Control Mechanism and Support Technology of Deep Roadway Intersection with Large Cross-Section: Case Study

Abstract

Conventional bolt–shotcrete support technology is usually single-layered, which does not meet the requirements of strength and stiffness for roadway support. Therefore, in this paper, new combined support technology, including a multiple-layered staggered dense arrangement of bolts, multiple-layered laying of steel meshes, multiple-layered pouring of shotcrete, strengthening support of long cables, and full cross-section grouting, is proposed. Specifically, the following new combined support technology process is proposed: first layer of shotcrete (80 mm), first layer of mesh, first layer of bolt, second layer of shotcrete (50 mm), second layer of mesh, second layer of bolt, reinforced cable, third layer of shotcrete (50 mm), and grouting. The results show the following: (1) In the system of a superimposed coupling strengthening bearing arch, compared to a cable bearing arch, changing the support parameters of the bolt bearing arch can significantly vary the bearing capacity. A range of bolt spacing between 0.4 m and 0.7 m is more conducive for a high performance of the bearing capacity of the superimposed coupling strengthening bearing arch. (2) With the increase in the single-layer shotcrete thickness (from 50 mm to 100 mm), the bearing capacity of the shotcrete structure increased rapidly in the form of a power function. (3) After the multi-level bolt–shotcrete support structure was adopted, the ring peak zone of the deviatoric stress of the surrounding rock at the roadway intersection was largely transferred to the shallow part, and the plastic zone of the surrounding rock of the roadway was reduced by 43.3~52.3% compared to that of the conventional bolt–shotcrete support. The field practice model showed that the final roof-to-floor and rib-to-rib convergences of the roadway intersection were 114 mm and 91 mm after 26 days, respectively. The rock mass above the depth of 3 m of the roadway’s roof and sides was complete, the lithology was dense, and there was no obvious crack. The new technology achieves effective control of a deep roadway intersection with a large cross-section.

1. Introduction

China’s “lack of gas, less oil and relatively rich in coal” resource occurrence characteristics have determined that coal will still be the leading source of energy in China for a long time in the future [1,2]. In order to ensure the energy supply, researchers have conducted a lot of research on the prediction and control of various disasters in the field of coal mining [3,4,5]. In addition, in China, the coal mining depth is increasing continuously with the exhaustion of shallow coal resources [6,7,8]. With the increase in the mining depth, the stress environment of the surrounding rock becomes more complex, and it presents the characteristics of a soft rock, which makes surrounding rock control of a deep roadway more difficult [9,10,11]. As the throat part of mine transportation, the cross-section of a roadway intersection is large [12,13]. Simultaneously, it is affected by stress concentrations and engineering disturbances, resulting in poor support conditions and frequent renovations of the roadway intersection [14,15,16]. Owing to the high transport capacity of a modern mine, there are numerous roadway intersections in a deep mine. Therefore, studying the stability of the surrounding rock at a deep roadway intersection with a large cross-section has become an urgent technical problem in modern mines.

As an essential part of deep roadway support technology, bolt–shotcrete support technology plays an important role in controlling the surrounding rock of a deep roadway [17,18]. In fact, in deep complex environments, when roadways are subjected to the superimposed effects of static and dynamic loads, one of the most common methods to ensure the stability of the surrounding rock of a deep roadway is to use bolt and anchor cable for reinforcement [19,20]. Shotcrete can quickly seal the surrounding rock, which is often prepared with admixtures, such as cement, sand, stone, and mineral admixtures, such as fly ash, ultra-fine slag, and silica fume [21,22]. On the basis of the above, a super early strength agent composed of fast-setting early-strength components and water-reducing components can be added to prepare fast-setting and rapid-hardening high-strength shotcrete [23]. In recent years, scholars have realized numerous beneficial achievements in deep roadway bolt–shotcrete support technology. Malmgren et al. investigated the influence of the following factors on bolt–shotcrete support: shotcrete thickness, bolt parameters, rock strength, and Young’s modulus, and the mechanical properties of the interface of the shotcrete, rock, and bolts [24]. Sjolander et al. proposed a numerical model for simulating the failure of bolt anchorage and shotcrete, which includes the pull-out failure of bolts, cracking of the shotcrete, and bond failure between the shotcrete and rock [25]. Naithani et al. implemented 3D engineering geological mapping for a project roadway and proposed combined support technology including bolt, steel fiber-reinforced shotcrete, drainage hole provisions, and grouting [26]. Basarir et al. analyzed the stability of the gateway of a deep mine and proposed a support system composed of a bolt, a cable, and shotcrete [27]. Panda et al. analyzed the rock mass properties for an optimum support system and proposed a combined reinforcement method for surrounding rock, including 3.0–4.4 m of bolts, 0.1–0.15 m of shotcrete, wire mesh, and a final concrete lining [28]. Yu et al. suggested a mechanical model of an overlap arch-bearing body, which was composed of a main compression arch (bolt support) and a second compression arch (cable support) [29]. The above scholars augmented the research results of deep mine bolt–shotcrete supports. However, bolt–shotcrete support technology has usually been a single-layered arrangement in previous studies. In addition, researchers have scarcely studied multiple-layered bolt–shotcrete superimposed coupling strengthening bearing arch support technology and its surrounding rock control mechanism. The difference between this technology and conventional single-layered bolt–shotcrete support technology lies in the multiple-layered staggered dense arrangement of bolts, multiple-layered laying of steel meshes, and multiple-layered pouring of shotcrete. The difference between this technology and conventional single-layered bolt–shotcrete support technology is shown in Figure 1.

Taking a roadway intersection with a large cross-section buried more than 800 m deep in the Xingdong coal mine as the engineering background, we comprehensively utilized several research methods, such as on-site geological investigation, numerical simulation, theoretical analysis, engineering practice, and on-site rock pressure measurement. The failure characteristics and mechanisms of the roadway intersection were investigated, and new combined support technology, including a multiple-layered staggered dense arrangement of bolts, a multiple-layered laying of steel meshes, multiple-layered pouring of shotcrete, the strengthening support of long cables, and full cross-section grouting is proposed. In addition, the supporting mechanism of the new technology is revealed, the numerical calculation models for roadway intersections under different supporting conditions are established, and the engineering applicability of the new technology is verified. The field engineering application results show that this technology can significantly limit the deformation of the roadway’s surrounding rock.

2. Project Background

2.1. Project Overview

The Xingdong coal mine in Xingtai City, Hebei Province, China, has two working levels. In this study, a −760-m-level roadway intersection was selected, as presented in Figure 2a.

The roadway intersection is arranged on the floor of the No. 2 coal seam, which is buried deeper than 800 m. A detailed strata histogram is illustrated in Figure 2b. The maximum cross-section of the roadway intersection has the shape of a straight wall and an arc arch with a width of 9.4 m and a height of 4.3 m (see Figure 2c).

2.2. Primary Support Scheme and Strata Behavior Characteristics

The primary support of the roadway intersection is a conventional single-layered bolt–shotcrete support. The support parameters are as follows: bolt: φ22 × 2000 mm, spacing: 800 × 800 mm; cable: φ21.8 × 6500 mm, spacing: 2000 × 1800 mm; grouting bolt: φ22 × 2000 mm, spacing: 1500 × 1500 mm; shotcrete: 120 mm. After the implementation of the primary support scheme and parameters, the roof-to-floor and rib-to-rib of the roadway underwent continuous large convergence (see Figure 3a). Numerous support materials were broken after their initial installation, as shown in Figure 3b, which severely affected the mine’s transportation and regular production. Therefore, it is urgent to develop a new support scheme for this roadway intersection.

2.3. Failure Mechanism Analysis of Roadway Intersection

Briefly speaking, a roadway intersection at a large buried depth has a large cross-section span. In addition, the surrounding rock mass is significantly affected by the engineering excavation. In the case of unreasonable primary support, deformation of the whole cross-section and failure of the surrounding rock at the roadway intersection are more prominent. The causes for the failure of a roadway intersection are shown in Figure 4.

3. New Combined Support Technology

Therefore, in view of the damage of the roadway intersection buried deeper than 800 m at the Xingdong coal mine under a conventional single-layered bolt–shotcrete support, the following new combined support technology process is proposed: first layer of shotcrete (80 mm), first layer of mesh, first layer of bolt, second layer of shotcrete (50 mm), second layer of mesh, second layer of bolt, reinforced cable, third layer of shotcrete (50 mm), and grouting. This new support technology comprises a multiple-layered staggered dense arrangement of bolts, multiple-layered laying of steel meshes, multiple-layered pouring of shotcrete, strengthening support of long cables, and grouting reinforcement of the surrounding rock.

3.1. Superimposed Coupling Strengthening Bearing Arch Support Technology and Mechanism

The technology of superimposed coupling strengthening bearing arch formation using multiple bolt–shotcrete layers is shown in Figure 5.

Specifically, first, workers apply shotcrete to plug the surrounding rock after the excavation of the roadway intersection, lay the first steel mesh, and install the first layer of bolts (this stage is the first bolt–mesh–shotcrete support, as shown in Figure 5a). Subsequently, the workers apply the second layer of shotcrete, lay a second layer of a steel mesh, install a second layer of bolts, and install a reinforced cable (this stage is the second bolt–cable–mesh–shotcrete support, as shown in Figure 5b). The support effect formed by two bolt–shotcrete layers is shown in Figure 5c. Finally, the third layer of shotcrete is implemented, as shown in Figure 5d. The support effect after the third layer of shotcrete is shown in Figure 5e. Compared to the primary support (see Figure 5f), in the new support (see Figure 5e), the thickness of the superimposed coupling strengthening bearing arch is larger, the influence range is wider, and the bearing capacity is higher. In addition, in this new combined support technology, pouring shotcrete thrice and laying the steel mesh twice can form a thick shotcrete structure with a two-layer steel mesh, which improves the shotcrete structure’s mechanical properties significantly and plays a role in maintaining a two-layer bearing arch. In this study, to distinguish between the action mechanisms of a bolt and a cable, the bearing arch formed by bolts is called a bolt bearing arch, and that formed by cables is called a cable bearing arch. The bearing arch formed by two layers of bolts is called a bolt-superimposed bearing arch.

When the first bolt–mesh–shotcrete support is implemented, the compressive stress zones between adjacent bolts are superimposed, forming a bearing arch structure. In the initial stage of roadway excavation, the bearing arch structure using a shotcrete layer could seal and support the broken surrounding rock in time. A sufficient prestress offered by the bearing arch structure improves the roadway’s confining pressure. FLAC3D software simulated the three-dimensional spatial stress and the profile stress distribution of the bearing arch under the first bolt–mesh–shotcrete support, as shown in Figure 6a–c.

After a certain degree of deformation, pressure is released from the surrounding rock, and the second bolt–cable–mesh–shotcrete support is implemented. The bearing arch structure formed by the bolts and reinforced cables in the second bolt–cable–mesh–shotcrete support (the three-dimensional spatial stress and profile stress distribution of this bearing arch are shown in Figure 7a–c) is superimposed, with the bearing arch formed by the bolts in the first bolt–mesh–shotcrete support. This will form a superimposed coupling strengthening bearing arch structure with high strength, strong bending, crack resistance, and reasonable stress distribution, which prevents further deformation of the surrounding rock.

Owing to the complexity of the superimposed coupling strengthening bearing arch structure in practical engineering, to facilitate the stress analysis of its bearing capacity, the following assumptions were made: (1) The surrounding rock at the roadway is approximately isotropic, homogeneous, and continuous. (2) After a two-layer bolt–shotcrete, the surrounding rock mass is still regarded as an elastic–plastic medium. The failure of the rock mass follows the Mohr–Coulomb criterion. (3) The arch section of the roadway is approximately semicircular. (4) The restraint resistance of each bolt (cable) is evenly distributed in the surrounding rock circle.

Combined with the above assumptions, specific mechanical models were established. The bearing arch mechanical models under the first bolt–mesh–shotcrete support and the second bolt–cable–mesh–shotcrete support are shown in Figure 6d and Figure 7d, respectively. P is the restraint resistance provided by the bolt, and $P^{\prime }$ is the restraint resistance provided by the cable.

Following the first bolt–mesh–shotcrete support, the roadway’s surrounding rock still follows the Mohr–Coulomb strength criterion under the limit equilibrium as follows:

$${\sigma }_1={\sigma }_3\frac{1+\sin \phi }{1-\sin \phi }+2c\frac{\cos \phi }{1-\sin \phi } (\mathrm{MPa})$$

(1)

where ${\sigma }_1$ and ${\sigma }_3$ are the maximum and minimum principal stresses in the rock mass (MPa), respectively, $\phi$ is the rock internal friction angle of the loosely broken zone in the surrounding rock (°), and c is rock cohesion of the loose zone in the surrounding rock (MPa). When the surrounding rock does not have a support, the rock in the loose zone is in a broken state. From the perspective of conservation and safety, c is 0. Therefore, Equation (1) becomes

$${\sigma }_1={\sigma }_3\frac{1+\sin \phi }{1-\sin \phi } (\mathrm{MPa})$$

(2)

when the stress state of any point of the surrounding rock in the bearing arch satisfies Equation (2), the surrounding rock will enter the failure state. The stress in the inner surface of the roadway’s surrounding rock is generally equal to the restraint resistance: ${\sigma }_3$ = P.

The P relationship with the bolt support resistance is as follows:

$$P=\frac{F_{\mathrm{b}}}{D^2} (\mathrm{MPa})$$

(3)

where $F_{\mathrm{b}}$ is the bolt support resistance (kN), and $D$ is the bolt spacing under the first bolt–mesh–shotcrete support (m).

To calculate the bearing resultant force, N, of the bearing arch on the unit length along the axial direction of the roadway, the following differential relationship is used:

$$d\mathrm{s}=(r+\frac{b}{2})\mathrm{d}\theta$$

(4)

where $d\mathrm{s}$ is the arc differential length unit of the bearing arch center, r is the roadway radius (m), $\mathrm{d}\theta$ is the angle differential unit of the bearing arch along the roadway center, and b is the maintaining bearing arch thickness (m).

$$b=\frac{l\tan \partial -D}{\tan \partial } (m)$$

(5)

According to Equations (1) and (4), N is obtained as follows:

$$N=P\frac{1+\sin \phi }{1-\sin \phi }b+\frac{1}{2}{k}^2{b}^2 (\mathrm{kN})$$

(6)

where k is the increasing slope of the radial stress, k is 0 in the unstable broken rock mass, l is the bolt effective length (m), and $\partial$ is the bolt control angle in a fractured rock mass (°). Therefore, Equation (6) can be rewritten as follows:

$$N=P\frac{1+\sin \phi }{1-\sin \phi }\frac{l\tan \partial -D}{\tan \partial } (\mathrm{kN})$$

(7)

For the ring-shaped bearing arch, using its symmetry, yields the following relations:

$$\sum Y=0;{\int }_0^{\mathsf{\pi }}q\sin \theta \mathrm{ds}=2N_0$$

(8)

where q is the bearing capacity of the bearing arch under the first bolt–mesh–shotcrete support, and N₀ is the circumferential direction axial force generated by the bearing arch. From Equations (4) and (8), the following results can be obtained:

$$N_0=(r+\frac{b}{2})q (\mathrm{kN})$$

(9)

when N is equal to N₀, the bearing arch is in the limit equilibrium state. Combining Equations (5), (7), and (9), we can obtain the following results:

$$q=\frac{2P\left(1+\sin \phi \right)\left(l\tan \partial -D\right)}{\left(2r\tan \partial +l\tan \partial -D\right)\left(1-\sin \phi \right)} (\mathrm{MPa})$$

(10)

By substituting the above equation into Equation (3), the bearing capacity of the bearing arch after the first bolt–mesh–shotcrete support is obtained as follows:

$$q=\frac{2F_{\mathrm{b}}(1+\sin \phi \left)\right(l\tan \partial -D)}{D^2(2r\tan \partial +l\tan \partial -D\left)\right(1-\sin \phi )} (\mathrm{MPa})$$

(11)

The support parameters of the bolts used in the second bolt–cable–mesh–shotcrete support are the same as those used in the first bolt–mesh–shotcrete support. Therefore, under the second bolt–cable–mesh–shotcrete support, the bearing capacity, $q^{\prime }$, of the bearing arch is as follows:

$$q^{\prime }=\frac{2F_{\mathrm{b}}(1+\sin \phi \left)\right(l\tan \partial -D)}{D^2(2r\tan \partial +l\tan \partial -D\left)\right(1-\sin \phi )}+\frac{2F_{\mathrm{c}}(1+\sin \phi \left)\right(l^{\prime }\tan \partial -D^{\prime })}{{D^{\prime }}^2(2r\tan \partial +l^{\prime }\tan \partial -D^{\prime }\left)\right(1-\sin \phi )} (\mathrm{MPa})$$

(12)

Subsequently, the bearing capacity of the superimposed coupling strengthening bearing arch formed by a two-layer bolt–shotcrete is as follows:

$$q^{\mathrm{toal}}=\frac{4F_{\mathrm{b}}(1+\sin \phi \left)\right(l\tan \partial -D)}{D^2(2r\tan \partial +l\tan \partial -D\left)\right(1-\sin \phi )}+\frac{2F_{\mathrm{c}}(1+\sin \phi \left)\right(l^{\prime }\tan \partial -D^{\prime })}{{D^{\prime }}^2(2r\tan \partial +l^{\prime }\tan \partial -D^{\prime }\left)\right(1-\sin \phi )} (\mathrm{MPa})$$

(13)

where $q^{\mathrm{toal}}$ is the bearing capacity of superimposed coupling strengthening bearing arch (MPa), $F_{\mathrm{c}}$ is the cable support resistance (kN), $D^{\prime }$ is the cable spacing (m), and $l^{\prime }$ is the cable effective length (m).

Combined with the actual production, the basic parameters are determined as follows: $\phi =30^{°}$, $\partial =45^{°}$, $l=2$ m, and $l^{\prime }=6$ m. According to Equation (13), the relationship between the bearing capacity of the superimposed coupling strengthening bearing arch and the spacing and support resistance of the bolts (cables) can be obtained, as shown in Figure 8.

It can be seen from Figure 8 and Equation (13) that the bearing capacity of the superimposed coupling strengthening bearing arch linearly increased with the increase in the support resistance of the bolts (cables). According to Figure 8a,b, when the bolt spacing was large (0.7–1.0 m), then even if the support resistance of the bolt or cable was high, the superimposed coupling strengthening bearing arch still maintained a low-stress state. With the decrease in the bolt spacing (from 0.7 m to 0.4 m), the bearing capacity of the superimposed coupling strengthening bearing arch increased rapidly in the form of a power function. Taking the bolt support resistance, F_b = 90 kN in Figure 8a, as an example, when the bolt spacing decreased from 1 m to 0.7 m, the bearing capacity of the superimposed coupling strengthening bearing arch increased from 0.32 MPa to 0.56 MPa, with an increase of only 75%. When the bolt spacing decreased from 0.7 m to 0.4 m, the bearing capacity of the superimposed coupling strengthening bearing arch increased from 0.56 MPa to 1.59 MPa, with an increase of 183.9%. It can be seen that when the bolt spacing was between 0.7 m and 0.4 m, it was more conducive for high performance of the bearing capacity of the superimposed coupling strengthening bearing arch.

The comparative analysis in Figure 8a,b (or Figure 8c,d) shows that the increase in the bolt support resistance caused the bearing capacity of the superimposed coupling strengthening bearing arch to increase significantly. However, the cable support resistance increase did not significantly change the bearing capacity of the superimposed coupling strengthening bearing arch. Taking the 0.7-m bolt spacing as an example, in Figure 8a, as the bolt support resistance, F_b, increased from 70 kN to 100 kN, the bearing capacity of the superimposed coupling strengthening bearing arch increased from 0.47 MPa to 0.6 MPa, with an increase of 27.7%. However, in Figure 8b, when the cable support resistance, F_c, increased from 160 kN to 190 kN, the bearing capacity curves of the superimposed coupling strengthening bearing arch were nearly coincident, and the bearing capacity was almost unchanged. The comparative analysis in Figure 8a,c (or Figure 8b,d) shows that compared to reducing the cable spacing, reducing the bolt spacing increased the bearing capacity of the superimposed coupling strengthening bearing arch more rapidly. Taking the bolt support resistance, F_b = 90 kN, as an example, in Figure 8a, when the bolt spacing decreased from 0.7 m to 0.4 m, the bearing capacity of the superimposed coupling strengthening bearing arch increased from 0.56 MPa to 1.59 MPa, with an increase of 183.9%. Nevertheless, in Figure 8c, when the cable spacing decreased from 1.6 m to 1.3 m, the bearing capacity of the superimposed coupling strengthening bearing arch increased from 0.56 MPa to 0.66 MPa, with an increase of only 17.9%. The above results show that in the superimposed coupling strengthening bearing arch system, compared to the cable bearing arch, changing the support parameters of the bolt bearing arch can significantly change the bearing capacity. Therefore, the setting of the bolt support parameters is particularly important.

Furthermore, in the new combined technology, the shotcrete layer and the bolts work together to produce restraint resistance at the surrounding rock, jointly resisting the roadway’s deformation. The thicknesses of the shotcrete layer and the bolt support parameters directly affect the level of restraint resistance. To further optimize the support parameters of the new combined technology, it was essential to determine the thickness of each shotcrete layer. Therefore, mechanical models of the single-layer shotcrete and bolts were established to explore the support principles of the shotcrete layer and bolts for a broken rock mass.

To facilitate the analysis and calculation, the following assumptions were made for this mechanical model: (1) the single-layer shotcrete structure (see Figure 9a) between adjacent bolts on the same level was regarded as the mechanical model of a clamped–clamped beam (see Figure 9b); (2) this clamped–clamped beam is a continuous, homogeneous, isotropic material, which accords with the assumption of elastic mechanics.

According to the Mohr–Coulomb strength criterion,

$${\sigma }_1={\sigma }_3\frac{1+\sin \phi }{1-\sin \phi }+2c\frac{\cos \phi }{1-\sin \phi } (\mathrm{MPa})$$

(14)

Because the uniform load $i$ is equal to ${\sigma }_3$, according to the material mechanics,

$$i=\frac{2h^2{\sigma }_{\mathrm{t}}}{D^2} (\mathrm{MPa})$$

(15)

where $h$ is the thickness of the single-layer shotcrete (mm), ${\sigma }_{\mathrm{t}}$ is the shotcrete strength (MPa), and $u$ is the Poisson ratio.

From Equations (14) and (15), we can obtain the following results:

$${\sigma }_{\mathrm{1}}=\frac{2h^2{\sigma }_{\mathrm{t}}}{D^2}.\frac{1+\sin \phi }{1-\sin \phi }+2c\frac{\cos \phi }{1-\sin \phi } (\mathrm{MPa})$$

(16)

Combined with the production practice, $c$ = 0.11 MPa, ${\sigma }_{\mathrm{t}}$ = 1.05 MPa, and $u$ = 0.3. By substituting these into Equation (16), the relationship between the bearing capacity of the single-layer shotcrete structure and the bolt spacing and the single-layer shotcrete thickness is obtained, as shown in Figure 10. It can be seen in Figure 10a that when the bolt spacing decreased from 1 m to 0.7 m, even if the thickness of the single-layer shotcrete was large, the bearing capacity of the shotcrete structure increased slowly with the decrease in bolt spacing. When the bolt spacing decreased from 0.7 m to 0.4 m, the bearing capacity of the shotcrete structure increased more obviously with the decrease in the bolt spacing. Taking the single-layer shotcrete thickness, h = 80 mm in Figure 10a, as an example, when the bolt spacing decreased from 1 m to 0.7 m, the bearing capacity of the shotcrete structure increased from 0.42 MPa to 0.46 MPa, with an increase of only 9.5%. However, when the bolt spacing decreased from 0.7 m to 0.4 m, the bearing capacity of the shotcrete structure increased from 0.46 MPa to 0.63 MPa, with an increase of 37%. Thus, it is clear that when the bolt spacing was between 0.7 m and 0.4 m, it was more conducive to the high performance of the shotcrete structure’s bearing capacity. This result is the same as the reasonable range of the bolt spacing in the superimposed coupling strengthening bearing arch structure, which also shows that the design of the bolt spacing in the primary support scheme is unreasonable. In addition, it can be seen in Figure 10b that the bearing capacity of the single-layer shotcrete structure maintained a low-stress value when the thickness of the single-layer shotcrete was small (from 20 mm to 50 mm). With the increase in the single-layer shotcrete thickness (from 50 mm to 100 mm), the bearing capacity of the shotcrete structure increased rapidly in the form of a power function. However, considering factors such as the rebound rate and economic benefits, the single-layer shotcrete thickness should not be extremely large.

According to the above analysis, the setting of the bolt support parameters is particularly important. A bolt spacing of 0.4–0.7 m and a single-layer shotcrete thickness of 50–100 mm are more conducive to the stability of the surrounding rock than in other cases.

3.2. Technology and Mechanism of Grouting Reinforcement for the Full Cross-Section of Surrounding Rock

The primary support of the roadway intersection is the conventional single-layered bolt–shotcrete support technology. The surrounding rock undergoes fracture repeatedly, and cracks continue to develop in deeper areas impacted by engineering disturbances. Grouting has the effect of plugging the cracks of the surrounding rock and increasing the cohesion between rock blocks, allowing soft and broken surrounding rocks to bond with a certain degree of strength. The ability of the broken surrounding rock to resist dynamic pressure is improved. Grouting makes the surrounding rock of the roadway maintain a high residual grouting pressure. A slurry with a high residual grouting pressure can fill the cracks in a shallow surrounding rock, the gap between a bolt (cable) and a hole in the wall, and deeper fracture areas, forming a network skeleton structure, including a shotcrete layer, dense high-strength bolts (cables), a consolidation slurry, and a rock mass. In addition, the grouting makes the dense high-strength bolts similar to a full-length anchorage, expanding the anchorage scope of the bolts, and further increasing the influence range of the superimposed coupling strengthening bearing arch, as shown in Figure 11a. Grouting also strengthens thick shotcrete structures with a two-layer steel mesh. Among the multiple-layered staggered dense arrangement of bolts, multiple-layered laying of steel meshes, multiple-layered pouring of shotcrete, strengthening support of long cables, and full cross-section grouting, the interaction mechanism of the arching and strengthening support is shown in Figure 11b.

4. Numerical Analysis for Primary Support and New Support

4.1. Parameters of Primary and New Supports

Under the action of high tectonic stress on the deep coal measure strata, the surrounding rock of the roadway intersection appears soft, broken, and loose and presents rheological features [30,31]. Considering the geological conditions of roadway intersections, combined with theoretical analysis, engineering analogies, and economic benefits, the new support scheme was determined as follows: The first-, second-, and third-layer shotcrete thicknesses were 80 mm, 50 mm, and 50 mm, respectively. The spacing of the bolts in the same layer was 700 × 700 mm. The bolts in different layers cooperated effectively in space, finally forming the new multiple bolt–shotcrete support technology with a bolt spacing of 350 × 350 mm and a shotcrete layer 180 mm thick. From the calculation, the bearing capacity of the bearing arch formed by the primary support scheme was 0.17 MPa, whereas that of the superimposed coupling strengthening bearing arch formed by the new support scheme reached 0.57 MPa, an increase of 235%.

In the primary support, the bearing capacity of the bolt bearing arch is low. Combined with the field engineering experience, it was decided to select high-strength bolts with dimensions of Φ 22 × 2400 mm and spacing of 700 × 700 mm. When long cables are arranged with small spacing, the compressive stress zone formed by the adjacent cables will overlap in a large area of the surrounding rock, forming a cable bearing arch. In the primary support, the compressive stress zones formed by the adjacent cables cannot be effectively overlapped. They cannot be coupled well with the bolt bearing arch in the shallow surrounding rock. Therefore, based on field engineering experience, it was decided to select cables with dimensions of Φ 21.8 × 8500 mm and spacing of 1600 × 1600 mm. Based on the above analysis, in the numerical simulation, the parameters of the bolt (cable) and shotcrete layers in the primary support and new support are listed in

Table 1. Parameters of bolt (cable) and shotcrete layer in primary and new supports.

	Bolt	Cable	Shotcrete	Grouting Bolt
Primary scheme	Φ22 × 2000 mm Spacing: 800 × 800 mm	Φ21.8 × 6500 mm Spacing: 2000 × 1800 mm	120 mm	Φ22 × 2000 mm Spacing: 1500 × 1500 mm
New scheme	Φ22 × 2400mm Spacing: 700 × 700 mm	Φ21.8 × 8500 mm Spacing: 1600 × 1600 mm	180 mm	Φ22 × 2000 mm Spacing: 1500 × 1500 mm

.

4.2. Numerical Model Generation

Based on the geological conditions of the −760-m-roadway intersection in the Xingdong coal mine, a numerical model (see Figure 12) was established to investigate the support effect of the surrounding rock in the roadway intersection under the different support conditions. A vertical stress of 19.3 MPa was applied at the top model boundary to simulate the overburden pressure. The bottom boundary of the model was restricted along the vertical direction. The lateral boundary of the model was restricted along the horizontal direction.

The cable and shell structures in the FLAC3D software were used to simulate the bolt (cable) and shotcrete layers, respectively. The Mohr–Coulomb constitutive model was used to simulate the failure of the rock masses.

Table 2. Rock strata properties used in numerical model.

Rock Strata	Density (kg/m3)	Bulk Modulus (GPa)	Shear Modulus (GPa)	Friction Angle (°)	Cohesion (MPa)	Tensile Strength (MPa)
Sandy mudstone	2400	4.87	2.58	30	1.2	1.10
Fine sandstone	2700	7.87	3.38	30	2.6	2.40
Siltstone	2750	8.82	4.84	31	3.0	2.57
2#Coal	1350	2.35	1.87	20	1.2	0.90
Sandstone	2620	7.50	4.10	32	1.7	1.20

lists the main rock parameters.

4.3. Numerical Model Results Analysis

4.3.1. Vertical Stress Analysis

The simulated evolutionary processes of the vertical stress around the roadway intersection under the different support conditions are shown in Figure 13.

The results show that the roadway intersection is located in a large stress relaxation area under the conditions of excavation without support. After adopting the conventional bolt–shotcrete support scheme, the shallow surrounding rock at the roadway intersection was still in a low-stress area; however, the stress level of the shallow surrounding rock of the roadway was greatly improved after the multi-level bolt–shotcrete support structure was adopted. The application of the new multi-level bolt–shotcrete support structure makes the shallow surrounding rock at the roadway intersection approximately present a triaxial compression state, and the bearing capacity of the shallow surrounding rock was greatly improved, which is conducive to maintaining the long-term stability of the roadway’s surrounding rock.

4.3.2. Deviatoric Stress Analysis

The simulated evolutionary processes of the deviatoric stress around the roadway intersection under the different support conditions are shown in Figure 14.

The results show that the roadway intersection formed a ring peak zone of deviatoric stress after excavation. With the increase in support strength, the ring peak zone of deviatoric stress gradually moved to the shallow part of the roadway’s surrounding rock. More importantly, the transfer distance of the ring peak zone of the deviatoric stress under the traditional bolt–shotcrete support was far less than that of the multi-level bolt–shotcrete support compared to the no-support conditions.

4.3.3. Plastic Zone Analysis

After roadway excavation, the surrounding rock mainly undergoes shear and tensile failure. The maximum plastic zone depth of surrounding rock was used to determine the plastic extent around the excavations (marked as magenta numerals in the following figure). The distribution of the plastic zone depth of each section of the roadway’s surrounding rock under different support conditions at the roadway intersection is shown in Figure 15 below:

The results show that due to the weak lithology, the large buried depth, and the large cross-section of the roadway intersection, the deformation and damage of the roadway intersection are serious under the conditions of excavation without support. After adopting the conventional bolt–shotcrete support scheme, the plastic zone of the surrounding rock of the roadway was only reduced by 23.5~29.8%. However, after the multi-level bolt–shotcrete support structure was adopted, the plastic zone of the surrounding rock of the roadway was reduced by 58~65.7% compared to that without support, and the plastic zone of the surrounding rock of the roadway was reduced by 43.3~52.3% compared to that of the conventional bolt–shotcrete support, indicating that the support effect was greatly improved.

5. Industrial Test

5.1. Technological Process for New Combined Support Technology

Considering the length of the article, this paper only describes the support scheme of the maximum section A-A profile of the roadway intersection in detail. Detailed steps for the new combined support technology are shown in Figure 16. The detailed technological process is depicted in Figure 17.

Step 1: After the roadway is excavated to the specified size, the first layer of shotcrete with a thickness of 80 mm should be sprayed immediately. After the shotcrete layer is solidified, the workers drill holes, lay the first layer of steel mesh, and install the first layer of bolt. This step is the first bolt–mesh–shotcrete support. In this step, Φ22 × 2400 mm high-strength bolts are used with a spacing of 700 × 700 mm. The pre-tightening torque of the bolts is not less than 300 Nm. After the implementation of Step 1, the cross-section support effect of the roadway intersection is as depicted in Figure 17a.

Step 2: A second layer of shotcrete with a thickness of 50 mm is resprayed. A second steel mesh layer is laid, and subsequently, a second layer bolt is installed. Following this, reinforced cables are installed in the roof and sides of the roadway. The second- and the first-layer bolts are staggered in space. This step is the second bolt–cable–mesh–shotcrete support (see Figure 17b). In this step, Φ21.8 × 8500 mm high-strength cables are used with a spacing of 1600 × 1600 mm. The prestress of the cable is 120 kN. The bolt support parameters are the same as those in Step 1. The cross-section effect after the double bolt–shotcrete support at the intersection is shown in Figure 17c.

Step 3: A third layer of shotcrete with a thickness of 50 mm is resprayed. Subsequently, the grouting holes are arranged in the third layer of shotcrete, and the grouting bolt is installed (see Figure 17d). In this step, Φ22 × 2000 mm grouting bolts are used with a spacing of 1500 mm × 1500 mm. For the grouting, #425 cement is used, the water–cement ratio is 0.7:1–1:1, and the grouting pressure is 1.5–2.5 MPa.

The support effect of the roadway intersection after grouting is shown in Figure 17e. Compared to the primary support effect (see Figure 17f), the thickness of the arch structure is larger and the bearing capacity is stronger. The roadway maintenance effects of the new and primary support schemes are illustrated in Figure 17g,h, respectively.

After the new combined support technology is implemented, the cross-section of the new support is as shown in Figure 18a, and the corresponding space diagram is depicted in Figure 18b.

5.2. Strata Control Observation at Test Roadway Intersection

(1)

Surface convergence

The surrounding rock convergence of the roadway was observed by using telescoping rods and flexible tape. Four pegs were installed in the roadway’s two sides, roof, and floor. The pegs in the roof and the floor were installed in the middle positions to monitor the roof-to-floor convergence. The pegs in the roadway’s two sides were installed 1.6 m away from the roadway floor to monitor the rib-to-rib convergence. The monitoring section was the A-A profile, as shown in Figure 2c. The monitoring results are presented in Figure 19. The results indicate that the roadway intersection convergences tended to be stable after 26 days. The final roof-to-floor and rib-to-rib convergences of the roadway intersection were 114 mm and 91 mm, respectively. Moreover, the roof-to-floor and rib-to-rib maximum convergence rates of the roadway intersection were 12.3 mm/d and 7.9 mm/d, respectively. The deformations were in the allowable range, which illustrates that the control of the surrounding rock with the new support scheme was effective, as seen in Figure 17g.

(2)

Borehole drilling results of roadway roof and side

The crack development degree of the roof and side of the roadway was tested by a borehole detector to determine the control effect of the roadway’s surrounding rock under the new support mode. The results are shown in Figure 20.

The rock mass above the depth of 3 m in the roadway’s roof and side was complete, the lithology was dense, and there was no obvious crack. It can be seen that under the new support mode, the control effect of the roadway’s surrounding rock was good, and effective control of the roadway’s surrounding rock can be achieved.

6. Conclusions

(1)

The staggered arrangement of the two layers of high-strength bolts forms a thick strengthening bearing arch, improving the stress state of the shallow surrounding rock and weakening the borehole disturbance. The multiple–layered pouring of shotcrete, multiple-layered staggered dense bolts, and interlayer steel meshes form a new type of high-strength bending and crack-resistant reinforced concrete supporting arch structure with built-in multi-layer steel meshes, providing radial overall support stress distribution for the surrounding rock. Grouting is carried out under the conditions of strong sealing of the surrounding rock with a thick reinforced concrete support arch, which further expands the reinforcement scope of the surrounding rock.

(2)

By establishing the mechanical model of the superimposed coupling strengthening bearing arch, the relationship between its bearing capacity and the bolt (cable) spacing and support resistance is obtained. By establishing the shotcrete-layer structure mechanical model, the relationship between its bearing capacity and the bolt (cable) spacing and the single-layer shotcrete thickness is obtained. Analysis of the above two equations shows that when the bolt spacing was between 0.4 m and 0.7 m, it was more conducive to the stability of the surrounding rock. With the increase in the single-layer shotcrete thickness (from 50 mm to 100 mm), the bearing capacity of the shotcrete structure increased rapidly in the form of a power function.

(3)

The numerical simulation results show that, after the multi-level bolt–shotcrete support structure was adopted, the overall stress level of the shallow surrounding rock at the roadway intersection was improved, the bearing capacity of the shallow rock mass was improved, the ring peak zone of the deviatoric stress of the surrounding rock at the roadway intersection was largely transferred to the shallow part, and the plastic zone of the surrounding rock of the roadway was reduced by 43.3~52.3% compared to that of the conventional bolt–shotcrete support.

(4)

After the new combined support technology process was adopted, the roadway intersection convergences tended to be stable after 26 days, the final roof-to-floor and rib-to-rib convergences of the roadway intersections were 114 mm and 91 mm, respectively. Moreover, the maximum roof-to-floor and rib-to-rib convergence rates of the roadway intersection were 12.3 mm/d and 7.9 mm/d, respectively. These results prove that the new combined support technology could achieve long-term stability of the surrounding rock of the deep roadway, avoid frequent expansions and renovations of the roadway, and also save a lot of maintenance costs, ensuring the normal production of the working face.

7. Discussion

Conventional bolt–shotcrete support technology is usually single-layered, which does not meet the requirements of strength and stiffness for roadway support. Therefore, the new combined support technology, including a multiple-layered staggered dense arrangement of bolts, multiple-layered laying of steel meshes, multiple-layered pouring of shotcrete, strengthening support of long cables, and full cross-section grouting, is proposed. Compared to the previous single-layered bolt–shotcrete support, this new technology has the following characteristics and advantages: (1) The staggered arrangement of the two layers of high-strength bolts forms a thick strengthening bearing arch, which greatly improves the stress state of the shallow surrounding rock. (2) The multiple-layered pouring of shotcrete, multiple-layered staggered dense bolts, and interlayer steel meshes form a new type of high-strength bending and crack-resistant reinforced concrete supporting arch structure with built-in multi-layer steel meshes, providing radial overall support stress distribution for the surrounding rock. (3) Grouting is carried out under the conditions of strong sealing of the surrounding rock with a thick reinforced concrete support arch, which further expands the reinforcement scope of the surrounding rock. In this study, we established a mechanical model of the superimposed coupling strengthening bearing arch and the shotcrete layer structure, analyzed the support mechanism of this new technology, and conducted research on the vertical stress, deviator stress, and plastic zone depth distribution of the roadway’s surrounding rock under different support conditions. Finally, this technology was successfully applied in the field. In future research work, we will focus on the research and development of the performance improvement of support materials related to this technology, such as the bolts, cables, mesh, and shotcrete.