Introduction

Coal is the basic energy and important raw materials in China, coal industry is an important basic industry related to national economic lifeline and energy security. In a long period of time, the dominant position of coal will not change. With the depletion of shallow coal resources, the depth of coal mine excavation in China is increasing, partial mining will become the normal state of coal resources development1.

The longwall mining method is mainly used in coal mining in China. Between the 1960s and 1980s, Academician Minggao Qian and Zhenqi Song successively put forward the theory of “masonry beam” and “transmission rock beam”, which laid the theoretical foundation of longwall mining and formed the “121 working method” of longwall mining. This innovation not only increased China’s coal production but also enhanced the level of automation in mining operations. As shallow coal reserves are depleted, mining operations are extending to greater depths, resulting in severe overburden pressures on the coal face. The protective coal pillars cause the stress concentration of surrounding rock of the lower horizontal working face, leading to serious deformation and damage along the goaf, which in turn can trigger severe safety incidents such as rock bursts and coal and gas outbursts. In 2008, Academician Manchao He proposed “roof-cutting short wall beam” and “roof-cutting and pressure relief automatic roadway forming coal pillar free mining technology” to realize coal pillar free balanced mining2,3. However, for the deep high stress roadway, the surrounding rock is loose and broken, The joints and fissures are developed and the bearing capacity of rock mass is weak, the interface bonding ability between the anchoring agent and the broken surrounding rock is weak, the bolt (cable) slippage failure, and the support potential is difficult to play, thus affecting the implementation effect of roof cutting and retaining roadway. Therefore, it’s difficult to implement the roof cutting and retaining roadway in deep broken surrounding rock, and further research is needed which can improve the stability of roof cutting roadway with deep broken surrounding rock (Fig. 1)2,3,4,5.

Fig. 1
figure 1

Detection and destruction of broken surrounding rock in roadway26.

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The current research on the support of the surrounding rock in gob-side entry retaining is primarily concentrated in two key areas. Firstly, there is a focus on constant resistance and large deformation bolt (cable) for support. This approach aims to provide a stable and reliable form of reinforcement that can adapt to the changing conditions within the mining environment. Zhang et al. presplits the roof along the strike by the advanced bidirectional shaped charge tension blasting, which reduces the blasting damage of the roof6; The stability of self formed roadway is increased by using constant resistance and large deformation bolt (cable), anchor cable at roadway side and hydraulic prop beside roadway to strengthen roof cutting combined support technology in roadway7; Wang et al. proposed to control the deformation of “pressure bearing structure” by using roof cutting seam and to control the deformation of pressure relief structure by using NPR anchor cable support with constant resistance and large deformation8.

The second area of focus is the implementation of bolt (cable) support systems. These systems are designed to enhance the structural integrity of the surrounding rock by providing additional reinforcement. The use of bolts or cables helps to maintain the stability of the rock mass, preventing deformation and potential collapse, which is crucial for ensuring the safety and efficiency of mining operations. Hua et al. established the mechanical model of gob side entry retaining considering the bearing effect of roadway side coal mass and the reinforcement of roadway side anchor cable, the mechanism of bolt support in roadway and anchor cable reinforcement support beside roadway are analyzed9. Yan et al. theoretically analyzed the stress distribution characteristics of roof surrounding rock of gob side entry retaining, and proposed the anchor cable coupling support technology based on bolt + W steel belt10. Zhang et al. expounded the transmission and bearing mechanism of the roof in the wedge-shaped area, and proposed the key technologies for surrounding rock control of gob side entry retaining, such as presplitting blasting pressure relief, zoning treatment, structural parameter optimization, “Trinity” surrounding rock control and rapid wall construction11. Kang et al. adopted high prestressing force, strong bolt and anchor cable as the basic support in the roadway, the single pillar with hinged top beam was used to strengthen support and paste filling roadway side support, which can effectively control the strong deformation of surrounding rock in deep gob side entry retaining12.

Bolt grouting support can effectively improve the strength of surrounding rock, increase the initial support strength of roadway, and effectively control the deformation of roadway. Bolt grouting support is widely used in the field. But the basic research of bolt grouting support is insufficient and the theory lags behind the practice, which restricts the application of roof cutting and roadway retaining technology in deep broken surrounding rock, therefore, carrying out the research on the bearing strength of bolt grouting complex is beneficial to promote the development of bolt grouting theory and roof cutting technology. For the deficiencies of common grouting technology, three-step grouting technology is proposed to reinforce the surrounding rock to get better results, the grouting diffusion depth of the surrounding rock is generally greater than 3 m, which greatly improves the effect of grouting reinforcement of surrounding rock, enhances the self-bearing capacity of rock mass, and effectively controls the overall stability of surrounding rock13.The combined support scheme of “bolt mesh cable shotcreting + U-shaped steel support + floor bolt grouting” can effectively control the large deformation and floor heave of deep high stress soft rock roadway14. Wang Lianguo et al. proposed the structural model of the expert system for bolt grouting support design of soft rock roadway, constructed the core reasoning engine of the expert system for bolt grouting support, and realized the optimization design of bolt grouting support scheme and parameters for soft rock roadway15,16. Based on the deep and shallow grouting reinforcement, the integrated support technology with bolt grouting support as the core and other supports coordinating with each other is adopted to ensure the stability of deep roadway17,18.

Due to the impact of high stress, underground corrosive water, and high temperatures in deep roadways, the anchor bolts react with the corrosive media in the surrounding environment, leading to degradation of bolt performance and structural damage. This results in frequent fracture and failure of the anchor bolts, reducing the synergistic effect between the bolts and rock, and compromising the stability of deep roadways. This phenomenon has garnered attention from scholars. Wu et al. specifically designed a stress corrosion testing system to study the stress corrosion resistance of various coatings on anchor cables and proposed mechanisms for the initiation, propagation, and catastrophic failure of stress corrosion cracking in anchor cables19,20. Chen et al. developed an in-situ testing method to determine the causes of failure in the installation environment of anchor cables21. Wang et al. analyzed the corrosion behavior and failure mechanism of anchor bolts from both macroscopic and microscopic perspectives, revealing the degree of corrosion at different parts of the bolts22.

Extensive research indicates that bolts can effectively control roadway deformation by supporting the surrounding rock along the gob and cut top. However, these methods have limitations when dealing with deep, fractured rock. Bolt grouting support, on the other hand, can enhance the load-bearing capacity of both deep and shallow fractured rock masses, effectively controlling the deformation of broken rock. Current research on the mechanisms of bolting and grouting in fractured surrounding rock of cut roof roadways is limited. In light of this, the present study leverages roof cutting technology to derive a formula for the bearing strength of the FRP anchoring cable. It further analyzes the bearing strength’s variation under various retaining areas, support parameters, and surrounding rock conditions. The paper offers practical engineering recommendations and applies these in field tests of bolt grouting support, thereby validating the bearing strength of the FRP anchoring cable.

Evolution of roof surrounding rock in deep broken surrounding rock

During the process of retaining roadway, the roadway roof’s surrounding rock exhibits a variety of structures at different stages. Based on the structural characteristics of the roof’s surrounding rock and the mechanical effects of seam cutting, the roadway is categorized into four distinct areas: preparation area of cutting seam, implementation area of cutting seam, influence area of cutting seam, and stability area of cutting seam, as illustrated in Figs. 2 and 3.


Slit preparation area: The roadway roof remains uncracked and unaffected by mining activities. It is only during the construction of the roadway that the surrounding rock stress is redistributed, causing disturbances to the roadway roof. Under the influence of high stress, the roadway roof becomes relatively fragmented, with the structure of the roadway roof exhibiting a symmetrical distribution.

Slit implementation area: The seam cuts through the roadway roof, severing the connection between the roadway roof and the direct roof of the working face. Although the working face has not been mined to this area, its direct roof exerts an oblique supporting effect on the roadway roof. This support helps to prevent significant deformation of the roadway roof due to the influence of the cutting seam. Additionally, the roadway in this area is subjected to the advance pressure from the working face, leading to observable strata pressure behavior.

Slit influence area: Once the working face has traversed the seam, coal seam mining is concluded. The central part of the goaf initiates a direct breakage and collapse, extending to both ends of the working face. The cutting seam severs the connection between the roadway roof and the direct roof of the working face, causing the direct roof to collapse along the seam. This collapse results in the area becoming directly unstable, with the basic roadway roof hanging in a precarious state. Upon reaching its ultimate bearing capacity, the basic roadway roof fractures, bends, and sinks. The presence of the cutting seam positions the basic roadway roof fracture outside the retained cut roof of the roadway. Post-fracture, the basic roof exerts a certain supporting effect on the roof cutting and retaining roadway, mitigating the impact of its fracture. In this area, the roof caving and the basic roof’s bending subsidence are unstable. The collapsed gangue in the goaf does not make contact with the direct roof, leaving an unfilled space. The roadway roof strata are in a state of constant movement, and the stability of the surrounding rock in this critical area is pivotal to the success of the roof cutting and retaining roadway.

Slit stability area: The progression of mining activities leads to a series of structural changes. Initially, the direct roadway roof caves in, followed by the bending and sinking of the basic roadway roof. As the basic roadway roof comes into contact with the gangue, a hinged connection is formed, and the system reaches an equilibrium state. This results in a significant reduction in movement. The gangue within the goaf becomes compacted, and the stress from the upper overburden is transferred to the direct roadway roof, the caving gangue, and the coal body. This transfer of stress marks the achievement of a basic stable state. At this stage, the temporary support structures in the roadway are safely removed, as described by Gao23.

Fig. 2
figure 2

Roof structure of self-formed roadway.

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Fig. 3
figure 3

Evolution law of roof in self-formed roadway.

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Analysis of bearing strength of the FRP anchoring cable in cutting roof and retaining roadway

Roof structure model of cutting roof and retaining roadway

Roadway roof seam cutting technology effectively severs the connection between the goaf roof and the roadway roof, thereby mitigating stress concentration in the surrounding rock. The slurry fills the cracks within the shallow broken surrounding rock, leading to the formation of a regenerated rock mass. This process enhances the self-forming bearing capacity of the shallow broken rock. High-strength the FRP anchoring cable, when subjected to high preload, increase the initial support strength of the roadway roof. They not only effectively control the shallow broken surrounding rock but also promote the formation of a deep anchor cable compression arch, improving its stress state24.

The composite structure, formed by the interaction and coupling of the bolt grouting body and the cutting seam structure, is integral to the stability of the system. To simplify the analysis of the bearing structure of the FRP anchoring cable in the cutting roof and retaining roadway, the following basic assumptions are made:

  1. (1)

    The surrounding rock of the roadway is assumed to be isotropic and homogeneous, simplifying the analysis by treating the rock as having uniform properties in all directions.

  2. (2)

    For the roof cutting retaining roadway supported by the FRP anchoring cable, the surrounding rock is considered an elastic-plastic medium, with the anchoring complex adhering to the Mohr-Coulomb failure criterion.

  3. (3)

    The working resistance exerted by the FRP anchoring cable is assumed to be uniformly distributed along the roadway surface, ensuring a consistent support force across the area.

The roof of the cutting and retaining roadway is supported by the FRP anchoring cable. Given the rectangular cross-section of the roadway, The grouting anchor cable, positioned at the shoulder of the roadway, has the angle of installation. Consequently, the geometric parameters of the FRP anchoring cable for roof cutting and roadway retaining encompass the radius R of the boundary curve of the roof grouting anchor cable and the corresponding center angle γ1 and γ2.

Based on the grouting anchor cable support parameters, the following dimensions are defined: the length of the grouting anchor cable is denoted as L, the thickness of the internal cone compression zone is L0, the thickness of uniform compression band is L1, and the cutting angle is ω, the geometric form of bolt grouting complex is shown in Fig. 4.

Fig. 4
figure 4

Structure model of the FRP anchoring cable.

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The thickness of the FRP anchoring cable is the sum of the internal conical compression band and uniform compression band.

The overall thickness of the FRP anchoring cable is constituted by the thickness of the internal conical compression band and the uniform compression band.

$$L_{c} = L_{0} + L_{1} = L - \frac{{D_{{\max }} }}{{2\tan \theta }}$$
(1)

In the equation, θ represents the compression angle of the grouting anchor cable, which is conventionally set at 45°. Dmax denotes the larger value of the row spacing between the FRP anchoring cable.

As depicted in the structural model of the anchoring and grouting complex in Fig. 4, the radius R of the boundary curve and the corresponding central angles γ1 & γ2 of the anchoring and grouting complex are subject to calculation.

$$ \left\{ \begin{gathered} R\sin \left( {\gamma _{1} } \right) = B + L_{C} \hfill \\ R\cos \left( {\gamma _{1} } \right) = R - L_{C} \hfill \\ \end{gathered} \right. $$
(2)

It is deduced from formula 2.

$$R = B + L_{C} + \frac{{B^{2} }}{{2L_{C} }}$$
(3)
$$\gamma _{1} = \arcsin \left( {\frac{{B + L_{C} }}{R}} \right)$$
(4)

It can be obtained from arc DE.

$$\frac{{\psi \pi R}}{{180}} = \frac{{L_{c} \pi \left( {\frac{\pi }{2} - \omega } \right)}}{{180}}$$
(5)
$$\psi = \frac{{L_{C} \left( {\frac{\pi }{2} - \omega } \right)}}{R} = \frac{{L_{C}^{2} \left( {\pi - 2\omega } \right)}}{{2L_{C} B + 2L_{C}^{2} + B^{2} }}$$
(6)

It is obtained by formulas 4 and 6.

$$\gamma _{2} = \arcsin \left( {\frac{{B + L_{C} }}{R}} \right) - \left( {\frac{{L_{C}^{2} \left( {\pi - 2\omega } \right)}}{{2L_{C} B + 2L_{C}^{2} + B^{2} }}} \right)$$
(7)

The starting angle corresponding to the boundary curve of grouting anchor cable in roof is \(\varepsilon = \frac{\pi }{2} - \arcsin \left( {\frac{{B + L_{c} }}{R}} \right)\), the ending angle is \(\gamma _{3} = \frac{\pi }{2} + \arcsin \left( {\frac{{B + L_{c} }}{R}} \right) - \frac{{L_{c}^{2} \left( {\pi - 2\omega } \right)}}{{2L_{c} B + 2L_{c}^{2} + B^{2} }}\).

Calculation of high strength bolt grouting bearing capacity of cut roof roadway

In accordance with the evolutionary principles of roof structures in cut-off roadways, the roadway roof strata within the influence area of the cutting seam exhibit significant movement, manifesting pronounced ground pressure behaviors. This area is crucial for influencing the stability of the surrounding rock mass and is decisive for the efficacy of the roof-cutting and retaining operations. The stable area of the cutting seam lags behind the working face, the roadway roof bends and sinks, making contact with the gangue, reaching an equilibrium state. In this region, the temporary supports are removed. Therefore, both the affected and stable sections are chosen for the analysis of strength of the FRP anchoring cable.

In the roof of the cut roof roadway, the FRP anchoring cable are installed. The FRP anchoring cable are used to inject grout into the roadway roof, filling the drill holes with grout to achieve full-length anchoring of the FRP anchoring cable. Concurrently, the grout consolidates the fragmented surrounding rock into a cohesive mass, thereby increasing the mechanical strength of the surrounding rock.

The FRP anchoring cable for roof cutting and roadway retaining is subjected to a uniform vertical load q, along with support strength from the grouting anchor cable denoted as pi and the strength of single pillar as F, as shown in Fig. 5, the vertical load on the cross section of roadway is pc + f(x) = Fn, the FRP anchoring cable of roof cutting and roadway retaining should ensure the vertical balance25.

  1. ①

    Calculation of vertical load Fn of anchor grouting complex.

Utilizing the Mohr-Coulomb strength criterion, it is assumed that the bearing capacity of the FRP anchoring cable reaches the maximum value under the influence of external loads, the tangential stress pc of roadway surface can be obtained.

$$p_{c} = p_{i} \frac{{1 + \sin \varphi ^{*} }}{{1 - \sin \varphi ^{*} }} + \frac{{2c^{\prime } \cos \varphi ^{*} }}{{1 - \sin \varphi ^{*} }}$$
(8)

In the formula, pc is the tangential stress of roadway, pi is the supporting strength of grouting anchor cable, φ* is the internal friction angle of bolt grouting complex, c′ is the cohesion of bolt grouting complex25.

Fig. 5
figure 5

Bearing capacity model of the FRP anchoring cable in implementation area of self-formed roadway.

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Calculation formula of support strength pi of grouting anchor cable:

$$p_{i} = \frac{{Q_{s} }}{{s_{l} s_{r} }}$$
(9)

In the formula, Qs is the working resistance of grouting cable, sl and sr are row spacing between the FRP anchoring cable.

The bearing capacity of the FRP anchoring cable is the limit state, therefore, the working resistance of grouting anchor cable is the yield strength value.

$$Q_{s} = \sigma _{s} \pi \left( {r_{b}^{2} - d^{2} } \right)$$
(10)

In the formula, σs is the yield strength of grouting anchor cable.

The FRP anchoring cable is subjected to a vertical load Fn on the cross section of roadway, where pc represents tangential stress on the roadway surface, and f(x) denotes the stress increment. Consequently, the vertical force of the anchoring and grouting complex can be expressed as follows:

$$F_{n} = \left( {p_{i} \frac{{1 + \sin \varphi ^{*} }}{{1 - \sin \varphi ^{*} }} + \frac{{2c^{\prime } \cos \varphi ^{*} }}{{1 - \sin \varphi ^{*} }}} \right)L_{C} + \int\limits_{0}^{{L_{C} }} {f\left( x \right)dx}$$
(11)
  1. ②

    Calculation of vertical component of uniformly distributed load on anchor grouting composite.

Fig. 6
figure 6

Overburden load distribution of the FRP anchoring cable.

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The unit arc block is selected for stress analysis, and the stress is integrated to obtain the size of the overlying load of the anchor grouting complex, as shown in Fig. 6.

$$ds = Rd\chi$$
(12)
$$F_{q} = \int\limits_{s} {q\sin \chi ds = \int\limits_{\varepsilon }^{{\gamma _{3} }} {q\sin \chi Rd} } \chi$$
(13)
  1. ③

    Equilibrium equation of bolt grouting complex

The force exerted by the compacted gangue in the goaf on the slot is denoted as q1. The calculation formula for the strength of the anchor-grouting composite body considering q1 is as follows:

$$F = q_{1} L_{C} \sin w$$
(14)

The equilibrium equation in the vertical direction of the FRP anchoring cable is shown in the following formula.

$$F_{n} = F_{q} - F_{p} - F$$
(15)

By substituting formulas 11 and 13 into formula 14, the following calculation results are obtained.

$$\left( {p_{i} \frac{{1 + \sin \varphi ^{*} }}{{1 - \sin \varphi ^{*} }} + \frac{{2c^{\prime } \cos \varphi ^{*} }}{{1 - \sin \varphi ^{*} }}} \right)L_{c} + \int\limits_{0}^{{L_{c} }} {f\left( x \right)} dx = \int\limits_{\varepsilon }^{{\gamma _{3} }} {q\sin \chi Rd\chi - 2p_{i} B - F}$$
(16)

In accordance with formula 15, the formula for calculating the uniformly distributed load exerted onthe FRP anchoring cable is derived as presented below.

$$q = \frac{{\left( {p_{i} \frac{{1 + \sin \varphi ^{*} }}{{1 - \sin \varphi ^{*} }} + \frac{{2c^{\prime } \cos \varphi ^{*} }}{{1 - \sin \varphi ^{*} }}} \right)L_{c} + \int\limits_{0}^{{L_{c} }} {f\left( x \right)} dx + 2p_{i} B + F}}{{R\left( {\cos \varepsilon - \cos \gamma _{3} } \right)}}$$
(17)

Formulas 1, 9, 10 and the starting and ending angles of grouting anchor cable boundary curve are brought into Formula 16, it is assumed that f(x) is a linear function and the slope is k, such that f(x) = kx, the formula for calculating the bearing strength of the FRP anchoring cable within the influence area is derived as follows.

$$q = \frac{{\left( {\frac{{\sigma _{s} \pi \left( {r_{b}^{2} - d^{2} } \right)}}{{s_{l} s_{r} }}\frac{{1 + \sin \varphi ^{*} }}{{1 - \sin \varphi ^{*} }} + \frac{{2c^{\prime } \cos \varphi ^{*} }}{{1 - \sin \varphi ^{*} }}} \right)\left( {L - \frac{{D_{{\max }} }}{{2\tan \theta }}} \right) + k\left( {L - \frac{{D_{{\max }} }}{{2\tan \theta }}} \right)^{2} + \frac{{2B\sigma _{s} \pi \left( {r_{b}^{2} - d^{2} } \right)}}{{s_{l} s_{r} }} + F}}{{R\left( {\frac{{B + L_{C} }}{R} + \sin \left( {\arcsin \left( {\frac{{B + L_{C} }}{R}} \right) - \frac{{L_{C}^{2} \left( {\pi - 2\omega } \right)}}{{2L_{C} B + 2L_{C}^{2} + B^{2} }}} \right)} \right)}}$$
(18)

In the same way, the formula for calculating the bearing strength of the stability area of the FRP anchoring cable can be derived.

$$q = \frac{{\left( {\frac{{\sigma _{s} \pi \left( {r_{b}^{2} - d^{2} } \right)}}{{s_{l} s_{r} }}\frac{{1 + \sin \varphi ^{*} }}{{1 - \sin \varphi ^{*} }} + \frac{{2c^{\prime } \cos \varphi ^{*} }}{{1 - \sin \varphi ^{*} }}} \right)\left( {L - \frac{{D_{{\max }} }}{{2\tan \theta }}} \right) + k\left( {L - \frac{{D_{{\max }} }}{{2\tan \theta }}} \right)^{2} + \frac{{2B\sigma _{s} \pi \left( {r_{b}^{2} - d^{2} } \right)}}{{s_{l} s_{r} }} + q_{1} L_{C} \sin \omega }}{{R\left( {\frac{{B + L_{C} }}{R} + \sin \left( {\arcsin \left( {\frac{{B + L_{C} }}{R}} \right) - \frac{{L_{C}^{2} \left( {\pi - 2\omega } \right)}}{{2L_{C} B + 2L_{C}^{2} + B^{2} }}} \right)} \right)}}$$
(19)

Analysis on bearing strength law of bolt grouting complex in cutting roof and retaining roadway

  1. (1)

    Influence of different parameters on bearing strength of the FRP anchoring cable

The factors that affect the stability of roof cutting and retaining roadway mainly include the supporting parameters of grouting anchor cable, the mechanical parameters of the FRP anchoring cable and the cross-section dimensions of roadway. When the bearing capacity of the the FRP anchoring cable under different factors is determined, it becomes feasible to propose rational parameters for grouting anchor cable support and grouting for the roof cutting and retaining roadway. This allows for the optimization of the design of grouting anchor cable support parameters.

According to the regulations of the People’s Republic of China’s coal industry standard “Fiber-glass reinforced plastics bar of bolt and accessories,” the tensile strength of the FRP anchoring cable is not less than 300 MPa, and the anchoring force is not less than 80kN.The article focuses on the influence area of the cutting seam in the roof cutting and retaining roadway as the subject of study. the factors such as length, diameter, row spacing, preload, cohesion of anchor grouting complex and internal friction angle are considered respectively, then studied the variation rule of bearing capacity of bolt grouting complex under the influence of different factors. It takes into account various factors, including the length, diameter, row spacing, preload, cohesion, and internal friction angle of the FRP anchoring cable. The research then examines the variation in the bearing capacity of the FRP anchoring cable under the influence of these different factors.

Fig. 7
figure 7

Variation of bearing strength of the FRP anchoring cable under different parameters. (a) Cohesion of anchoring and grouting complex. (b) Internal friction of anchoring and grouting complex. (c) Preload of grouting anchor cable. (d) Row spacing of the FRP anchoring cable; (e) Length of grouting anchor cable. (f) Diameter of grouting anchor cable.

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The following results have been obtained through comparative analysis as illustrated in Fig. 7:

  1. ①

    As cohesion and the internal friction angle increase, so does the bearing strength of the FRP anchoring cable, the relationship between the bearing strength and cohesion is linear and positive, while a quadratic function describes the relationship with the internal friction angle. For instance, When the cohesion c’ increases from 0.71 MPa to 1.74 MPa, the bearing strength of the FRP anchoring cable increases from 3.27 MPa to 4.85 MPa, reflecting a 48% improvement. Similarly, when the friction angle of the composite increases from 21.80° to 51.24°, the bearing capacity of he bolt grouting complex jumps from 3.27 MPa to 7.14 MPa, indicating an impressive 118% improvement.

Therefore, employing high-strength anchoring and grouting to reinforce fractured surrounding rock is beneficial. It consolidates the broken rock into a cohesive unit, enhancing the integrity of the surrounding rock. By improving the cohesion and internal friction angle of the grouted rock mass, the bearing strength of the FRP anchoring cable is significantly enhanced.

  1. ②

    The bearing strength of the FRP anchoring cable is positively correlated with the preload applied. As the row spacing between the FRP anchoring cable increases, the bearing strength of the FRP anchoring cable decreases, following a quadratic function relationship. For example, when the preload of the grouting anchor cable increases from 100 kN to 200 kN, the bearing strength of the FRP anchoring cable increases from 3.27 MPa to 3.57 MPa, which is a 9% improvement. Conversely, when the row spacing of the FRP anchoring cable increases from 0.8 m to 2 m, the bearing strength of the FRP anchoring cable decreases from 3.99 MPa to 2.25 MPa, representing a 44% reduction.

Therefore, applying a high preload to the grouting surrounding rock with the FRP anchoring cable is effective in enhancing the bearing strength of the the FRP anchoring cable. Reducing the row spacing between the FRP anchoring cable and increasing the support density can further improve this strength. By adjusting the parameters of the grouting anchor cable, such as preload and spacing, reasonable support parameters can be determined. These adjustments aim to enhance the stability of the roof cutting and retaining roadway.

  1. ③

    The bearing strength of the FRP anchoring cable increases linearly with the length and diameter of the grouting anchor cable. For instance, extending the length of the grouting anchor cable from 4 m to 10 m results in an increase in the bearing strength of the anchor grouting composite from 2.58 MPa to 4.72 MPa, reflecting an 82% improvement. Similarly, increasing the diameter of the grouting anchor cable from 22 mm to 30 mm raises the bearing strength from 3.27 MPa to 4.14 MPa, which corresponds to an 87% increase.

Thus, augmenting the length and diameter of the grouting anchor cable enhances the bearing strength of the FRP anchoring cable, contributing to the increased stability of the cutting roof and retaining roadway. Reasonable parameters for grouting anchor cable support should be compared and selected based on field conditions and economic viability.

In conclusion, the supporting parameters of grouting anchor cable (preload and row spacing), grouting anchor cable properties (length and diameter) and mechanical parameters of anchor grouting composite have significant influence on bearing strength. The results show that the high preload of grouting anchor cable can increase initial support strength and the stability of roof cutting and retaining roadway, which can ensure the roadway completion effect.

Control mechanism of FRP anchoring cable in deep roadway

In order to ensure the stability of the roadway roof, high-strength anchoring cable must be used to support the roadway roof. The FRP anchoring cable is composed of glass fiber and resin matrix composites. Glass fiber is an inorganic non-metallic material with excellent mechanical properties, possessing good insulation, heat resistance, and corrosion resistance. The resin matrix employs materials such as epoxy resin and vinyl ester resin, and through processes such as pultrusion and winding, the FRP anchoring cable is formed. The FRP anchoring cable is mainly composed of a rod body, threads, a grouting port, and a tray. In recent years, it has been applied in various engineering fields due to its advantages in tensile strength, costs, corrosion resistance ability, etc. The tensile strength of anchoring cables, bolts or reinforcements made of FRP can reach 3–5 times that of steel, with the weight of only 1/4 of steel. Therefore, the tensile strength of a single anchor cable is high and the material cost is low. At the same time, FRP material can resist groundwater corrosion and significantly enhance the durability of underground engineering structures, making it a highly advantageous raw material for anchoring cables. The mechanical characteristics of anchoring cables used in the relying case are shown in Table 1.

Table 1 Mechanical characteristics of anchoring cable.
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For the study of the anchoring performance of FRP anchor bars, FRP anchor bar anchoring specimens is made, with the dimensions of the anchoring specimens being 150 mm × 150 mm × 150 mm (length × width × height), and the anchoring length of the FRP anchor bar being 150 mm. After curing, anchoring specimens of the FRP anchor bar is placed into the anchoring system, with one end of the anchoring system clamps in the test machine’s jaws and the other end of the FRP anchor bar clamps in the test machine’s jaws. The test machine is used to conduct tests on the FRP anchor bar anchoring specimens. The tests reveals that the anchoring force of the FRP anchor bar is 36.3kN, with a peak displacement of 12.1 mm, indicating that the FRP anchor bar has a high anchoring strength in Fig. 8.

Fig. 8
figure 8

The FRP anchoring cable anchorage strength and tensile strength testing.

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Using a Scanning Electron Microscope (SEM), a microscopic test is conducted on the interface between Fiber Reinforced Polymer (FRP) rebars and cement. The test reveals that a transition zone is formed at the interface between the FRP rebars and the cement, which includes the surface of the FRP rebar, the cement paste, and the interlayer between them. This interlayer is found to be free from micro-cracks, voids, and other defects in Fig. 9, indicating that the bonding performance between the FRP rebar and the cement interface is good.

Fig. 9
figure 9

Microscopic images of the interface between FRP bars and cement.

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Engineering application

The Suncun coal mine, located in Tai’an City, China, boasts a maximum mining depth of 1501 m, making it the deepest coal mine in the country. With a designed production capacity of 1.2 million tons per annum (mt/a), it is a quintessential example of a kilometer-deep mine. The mine’s structural form is predominantly monoclinic, characterized by folds. The predominant geological structure is a normal fault. Despite the complexity of the structure, the coal seams are stable. The coal seams that are primarily targeted for mining are seams 2, 4, and 11.

2217 working face is located in the second coal mining area of the second floor at the level of -800 m. The strike length of the working face is 323.2–3124.3 m, and the inclined length of this working face is 178.3–204.6 m. The average coal seam thickness of 2217 working face is 2.41 m, the average dip angle of 2217 working face is 14.6° and the coal seam is relatively stable. The direct roof and bottom are siltstone with thickness of 2.1 m.

The 2217 roadway has a clear width of 4000 mm and a clear height of 2900 mm. The roof support system for the roadway utilizes a combination of the FRP anchoring cable and steel mesh. The specific parameters for the FRP anchoring cable are as follows: a diameter of Φ 22 mm and a length of 6.1 m. The cables are arranged in a grid pattern with a row and line spacing of 1100 mm by 1100 mm, and each cable is preloaded to 120 kN. The mechanical properties of the bolt grouting complex include cohesion of 0.82 MPa and an internal friction angle of 21.8°. Additionally, the cutting angle is set at 20°, and the vertical stress exerted by the surrounding rock on the roadway roof is 2 MPa, the horizontal stress applied by the gob pile is 1.6 MPa.

To determine the bearing strength of the FRP anchoring cable, Eqs. 17 and 18 are applied. The calculated bearing strength in the influence area of the cutting seam is 3.27 MPa, and in the stability area of the cutting seam, it is 3.59 MPa. These values indicate that the bearing strength provided by the FRP anchoring cable exceeds the stress from the surrounding rock, thus satisfying the design specifications.

Evolution law of bearing strength of bolt grouting complex

The characteristics of slit the stability area and slit influence area of the roadway are as follows.


Slit influence area: After the working face passes through the pre-cracked cutting seam, the top of the working face directly collapses along the pre-cracked cutting seam, and the basic top is in a suspended state, resulting in fracture and bending settlement. The roof collapse and basic roof bending settlement in this area are not stable, the collapsed gangue in the goaf is not in contact with the direct roof, and there is a certain unfilled space.


Slit stability area: As the working face is mined, the top of the working face directly falls, the basic top bends and sinks, and the basic top contacts the gangue. The basic top is hinged and connected to reach a balanced state, and its movement slows down. The fallen gangue in the goaf is compacted, and the stress in the upper overlying rock shifts.

It is essential to investigate the evolution of the bearing strength of the FRP anchoring cable in both the influence and stable areas of the cutting seam. Understanding these dynamics will illuminate the bearing characteristics of the FRP anchoring cable in self-formed roadways, which is vital for the design and operational effectiveness of mining operations.

The evolution law of bearing strength of anchoring grouting complex in different areas is as follows: the bearing strength of bolt grouting complex in the stable area of cutting seam is greater than that in the influence area of cutting seam. The main reasons are analyzed as follows: gangue in the goaf of self-formed roadway is compacted, the stress of surrounding rock is balanced again, the cutting seam surface is affected by the surrounding rock stress, and the stress of surrounding rock plays an oblique supporting role on the anchoring grouting complex, the bearing strength of the composite is increased. The bearing strength of influence area of cutting seam is 3.27 MPa, the bearing strength of stability area of cutting seam is 3.59 MPa, and the bearing strength of the two areas is relatively large in Fig. 10.

Fig. 10
figure 10

Bearing strength of the bolt grouting complex in different areas of self-formed roadway.

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Verification test of row spacing between the FRP anchoring cable

In order to verify whether the row spacing of 1100 × 1100 mm between the FRP anchoring cable is reasonable, the field test of grouting diffusion range is carried out to study whether the cracks are filled by slurry tightly when the spacing between the FRP anchoring cable is 1100 × 1100 mm.

After grouting, grouting liquid diffusion peepholes shall be constructed at 0.4 m, 0.8 m, 1.2 m and 1.6 m away from grouting anchor cable. As shown in Fig. 10a, four slurry diffusion peepholes are detected by high-definition borehole peep to detect the scope of slurry diffusion.

According to peep video of 4 boreholes which are 0.4 m, 0.8 m, 1.2 m and 1.6 m away from grouting anchor cable, the effect diagram of grouting anchor cable slurry diffusion range is drawn. In the figure, red indicates that the grout has filled the cracks and blue indicates that the grout has not filled the cracks, which can be obtained from Fig. 11b.

  1. (1)

    When the distance from grouting anchor cable is 0.4 m, the crack filling is relatively dense and the grouting effect is good.

  2. (2)

    When the distance is 0.8 m away from the grouting anchor cable, the crack filling area decreases, but most of the cracks are still well-filled, the diffusion of slurry begins to decline, unfilled area appears in shallow fracture.

  3. (3)

    When the distance is 1.2 m from the grouting anchor cable, the cracks are filled and there are unfilled cracks, the slurry diffusion attenuation increases, and the unfilled areas appear in the shallow and deep parts of the surrounding rock.

  4. (4)

    When the distance is 1.6 m from the grouting anchor cable, there is almost no filling slurry in the fracture, only a small amount of slurry trace is found at the distance of 1 m from the surrounding rock, so the slurry diffusion range is about 1.2 m.

Row spacing between the FRP anchoring cable is 1100 × 1100 mm, which can give full play to the potential of grouting and obtain the maximum grouting thickness. There is no “window” in the overlap of grout diffusion between two rows of the FRP anchoring cable, the grouting is dense, as shown in Fig. 11.

Fig. 11
figure 11

Grouting slurry diffusion monitoring (a) Grout diffusion peephole. (b) Slurry diffusion area (c) Slurry diffusion effect.

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Field monitoring of surrounding rock of roof cutting and roadway retaining

A number of monitoring section is arranged in 2217 roof cutting and roadway retaining with high strength bolt grouting, the convergence ruler is used to monitor the convergence of roof and solid side, and the single pillar pressure gauge is used to monitor the stress of roadway roof to verify the stability of roof cutting and roadway retaining.

Fig. 12
figure 12

Force change of the FRP anchoring cable.

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Within the range ahead of the working face, as the working face retreats during mining, the force of the FRP anchoring cable shows an increasing trend. In the range of 9 to 24 m behind the working face, the force of the FRP anchoring cable experiences a sudden drop due to the influence of the pre-split roof collapse. After about 24 m behind the working face, the force of the FRP anchoring cable tends to stabilize overall. Among them, the maximum force of the FRP anchoring cable is 137.5 kN (Fig. 12).

Figure 13 shows a group of typical curves of convergence and pressure of single pillar during on-site monitoring.

Fig. 13
figure 13

Field monitoring data curve (a) Convergence curve of cutting roof and retaining roadway. (b) Single pillar pressure of cutting roof and retaining roadway.

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  1. (1)

    The pressure of single pillar increases suddenly in the influence area of cutting seam, the maximum pressure of single pillar is 10.2 MPa. When the pressure of single pillar reaches the maximum value, the pressure of single prop decreases from 10.2 MPa to 5.9 MPa. When the working face lags 84 m, the pressure of single pillar tends to be constant. The main reason for this phenomenon is that direct roof of working face collapse and goaf area increases with the working face mining. When the pressure of overlying strata reaches the ultimate bearing capacity of the main roof, the main roof of the working face breaks, which result in a sudden increase in the pressure of single pillar. When the roof of the working face falls, the gangue in the goaf is compacted and the main roof is broken, pressure of working face is reduced, the goaf area tends to be stable.

  2. (2)

    The convergence and deformation rate of the roadway in the influence area of the cutting seam are large, while the convergence of the roadway in the stable area of the cutting seam is small and tends to be stable basically. The overall deformation law of roadway is “roof > entity coal beside the roadway “. The main reason for this phenomenon is that there is no support on the side of the short arm beam n the influence area of the cutting seam, the bearing strength of the FRP anchoring cable is small, and deformation of roadway is large. In the stable area of the cutting seam, the gangue is compacted, the short arm beam is supported by gangue, the bearing strength of bolt grouting complex is large, and the convergence of roadway is small.

Fig. 14
figure 14

Effect of bolting and grouting roadway in self-made roadway.

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To sum up, the field test monitoring results are consistent with the evolution law of the bearing strength of the FRP anchoring cable, which verifies the rationality of the bearing strength of the FRP anchoring cable. The high-strength bolt grouting support can effectively control the deformation of the broken surrounding rock and ensure the implementation effect of the roof cutting retaining roadway (Fig. 14).

Conclusions

  1. (1)

    Considering the structural characteristics of the surrounding rock in the roadway, the area is categorized into four distinct areas: the preparation area, the implementation area, the influence area, and the stability area. An analysis of the structural evolution within the roof surrounding rock of these as has been conducted. Based on this analysis, a structural model for the FRP anchoring cable has been developed, and the geometric parameter calculation formula for the complex has been derived.

  2. (2)

    In accordance with the evolutionary patterns of the surrounding rock structure in the roof cutting retaining roadway, the mechanical model depicting the bearing strength of the FRP anchoring cable has been established. From this model, the calculation formula for the FRP anchoring cable has been deduced, revealing the variation law of the complex’s strength under various influencing factors. By integrating field data on surrounding rock parameters and roof cutting parameters, a set of rational bolt grouting support parameters has been proposed.

  3. (3)

    The bearing strength of the composite is influenced by its cohesion and internal friction angle, showing an increase as these properties rise. Specifically, the bearing strength is directly proportional to the cohesion and follows a quadratic function in relation to the internal friction angle. Additionally, the bearing strength of the FRP anchoring cable exhibits a linear positive correlation with the preload. However, it decreases significantly as the row spacing of the FRP anchoring cable increases, with the relationship between these two factors being quadratic. Conversely, as the length and diameter of the grouting anchor cable increase, the bearing strength of the FRP anchoring cable also trends upward, demonstrating a linear relationship.

  4. (4)

    Field application results of high-strength bolt grouting support demonstrate that the overall deformation of the roof cutting roadway is minimal, with the deformation pattern following the law ‘roof > solid side’. The maximum single support pressure recorded is 10.2 MPa, and the area of single pillar pressure stability extends 84 m behind the working face, which corresponds to the cutting seam stability area. The slurry diffusion range within the fractured surrounding rock is 1.2 m, and with a row spacing of 1100 × 1100 mm between the FRP anchoring cable, the full potential of the grout is realized, achieving the maximum thickness of the grouting body. The overlap of slurry diffusion between adjacent rows of the FRP anchoring cable is seamless, ensuring dense grouting. The use of high-strength bolt grouting effectively controls the deformation of the surrounding rock and ensures the stability of the roof cutting and retaining roadway.