Selection of an Optimum Anchoring Method of Composite Rock Stratum Based on Anchor Bolt Support Prestress Field

Abstract

In order to make the anchor bolt support prestress field fully diffuse in the composite rock stratum, improve the overall bearing capacity of surrounding rock, and give full play to the role of active support of the anchor bolt, a self-made 1:1-scale composite rock stratum similarity simulation test bed was used to compare and analyze the distribution of the anchor bolt support prestress field using different anchoring surrounding rock lithology and anchorage lengths, and the principle for optimum selection of anchoring parameters of composite rock stratum was proposed based on the test results. Considered from the point of view of stress diffusion, the effect of prestress diffusion of end anchorage bolts is better than that of lengthening anchorage; at the same time, the anchorage section should be preferentially arranged in hard rock, and the area of anchorage section near the free section should avoid the structural plane of surrounding rock. In conclusion, an industrial test was carried out under the conditions of a deep composite roof of the 2# coal seam in Qinyuan Mining Area, which determined a reasonable anchoring method and position of the composite roof under different conditions and achieved good results.

1. Introduction

At present, anchor bolt support has become recognized as the most effective way to achieve safety in roadway supports [1]. Through the anchor bolt support, the shallow surrounding rock stratum can form a stable bearing structure to resist deep deformation and ensure the stability of the roadway [2,3]. Prestress is the most important and core factor of anchor bolt support, and this has become the consensus of researchers.

There is much research on the prestress diffusion of anchor bolts in China. On the basis of analyzing the existing problems of anchor bolt support in complex and difficult roadways, Kang Hongpu et al. [4] discussed the importance of prestress in supporting effectiveness, putting forward the concept of an anchoring bolt support stress field [5,6,7,8], and carrying out simulation research and tests on the anchor bolt support stress field in their laboratory [9]. Lin Jian et al. [10] studied the support stress field characteristics of single bolts under different working loads and the distribution characteristics of the end anchor bolt support prestress field through laboratory similar simulations and developed the high-efficiency antifriction gasket to improve the pretightening force of anchor bolts. Fan Mingjian [11] studied the relationship between anchor bolt prestress and roadway support effect and pointed out that roadway deformation decreases with the increase in anchor bolt prestress. Shi Yao [12] simulated and studied the influence of different composite components on the bolt support stress field through a large-scale similar material test bed, obtained the influence law of different bolt combination components on the diffusion effect of the bolt support prestress field, and presented the order of advantages and disadvantages. Wang Xiaoqing et al. [13] studied the support effect of bond stiffness on prestressed anchor bolts through numerical simulation. Wang Qiang et al. [14] proposed a reasonable range of bolt pretightening torque for commonly used anchor construction pretightening machines in coal mines, effectively eliminating the phenomenon of bolt breakage. Zhang Wenlong [15] analyzed the deformation and failure characteristics of fractured roofs under the influence of bedding and proposed that high prestressed anchor rods have a good effect on supporting fractured roof tunnels with deteriorating bedding. Yang Yongliang et al. [16] explored the real-time monitoring of the load status of service anchor bolts in tunnels under the influence of high-intensity mining and analyzed the real-time safety status of tunnel anchor bolts based on on-site monitoring data.

The above research results have played a significant role in driving the large-scale promotion of bolt support in China. As most of the coal measure strata are sedimentary rocks and laminar strata, composite rock strata exist widely in the underground surrounding rocks of coal mines and are common in the roof structure of roadways [17]. The common performance of such roadways is a weak interlayer roof, soft–hard intercalated roof, and so on. Due to the large difference in physical and mechanical parameters among the layers and the low cohesion between layers, it is easy for deflection and subsidence to occur after mining, which leads to mutual separation and falling off between strata, and even further causes roof fall accidents. In view of the common composite rock strata in the mine, the thickness and lithology of each layer vary greatly under different conditions. However, the anchoring parameters currently used in the design of bolt supports are relatively singular, and the anchoring parameters used in different situations of composite roofs are basically the same. Therefore, this paper hopes to optimize the anchorage parameters for a composite rock stratum by studying the distribution law of the prestress field of anchor bolt supports under different rock lithologies and anchorage lengths from the perspective of the prestress field of anchor bolt supports.

2. Establishment of a Similarity Simulation Test Bed of a Composite Rock Stratum and Test Scheme

As the physical model is a real physical entity, the similarity simulation test can accurately reflect the field situation of underground engineering and give more intuitive test results under the condition of basically meeting the similarity principle. With the further development of similar materials and the progress of monitoring methods, similarity simulation testing has become an indispensable research method for geotechnical engineering problems, especially for underground engineering problems. The self-developed stress field test bed of anchor bolt supports is used for the purpose of the test. The geometric parameters of the test bed are length × width × height = 3000 mm × 2000 mm × 2000 mm. Four anchor bolts can be arranged in the middle of the test bed, with a row spacing of 1000 mm between the anchor bolts, and the distance between each bolt and the boundary is 500 mm. The materials used in this simulation experiment are all the longest support material models used underground in major mining areas in China. The diameter of the anchor rod is 22 mm, and the material is MG-500 (Yongqing Branch, CCTEG Coal Mining Research Institute, Langfang, China), with a yield strength of no less than 500 MPa. The tray specification is 150 mm × 150 mm × 10 mm. The test bed for composite rock stratum is divided into two parts with the same volume but different mechanical properties. Among them, the compressive strength σ_hard = 2.65 σ_soft (σ_soft = 16.56 MPa, σ_hard = 43.56 MPa); the elastic modulus is E_hard = 3.51 E_soft (E_soft = 3.27 GPa, E_hard = 11.47 GPa); and the hard and soft interface is placed in the middle of the model. The layout of the composite rock stratum test bed and sensors is shown in Figure 1 and Figure 2. The data acquisition system consists of two parts: one is the stress acquisition system which applies pretightening force to the bolt, and the other is the stress acquisition system inside the test bed. The pretightening force of anchor bolt is collected by ZHC-35 load sensor (Dandong Electronic Instrument Factory, Dandong, China), and the internal stress of the test bed is collected by the XYJ-2 vibrating wire sensor (Dandong Electronic Instrument Factory, Dandong, China). The technical indicators of embedded strain sensors are shown in

Table 1. Technical index of embedded strain sensor.

Model	Working Length/mm	Flange Diameter/mm	Strain/με	Resolution	Non-Repeatability	Nonlinear	Comprehensive Error	Temperature Range
XYJ-2	150	φ19.5	3000	<2%	<0.5%	<0.5%	<2%	−20~80 °C

.

In order to better simulate the actual working state of the mine, the most commonly used anchor bolt in the construction site is the left-screwing threaded steel anchor bolt without longitudinal rib, which has a diameter of 22 mm and a length of 2400 mm. The W steel guard plate and metal mesh commonly used in underground works are used as surface protection components. Two anchor bolts are installed on the stress field test bed of anchor bolt support. Under the condition of applying 400 N·m pretightening torque, the stress field distribution of anchor bolt support is tested for end anchorage and lengthening anchorage, and the influence of the length of the anchoring section on the prestress field of the anchor bolt support is explored. One MSZ2335 (Shanxi Qinxin Coal Industry Co., Ltd., Changzhi, China) resin anchoring cartridge is used at the end, and the anchoring length is 445.1 mm. A two-speed resin anchoring cartridge is used in lengthening anchorage. The anchoring cartridge is composed of one MSCK2340 and one MSZ2360 (Shanxi Qinxin Coal Industry Co., Ltd., Changzhi, China). The anchorage length is 1271.7 mm. At the same time, the distribution of prestress field of anchor bolt support in soft rock and hard rock in the anchoring section is tested under the condition of end anchorage, and the influence of anchoring surrounding rock lithology on the prestress field of bolt support is explored. The specific test scheme is shown in

Table 2. Test scheme.

Scheme No.	Crossing Direction of Anchor Bolt	Anchoring Length	Test Variables
1	Soft-hard	445.1 mm	/
2	Hard-soft	445.1 mm	Properties of anchoring surrounding rock
3	Soft-hard	1271.7 mm	Anchoring length

.

3. Test Scheme and Result Analysis

The measured prestress of bolt support is visually displayed using the drawing software Surfer 8.0, which facilitates better analysis of the distribution law of prestress field in composite rock layer anchor rod support. The prestress field of anchoring bolt support in hard rock is shown in Figure 3; the prestress field of anchoring bolt support in soft rock is shown in Figure 4. By comparing and analyzing the compressive stress area (Y = 1000~2100 mm) near the anchorage section of the two bolts, it can be seen from Figure 3 and Figure 4 that this range includes the structural plane at the junction of soft and hard rocks. It can be seen that when the anchorage section is in hard rock, the compressive stress concentration degree and diffusion range are higher than that of anchorage section in soft rock, and its stress distribution along the direction of stress diffusion is wider than that of soft rock. The comparison of compressive stress area and tensile stress area at the upper end of the free section is shown in

Table 3. Comparison of compressive stress area at the upper end of free section.

Properties of Anchoring Surrounding Rock	Hard	Soft	Properties of Anchoring Surrounding Rock
Peak value of compressive stress/MPa	−0.35	−0.2	Peak value of compressive stress/MPa
−0.2 MPa range area/m2	0.26 × 2	0.021 × 2	−0.2 MPa range area/m2
−0.15 MPa range area/m2	0.42 × 2	0.13 × 2	−0.15 MPa range area/m2

.

The stress distribution of surrounding rock near the structural plane is shown in Figure 5. When the anchorage section is in hard rock, the stress values at the soft and hard sides of the structural plane are quite different, and the stress value of the hard rock side is significantly higher than that of the soft rock side. The compressive stress near the structural plane decreases rapidly along the anchor bolt diffusion direction. Taking the compressive stress at the X = 125 mm location at the hard rock side of the structural plane as the benchmark, the compressive stress at 250 mm, 375 mm, and 500 mm locations from the bolt body decreases by 27.1%, 50.4% and 73.9%, respectively, showing a law of linear attenuation. Taking the stress at the x = 125 mm location at the soft rock side as the benchmark, the compressive stress at 250 mm, 375 mm and 500 mm locations from the bolt body decreases by 26.5%, 39.4% and 46.9%, respectively, and the reduction amplitude decreases rapidly near the bolt and slowly far from the bolt, showing a law of exponential attenuation. Considering the diffusion range of compressive stress, the anchoring effect of the anchoring section in hard rock is better than that in soft rock.

This phenomenon can be analyzed by using the R. Mindlin [18] stress solution of a concentrated force acting in the interior of the foundation in elastic space problems. Assuming that the distance from a location in the anchorage section to the surrounding rock surface is $\omega$, the micro element with the length of $d\omega$ is taken at this position, and the corresponding concentration force is $dp=2\pi b\tau (\omega )d\omega$. In the formula, $b$ is the radius of the anchor rod body, and $\tau$ is the anchorage force between the anchor rod and the surrounding rock. When the anchoring length along $\left(z_0,z_0+L\right)$ in the anchoring section is $L$, the vertical stress generated by the shear stress load at the interface of the anchor body at any internal point $\left(x,y,z\right)$ is

$${\sigma }_{\mathrm{z}}={\displaystyle \frac{kdf}{8(1-\mu )}}$$

(1)

where μ is the Poisson’s ratio of the surrounding rock, $f$ is the parameter relating to coordinate location, $d$ is the diameter of anchoring bolt, and $k$ is the Shear stiffness of interaction interface between bolt and surrounding rock. According to the research on the interfacial shear stiffness of the bolt [19,20], the interfacial shear stiffness of the bolt can be determined by the following formula:

$$k={\displaystyle \frac{G_{\mathrm{g}}{G}_{\mathrm{m}}}{[G_m\ln (r_g/r_b)+G_g\ln (R/r_g)]r_b}}$$

(2)

where $G_g$ and $G_m$ represent the shear modulus of anchoring cartridge and surrounding rock, $r_b$ is the radius of the anchor rod, $r_g$ is the radius of the anchor body, and $R$ is the radius of the influence of the anchor rod. It can be seen from the formula that the higher the shear modulus of anchoring cartridge and surrounding rock is, the larger the shear stiffness is.

According to the above two formulas, the vertical stress generated by the anchoring force in the surrounding rock is affected by the shear stiffness of the anchoring interface. Due to the high shear stiffness (high shear modulus) of the hard rock, the stress values of its anchoring force acting on each point in the surrounding rock are higher than those in the soft rock.

The prestress field of lengthening anchor bolt support is shown in Figure 6. The comparison of Figure 3 and Figure 6 yields the change of surrounding rock stress near the bolt body along the bolt axis (shown in Figure 7). In the front area of bolt action (Y = 0~1050 mm), the overall stress size and change trend of the two are basically consistent. In the middle and rear parts of the bolt action (Y > 1050 mm), the stress distribution of the end anchorage and the lengthening anchorage is obviously different. The surrounding rock of the lengthening anchorage is basically in the range of tensile stress in the middle and rear parts of the bolt, and the tensile stress increases first and then decreases slowly along the bolt body. The surrounding rock of the end anchorage shows a high compressive stress effect in the middle of the bolt and is only tensioned at the rear part of anchoring bolt, but the peak value of the tensile stress zone is higher than that of the lengthening anchorage and decreases rapidly along the bolt body.

The stress diffusion of surrounding rock at the hard rock side and the soft rock side of the soft–hard interface is analyzed. Figure 8 shows the variation law of stress near the structural plane along its diffusion radius. The stress properties of surrounding rock near the structural plane of end anchorage and lengthening anchorage are different. In the case of end anchorage, both the hard rock side and the soft rock side of the structural plane are subject to compressive stress; the compressive stress gradually decreases along the stress diffusion direction (X direction), and the surrounding rock pressure on the hard rock side is generally higher than that on the soft rock side. In the case of lengthening anchorage, both sides of the structural plane are subject to tensile stress, the tensile stress gradually decreases along the stress diffusion direction (X direction), and the tensile stress value of the soft rock side of the structural plane is higher than that of the hard rock side. According to the stress diffusion effect in the composite rock stratum and the structural plane position and anchorage length designed by the model, the prestress diffusion effect of end anchorage is obviously better than that of lengthening anchorage.

From the perspective of stress diffusion effect in composite rock layers, the prestress diffusion effect of end anchorage is significantly better than that of lengthening anchorage. When anchoring at the end, a compressive stress distribution zone is formed near the structural plane, which can effectively suppress the phenomenon of delamination and collapse in weakly bonded structural planes. However, when the anchorage is lengthened, the stress of the surrounding rock at the position of the structural plane is manifested as tensile stress, which is not conducive to the stability of the structural plane from the perspective of prestressing alone.

4. Selection Principle of the Optimum Anchoring Method for a Composite Rock Stratum

Based on the above comparison’s conclusions about the different anchoring surrounding rock lithology and prestress field of anchor bolt support under different anchoring methods for the composite rock stratum, it can provide a certain reference for underground field support.

4.1. Reasonable Control of Anchoring Length

In the composite rock stratum, the effect of the pretightening force of the anchor bolt is mainly reflected in the restraint ability of the surrounding rock in the free section. In the anchorage section, the restraint effect of pretightening force of the anchor bolt on the surrounding rock is relatively limited, so it is difficult to generate a certain range of high compressive stress near the anchorage section, especially in the deep part of the anchorage section. Therefore, shortening the anchorage length properly and leaving a certain length of free section are conducive to giving the full effect of pretightening force in the surrounding rock. However, if the length of the anchorage section is too small, it is not conducive to the transmission of compressive stress in the axial direction of anchor bolt; meanwhile, the reliability of anchorage will be reduced. The calculation of anchorage length is based on the following formula:

$$L={\displaystyle \frac{D^2\times L_1}{D_1^2-D_2^2}}$$

(3)

In the formula,

L—anchoring length of anchoring agent, cm;

D—anchoring agent diameter, mm;

L₁—anchoring agent length, cm;

D₁—drilling diameter, mm;

D₂—anchor rod diameter, mm.

By using this formula, the anchorage length of any anchor rod can be quickly calculated. By combining rock strength testing and rock observation, a comprehensive analysis of the rock properties can be conducted to further optimize the anchorage parameters based on the rock properties.

4.2. Influence of Tensile Stress of Surrounding Rock in the Anchorage Section on Composite Rock Stratum

Based on the analysis of the prestress field of the anchor bolt, at the end position of the side where the pretightening force is applied in the anchoring section of the bolt, the surrounding rock close to the bolt body will be subject to high tensile stress due to the effect of the anchoring force of the bolt on the surrounding rock. In general rock strata, the tensile stress will not have a great impact on the surrounding rock control, but for the composite rock stratum, if the structural plane is located in the tensile stress concentration area under the action of high tension, a separation layer is likely to occur at the structural plane because of the low cohesive force between the rock strata, which is not conducive to the overall control of the composite rock strata. Therefore, the relative position between the anchorage section of the anchor bolt and the structural plane should be reasonably determined. If the effect of horizontal stress on the surrounding rock is not considered, the structural plane can be in the range of high compressive stress by shortening the anchorage length; if the influence of horizontal stress on the interlayer dislocation of composite rock stratum is considered, and the anchorage section is needed to improve the overall shear capacity of the bolt, the anchorage length should be appropriately extended to make the structural surface located in the middle and rear part of the anchorage section so as to avoid the area of concentrated tensile stress and reduce the influence of tensile stress on the structural plane.

4.3. Selection of Lithology of Surrounding Rock for Anchoring in the Composite Rock Stratum

In the composite rock stratum, the anchorage section should be arranged in hard rock as a priority, which is beneficial to the enhancement and diffusion of the stress peak value in the compressive stress zone in the free section of the free section of the bolt body, and to a certain extent, increases the restraint ability of the free section to the surrounding rock, and further improves the surrounding rock control function of the free section area.

5. Field Industrial Test

The main mined No.2 coal seam in Qinyuan Mining Area is a composite roof with large burial depth and severe mine pressure. The deformation of roadways affected by dynamic pressure is difficult to control. The average maintenance frequency of the roadways subject to dynamic pressure is 2–3 times during the service period.

Through the in situ test of geomechanical parameters in Qinyuan Mining Area, the distribution law of crustal stress in the mining area and the strength structure characteristics of surrounding rock are analyzed. Through the overburden structure detection and strength testing of the roof of No.2 coal seam in the mining area, it is concluded that the composite roof of No.2 coal seam in Qinyuan Mining Area has two composite modes in the anchor bolt control area, as shown in Figure 9, namely, a “soft intercalated” composite roof and a “hard intercalated” composite roof. The roof is supported by an anchor bolt and cables, the length of the bolt is 2400 mm, and a W steel panel is used with a steel joist.

For the hard intercalated roof, 0~0.9 m above the roof is mudstone with an average strength of 21 MPa, 0.9~1.8 m above the roof is fine-grained sandstone with an average strength of 57 MPa, and 1.8 m above the roof is mudstone with an average strength of 29 MPa, which are distributed in the form of “soft–hard–soft” along the direction of anchor bolt. According to the research results, the anchorage section should be located in the hard rock, which can ensure the anchoring force of the bolt and increase the prestress diffusion range of the anchor bolt. Moreover, the lower part of anchorage section should avoid the structural plane of the surrounding rock. The drilling length of anchor bolt is 2300 mm, and the anchorage section should be more than half of the range of fine sandstone. Therefore, the anchorage length should be 1000 mm−1400 mm. MSK2335 and a MSZ2360 anchoring cartridges are used for anchoring according to the design, and the anchoring length is 1210 mm.

For the soft intercalated roof, 0~1.3 m above the roof is mudstone with an average strength of 29 MPa, 1.3~1.8 m above the roof is coal with an average strength of 13 MPa, and 1.8 m above the roof is mudstone with an average strength of 34 MPa. Along the direction of the bolt body, it presents a “hard–soft–hard” distribution. Because the roof has a low overall strength, the anchoring parameters are optimized according to the dynamic pressure of the roadway.

5.1. Roadway with Solid Coal on Both Sides

For the solid coal roadway, appropriately reducing the anchorage length is conducive to the diffusion of the anchor bolt prestress field according to the research results under the condition that the roadway is less affected by dynamic pressure. Therefore, the end anchorage is adopted, the end position of the anchorage section should be located above the soft interlayer, and the anchorage length should be controlled within 500 mm. Therefore, a MSK2335 anchoring cartridge is used, and the anchoring length is 445 mm.

5.2. Roadway Affected by Secondary Dynamic Pressure

These kinds of roadways have two characteristics: weak and dynamic pressure. First of all, the overall strength of rock strata in the soft intercalated roof is low, as it is composed of mudstone and coal line. Secondly, the roadway is not only affected by the mining of this working face but also affected by the recovery of adjacent working face, so it is very difficult to support. This is the most problematic type of roadway in Qinxin mining area. For the secondary dynamic pressure roadway with this kind of roof, the influence of the horizontal stress of the composite rock strata should be fully considered. Under the influence of dynamic pressure and horizontal stress, the anchor bolt in composite rock is generally subject to high stress and is prone to horizontal shear fracture. Although the end anchoring is conducive to the diffusion of anchor bolt prestress, the anchoring force can easily fail to meet requirements under high-stress conditions, resulting in bolt support failure. Moreover, the anchorage section can improve the shear stiffness of the bolt, which is beneficial to alleviating the surrounding rock dislocation of the structural plane under horizontal stress. Therefore, combined with the anchoring conclusion, the anchorage length should be appropriately lengthened in this case. Under the condition that the anchor length is long and the anchoring reliability is guaranteed, the front section of the anchorage section should avoid the structural plane of the surrounding rock. According to the roof structure, the anchorage length should be controlled in the range of 1400–1600 mm. MSCK2340 + MSZ2380 double-speed anchoring cartridges are used in the design, and the anchorage length is 1520 mm.

Roadway support and mine pressure monitoring are shown in Figure 10 and Figure 11. According to the stress situation of the anchor bolt, the initial pretightening force of the bolt is within the range of 80~92 kN, which basically meets the design requirements; after applying high pretightening force to the bolt, the stress is basically stable with a very small change. The maximum stress of the bolt is no more than 168 kN, reaching 88% of the yield strength of the bolt, which is within the reasonable range of the bolt stress. The roadway deformation increases gradually with the increase in driving distance. Within 50 m from the heading face, the increase in amplitude is relatively large, the average increment of the two sides is 7 mm/d, and the average increment in the roof and floor is about 6 mm/d. The deformation tends to be stable with the increasing distance away from the heading face. The maximum convergence of roof and floor is 35 mm, the maximum convergence of the two sides is 38 mm, and the maximum convergence of the left side is 21 mm. Through analyzing the observation curve of roadway surface displacement, it can be seen that the overall deformation during crossheading is small, which indicates that the deformation of roof and surrounding rock at both sides is controlled in the initial stage, and the supporting measures are effective.

6. Discussion

This article mainly studies events under the condition of zero geostress, without considering the interaction between the actual original rock stress field, mining stress field, and support stress field underground. The next step is to combine the hydraulic loading system of the laboratory test bench to further explore the distribution law of the prestress field of anchor rods under confining pressure and to study the in situ testing method of the stress field of anchor rod support underground in order to obtain the stress situation of the surrounding rock under the interaction of the three fields underground and provide a strong basis for the quantitative design of on-site support in coal mines.

7. Conclusions

Through the simulation test bed of a composite rock stratum, the distribution law of the prestress field of bolt support under different surrounding rock lithology and anchorage lengths is obtained, and the optimization of the anchoring method and anchoring position is proposed. The main conclusions are as follows.

(1) The main action area of anchor bolt prestress on surrounding rock is in the range of the free section of bolt body. Shortening the anchorage length properly and setting a certain length of free section is conducive to the full effect of pretightening in the surrounding rock. However, if the length of anchoring section is too small, it is not conducive to the transmission of compressive stress in the axial direction of the anchor bolt, and the reliability of anchoring will also be reduced.

(2) In the composite rock stratum, the anchorage section should be arranged in hard rock as a priority, which is beneficial to the enhancement and diffusion of the stress peak value in the upper compressive stress zone of the free section of the bolt body and, to a certain extent, increases the restraint ability of the free section on the surrounding rock, further improving the surrounding rock’s control of the free section area.

(3) The relative position between the anchorage section of the anchor bolt and the structural plane should be reasonably controlled in the composite rock stratum. If the effect of horizontal stress on the surrounding rock is not considered, the structural plane can be in the range of high compressive stress by shortening the anchorage length; if the influence of horizontal stress on the interlayer dislocation of the composite rock stratum is considered and the anchorage section is needed to improve the overall shear capacity of the bolt, the anchorage length should be appropriately extended to ensure that the structural surface is located at the middle and back of the anchorage section so as to avoid the area of concentrated tensile stress and reduce the influence of tensile stress on the structural plane.

(4) These conclusions apply to the “hard intercalated” and “soft intercalated” composite roofs in Qinyuan Mining Area; they may better ensure the stress state of the bolt and effectively control the deformation of the roadway, achieving good results.