Optimization of the Flexible Mesh Support Processing Parameters in Downward Approach Mining Drift by Numerical Simulation

Abstract

The support methods in downward approach mining drift have always suffered from problems of inflexible operation, with high substantial costs and poor supporting efficiency under high ground stress and repeated engineering disturbances. In this work, a novel flexible mesh support schematic was designed and introduced in downward approach mining drift. Based on extensive field investigations and sampling experiments conducted within the mining region, material models and contact models for defining the complex hidden joint structure in a metal mine were established and developed to simulate the stress distributions of the designed flexible mesh support. The deformation and failure behavior of the flexible mesh support under the effect of high ground stress were investigated and its feasibility was evaluated. The results show that the flexible mesh support system exhibits a distinct control effect on the deformation of perimeter rock. The perimeter rock deformation decreases by nearly 46.0% after the flexible mesh support, and the maximum horizontal displacement is 60 mm. An optimized flexible mesh support scheme for downward approach mining drift was obtained and confirmed by industrial tests. This work provides a flexible mesh support technology for downward approach mining drift, which can not only enhance the safety of downward approach mining drift operations, but also significantly improve construction efficiency.

1. Introduction

Mineral resources have a pivotal position in economic development and national security [1,2,3]. The increasing demand for mineral resources, coupled with the limited availability of shallow deposits, necessitates the exploration of deeper resources beneath the earth’s surface. In the underground environment, rock masses under the effect of high ground stress and engineering disturbances may suffer from rock bursts and instability in the surrounding rock, which is a significant threat to the safety of mine workers and equipment [4,5,6]. Consequently, mine support has emerged as a crucial issue that must be seriously addressed in mines [7]. With increasing underground mining depth, the increasingly complex geological conditions within the mine pose great challenges to the design of the support and the evaluation of its effects [8]. The conventional methods of roadway support have become inadequate for modern mining operations, because of their inflexible operation with high substantial costs and poor supporting efficiency under high ground stress and repeated engineering disturbances. Therefore, it is necessary to explore innovative roadway support technologies to accommodate the evolving geological complexities.

Recently, many studies have been conducted on support protection, such as yielding rock bolts, flexible mesh, and fiber-reinforced shotcrete [9,10,11]. Among them, the flexible mesh support technology, characterized by its excellent toughness, good impact resistance, high deformation ability, construction simplicity, and adaptability to varying terrains, has been widely employed in various engineering projects [12]. Yuan et al. [13] proposed a dynamic model of the rock system surrounding the support in mining processing with a large mining height face. Xu Hu et al. [14] employed an energy allocation-based design approach for flexible rockfall protection barriers. The results demonstrate the effectiveness of this approach in preventing stones from rolling onto the railway line, maintaining slope stability, and ensuring the smooth and safe operation of the railway. Qitao Duan et al. [15] used a high-strength polyester fiber flexible net for mining through the end coal, resulting in an enhanced safety factor and ultimate mining efficiency. Shanchang Bao [16] optimized the traditional support methodology for a fully mechanized caving face in an extra-thick coal seam. By changing the metal mesh of the traditional support to a “flexible mesh and wire rope”, remarkable technical and economic benefits can be achieved. Zhigang Tao et al. [17] used a negative Poisson’s ratio anchor and a flexible anchor mesh to reinforce the rock slope. Monitoring of the surrounding rock stress revealed that the combined support of the negative Poisson’s ratio anchor and the large deformation flexible anchor mesh provide a good reinforcement effect on the rock slope. Due to the influence of deep ground pressure, pyrite enrichment and the complex hidden joint structure in a metal mine, roof caving and rib spalling phenomena in the underground mining face are becoming more and more frequent. The traditional anchor net support system is always employed on both sides of the underground mining approach, aiming to mitigate safety hazards stemming from roof collapse and spalling. However, for the waistline of the sidewall, the question of whether it may be beneficial to employ an anchor net support in the excavation project is determined by the stability of the surrounding rock, the specific engineering service type, and the age of the mine. This kind of anchor net support operation, to a significant extent, not only incurs substantial support costs, but also decreases the efficiency of subsequent processes. From the available literature, it can be concluded that the support methods in downward approach mining drift have always suffered from problems of inflexible operation, with high substantial costs and poor supporting efficiency under high ground stress and repeated engineering disturbances.

In this study, a flexible mesh support technology was introduced in downward approach mining drift in a metal mine. The flexible mesh support scheme has been designed based on extensive field investigations and sampling experiments conducted within the mining region. The feasibility of the flexible mesh support system has been evaluated through numerical simulations and validated by an industrial-scale test. This work provides a flexible mesh support technology for downward approach mining drift in a metal mine, which can not only enhance the safety of downward approach mining drift operations, but also significantly improve construction efficiency.

2. Experimental and Numerical Methods

2.1. Geological Characteristics of the Mining Area and the Deposit

For a metal mine, the ore body is always situated within coarse-grained dolomite and limestone, exhibiting a distinct boundary with the overlying, underlying, and surrounding rocks. The ore body, in its natural state, displays configurations characterized by lenticular formations, layers, and cystic structures. The ore minerals are mainly composed of galena, sphalerite, and pyrite. Enriched sphalerite is primarily localized in the central and lower regions of the ore body, whereas pyrite is predominantly distributed at the top of the ore body. The presence of loosely packed pyrite contributes to a decrease in the overall strength of the ore body. Moreover, joint fissures and minor faults may develop, intersecting the ore body and compromising its structural integrity. All in all, there is an uneven distribution of strength within the ore body due to its complex structure.

The roof and floor of the ore body are limestone or dolomite, aligned with the surrounding rock. The surrounding rock primarily consists of substantial layers ranging from 0.5 m to 2 m in thickness. These rocks exhibit a certain degree of compressive strength, and their stability is influenced by the extent of fracture development.

Based on previous investigations and research, the exposed area of the ore body can reach up to 50~200 m². Furthermore, there are some unstable oxidized ores in the hanging wall. Therefore, it is necessary to reinforce the support when undertaking mining activities in unstable rock mines.

2.2. Support Status

The downward approach mining drift of the mine is rectangular in shape, with a width of 3 m, height of 3 m, and length ranging from 15 to 45 m. The drift is divided vertically or along the ore trends according to the thickness of the ore body. The stope mainly adopts the anchor mesh support technology. The anchor mesh is welded and processed using six round steel rods. The primary dimensions are mainly 1.2 m × 1.8 m and 1.2 m × 2.0 m, and the mesh opening size measures 10 cm. The anchor bolt is a pipe-seam type anchor bolt with a diameter of 42 mm and a length of 1.8 m. The anchor support spacing is mainly 1.0 m × 1.0 m and 0.8 m × 0.8 m. The support area is the side wall from 1.2 m above the floor on both sides of the approach in the ore body to the false bottom, and some false bottoms with serious deformation and failure are also supported by the anchor mesh. In addition to maintaining the stability of the two sides of the roadway, the support process is mainly to prevent small ore blocks from falling and injuring people, as shown in Figure 1. The traditional anchor mesh support method can ensure the safety of the mining approach. However, the high support cost, 327.20 yuan per m², cannot be neglected. Moreover, the anchor mesh support method is relatively conservative for ore with medium stability, and is not conducive to the efficiency of the subsequent drilling, blasting, and ore removal. For mining and cutting exploration projects involving medium and stable surrounding rock, this support method will seriously hinder the progress of the excavation and increase the burden on the material transportation capabilities.

Based on the authors’ previous work in the construction of a large drilling chamber in Bolivia [18,19,20] and the different characteristics of the rock surrounding the ore body in the metal mine, a flexible mesh support scheme has been meticulously designed. The downward approach mining drift support method adopts the anchor mesh support method under the false bottom. However, the steel mesh is replaced with a flexible mesh (8# galvanized iron wire, diamond mesh, mesh 50 mm, as shown in Figure 2). The anchor bolt style of the pipe joint is the same as that currently used, but there are two kinds of anchor bolt spacing: 1.0 m × 1.0 m, and 1.2 m × 1.2 m, and three kinds of drilling depths: 1.0 m, 1.2 m, and 1.4 m, respectively. There are a total of six kinds of support methods, as shown in

Table 1. Different flexible mesh support technology schemes designed for the downward approach mining drift.

Stope Approach Section	Bolt Parameter	Support Scheme
3 m × 3 m	anchor bolt spacing: 1.0 m × 1.0 m, 1.2 m × 1.2 m; anchor bolt length: 1.0 m, 1.2 m, 1.4 m	1#: anchor bolt length: 1.0 m, spacing: 1.0 m × 1.0 m
		2#: anchor bolt length: 1.4 m, spacing: 1.0 m × 1.0 m
		3#: anchor bolt length: 1.2 m, spacing: 1.2 m × 1.2 m
		4#: anchor bolt length: 1.0 m, spacing: 1.2 m × 1.2 m

. The supporting length aligns with the one-time blasting footage, ranging from 2.2 to 4 m, and can further extend from the head by 20 to 30 cm. It is required that the flexible mesh be placed close to the wall, with 10 cm reserved at the end for seamless integration with the second anchor mesh. Besides the anchor bolt connection, the connection between the two meshes is securely fastened using iron wire, as shown in Figure 3.

2.3. Numerical Models

2.3.1. Geometric Models

In order to verify the feasibility of flexible mesh support technology, the ABAQUS finite element software was used. According to the on-site joint fissure investigation and a comprehensive analysis of the property characteristics of the ore, as well as previous investigations [18], the numerical models considered the stope surrounding rock area (Figure 4a) with dimensions of 50 m in length, 4 m in width, and 30 m in height. The surrounding rock is mainly composed of thick layers of 1 m. The model considers the ore body, the surrounding rock, and the backfill. The false bottom (as indicated by the yellow line) and supporting structure are shown in Figure 4b. The geometric models of the flexible mesh support in the downward approach mining drift were meshed and divided into 58,285 elements and 57,779 nodes, as shown in Figure 4c.

Based on an integrated analysis of field surveys of joint networks and a comprehensive characterization of ore properties, the rock mass quality of the ore body is classified as Category III. The mechanical parameters of the surrounding rock, the ore body, and the backfill are given in

Table 2. Mechanical parameters of the surrounding rock, the ore body, and the backfill [18].

Type	Density /kg/m3	Elastic Modulus/GPa	Poisson Ratio	Angle of Internal Friction/(°)	Cohesion/MPa	Tensile Strength/MPa	Rock Mass Rating (RMR)
Surrounding rock	2720	10.20	0.27	35.30	3.30	0.19	60
Ore body	2590	7.80	0.28	34.30	3.20	0.22	59
Backfill	1970	8.42	0.26	23.10	0.08	0.86	60

. The mechanical parameters of the supporting structure materials, such as the flexible mesh, the rebar, and the anchor bolt are listed in

Table 3. Mechanical parameters of the supporting structure materials.

Type	Natural Density/(kg/m3)	Elastic Modulus/GPa	Poisson Ratio	Tensile Strength/MPa
Flexible mesh	7850	200.70	0.30	300.00
Rebar (diameter 14 mm)	7850	200.70	0.30	335.00
Anchor bolt	-	200.70	0.25	450.00

.

2.3.2. Damage Criterion

The selection of a constitutive model is dictated by the material behavior and the research objectives. The Mohr–Coulomb theory describing the stress state of rock under compression, tension, and shear for different strength conditions was used to reveal the relationship between the tensile and shear properties of the materials in this work [21,22]. The Mohr–Coulomb criterion is as follows:

$$\tau \ge S_s=\sigma tg\phi +C,$$

(1)

where $\tau$ is the shear stress in the diagonal section; $S_s$ is the shear strength of the material; $\sigma$ represents the positive stress in the diagonal section; $\phi$ is the angle of internal friction of the material, and $C$ is the cohesive force.

In this work, the M–C criterion is theoretically more appropriate for capturing the shear-dominated failure mechanisms in rock masses. The von Mises criterion is applied to characterize the strength of the rebar. The von Mises equivalent stress is ubiquitously plotted in numerical simulations as a convenient scalar measure to visualize the overall intensity of the stress field and identify regions where the stress level approaches the material’s yield capacity.

The von Mises criterion was given as follows [23]:

$$\sqrt{{({\sigma }_1-{\sigma }_2)}^2+{({\sigma }_2-{\sigma }_3)}^2+{({\sigma }_3-{\sigma }_1)}^2}=k,$$

(2)

where K is a material constant. The left side of Equation (2) is the second invariant of the principal stress, which is of great significance when considering the yield of materials. The ultimate criterion for instability is determined by the actual constitutive model used in the calculation (e.g., M–C plasticity), not merely by the contour of von Mises stress. The widespread visualization of von Mises stress stems from its computational robustness and its role as an effective indicator of high-stress regions where yielding is likely to occur according to the model’s inherent failure law. According to the maximum allowable value of roadway deformation [21], once the strain rate exceeds 1%, large deformation would occur in the tunnel. However, the mining problem is different from that of tunneling projects; i.e., the emergence of some small deformations is permissible. Therefore, a strain of 2.5% was defined as the critical value for deformation in tunnels without support.

2.3.3. Numerical Simulation of the Flexible Mesh Support Scheme

The model adopts a generalized analysis process. Firstly, ground stress was applied and an initial ground stress equilibration was carried out. Secondly, the roadway was excavated, and then the deformation and stress distribution was analyzed. Thirdly, the support of the surrounding rock and filling body was evaluated, and then the deformation and stress field of the filling body and the surrounding rock after taking the support into consideration were calculated and analyzed.

In order to ensure the efficiency of the calculation, the contact between the anchor bolt and the anchor mesh joint support model and the surrounding rock was simplified. The contact model selected the face-to-face contact between the paste-filled body and the mesh shell structure. To achieve a balance between computational fidelity and efficiency, the interaction between the flexible mesh shell elements and the rock surface was defined using a “hard” contact in the normal direction and a “penalty” friction formulation in the tangential direction, with a friction coefficient of 0.3 to simulate the mechanical interlock and frictional load transfer. The mesh shell face was divided into nodes, and each node interacted with the nodes on the paste-filled body.

For the geostress, the maximum principal stress (σ₁) and intermediate principal stress (σ₂) is oriented sub-horizontally, while the minimum principal stress (σ₃) is sub-vertical. According to our previous investigations [18], the mean magnitude of σ₁ is 63.13 MPa in this work, and the measured vertical stress is moderately higher than the theoretical gravitational stress. In this work, the model was also subjected to the gravity force. It is acknowledged that this represents a simplification. However, the primary goal of this study was to conduct a comparative analysis of different support schemes under the same initial conditions, for which this approach remains valid. Assuming that the interior of the rock layer is a continuous and uniform medium, the model applies horizontal displacement constraints on the two side boundaries, and three-direction fixed constraints on the bottom boundary, as shown in Figure 4d.

3. Results and Discussion

3.1. The Support Effect for the Surrounding Rock

Because the left support situation is basically similar to that of the right side, only the left support situation was studied. This can also help avoid repeated expression. Figure 5a shows the stress distribution of the surrounding rock without support. It can be seen that the periphery of the three-centered arch stope and the paste filling body without support are subjected to tensile stress. The maximum tensile stress value is 0.55 MPa. When there is a support with an anchor bolt with a length of 1.0 m and a spacing of 1.0 m × 1.0 m, as shown in Figure 5b, the axial stress on the anchors is a tensile stress, and the maximum tensile stress is 154 MPa, revealing that the anchors add a certain amount of suspension to the support in the perimeter rock. For the surrounding rock, as shown in Figure 5(b-1), there is no tensile stress action in the left wall part of the three-center arch stope, and part of the stope roof and the right wall are subjected to tensile stress, ranging from 0 to 3.49 MPa. Figure 5(a-1) shows the displacement distribution of the surrounding rock without support; the maximum horizontal displacement deformation of the model without support is 59 mm. Figure 5(c,c-1) displays the displacement distribution of the surrounding rock under support. It can be seen that the maximum horizontal displacement deformation of the model with support is 34 mm, which is reduced by 25 mm compared with the situation without support.

3.2. Effect of the Anchor Bolt Length

Figure 6 depicts the simulated displacement field of the anchor bolt, the mesh, and the rock mass for different anchor bolt lengths. From Figure 6(a-1), when the anchor bolt length and spacing are 1.4 m and 1.0 m × 1.0 m, respectively, the maximum horizontal displacement deformation of the model is 32 mm, which is 28 mm less than that of the unsupported condition. When the anchor bolt length is decreased to 1.2 m, as shown in Figure 6(b,b-1), the maximum horizontal displacement deformation is 34 mm. Apparently, the maximum horizontal displacement deformation of the model decreases with decreasing anchor bolt length.

Figure 7 displays the calculated stress field of the anchor bolt, the mesh, and the rock mass for different anchor bolt lengths. Tensile stress is identified as positive values. From Figure 7a, when the support has an anchor bolt length of 1.4 m and a spacing of 1.0 m × 1.0 m, the axial stress exposed on the anchor is a tensile stress, and the maximum tensile stress is 134.2 MPa, revealing that the anchor adds a certain amount of suspension to the support in the perimeter rock. From Figure 7(a-1), it can be seen that compared with the situation without support in Figure 5a, which is subjected to tensile stress (the maximum tensile stress value is 0.55 MPa), there is no tensile stress in the left wall part of the three-centered arch stope. The top plate of the stope and the right wall are subjected to part of the tensile stress, and the maximum tensile stress ranges from 0 to 1.11 MPa. When the length of the anchor bolt decreases from 1.4 m to 1.2 m, the axial stress acting on the anchor is a tensile stress, and the maximum tensile stress is 151.3 MPa, as shown in Figure 7b. Obviously, the anchor adds a certain amount of suspension to the support in the perimeter rock. In addition, there is no tensile stress in the left wall part of the three-center arch stope, and part of the roof and right wall of the stope are subjected to tensile stresses ranging from 0 to 2.40 MPa, as shown in Figure 7(b-1).

3.3. Effect of the Anchor Bolt Spacing

Compared with the findings presented in Figure 6(a,a-1), when the anchor bolt length remains 1 m while the anchor bolt spacing increases from 1.0 m × 1.0 m to 1.2 m × 1.2 m, the maximum horizontal displacement deformation of the model is 37 mm, as shown in Figure 8(a,a-1). Tensile stress acts on the anchor, and the maximum tensile stress is 117 MPa, as shown in Figure 8b. Obviously, there is no tensile stress on the left wall of the three-center arch stope, and part of the roof plate and the right wall of the stope are subjected to tensile stresses ranging from 0 to 2.40 MPa, as shown in Figure 8(b-1).

3.4. Optimization

Based on the above analysis, the trend of surrounding rock deformation under different schemes is shown in Figure 9 and

Table 4. Comparison of the maximum displacement of the sidewalls of the stope under different schemes, where displacement rate (%) is defined as a normalized measure of geometric deformation, representing the magnitude of displacement relative to an original height.

Support Scheme	Anchor Bolt Length	Spacing	Displacement Rate (%)
No support	--	--	1.986
1#	1.0 m	1.0 m × 1.0 m	1.155
2#	1.4 m	1.0 m × 1.0 m	1.064
3#	1.2 m	1.2 m × 1.2 m	1.143
4#	1.2 m	1.2 m × 1.2 m	1.221

. When downward approach mining drift is adopted, the maximum horizontal displacement is 60 mm without support, and the deformation of the surrounding rock after support decreases by nearly 46.0%. Thus, the flexible mesh support has the obvious effect of controlling the deformation of the surrounding rock. However, the spacing between the anchor bolts and the length of anchor bolts do not significantly change the effect of the support in the peripheral rock. For example, when comparing the strongest support (2# with an anchor bolt length of 1.4 m and a spacing of 1.0 m × 1.0 m) to the weakest support (4# with an anchor bolt length of 1.0 m and a spacing of 1.2 m× 1.2 m), control of the peripheral rock is only improved by 5 mm. It can be safely concluded that all schemes can meet the support requirements after support. Moreover, 2# (anchor bolt length 1.4 m, anchor bolt spacing of 1.0 × 1.0 m) can also better meet the support requirements from an economic and safety point of view.

3.5. Industrial Tests

The theoretical feasibility of the flexible mesh support scheme was demonstrated by numerical simulation; the 2# condition with an anchor bolt length of 1.4 m and a spacing of 1.0 × 1.0 m was selected for industrial tests in downward approach mining drift, while the flexible mesh had a length of 8 m and a width of 2 m.

The optimized flexible mesh support replacing the traditional anchor steel mesh support after testing is shown in Figure 10. The industrial test results show that the longest test drift is more than 35 m, and the unfilled time of the mining drift is more than 30 days. The implemented support system effectively ensured sidewall stability and successfully intercepted dislodged rock blocks, which also validates the accuracy of the above established models. Moreover, the flexible mesh support technology is conducive to improving construction efficiency and reducing construction costs. Firstly, due to the large area of the one-time flexible mesh support, the indirect head of the mesh is smaller. Secondly, the anchor bolt length is reduced from 1.8 m to 1.4 m; each support can save 6 working hours, and the efficiency of the construction anchors is improved, thus significantly improving the construction efficiency of the whole support operation. Moreover, the cost of the flexible mesh support is 213.26 yuan per meter compared with 327.20 yuan per meter for the traditional support, thereby decreasing the cost by 113.94 yuan per meter. Hence, the cost is reduced by 34.82%. These findings reveal that the application of the flexible mesh support technology in downward approach mining drift in the industrial tests was successful.

4. Conclusions

In order to prevent roof caving and rib spalling in deep underground mining operations, as well as to optimize conservation and the substantial costs incurred by downward approach mining drift filling mining techniques, a novel flexible mesh support schematic was designed and introduced in downward approach mining drift.

(1) Based on extensive field investigations and sampling experiments conducted within the mining region, a flexible mesh support scheme has been meticulously designed, and its feasibility has been synchronously validated through numerical simulations in terms of displacement and stress distribution. Subsequently, industrial-scale tests of the support system were undertaken. The results show that the flexible mesh support technology applied to the side support of the downward approach mining drift can effectively ensure the stability of the side during the mining period.

(2) Based on extensive field investigations and sampling experiments conducted within the mining region, material models and contact models for defining the complex hidden joint structure in a metal mine were established and developed to simulate the stress distributions of the designed flexible mesh support. The deformation and failure behavior of the flexible mesh support under the effect of high ground stress were investigated and the feasibility of the mesh was evaluated. The results show that the flexible mesh support system exhibits a distinct control effect on the deformation of perimeter rock.

(3) The maximum horizontal displacement is 60 mm without support, and the perimeter rock deformation decreases by nearly 46.0% after applying the support, revealing the efficiency of the flexible mesh support effect on controlling perimeter rock deformation. Considering safety and economic benefits, 2# with an anchor bolt length of 1.4 m and a row spacing of 1.0 × 1.0 m can better meet the support requirements. An optimized flexible mesh support scheme for downward approach mining drift was obtained and confirmed by the industrial tests.

(4) The simulation and the industrial test results demonstrate that the flexible mesh support technology for downward approach mining drift not only enhances the safety of downward mining operations, but also significantly improves construction efficiency and reduces costs.