Study on Asymmetric Support of Anchor Cable with C-Shaped Tube in Inclined Coal Seam Roadway

Abstract

In view of the complex asymmetric deformation characteristics of inclined coal seam roadways and the tensed shear failure of anchor cable supports, the asymmetric support scheme of an anchor cable with a C-shaped tube is proposed. In order to study its supporting effect on an inclined coal seam roadway, this paper first explores the difference in shear performance between an anchor cable with a C-shaped tube and an anchor cable through double shear tests. Then, based on the asymmetric deformation characteristics of an inclined coal seam roadway in the Pangpangta Mine, a numerical simulation is used to study the asymmetric support effect of an anchor cable with a C-shaped tube in an inclined coal seam roadway. The results of the double shear test show that the anchor cable with the C-shaped tube has stronger resistance to shear load than that of the anchor cable. Through the results of the numerical simulation, the original stress field distribution on both sides of the roadway was found to be uneven due to the influence of the coal seam dip angle, and after the excavation of the inclined coal seam roadway, the displacement and plastic zone distribution on both sides showed obvious asymmetric characteristics. Compared with the symmetric support, the asymmetric support can obviously alleviate the asymmetric deformation characteristics of the two sides and effectively control the deformation and plastic failure zone of the roadway. The anchor cable with the C-shaped tube has better resistance to shear deformation than that of the anchor cable. The anchor cable with the C-shaped tube can reduce the deformation and plastic area of the roadway more effectively.

1. Introduction

The coal resource is one of the indispensable basic energies in our social development and economic construction [1,2,3]. At present, China has proven to have large reserves of inclined coal seams, which have a very high mining value [4,5]. However, due to the influence of underground inclination and angle, the deformation features of inclined coal seam roadways are more complicated, which brings great difficulties to roadway shoring [6,7,8].

The support of inclined coal seam roadway research is now making significant strides both domestically and internationally. Rong et al. [9] proposed measures for controlling the stability of the surrounding rock in severely inclined and ultra-thick coal seams. They suggested that full-length anchoring should be implemented in the support of inclined coal seam tunnels, increasing the anchoring length of the anchor cables to enhance the integrity of the roof. Wu et al. [10] investigated the stress distribution mechanism of rock bolts in steeply inclined rock formations and suggested that longer rock bolts should be used during support to suppress further deformation of the steeply inclined rock formations. Xiong et al. [11,12,13] analyzed the deformation law of a tunnel in an inclined coal seam through laboratory tests and a numerical simulation and proposed the cyclic deformation and failure theory of a roadway. In light of the rock deformation issue along goaf coal roadways in an inclined coal seam, Liu et al. [14] proposed using a combination of an anchor cable, shotcrete, and U-shaped steel for support. The study conducted by Das et al. [15,16,17,18] investigated the influence of different inclinations on the stability of inclined coal seam roadways. Through theoretical analysis, a numerical simulation, and field experiments, they explored the failure mechanism of inclined coal pillars, providing a significant reference value for the safe mining of inclined coal seams. Through the use of finite element analysis software, Jiang et al. [19] discovered that the combination of several forms of anchor cable reinforcement can increase the stability of the rock around them in inclined coal seams.

Anchor cables are one of the most common methods used to reinforce cracked and dynamically loaded rock masses due to their flexibility and strength parameters. Due to the complex deformation characteristics of the surrounding rock in inclined coal seam roadways, anchor cables play a crucial role in the design of support systems for these roadways. However, studies [20,21,22] show that when the roadway is driven, the surrounding rock will shift and slip around the joint surface; at the coal roadway support site, tensed shear failure occurs in some anchor cables, mostly in the free section, affecting the stability of the roadway. There are many research studies on the shear performance of anchor cables at home and abroad. Tahmasebinia et al. [23,24,25] conducted a study using the finite element software ABAQUS/Explicit to establish static and dynamic double shear test models. They investigated the effects of bolt diameter, yield strength of the steel material, loading rate, and dynamic load mass on the shear force and energy absorption of the cable bolts. Mirzaghorbanali et al. [26] developed a frictionless double shear test apparatus for concrete and studied the effect of pre-tensioning on the shear strength of different types of anchor cables. Li et al. [27] compared the contribution of fiber glass bolts, rock bolts, and anchor cables to the shear strength of the concrete surface and analyzed the failure modes of the different bolts. Wang et al. [28] conducted a series of single shear tests to analyze the tensile, bending, and shear characteristics of anchor cables. They also developed an anchor cable model using the finite element software ABAQUS, accurately simulating the interaction and failure modes between the anchor cable and the grouting body. Li et al. [29] investigated the mechanical properties of Sumo and TG cable bolts under constant normal stiffness through pull-out tests. They also developed an analytical model that effectively predicts the load displacement behavior of anchor cables. Spang et al. [30] conducted shear tests on anchor bolts and found that the shear resistance of the anchor bolts is influenced by the strength of the surrounding rock. Aziz et al. [31,32,33] claim that single and double shear beams are available to evaluate the anchor cable’s shear ability and that the anchor cable’s shear strength is correlated with its preload and installation angle.

It is evident that there have been many advances in the study of inclined coal seam roadway supporting techniques, but the asymmetric deformation properties of the inclined coal seam roadway’s surrounding rock are complicated, and there is a dearth of research on its asymmetric deformation process. Because of the inadequate shear strength of its free portion when an anchor cable support is being used, it cannot play a stronger supporting role. However, the anchor cable with a C-shaped tube (hereinafter referred to as the “ACC”) developed by Shan et al. [34,35,36] has the characteristics of high prestress and high shear performance, and is a new type of mining supporting component material. Currently, the ACC has been applied in the engineering field, but there is little research on its mechanical properties and the numerical simulation of its engineering applications. Therefore, studying the support effect of the ACC on inclined coal seam roadways has important reference value for the selection of support materials for similar roadways. The difference in shear performance between the ACC and an anchor cable is first investigated in this paper using a double shear test. Then, based on the background of the 9-8032 roadway of an inclined coal seam in the Pangpangta Mine, the asymmetric deformation characteristics are analyzed based on a numerical simulation test. The supporting impact of the asymmetric support of the ACC on the roadway of an inclined coal seam is investigated in conjunction with the findings of the double shear test.

2. Research on the Mechanical Characteristics of Anchor Cable with C-Shaped Tube

2.1. An Introduction

An ACC consists of a C-shaped pipe installed at the free end of the prestressed anchor cable. To fully utilize the effectiveness of the C-shaped pipe, it is recommended that the length of the C-shaped pipe match the range of loose surrounding rock in the roadway. Please refer to Figure 1 for an illustrative diagram of the ACC structure.

The C-shaped tube progressively closes beneath the force applied during the ACC support process when the joint plane slides and moves incorrectly, and the anchor cable is then firmly wound. The two form a unified assembly to resist transverse deformation and axial deformation together, which not only increases the transverse bearing ability of anchor cable but also increases its axial bearing ability. Figure 2 depicts the deformation process of the ACC under stress in surrounding rock.

2.2. A Brief Analysis of the Shear Strength of ACC Anchored Joint Plane

When there is relative displacement occurring in the rock mass near a rock joint plane, the ability of the joint plane to resist rock mass movement is known as the shear strength of the joint plane itself. Extensive research has shown that the shear strength of the joint plane follows the Mohr–Coulomb criterion [37].

$$\tau =\sigma \tan \phi +c$$

(1)

where $\tau$ represents the shear strength of the joint plane; $\sigma$ represents the normal stress acting on the joint plane; $\phi$ represents the internal friction angle of the joint plane; and $c$ represents the cohesion strength of the joint plane.

When an ACC is employed as a support system, during sliding displacement along the joint plane, the C-shaped tube undergoes compression, generating radial resistance to counteract the deformation of the joint plane. This resistance continues until the C-shaped tube fully encloses the anchor cable. The resulting circumferential load $q_c$ provided by the C-shaped tube can be determined [38].

$$q_c=\frac{E_c{I}_c}{3\pi r_c^4}u$$

(2)

In the equation, $E_c$ denotes the elastic modulus of the C-shaped tube, $I_c$ represents the moment of inertia of the cross-section of the C-shaped tube, $r_c$ represents the outer diameter of the C-shaped tube, and $u$ represents the closure displacement of the C-shaped tube.

Consequently, the resistance $Q_c$ provided by the C-shaped tube per unit length is

$$Q_c={\displaystyle {\int }_0^{\pi }{q}_c}\sin \theta ·d\theta ={\displaystyle {\int }_0^{\pi }\frac{E_c{I}_c}{3\pi r_c^4}u\sin \theta ·d\theta =\frac{2E_c{I}_c}{3\pi r_c^4}u}$$

(3)

Since the contact area between the rock mass on either side of the joint plane and the C-shaped tube is limited, an influence range coefficient $f$ is introduced to represent the extent of the joint plane’s impact on the C-shaped tube. This allows us to determine the shear strength generated by the closure of the C-shaped tube within this specific influence range.

$${\tau }_c=\frac{2E_c{I}_{c}f}{3\pi r_c^{4}A}u$$

(4)

where $A$ is the area of the joint surface being anchored.

The shear strength of the joint plane with anchor reinforcement consists of three components: the inherent shear strength of the joint plane, the shear strength induced by the “nail” action of the support component, and the shear strength caused by the effect of the axial force of the support component on the joint plane [39,40]. By integrating the aforementioned analysis and incorporating further improvements, the shear strength of the ACC anchored joint plane can be determined.

$${\tau }_{ACC}={\tau }_0+{\tau }_c+{\tau }_{aj}+{\tau }_{az}$$

(5)

$${\tau }_0={\sigma }_0\tan \phi +c$$

(6)

$${\tau }_c=\frac{2E_c{I}_{c}f}{3\pi r_c^{4}A}u$$

(7)

$${\tau }_{aj}=\frac{N_{ACC}}{A}\tan \phi$$

(8)

$${\tau }_{az}=\frac{T_{ACC}}{A}$$

(9)

where $N_{ACC}$ is the normal force applied by the ACC to the joint plane perpendicular and $T_{ACC}$ is the component of the resultant force of the axial force and shear force of the ACC in the direction parallel to the joint plane.

2.3. Preparation for Test

To compare the shearing ability of the prestressed ACC and the prestressed anchor cable under shear load, the self-developed tension shear test equipment was employed in a double shear test. In the test, a 21.8 mm ACC (composed of a 21.8 mm anchor cable and a C-shaped tube having an internal diameter of 24 mm and an exterior diameter of 28 mm) and a 21.8 mm anchor cable commonly used in roadway support were selected. The length of the anchor cable and the embedded anchor cable in the ACC was 1200 mm, while the length of the C-shaped tube was 850 mm. The steel grade of the anchor cables was 82B carbon steel, while the steel grade of the C-shaped tubes was Q345B.

Table 1. Test scheme.

Number	Material Type	Design Prestress (kN)	Actual Prestress (kN)
S1	ACC	100	100
S2	Anchor cable	100	100
S3	ACC	200	223
S4	Anchor cable	200	211

shows the test scheme.

A concrete test block was used as the supporting material in the test, which was 300 mm by 300 mm by 300 mm in dimension, with 32 mm holes set aside in the center. Water was the raw material, and the proportion of cement, fine sand, and gravel was 1:2:4:4. When making each set of concrete test blocks at the same time, small test blocks were reserved with a size of 100 mm by 100 mm by 100 mm for uniaxial compression testing. Once the cured concrete test specimen reached its design strength, it was placed inside a shear box. The ACC was then inserted into prefabricated holes, and an axial force sensor was placed on the exposed portion of the ACC. The ACC was then tensioned using a prestressing jack to achieve the design prestress. Then, the shear box was placed within a vertical loading system, with the side shear boxes fixed using transverse beams while the middle shear box remained suspended. At the beginning of the experiment, the vertical loading system applied a loading rate of 2 mm/min. The internal system was equipped with stress sensors and displacement sensors to record variations in shear load and shear displacement throughout the test. The test concluded when the ACC fully ruptured. The processes of test material assembly and test loading are shown in Figure 3. The findings of the uniaxial compression test revealed that the average strength of the test blocks for the double shear test was 40 MPa.

2.4. Analysis of the Outcomes

Figure 4 depicts the typical ACC shear load/axial force–shear displacement curve.

The curve of “shear load-shear displacement” of ACCs may be separated into three phases, as depicted in Figure 4: rapid-rising stage, slow-rising stage, and stepped-falling stage. There are three phases to the “axial force-shear displacement” curve as well: gentle stage, ascending stage, and stepped-falling stage. Before the test, prestress was applied to the ACC passing through the concrete test blocks, which squeezed each other, and there was a large friction force between the two joint planes. When the test first began, the shear load quickly overcame the friction and rose, and the ACC axial force was in the gentle stage. Then, the shear load rose slowly, the ACC gradually underwent elastic and plastic deformation, and the axial force reached the rising stage. When the shear load and axial force reached their maximums at the same time, the steel strands in the ACC were broken one by one, and then the shear load and axial force entered the step decline stage.

Figure 5 displays the shear load–shear displacement changes of the ACC and anchor cable underneath various prestresses.

Figure 5 illustrates that in the shear process, before the shear load reaches its maximum value, when the displacements of the ACC and the anchor cable under the same level of prestress are identical, the shear load borne by the ACC is higher than that borne by the anchor cable. The peak shear capacities of the ACC with 100 kN of prestress were 19.4% higher than those of the anchor cable with 100 kN of prestress, and with 223 kN of prestress, the ACC’s peak shear capacities were 4.1% higher than those of the anchor cable with 211 kN of prestress. The conclusion is that, under the same circumstances, the ACC has a much stronger shear load resistance capability than the anchor cable.

3. Roadway Deformation Law in Inclined Coal Seam

3.1. Initial Support Scheme

In this paper, the research object is at Pangpangta Mine’s 9-8032 roadway in Shanxi Province, and Figure 6 shows the specific location of the Pangpangta Coal Mine. The roadway segment is a rectangle portion, measuring 4.5 m by 3.6 m in sectional area. The estimated excavation depth of the roadway is 2050 m. The No. 9 coal seam is stable, and the lithology is mostly carbonaceous mudstone with a small amount of local mudstone. The coal seam on the working face has a monocline structure and is 12 m thick. The average seam inclination is 25°, ranging from 17 to 30 degrees, making it an inclined coal seam.

The initial support scheme for the 9-8032 roadway adopts the combined support of rock bolts and anchor cables. During the support process, both anchor cables and rock bolts are employed in an end anchoring configuration. The anchor bolts and anchor cables consist of free sections and anchoring sections, with the anchoring sections being secured using resin anchoring agents. The maximum bearing capacity of the rock bolt is 216 kN, and the maximum bearing capacity of the anchor cable is 550 kN. Six 22 mm by 2400 mm rock bolts are installed in each row of the roadway roof, with a spacing of 800 mm and a row spacing of 800 mm. Each row contains three anchor cables, each measuring 10,300 mm in length and 21.8 mm in diameter, with a spacing of 1500 mm and a row spacing of 2400 mm.

Each row on the side has five rock bolts, each measuring 2000 mm in length and 22 mm in diameter, with a spacing of 800 mm and a row spacing of 800 mm. The prestress of the rock bolts in the roof is 30 kN, and the prestress of the anchor cables ranges from 210 kN to 290 kN. The anchor cables’ prestress in the numerical simulation is 200 kN, taking prestress loss into account. The initial support scheme is displayed in Figure 7.

3.2. Building Numerical Models

Based on the geological conditions of the 9-8032 roadway in the Pangpangta Mine, a numerical model was established using FLAC 3D software. The model has dimensions of 45 m in length, 5 m in width, and 35 m in height, comprising a total of seven rock layers with a dip angle of 25°. The simulated roadway section has a rectangular shape with a width of 4.5 m and a height of 3.6 m. The model is constrained for horizontal movement around its perimeter and fixed at the bottom. The Mohr–Coulomb model was utilized in the numerical simulations.

During the model creation, smaller element sizes were used near the roadway area, gradually increasing the element sizes further away from the roadway. This approach ensures more accurate simulation results while improving computational efficiency. The model consists of a total of 47,310 elements and 53,339 nodes. Considering a roadway depth of 500 m, a vertical load of 12 MPa was applied on the upper surface of the model, with a lateral pressure coefficient of 1 [41]. The numerical model is illustrated in Figure 8.

Table 2. Table of strata parameters.

	Density (kg/m3)	Bulk Modulus (GPa)	Shear Modulus (GPa)	Tensile Strength (MPa)	Cohesion (MPa)	Friction Angle (°)
Coal	1370	1.25	0.49	0.18	1.4	25
Mudstone	2300	1.4	1.5	0.49	0.9	27
Sandy mudstone	2860	1.25	0.76	0.49	0.7	25
Limestone	2690	2.3	1.2	0.62	1.2	29

displays the simulation parameter settings for the roadway’s surrounding rock. Cable elements were employed to simulate both the rock bolts and anchor cables, while a combination of cable and pile elements were employed to simulate the ACCs. The parameters used in the numerical simulations are presented in

Table 3. Pile unit-related parameters.

	Emod (GPa)	Xcarea (mm2)	Per (mm)	Cs_sk (MPa)	Cs_scoh (MN/m)	Cs_nk (MPa)	Cs_ncoh (MN/m)	Cs_sfric (MPa)	Nu
pile	200	4.02	100	13	13	13	0	25	0.3

and

Table 4. Support member parameters.

Type	Anchor Cable	Bolt
Diameter (mm)	21.8	22
Xcaera (mm2)	373	380
Emod (GPa)	200	200
Breaking load (kN)	550	216
Gr-per (mm)	100	100
Gr-k (MN/m/m)	17.5	17.5
Gr_coh (MN/m)	0.44	0.44

.

On both sides of the roadway, a certain number of displacement monitoring stations were placed, and stress monitoring points were placed in some of the same locations. The specific locations and numbers of the measuring points are shown in Figure 9.

3.3. Asymmetric Deformation Law of the Initial Support System

Figure 10 displays the simulation results of roadway deformation under the initial support scheme. It can be seen from Figure 10 that the maximum settlement of the roof is 9.23 cm, and that the majority of the settlement occurs in the middle of the roof. The maximum displacement on the right side is 7.93 cm, and that on the left side is 6.77 cm. It can be seen that the right side has undergone significantly more deformation than the left. In addition, by observing the nephogram of the plastic zone, it can be found that shear and tensile failure occur around the roadway, with mostly shear failure. The plastic zone is large where it shows a tendency to develop along the coal seam, the right and left of which are asymmetrically distributed.

Table 5. Horizontal displacements on both sides of the displacement monitoring points under the initial support scheme.

Distance from Roadway Surface	The Side Is 2.4 m High		The Side Is 1.2 m High
	Right Displacement/cm	Left Displacement/cm	Right Displacement/cm	Left Displacement/cm
0	−7.83	6.2	−7.13	6.59
0.6	−6.85	4.94	−6.33	5.09
1.2	−5.26	3.67	−5.49	3.27
1.8	−3.11	2.57	−3.48	1.98
2.4	−2.5	2.57	−2.81	1.98

displays the horizontal displacement of the monitoring points for roadway displacement under the initial support scheme. Table 3 shows that the horizontal displacement of the monitoring point was gradually decreasing from surface to depth. When compared to the left, the right side has a displacement that is noticeably greater, indicating that the influence caused by roadway excavation gradually waned with increasing separation from the roadway surface, and the two sides of the roadway also clearly exhibited asymmetric deformation. In order to conduct a quantitative study on the asymmetric deformation properties of roadways, the two-side asymmetry degree t is defined in this paper. Considering the displacement direction of the two sides of the roadway, the calculation method is as follows:

$$t=\frac{\left|A+B\right|}{\left|A\right|+\left|B\right|}$$

(10)

where the horizontal displacements of the right side are represented by A and the left side by B.

Underneath the initial support scheme, the variation law of the asymmetry of the displacement monitoring points on either side with respect to the distance from the road surface is shown in Figure 11. Figure 11 illustrates that the degree of asymmetry initially rises and then falls with increasing separation from the roadway surface and that the asymmetry characteristics become quite visible at a specific depth of roadway. In addition, Table 3 and Figure 11 demonstrate that serious deformation and large asymmetry occur at the shallow coal seam on the high side of the roadway, where it should be the key point of support.

Figure 12 shows that the vertical stress increases as one moves away from the roadway surface. The stress distribution on both sides of the shallow seam is uneven, with characteristics of asymmetric stress distribution, specifically manifested as the vertical stress of the right side being greater than that of the left side.

It is revealed that the inclined coal seam’s roadway deformation is considerable, and the plastic zone tends to develop along the coal seam. The two sides clearly exhibit asymmetric deformation characteristics, and when the separation from the roadway surface increases, the degree of asymmetry initially rises and then falls.

4. Design of Anchor Cable with C-Shaped Tube Supporting Reinforcement Based on Asymmetric Deformation of the Roadway

4.1. Brief Analysis of Roadway Deformation

The original condition of stress in coal rock is shattered during roadway excavation, and the stress is redistributed. The tangential stress increases gradually as the radial stress decreases. During the secondary stress distribution process, some surrounding rocks experience concentrated stress. When the concentrated stress exceeds the strength of the surrounding rock, deformation or even failure occurs, which forms a circular or elliptical distribution area consisting of a plastic zone, an elastic zone, and an original stress field zone from the inside out. The stress state of the unit in the surrounding rock of the rectangular roadway is shown in Figure 13.

(1) The rock mass in the elastic zone satisfies Hooke’s law. According to the theory of elastic mechanics, the stress at a point in the elastic zone is given by [42]:

$$\left\{\begin{array}{l}{\sigma }_{\mathrm{r}}=p-[\frac{2p(2\sin \phi -1)+{\sigma }_{\mathrm{c}}(1-\sin \phi )}{2\sin \phi }]·(\frac{R_{\mathrm{p}}^2}{r^2})\\ {\sigma }_{\mathsf{\theta }}=p+[\frac{2p(2\sin \phi -1)+{\sigma }_{\mathrm{c}}(1-\sin \phi )}{2\sin \phi }]·(\frac{R_{\mathrm{p}}^2}{r^2})\end{array}\right.$$

(11)

where ${\sigma }_{\mathrm{r}}$ is the radial stress at the point, ${\sigma }_{\mathsf{\theta }}$ is the tangential stress at the point, $r$ is the distance from the point to the center of the circle, $p$ is the stress before the excavation of the roadway at the point, $\phi$ is the angle of the rock internal friction,$R_p$ is the radius of the plastic zone, and ${\sigma }_{\mathrm{c}}$ is the uniaxial compressive strength of the rock.

The displacement of the surrounding rock mass caused by the excavation of the roadway in the elastic zone is given by

$$u_1=\frac{1+\mu }{2E}·[2p\sin \phi +{\sigma }_{\mathrm{c}}(1-\sin \phi )]-\frac{R_{\mathrm{p}}^2}{r}$$

(12)

(2) Assuming that the rock mass follows the Mohr–Coulomb criterion at yielding, the plastic criterion can be written as

$${\sigma }_{\mathsf{\theta }}=\frac{1+\sin \phi }{1-\sin \phi }{\sigma }_{\mathrm{r}}+{\sigma }_{\mathrm{c}}$$

(13)

Assuming that the rock mass in the plastic zone satisfies the condition of volume conservation under small deformation, it can be expressed as

$$\left\{\begin{array}{l}{\epsilon }_{\mathsf{\theta }}=\frac{u_2}{r}\\ {\epsilon }_{\mathrm{r}}=\frac{du}{dr}\\ {\epsilon }_{\mathsf{\theta }}+{\epsilon }_{\mathrm{r}}=0\end{array}\right.$$

(14)

where ${\epsilon }_{\mathsf{\theta }}$ is tangential strain at any point, ${\epsilon }_{\mathrm{r}}$ is radial strain at any point, and $u_2$ is displacement at any point in the plastic zone.

By combining Equations (12) and (14) and considering the displacement boundary continuity at the intersection of the elastic zones and plastic zones, the displacement of the surrounding rock at any point in the plastic zone can be obtained by solving the system of equations:

$$u_2=\frac{(1+\mu )R_{\mathrm{p}}}{2Er}·[2 p\sin \phi +{\sigma }_{\mathrm{c}}(1-\sin \phi )]-\frac{R_{\mathrm{p}}^2}{r}$$

(15)

Therefore, it can be seen from Equations (12) and (15) that after the excavation of the roadway, the displacement of a point in the surrounding rock not only depends on the mechanical properties of surrounding rock, such as elastic modulus, Poisson’s ratio, uniaxial compressive strength, and internal friction angle, but also on the original stress at that point before the excavation of the roadway. The greater the stress in the stress field at a certain point before the excavation of the roadway, the greater the deformation of the surrounding rock.

The contour map of the stress distribution in the surrounding rock of the roadway before the excavation of the coal seam is shown in Figure 14. It can be seen from Figure 14 that due to the influence of the inclination angle of the coal–rock layer, the vertical pressure borne by two sides of the roadway at the same depth is different, so the stress field before the excavation of the roadway is asymmetric, with the vertical stress on the right side being greater than that on the left.

The reason for the asymmetric deformation of the two sides of the inclined coal seam roadway after excavation is that, due to being affected by the inclination angle of the coal seam, the shallow surrounding rock unloads and undergoes damage during the excavation process, while stress concentration occurs in a certain range of the deep coal and rock mass. According to Equation (6), under the same conditions, the greater the stress at a certain point before excavation, the greater the displacement of that point after excavation. Therefore, the coal on the right side of the roadway undergoes more severe fragmentation, and after the deformation of the roadway becomes stable, the deformation and plastic zone of both sides of the roadway show obvious asymmetric characteristics.

4.2. Supporting Reinforcement Scheme

The coal seam primarily experiences the shear failure phenomenon of the 9-8032 roadway, combined with the ACC’s high shear performance. The ACC asymmetric support reinforcement scheme is proposed, and the supporting effects of asymmetric support and ACC supporting components are investigated. The ACC symmetric support reinforcement scheme and the anchor cable asymmetric support reinforcement scheme were designed to compare the supporting effects.

Two types of ACCs were used in the reinforcement scheme, which are as follows:

①

A Φ 21.8 mm × 10,300 mm ACC: it consists of a 21.8 mm × 10,300 mm anchor cable and a 2000 mm C-shaped tube.

②

A Φ 21.8 mm × 4300 mm ACC: it consists of a 21.8 mm × 4300 mm anchor cable and a 2000 mm C-shaped tube.

(1)

Supporting reinforcement Scheme 1

To keep the initial support method unchanged, four ACCs are added in each row on the top plate. The middle two ACCs are 10,300 mm in length, and the ACCs near both sides of the side are 4300 mm in length; the spacing is 1200 mm, and the row spacing is 800 mm.

On the right side of the roadway, each row receives an addition of four ACCs, while on the left, each row receives an addition of two ACCs. The length of the ACCs is 4300 mm, and the row spacing is 800 mm.

The ACC prestress is 200 kN. The first reinforcement scheme is shown in Figure 15a,c.

(2)

Supporting reinforcement Scheme 2

On the basis of unchanged initial support, the position and arrangement of the roof support are the same as in Scheme 1.

On each row on either side of the roadway, three ACCs with a length of 4300 mm are added, with a spacing of 800 mm and a row spacing of 800 mm. The angle between the ACC near the roof and the floor and the horizontal direction on both sides is 15°.

The anchor cable prestress is 200 kN. The second reinforcement scheme is shown in Figure 15b,d.

(3)

Supporting reinforcement Scheme 3

The supporting position and arrangement of roof and side are the same as in Scheme 1. The ACC is replaced with an anchor cable of the same length.

5. Support Effect of Different Support Schemes

A numerical simulation was carried out for different support reinforcement schemes, and the numerical simulation cloud diagram under each support reinforcement scheme is shown in Figure 16. The roadway’s maximum displacement and volume of the plastic zone under the initial support scheme and different support reinforcement schemes are shown in

Table 6. Maximum displacement and plastic zone volume under different schemes.

	Initial Support Scheme	Scheme 1	Scheme 2	Scheme 3
Roof/cm	−9.23	−7.93	−7.94	−8.31
Right side/cm	−7.93	−6.54	−6.88	−6.94
Left side/cm	6.77	5.97	5.66	6.15
Plastic zone volume/m3	457.8	330.9	370.3	384.6

.

5.1. Comparison of Supporting Effect between Asymmetric Scheme and Symmetrical Scheme

Scheme 1 is the ACC asymmetric support scheme, and Scheme 2 is the ACC symmetric support scheme. As shown in Table 6, using Scheme 1 on the basis of the original support scheme, the maximum displacement of the roof and the total maximum displacement of the two sides were reduced by 14.1% and 14.9%, respectively. Using Scheme 2 on the basis of the original support scheme, the maximum displacement of the roof and the total maximum displacement of the two sides were reduced by 14% and 14.7%, respectively. It can be seen that using the ACC asymmetric reinforcement scheme on the basis of the original support scheme can better control the deformation of the roadway compared with the ACC symmetric reinforcement scheme.

According to Figure 16c,f,i, it can be seen that different reinforcement schemes result in similar distributions of plastic failure zones to the original support scheme, but with significantly reduced plastic zone ranges, particularly in the vicinity of the roof and the left and right sides. There was a noticeable improvement in the plastic zone at the top corner of the right side, and the asymmetry in the left and right regions was reduced to some extent. As shown in Table 6, when Scheme 1 was adopted on the basis of the original support scheme, the volume of the plastic zone was reduced by 27.7%, and when Scheme 2 was adopted on the basis of the original support scheme, the volume of the plastic zone was reduced by 19.1%. This indicates that the ACC asymmetric reinforcement scheme can effectively control the plastic failure zone and improve the stability of the roadway compared to the ACC symmetric reinforcement scheme.

Ten monitoring points of displacement (points 1–10) located on the sides of the roadway, as shown in Figure 9, were selected as observation objects to study the effects of different reinforcement schemes on the horizontal displacement of the roadway sides. The results are presented in

Table 7. Horizontal displacement of monitoring points under different reinforcement scheme.

Distance from Roadway Surface	Scheme 1		Scheme 2		Scheme 3
	Right Displacement/cm	Left Displacement/cm	Right Displacement/cm	Left Displacement/cm	Right Displacement/cm	Left Displacement/cm
0	−6.09	5.29	−6.5	4.99	−6.42	5.43
0.6	−5.41	4.27	−5.62	3.99	−5.68	4.39
1.2	−4.24	3.11	−4.43	3.05	−4.43	3.22

.

As shown in Table 7, the horizontal displacement of the sides of the roadway decreased significantly for all monitoring points with the application of different reinforcement schemes, compared to the original support scheme. The trend of the horizontal displacement with respect to the distance from the roadway surface was found to be similar to that of the original support scheme. The asymmetry of the sides of the roadway with respect to the distance from the roadway surface for different reinforcement schemes is shown in Figure 17. It can be observed from Figure 17 that the asymmetry of the sides of the roadway increased with the distance from the roadway surface, and then decreased for all the different reinforcement schemes. In the case of Scheme 2, the asymmetry of the sides of the roadway increased significantly with depth, indicating that the symmetrical reinforcement scheme aggravated the asymmetric deformation characteristics of the sides. On the other hand, in the case of Scheme 1 or 3, the asymmetry of the shallow coal seam on both sides decreased significantly, and the asymmetry of the deep coal seam did not change significantly, resulting in a decrease in the overall asymmetry of the sides of the roadway. From Table 7 and Figure 17, it can be concluded that, compared with Scheme 2, the total displacement of the sides of the roadway at the same position is the smallest and the overall asymmetry of the sides of the roadway is also the smallest when Scheme 1 is applied. This indicates that the ACC asymmetric reinforcement scheme not only effectively controls the deformation of the roadway, but also effectively reduces the overall asymmetry of the sides of the roadway, alleviating the asymmetry deformation characteristics of the roadway. Therefore, the asymmetric support effect is better than that of symmetrical support.

5.2. Comparison of ACC and Anchor Cable Support Effect

The reinforcement components in Scheme 1 and Scheme 3 have the same supporting position and arrangement. The difference is that the reinforcement components in Scheme 1 are anchor cables and the ACC, while those in Scheme 3 replace all ACCs with anchor cables. From Table 6, it can be seen that compared with the original support scheme, the maximum displacements of the roof, right side, and left side are reduced by 10%, 12.5%, and 9.2%, respectively, by using Scheme 3. Compared with Scheme 3, using Scheme 1 on the basis of the original support scheme results in the maximum displacements of the roof, right side, and left side being reduced by 4.1%, 5%, and 2.6%, respectively. It can be found that under the same supporting parameters, the ACC support can effectively reduce the displacement of the roof and both sides of the roadway compared to the anchor cable support. This demonstrates that the ACC support can greatly increase the shear performance of the rock, increase the support strength of the roof and both sides, and increase the stability of the roadway when compared to the anchor cable.

From Table 6, it can be seen that the plastic zone volume is reduced by 16% by using Scheme 3 on the basis of the original support scheme. Compared with Scheme 3, using Scheme 1 on the basis of the original support scheme can reduce the plastic zone volume by 11.7%. This indicates that the ACC support can effectively improve the three-dimensional stress state of the surrounding rock, enhance the overall mechanical properties of the surrounding rock, reduce the plastic failure range of the surrounding rock, and improve the stability of the roadway.

Selecting the ACC at a distance of 1.75 m from the left side of the roof in Scheme 1 and the anchor cable in Scheme 3, which are located in the same position, two reinforcement schemes were adopted, and the horizontal displacements of the ACC and anchor cable are shown in Figure 18. Figure 18 provides the orientation of the coordinate system. When the model’s deformation aligns with the direction of the coordinate system, the numerical value is positive. Conversely, when the model’s deformation deviates from the direction of the coordinate system, the numerical value is negative. It can be seen from Figure 18 that the closer to the surface of the roadway, the greater the horizontal displacement of the ACC and anchor cable. The maximum horizontal displacement of the ACC is 2.66 mm, and the maximum horizontal displacement of the anchor cable is 2.81 mm. Compared with the anchor cable, the maximum horizontal displacement of the ACC can be reduced by 5.3%. Shan et al. [34] utilized the ACC in the deep straight-wall semi-circular arch roadways of the Zhengling Mine and found through field monitoring that the roadway deformation was effectively controlled, with no shear failure observed in the ACC. In response to issues such as anchor cable fractures, the Nanguan Mine adopted a full-section ACC support scheme. Field investigations revealed a smooth surface along the entire supporting section, and no anchor cable failure occurred within the ACC [35]. Combining the results of numerical simulations and field research, it can be concluded that the ACC exhibits superior resistance to shear deformation compared to anchor cables, making it an effective solution for controlling roadway deformation.

6. Conclusions

This paper first compares the shear performance of an ACC with that of an anchor cable through a double shear test. Then, aiming at the roadway’s asymmetrical deformation features in the Pangpangta inclined coal seam, according to its asymmetrical deformation law and combined with the high shear performance of the ACC, the asymmetrical support reinforcement scheme of the ACC is proposed. The numerical simulation test was conducted using FLAC 3D, and the following key findings were made:

(1) We derived the shear strength calculation formula for the ACC anchorage joint surfaces through theoretical analysis. There are three stages to the shear load–shear displacement curve of the ACC: rapid rising, slow rising, and stepped falling. The gentle stage, ascending stage, and stepped-falling stage of the axial force–shear displacement curve of the ACC can be separated. The stages divided by the shear load/axial force–shear displacement curve of the anchor cable are consistent with those of the ACC. Compared to the anchor cable, the ACC has a much better shear capability. The peak shear capacity of the ACC is 19.4% greater than that of the anchor cable at the same degree of prestress.

(2) Due to the influence of the dip angle, the stress of the surrounding rock on both sides of the inclined coal seam is asymmetric at the same depth, and the side with higher stress undergoes greater deformation after the excavation of the roadway, resulting in an asymmetric deformation of the sides. Symmetrical support exacerbates the asymmetric deformation characteristics of the inclined coal seam roadway on both sides. Compared with symmetrical support, asymmetrical support can not only effectively reduce the deformation and plastic failure zone of the roadway, but also significantly reduce the asymmetry of both sides, alleviate the asymmetric characteristics of both sides, and have a better supporting effect.

(3) Under the same support mode, compared with the anchor cable support, the roadway deformation and plastic zone volume are smaller when the ACC support is adopted. Shear resistance is higher in the ACC than in the anchor cable. The ACC support can enhance the shear performance of the surrounding rock, enhance the support strength of the roof and two sides, reduce the plastic zone, and enhance the ability of the coal seam to support itself.