creep, and high deformation properties. Overall, dispersed reinforcement with 1 to 3% steel fibers can increase the compressive strength by up to 40% and flexural strength by up to 150%, significantly improving the resistance to mechanical and thermal shocks, as well as wear resistance.

Frames made from the special SVP-22 profile are used as arched metal supports in the form of 27-metal, three-bar (or five-bar) arches (referred to as SP at the mine). The roof and sidewalls of the workings are tightened between the support frames using timber. Backfilling of the voids created after roof collapse is carried out using either timber supports or modern chemical materials that expand in volume (Figure 1).

In “very weak” and “exceptionally weak” rock formations, the distance between arches should be no greater than 0.5 and 0.3 m, respectively. These distances can be confirmed through regulatory documentation when using arched metal supports. The use of arched metal supports in rocks of stability category IV ensures the preservation of their load-bearing capacity and has a positive effect on the geomechanical state of the rock mass, thereby enhancing work safety. However, it was observed that in certain areas of the mine, the metal frame support was unable to withstand the loads, leading to support deformation.

In rocks of stability category V, the safe use of arched metal supports is not always possible due to the development of metasomatic processes and rockfalls within 0.5–1 work shifts. To improve rock stability under these conditions, advance supports are implemented. Advance supports are used during workings in unstable rocks. These supports have a length of 3 m and a working width of 3.5 m, with 8 supports mounted on the roof of a working, and up to 12 supports for wider workings (with a width of 4.5 m) (Figure 2).

Drive-in supports are employed in soft or crushed rock formations. The driving depth of the advance supports should be at least 1.5 times the depth of the face per cycle. The use of this type of advance support has demonstrated excellent results in highly fractured cohesive masses. However, the spillage of crushed rock between the supports has been observed. In such conditions, advance supports and injection hardening of the rock mass are used. To increase rock mass stability during workings in very challenging mining and geological conditions, drill-injection anchors are used to inject polyurethane resin [23,24,25,26,27].

Organomineral and polyurethane resins, including Geoflex, Bevedol VFA-Bevedan, and Bevedol S-Bevedan, were tested to reinforce disturbed or unstable zones in the rock mass. A large number of options for stabilizing mine workings in unstable zones have been tested under mining conditions. While they ensure the stability and functionality of the workings according to their intended purpose, they significantly reduce the excavation speed and increase the cost of support.

3. Results of Modeling of Stress–Strain State and Rock Mass-Support Interaction

The stress state of the rock mass and the mine working support were assessed via stress–strain modeling using the RS2 software (https://www.rocscience.com/software/rs2, accessed date 30 August 2024) package (Rockscience package), a finite element analysis (FEA) system that was developed to solve linear and nonlinear, stationary and non-stationary spatial problems involving solid mechanics, structural mechanics, and the mechanics of coupled fields [28,29]. This software allows for the determination of three-dimensional stresses and loads generated in multiple directions, which are summed to obtain an equivalent stress known as the von Mises stress [30,31]. The results of these calculations are presented as indicators of the stability coefficient of both the rock mass and the support system. RS2 also supports a range of constitutive models, such as the Mohr–Coulomb and Drucker–Prager models, as well as elastic behavior models. However, the focus of this study was not on applying these models directly but rather on analyzing the interaction between the rock mass and the support system over time. This analysis was conducted using diagrams that reflect how the rock stability strength factor evolves under various reinforcement technologies. These diagrams provide insights into stress redistribution patterns, deformation, and load transfer between the rock mass and the support structure, helping to identify critical stress points and guide reinforcement strategies. This approach allows for a comprehensive understanding of the time-dependent behavior of the rock mass and support interaction, ensuring that the selected technologies can maintain long-term stability under complex geological conditions.

The calculation results are presented as indicators of the factor of safety for rock stability and support. The calculation pattern is presented in Figure 3. The value of rock pressure is accepted for the mining depth of 550 m at stresses σ₁ = σ₂ = σ₃ = 16.2 MPa.

The physical and mechanical properties of the rocks used during modeling are presented in

Table 1. Physical and mechanical properties of rocks.

Ultimate Tensile Strength sp, МPа	Internal Friction Angle r, deg.	Adhesion С, МPа	Dynamic Modulus of Elasticity Е’10−4, МPа	Poisson Factor m	Hours
7	33	17	9.9	0.26	0
5.3	30.0	12.8	7.4	0.25	6
3.9	27.0	9.6	5.6	0.24	12
3.0	24.0	7.2	4.2	0.23	24

.

The fractures comprised two mutually perpendicular systems of fractures with an average distance between fractures of 500 mm.

For modeling, the following supporting options were adopted:

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Shotcrete, 50 mm thick;

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Friction anchors, SZA, 48 mm + reinforced support;

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Friction anchors, SZA, with a diameter of 48 mm + reinforced support + shotcrete with a thickness of 50 mm;

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Friction anchors, SZA, with a diameter of 48 mm + reinforced support + shotcrete with a thickness of 100 mm;

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SZA, friction anchors, with a diameter of 48 mm + reinforced support + shotcrete with a thickness of 150 mm;

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Insulating layer of shotcrete–SZA friction anchors with a diameter of 48 mm + reinforced support + shotcrete with a thickness of 150 mm.

The distribution of the factor of safety for rock stability immediately and 24 h after the excavation is exposed and subsequently reinforced with a 50 mm thick layer of shotcrete around the excavation is shown in Figure 4 and Figure 5. Figure 4 shows the distribution of the factor of safety for rock stability immediately after the excavation is exposed and reinforced with a 50 mm thick layer of shotcrete. Figure 5 shows the change in the distribution of the factor of safety 24 h after the excavation is exposed and reinforced with a 50 mm thick layer of shotcrete.

An analysis of the distribution of the factor of safety for rock stability in Figure 4 and Figure 5 shows that after the rock is exposed and reinforced with a 50 mm thick layer of shotcrete, the rock maintains its stability (Figure 4). However, after 24 h, the factor of safety in the sidewalls of the excavation decreases to a value of 1.3 (Figure 5), which could lead to the failure of the shotcrete lining. This indicates that using only a 50 mm layer of shotcrete is insufficient for long-term excavation stability, and additional reinforcement measures are necessary. The distribution of the factor of safety for rock stability immediately and 24 h after the excavation is exposed and subsequently reinforced with SZA anchors and reinforced frames is shown in Figure 6 and Figure 7. Figure 6 shows the distribution of the factor of safety for rock stability immediately after the excavation is exposed and reinforced with SZA anchors and reinforced frames. Figure 7 reflects the changes in the factor of safety 24 h after the exposure, with additional reinforcement using a 50 mm thick layer of shotcrete. This helps to assess the impact of time and additional measures on the stability of the excavation.

The values of the factor of safety, which are visible in these figures, vary, showing the dynamics of rock stability. An analysis of the factor of safety in Figure 6, which is above average, shows that the rock around the excavation is in a stable condition immediately after exposure thanks to the use of SZA anchors and frame elements. The absence of shotcrete in the initial stage does not cause critical changes, but there are already areas that require further reinforcement for long-term stability. After 24 h, some areas in Figure 7 show a decrease in the factor of safety below the recommended level (e.g., below 1.3). This indicates the possibility of inelastic deformations, necessitating the addition of a shotcrete layer to enhance the reliability of the support.

The analysis shows that the initial reinforcement using SZA anchors and frames provides satisfactory excavation stability. However, after 24 h, the stability may decrease, especially in the sidewalls of the excavation, despite the application of shotcrete. This emphasizes the importance of not only timely reinforcement but also regular monitoring and potential adjustments to reinforcement measures to maintain safety over time. Figure 8 and Figure 9 show how the factor of safety changes immediately after the excavation is exposed when using different types of support. Figure 8 demonstrates how the factor of safety appears after exposure with the installation of contour support, including SZA anchors, reinforced frames, and a 100 mm thick layer of shotcrete. Figure 9 shows how the factor of safety changes after the excavation is exposed with the application of an initial 50 mm thick layer of shotcrete, followed by reinforcement with the same anchors and frames, with additional layers of shotcrete added for a total thickness of 150 mm.

The distribution of the factor of safety for rock stability immediately after the excavation is exposed, with support provided by SZA anchors, reinforced frames, and a 100 mm thick layer of shotcrete, is presented. The analysis of the factor of safety in Figure 8 shows that this combination of reinforcement creates stable support in most areas. Figure 9 demonstrates the effect of applying a 50 mm isolating layer of shotcrete, followed by the addition of an extra shotcrete layer, bringing the total thickness to 150 mm. An improvement in structural stability is observed: an increase in the factor of safety as a result of the multi-layer approach. Both figures display zones with a factor of safety above 2, indicating the high stability of the applied support structures. In the case of the additional layer (Figure 9), the factors of safety are higher, indicating reliable support even in rocks of categories IV–V.

The analysis results show that the use of a 100 mm thick shotcrete layer combined with SZA anchors and reinforced frames (Figure 8) provides sufficient stability but may be less reliable for long-term use. The addition of a 50 mm isolating shotcrete layer followed by layers up to 150 mm thick (Figure 9) significantly increases the stability and offers more reliable protection against metasomatic processes. The application of a thicker shotcrete layer and a combined support system is better adapted for operation in complex geological conditions, providing more stable and safer mining excavations. Thus, the results show that the multi-layer support system (Figure 9) is more effective than the single-layer system (Figure 8), especially for rocks with low stability.

The research results indicate that changes in the strength factor of the chamber’s edge depend on the type of supporting technology used (Figure 10). Figure 10 illustrates how the strength factor changes depending on the type of support technology. The red horizontal curve represents the safety margin of the surrounding rock mass (the ratio of the nominal strength of the rocks to the operational stresses), showing a 20% margin before failure. The dotted vertical lines at values of 1.5, 2.0, and 3.0 indicate the extent of deformations within the surrounding rock mass at different depths, corresponding to the various recommended support technologies for the mine workings.

The application of an insulating shotcrete layer 50 mm thick prevents the occurrence and development of metasomatic processes. In combination with SZA anchors, reinforced frames, and a second (bearing) layer of shotcrete, this approach ensures the stability of mine workings in rocks belonging to stability categories IV and V. The safety margin factors in these conditions exceed 2, demonstrating the reliability of the recommended support methods.

4. Discussion

Analyzing the mining, geological, and mining engineering conditions of the development of the Kazakhstan deposit shows that the deposit is characterized by complex geological and mining conditions. The presence of intensely disturbed sericite–chlorite–carbonate metasomatic rocks in the bottom of the deposit and at its contacts significantly worsens the conditions for conducting and maintaining mining operations in these zones. These metamorphosed rocks require the development of special measures to prevent their interaction with the aggressive mine atmosphere, as well as the use of heavy types of fastening and advanced fastening systems. At the lower levels of the deposit, areas with elevated air and mass temperatures act as catalysts for the development of additional thermal stresses, metamorphic processes, and chemical reactions, all of which lead to rock collapse. Solid barite–polymetallic, copper–zinc, and disseminated copper–pyrite ores, along with some varieties of rocks (such as siliceous siltstones), exhibit high strength properties. Thus, during driving and supporting operations in these zones, it is essential to account for the likelihood of dynamic rock pressure and apply flexible types of support.

The numerical modeling results obtained through RS2 were calibrated by comparing them with field measurements and observational data from similar mining conditions. Key input parameters, such as the cohesion, internal friction angle, and modulus of elasticity, were adjusted based on laboratory testing results and previous case studies from mines with comparable geological settings. To ensure accuracy, the model was validated by comparing simulated stress and displacement patterns with in situ measurements of the deformations observed in the mine workings.

In the lower sections of the deposit, rocks and ores of stability categories III-IV and IV-V predominate. Exposure periods for outcrops (up to 10 m²) in such zones are limited: up to 3 days for unstable rocks and 1 day for very unstable rocks. Consequently, for workings driven through these zones, it is essential to minimize the time required to install temporary supports. Additionally, the use of various reinforcement strategies, including SZA anchors and multi-layer shotcrete, was modeled and compared with real-world performance to further refine the model parameters. The calibrated RS2 model enabled accurate predictions of stress redistribution and deformation over time, guiding the selection of optimal support systems for maintaining stability in challenging conditions, particularly in rocks of stability categories III–IV and IV–V. The applied reinforcement strategies demonstrated their effectiveness under dynamic stress conditions. However, in some areas of the mine, high lateral pressures resulted in sidewall collapse and floor heaving, necessitating adjustments to the reinforcement approach. Through careful calibration, the selected technologies ensure long-term stability of the workings, significantly enhancing operational safety.

5. Conclusions and Recommendations

The analysis of design solutions for supporting mine workings in difficult mining, geological, and engineering conditions (IV–V categories) shows that the regulatory framework in Kazakhstan provides clear and reliable guidelines. These standards regulate the selection of reinforcement technologies and the design of support systems, ensuring the stability of mine workings even under challenging geological conditions.

The calculation results indicate that the maximum compressive stresses σ₁ in the roof of the mine working reach 40–42 MPa, while in the sidewalls, they are 27–30 MPa. The minimum stress component σ₃ in both the roof and sides of the working is approximately 1 MPa. Generalizing the results for other rock categories demonstrates that the stress–strain state of the rock mass is significantly influenced by the rock category. In weak and exceptionally weak rocks, there is a 5–10% increase in the maximum compressive forces (on average, 7%) in the roof compared to more stable rocks (categories I–IV, according to Barton). However, the minimum stress component σ₃ remains practically unchanged, regardless of the rock category.

The analytical modeling confirms that throughout the mining process, geomechanical processes remain active at all stages of mining. The stresses developing in the rock mass are generally lower than the natural strength of the rocks. Therefore, the timely installation of support structures can effectively prevent the development of metasomatic processes and the risk of collapse. The stability analysis of mine workings reinforced with SZA anchors and shotcrete shows that the factor of safety for the roof ranges from 1.02 to 1.12. Although this range indicates a stable state, it approaches the minimum acceptable threshold for safe underground operations. For long-term stability, particularly in IV–V category rocks, increasing the factor of safety to 1.3–1.5 is recommended to provide an adequate safety margin. Based on the simulation results, SZA anchors combined with shotcrete are recommended as effective support structures for ensuring the stability of mine workings under challenging mining conditions.

Further research will focus on assessing the performance of composite support systems under different environmental conditions and at varying production depths. Additionally, the support materials will be optimized by exploring the potential of new composite materials and advanced reinforcement technologies. Expanding geomechanical modeling will provide deeper insights into the stress–strain interactions between the rock mass and the support system. This will allow for further refinements in support design and contribute to enhanced safety and economic efficiency in mining operations.