Mechanism and Control Technology of Lateral Load-Bearing Behavior of a Support System Adjacent to Empty Roadways

Abstract

Currently, research on the stability of roadway-side supports in gob-side entry techniques primarily focuses on vertical stress, neglecting the lateral effects induced via roof collapse and waste rock compaction in the mined-out area. This paper systematically investigates the effect of roof rotation and the compression of waste gangue on the lateral load-bearing behavior of the roadway-side support system, combining theoretical analysis with FLAC3D numerical simulations. The results indicate that the lateral load-bearing capacity of the support system is positively correlated with both mining height and the width of the roadway-side support. When the mining height or the support width is small, the lateral load-bearing capacity of the support system is weaker, making it more prone to sliding failure. Furthermore, lateral load control technology for the roadway-side support system is proposed, which includes “roof cutting + increasing width”. When the stress transfer path of the roof is blocked, as the support system width increases from 1 m to 2 m, the lateral load-bearing capacity of the roadway-side support significantly increases and then stabilizes. This results in different extents of expansion in the elastic region within the support system, providing valuable insights for the design of roadway-side supports.

1. Introduction

In the process of coal mining, in order to alleviate the tight situation of mining and excavation succession, gob-side entry technique has been widely applied. During the face recovery process, the original roadway is preserved through the effective filling of roadway-side support and support technology, and it is used as a passage for the adjacent working face. Compared with other mining techniques, the gob-side entry eliminates the section coal pillar, reducing the loss of coal resources and solving the issue of excessive gas concentration in the working face [1,2,3,4]. Meanwhile, for semi-coal rock tunnels, the excavation rate is significantly improved. However, due to the influence of mining-induced stress from the working face, the mining pressure in gob-side entry is more pronounced, making maintenance of the roadway more challenging. The prerequisite for safely extracting coal from a gob-side entry is the stability of the roadway-side support, and its deformation control technology is increasingly being emphasized.

In recent years, many scholars have conducted extensive research on the roadside support bodies of gob-side entry retention. Bai Jianbiao [5] proposed the time-division and zoning reinforcement mechanism of the surrounding rock in the gob-side entry by analyzing the stress and deformation characteristics of the surrounding rock during four stages: roadway excavation, advance mining activities of the working face, the stabilization of the surrounding rock, and reuse of the adjacent working face. He also determined the calculation formulas for the reasonable support resistance of the support system, the roof abscission layer, and the strength of the coal-side support strength. Wang Kai [6] studied the time characteristics of the fracture rotation of key blocks in the basic roof and the deformation mechanism of the surrounding rock, proposing a coordinated support system for deformation along the coal seam in soft and thick coal seams. Zhang Zizheng [7] analyzed the ratio of the bearing capacity of solid coal walls and roadside support system’s bearing capacity, revealing the impact of different unbalanced bearing coefficients on the stability of surrounding rock and the support effect in gob-side entries, as well as the support effect of high-water materials. Xu Jun [8] used a combination of numerical simulation and field experiments to analyze the stress evolution and deformation characteristics of surrounding rock in traditional gob-side entries and cooperative bearing gob-side entries under roof cutting, revealing the cooperative bearing mechanism of the support body and gangue. Hou Gongyu [9] investigated the relationship between the support resistance of the support wall and the length of the exposed roof, indicating the destructive effect of the laterally exposed roof on the support wall. Li Meng [10] improved the Salamon and Terzaghi models in numerical simulations using UP and CDM methods, obtaining the stress–strain relationship of waste rock backfill materials. Based on the test results of a specially designed waste backfill compaction device, he provided a new method for accurately determining the internal stress distribution of the WRBM. Li Yongming [11,12] conducted a systematic study on the filling problem of goaf in inclined coal seams and found that the deformation of the surrounding rock roof of the goaf without filling was large and showed a parabolic change trend. After the goaf was filled with gangue, the sinking deflection of the working face roof was effectively reduced, and the deformation of the surrounding rock of the goaf was effectively controlled. Zhang Jixiong [13], based on solid dense-filling coal mining technology, regarded one side of the goaf as a uniform and dense support body, conducted an in-depth theoretical analysis of the deformation characteristics of the surrounding rock of the goaf, and discussed the action mechanism of the side-support body. Combined with the on-site engineering practice, the mechanical model of the stability of the side-support body was verified, and the corresponding width calculation formula was given. Daria Chepiga [14], based on the physical properties of the lateral expansion and compressive stress of backfill gangue, established the duality of the deformation effect of the gangue protective structure. They discussed the bearing capacity and stress–strain state of the protective structure and explored the binary nature of this deformation effect and its influence on the mechanical performance of the protective structure. L D Pavlova [15] summarized the distribution characteristics of the roof suspension length on vertical displacement and stress near the stope. By controlling the hanging length of the roof strata within 25 m after the support section, it was found that the pre-failure area of the coal body in the advance area increases by 1.4 times before and after the control, and the vertical stress concentration factor increases by 1.25 times. Based on the strain hardening characteristics of goaf materials, Yadav, A. [16], solved the problem of the stress in goaf exceeding the in situ level in the model by simulating the double-yield constitutive parameters, which is helpful to reasonably estimate the redistribution of surrounding rock stress in goaf and improve the accuracy of the model. Mikhail Eremin [17] used the finite difference method, combined with continuous damage mechanics (CDM), to estimate the initial weighting and periodic weighting step of the immediate roof in stages and simulated the stress–strain evolution of the rock mass with the underground opening during the mining process. Faham Tahmasebinia [18], using finite element analysis (FEA) software and employing quasi-static loading, constructed bolt models with varying diameters and strengths to investigate the shear effects of static loads on bolts. The study found a positive correlation between the yield strength of bolts and their maximum force and displacement.

The aforementioned research results have effectively revealed the load-bearing characteristics of support systems and provided methods for the rational design of parameters. However, most scholars have not considered the significant impact of the collapse and compaction of the overlying rock in the mined-out area on the roadway-side support, specifically neglecting the lateral extrusion effect of the waste rock in the mined-out area. Under the lateral thrust of the waste rock in the mined-out area, the collapsed waste rock deforms by extruding into the roadway, thereby reducing the stability of the roadway-side support.

Therefore, based on the engineering background of the leave-the-air roadway working face, this study investigates the impact of roof breakage on the migration behavior of the “support system-basic roof” composite structure and its relationship with the surrounding rock. A mechanical model for the temporal and spatial evolution of the roadway-side support in the gob-side entry retaining is established under the condition of surrounding rock stability. The model derives an expression for the lateral load applied to the support system via the roof movement and determines the coupling relationship between the influencing factors of the roof and the lateral load on the support system. Based on the strain response characteristics of waste rock, the maximum lateral stress exerted via the waste rock on the roadway-side support is calculated through theoretical analysis. Furthermore, using the lateral stability model of the roadway-side support, the lateral load-bearing characteristics of the support system under the combined action of the roof and gangue are explored. Numerical simulation results are used to validate the lateral load-bearing characteristics of the roadway-side support. A load-reduction and stability strategy for the gob-side entry retaining support system is proposed, and its control mechanism is simulated and analyzed. In response to the roof movement characteristics of the gob-side entry retaining and the lateral load-bearing mechanism of the support system, a load-reduction control scheme for the roadway-side support is presented. The study explores the deformation patterns of the support system, as well as the development of the elastoplastic zone, and it proposes lateral load-reduction control techniques for the support system to prevent its migration into the roadway. The research results provide important modeling methods and theoretical foundations for the stability analysis and structural control of the roadway-side support.

2. Lateral Load-Bearing Behavior of the Roadway-Side Support

When the bearing limit is exceeded, the basic roof beam of the gob-side entry retaining undergoes rapid fracture under external force greater than the limit bending moment. The fractured blocks rotate relatively, forming an “O-X” fracture pattern [19,20], and an arc-shaped triangular block, B, forms at the end of the working face. To ensure the stability of the surrounding rock, key block B drives the rotation and extrusion of the lower roof, which causes the roadway-side support to tilt inward, as shown in Figure 1a. Meanwhile, under the compression of key block B, the gangue is spatially constrained, and it laterally acts on the roadway-side support under roof pressure, causing the horizontal sliding of the support, as shown in Figure 1b.

After the surrounding rock deformation in the gob becomes stable, the roadway-side support is subjected to lateral forces, including the support resistance from the roadway, the rotational force from the immediate roof, lateral extrusion stress from the gangue, and frictional resistance from the floor.

2.1. Roof Rotation Side Loading Characteristics

Based on the “S-R” stability principle of masonry beam structures [21,22], during the process of rotational settlement, adjacent blocks interlock to form a stable masonry beam structure. By constructing a superimposed simply supported beam hinge model, with the assumption that each block of the arcuate triangular masonry acts as a plastic hinge, the lateral impact on the roadway-side support during the rapid rotational settlement phase of the overlying strata is investigated. The force analysis of key block B is shown in Figure 2, and the force analysis of the immediate roof block is shown in Figure 3.

Assuming that no compressive deformation occurs in the strata at the roof, and that the weight of the overlying strata acts uniformly on the basic roof, ${\mathit{T}}_{\mathit{a}\mathit{b}}$, ${\mathit{T}}_{\mathit{c}\mathit{b}}$, ${\mathit{F}}_{\mathit{a}\mathit{b}}$, and ${\mathit{F}}_{\mathit{c}\mathit{b}}$ represent the horizontal thrust and shear forces provided via key blocks A and C to key block B, respectively. ${\mathit{G}}_{\mathit{b}}$ is the self-weight of key block B, q is the uniformly distributed load applied to key block B via the overlying weak strata, and ${\mathit{F}}_{\mathit{g}\mathit{j}}$ is the supporting force exerted on key block B via the fallen gangue. ${\mathit{\varphi }}_1$ is the deflection angle of key block B, and ${\mathit{L}}_{\mathit{b}}$ is the lateral length of the key block. ${\mathit{h}}_{\mathit{j}}$ is the thickness of the basic roof strata, and ${\mathit{F}}_{\mathit{z}\mathit{j}}$ is the support resistance provided via the immediate roof to key block B.

When the roof stops rotating, the surrounding rock structure is in a state of equilibrium. For key block B, there are the following:

$$\left\{\begin{array}{l}{T}_{ab}=T_{cb}\\ F_{ab}+F_{zj}+F_{gj}=F_{cb}+G_b+q{L}_b\cos {\varphi }_4\\ M_{F_{zj}}+M_{F_{gj}}+M_{T_{cb}}=M_{G_b}+M_{F_q}+M_{F_{cb}}+M_{T_{ab}}\end{array}\right.$$

(1)

As shown in Figure 4, the settlement of key block B consists of two components: the overall translational settlement, s₁, and the rotational settlement, s₂, at the fracture point on the coal-wall side [23]. When point O is taken as the origin and the gob side as the positive direction of the X-axis, the stresses the points A and B, where key block B contacts the gangue pile at the compression interface, are denoted as f_A, f_B, and the bending moment M_gj, respectively. These are expressed as follows:

$$\left\{\begin{array}{l}{f}_A=\xi \times {\displaystyle \frac{{\epsilon }_A}{\left(K_g-1\right)-K_g{\epsilon }_A}}\\ f_B=\xi \times {\displaystyle \frac{{\epsilon }_B}{\left(K_g-1\right)-K_g{\epsilon }_B}}\\ M_{gj}={\displaystyle \frac{1}{6}}\left(f_A+2f_B\right)l_{s2}^2\end{array}\right.$$

(2)

During the rotation and sinking process of key block B, it is subjected to vertical shear forces ${\mathit{T}}_{\mathit{a}\mathit{b}}$ and ${\mathit{T}}_{\mathit{c}\mathit{b}}$ generated via horizontal extrusion forces ${\mathit{F}}_{\mathit{a}\mathit{b}}$ and ${\mathit{F}}_{\mathit{b}\mathit{c}}$, respectively. When it is assumed that the friction factor is ${\mathit{\mu }}_{\mathit{B}}$ and the hinge contact height is s, we can get the following.

In the above equation, σ_c represents the initial in situ stress of the coal seam, and K_g is the swelling coefficient of the gangue, which reflects its compressibility characteristics.

During the rotational settlement of key block B, it is subjected to vertical shear forces T_ab and T_cb, which are generated via the horizontal pressing forces F_ab and F_bc, respectively. Let the friction coefficient be μ_B, and let the hinge contact height be s. The following relationship can be derived:

$$\left\{\begin{array}{l}{F}_{ab}={\mu }_B{T}_{ab}\\ F_{cb}={\mu }_B{T}_{cb}\\ s={\displaystyle \frac{1}{2}}\left(h_j-L_b\sin {\varphi }_1\right)\end{array}\right.$$

(3)

Calculate the moments and get the following:

$$\left\{\begin{array}{l}{M}_{F_{zj}}={\displaystyle \frac{F_{zj}{l}_{s1}}{2}}\\ M_{T_{ab}}={\displaystyle \frac{T_{ab}s}{2}}\sin {\varphi }_1\\ M_{T_{cb}}=T_{cb}\left(h_j-L_{b2}\sin {\varphi }_1-{\displaystyle \frac{s}{2}}\cos {\varphi }_1\right)\\ M_b=G_b{L}_b\cos {\varphi }_1\\ M_q={\displaystyle \frac{q{L}_b^2{\cos }^2{\varphi }_1}{2}}\\ M_{F_{cb}}=F_{cb}\left({\displaystyle \frac{L_b}{\cos {\varphi }_1}}+\left(h_j-L_b\sin {\varphi }_1-{\displaystyle \frac{s}{2}}\cos {\varphi }_1\right)\tan {\varphi }_1\right)\end{array}\right.$$

(4)

Combining Formulas (1)–(4), we can get the following:

$$\left\{\begin{array}{l}{T}_{ab}=T_{cb}={\displaystyle \frac{G_b{L}_b\cos {\varphi }_1+\frac{1}{2}q{L}_b^2{\cos }^2{\varphi }_1-\frac{1}{6}\left(f_A+2f_B\right)l_{s2}^2-\frac{1}{2}{F}_{zj}{l}_{s1}}{\frac{s}{2}\sin {\varphi }_1-{\mu }_B{L}_b-\left(1+{\mu }_B\tan {\varphi }_1\right)\left(h_j-L_b\sin {\varphi }_1-\frac{s}{2}\cos {\varphi }_1\right)}}\\ F_{zj}=G_b+q_s{L}_b\cos {\varphi }_1-{\displaystyle \frac{1}{2}}\left(f_A+f_B\right)l_{s2}\end{array}\right.$$

(5)

During the fracture and rotation of the basic roof, the underlying direct roof beam is also squeezed and rotated synchronously. The mutual compression and interlocking between the broken blocks of the articulated roof have a certain bearing effect on the overlying rock strata [24]. At the same time, due to the bending and sinking of the roof, the articulated roof structure will also generate a certain force on the lower non-articulated roof structure.

Taking point O₂ as the coordinate origin, according to mechanical equilibrium, we can get the following:

$$\left\{\begin{array}{l}{\displaystyle \frac{T_{sl}}{\sin {\varphi }_2}}+F_{zs}+G_s=F_c+F_z+F_{gsm}\\ {\displaystyle \frac{F_{sm}}{\cos {\varphi }_2}}+F_c+F_z+F_{gsm}=F_{zs}+G_s\\ M_s+F_{zj}\left({\displaystyle \frac{l_s\cos {\varphi }_2}{2}}+h_s\sin {\varphi }_2\right)+G_s{x}_2={\displaystyle \frac{F_c\left(x_0-h_s\tan {\varphi }_2\right)\cos {\varphi }_2}{2}}+F_z\left(x_0+a+{\displaystyle \frac{b}{2}}-{\displaystyle \frac{h_s\tan {\varphi }_2}{2}}\right)\cos {\varphi }_2+F_{gsm}{x}_3\end{array}\right.$$

(6)

Take segment CD to calculate the gangue support resistance F_gsm, which is as follows:

$$\left\{\begin{array}{l}{F}_{gsm}={\displaystyle \frac{1}{2}}\left(f_C+f_D\right){\displaystyle \frac{h_s\cos \left({\varphi }_2-{\varphi }_3\right)}{\sin {\varphi }_3}}\\ x_3={\displaystyle \frac{\left(f_C+2f_D\right)}{3\left(f_C+f_D\right)}}\left({\displaystyle \frac{h_s\sin \left({\varphi }_3-{\varphi }_2\right)}{\sin {\varphi }_3}}\right)+\left(l_s-{\displaystyle \frac{h_s}{\tan {\varphi }_3}}\right)\sin {\varphi }_2\end{array}\right.$$

(7)

Taking $O_1$ as the origin, we can find the bending moment:

$$T_{sl}={\displaystyle \frac{1}{h_s}}\left\{\begin{array}{l}{F}_{zj}{l}_s\cos {\varphi }_2+2G_s\left(x_2-h_s\tan {\varphi }_2\right)-F_c\left({\displaystyle \frac{x_0}{2}}-h_s\tan {\varphi }_2\right)\cos {\varphi }_2\\ -F_{gsm}(x_3-h_s\tan {\varphi }_2)-F_z\left(x_0+a+{\displaystyle \frac{b}{2}}-h_s\tan {\varphi }_2\right)\cos {\varphi }_2\end{array}\right\}$$

(8)

The self-weight of the direct top block G_s is as follows:

$$G_s={\displaystyle \frac{{\gamma }_s{L}_{b1}{h}_s{l}_s(2-\sin {\varphi }_3)}{2}}$$

(9)

In the formula, γ_s is the basic top rock beam density; L_b1 is the basic top block length.

Therefore, the lateral horizontal force F_z−x exerted via the direct top block on the roadside support is as follows:

$$F_{z-x}={\displaystyle \frac{h_s\sin {\varphi }_2}{2h_s\tan {\varphi }_2-\left(x_0+a+\frac{b}{2}\right)}}\left\{\begin{array}{l}{F}_c\left({\displaystyle \frac{x_0}{2h_s\tan {\varphi }_2}}\right)-F_{zj}\left(1+{\displaystyle \frac{l_s}{h_s\tan {\varphi }_2}}\right)\\ -G_s\left[1+{\displaystyle \frac{2\left(x_2-h_s\tan {\varphi }_2\right)}{h_s\sin {\varphi }_2}}\right]+F_{gsm}{\displaystyle \frac{(x_3-h_s\tan {\varphi }_2)}{h_s\sin {\varphi }_2}}\end{array}\right\}$$

(10)

2.2. The Lateral Loading Characteristics of Gangue

2.2.1. The Strain Response Characteristics of Gangue

Under the influence of the overlying strata load and self-weight, the immediate roof behind the gob area undergoes fracturing and collapse, forming fallen gangue, which exhibits typical swelling and compressive deformation characteristics similar to those of general rocks. The voids within the fallen gangue in the gob gradually close under the compaction effect of the overlying strata, and the gangue particles experience flow and rearrangement. Relative shearing and sliding occur between the particles [25,26], which can be considered ideal non-cohesive particles. During the slow compaction phase, influenced by the bending and settlement of the overlying strata and the restrictions imposed via the roadway-side support structure, the bearing capacity of the gangue gradually increases, and the lateral pressure exerted on the roadway-side support transitions into active lateral pressure. The progressive compaction process of the fallen gangue in the collapse zone and its bearing mechanical properties have a critical impact on the stress state of the roof of the gob and the roadway-side support structure.

Assuming that the compression strain of the gangue occurs continuously, during the phase when the roof has not yet touched the gangue, the effect of its own gravity is neglected, and the deformation of the gangue in the gob is represented as follows: ε = σ = 0.

During the gangue compaction phase, the stress–strain relationship is described as follows [27,28]:

$$\sigma ={\displaystyle \frac{10.39{{\sigma }_c}^{1.042}}{K^{7.7}}}\times {\displaystyle \frac{\epsilon }{1-\frac{K}{K-1}\epsilon }}$$

(11)

Among them, E₀ is the initial elastic modulus of the collapsed gangue, ε_m is the maximum strain reached after the rock mass is broken and expanded, K is the expansion coefficient of the collapsed gangue in the goaf, and σ_c is the initial vertical stress at the top of the coal seam.

Based on the mass ratio of gangue particles of various particle sizes, the gangues with the same original gradation, porosity, and mechanical properties at different particle sizes in the gob are selected [29], as shown in

Table 1. The mass ratio of gangue in each particle size grade.

Particle Size Grade/mm	Grading Total Weight/kg	Proportion/%	Particle Size Grade/mm	Grading Total Weight/kg	Proportion/%
0~5	47.988	15.48	20~25	79.67	25.7
5~10	55.304	17.84	25~31.5	26.567	8.57
10~16	32.581	10.51	31.5~40	13.516	4.36
16~20	36.704	11.84	40~50	17.67	5.7

. The gangue compression device specifications are chosen to be an outer diameter of 0.13 m and a cylinder depth of 0.5 m. Fine gangue particles are placed into the test container with a gangue height of approximately 0.468 m. The test process is shown in Figure 5. The results of the gangue compression test are presented in Figure 6.

As shown in Figure 6b, the stress–strain curve during the compression of gangue generally follows the trend of Equation (4). The axial strain of the gangue exhibits strong segmental behavior, namely the compaction phase and the stable consolidation phase.

When the strain is less than 0.17 (the initial compression phase of the gangue), the aggregate in the gangue-bearing structure within the test device is composed of larger-sized gangue particles. The voids between the gangue blocks are relatively large, providing ample deformation space. Under load, the larger gangue blocks primarily bear the load and compress and interlock with each other. The gangue experiences rigid failure, resulting in deformation, fragmentation, and rotation, while the voids between the blocks decrease.

When the strain is between 0.17 and 0.36 (the mid-compression phase), the strain rate gradually slows down. Under load, the voids within the blocks are continuously compressed, and the fine particulate gangue generated via this compression gradually fills the voids between the blocks. The load-bearing structure gradually transitions from being dominated by large-sized gangue particles to a coupled structure of large particles and fine particulates.

When the strain exceeds 0.36, the voids within the block structure are almost completely compressed and filled, resulting in a significant increase in the gangue’s bearing capacity. As the load increases, the strain rate slows down noticeably, indicating that the gangue has been fully compacted and is in a stable state.

2.2.2. Study on the Lateral Compression Characteristics of Gangue

Assuming the coal seam is nearly horizontal, the gangue in the gob area is affected by the dip angle and fills the gob accordingly. Based on the fundamental assumptions of Coulomb’s earth pressure theory [30], the fallen gangue is divided into two regions: the compaction phase (gangue region A) and the stable consolidation phase (gangue region B). Under the influence of the critical layer, gangue region A is subjected to lateral confinement from adjacent gangue and overlying strata load, with a corresponding unit weight denoted as γ₁. In contrast, gangue region B is influenced by the self-weight of gangue region A, the overlying load, and the stress from the base plate in the vertical direction, with its unit weight denoted as γ₂. Gangue region B is selected as the object of study. According to the theory of retaining walls, when the overlying load exceeds the internal shear strength of the compacted gangue pile, a sliding shear plane will form within the compacted gangue. The lateral resistance of the roadway-side support then acts on the gangue, resulting in the lateral pressure exerted on the support structure under the equilibrium state of the gangue. In line with typical engineering practices, the following is assumed [31]:

(1)

In the compressed state, the fractured gangue is considered the primary object of study, and the inter-particle cohesion, c, is neglected.

(2)

The roadway-side support structure is made from grout material, which can be treated as a rigid barrier to support the gangue pile. After the grout material consolidates, it can be considered a homogeneous, non-cohesive body with no vertical friction between it and the gangue.

(3)

When the gangue in the gob and the surrounding rock reach the ultimate equilibrium state, a sliding failure surface exists within gangue zone B. This surface is an inclined plane passing through the heel of the wall, with a sliding wedge-shaped triangular body, ABC, forming. The triangular body is considered a rigid body.

(4)

The load, q, applied to the gangue wedge can be approximated as a linear distribution.

(5)

The soil pressure problem is treated as a two-dimensional problem, and calculations are performed using a unit wall length, l_m.

The basic model of the Coulomb soil pressure distribution is shown in Figure 7a. Figure 7b illustrates the force distribution of the wedge-shaped triangular body ABC within gangue zone B. When the gangue wedge reaches the ultimate equilibrium state, the force equilibrium of the system can be analyzed using the rigid equilibrium method in theoretical mechanics, allowing the calculation of the lateral pressure on the barrier (AB) in the gangue equilibrium state.

In addition, P_a is the lateral support resistance required to maintain the stability of the wedge-shaped body. ϕ₄ is the angle between the load q_B and the horizontal plane. ϕ₅ is the angle between P_a and the horizontal plane. ϕ₆ is the wedge angle, which is the angle between the hypothetical sliding surface of the wedge-shaped body and the horizontal plane. F_n is the support force exerted via the gangue pile at the bottom of gangue zone B on the wedge-shaped triangular body. β is the internal friction angle of the gangue. G_MB is the self-weight of the wedge-shaped triangular body. h_m is the mining height of the coal seam. If the equilibrium of the wedge-shaped body is to be maintained, the following condition holds:

a.

The self-weight, G_MB, of the wedge-shaped triangular body.

Under the influence of the wedge-shaped sliding surface and the constraints of the roadway-side support structure, the self-weight, G_MB, of the wedge-shaped triangular body can be calculated using the following equation:

$$G_{MB}={\gamma }_2{V}_{MB}={\displaystyle \frac{{\gamma }_2{h_m}^2\cos {\varphi }_4\times \cos {\varphi }_6}{2\sin ({\varphi }_6-{\varphi }_4)}}$$

(12)

In the equation, h_m represents the length of the wedge-shaped triangular body in the vertical direction.

b.

The calculation of the overlying load, F_D, on the triangular block.

The overlying load, F_D, on the triangular block is composed of two parts: the self-weight, G_AM, of the gangue above the triangular block and the load, F_d, from the roof. The relationship is as follows:

$$\left\{\begin{array}{l}{F}_d=F_{gj}+F_{gsm}\times \sin {\varphi }_3\\ G_{MA}={\displaystyle \frac{{\gamma }_1(h_s+H_m-h_m)h_m\cos {\varphi }_4\times \cos {\varphi }_6}{\sin ({\varphi }_6-{\varphi }_4)}}\end{array}\right.$$

(13)

c.

The lateral support resistance, P_a, of the roadway-side filling wall.

d.

The support force, F_n, exerted via the lower gangue pile.

By solving the system of Equations (12) and (13), the solution can be obtained as follows:

In the horizontal direction,

$$P_a\cos {\varphi }_5=F_n\sin (\beta -{\varphi }_6)$$

(14)

In the horizontal direction,

$$G_{MB}+F_D=P_a\sin {\varphi }_5+F_n\cos (\beta -{\varphi }_6)$$

(15)

When β = ϕ₆, i.e., when the support force F exerted via the lower gangue pile is directed vertically upwards, the following Equation (14) can be obtained:

$$P_a=0$$

(16)

When β ≠ ϕ₆, solving the system of Equations (12)–(15) yields the following:

$$P_a={\displaystyle \frac{\tan {\varphi }_5\times \tan (\beta -{\varphi }_6)}{2\times \left[\tan {\varphi }_5\times \tan (\beta -{\varphi }_6)+1\right]}}\times \left[\begin{array}{l}{\displaystyle \frac{h_m\cos {\varphi }_4\times \cos {\varphi }_6\left[2{\gamma }_1\left(h_s+H_m-h_m\right)+{\gamma }_2{h}_m\right]}{\sin ({\varphi }_6-{\varphi }_4)}}\\ +\left(f_A+f_B\right)+{\displaystyle \frac{\left(f_C+f_D\right)h_s\cos ({\varphi }_2-{\varphi }_3)}{\sin {\varphi }_3}}\end{array}\right]$$

(17)

For the convenience of calculation, assume ϕ₄ = ϕ₅ = 0, meaning that the lateral support resistance, P_a, exerted via the roadway-side filling wall acts vertically on the edge (AB), and the gangue wedge edge (AC) is horizontal. Equation (17) simplifies to the following:

$$\begin{array}{l}\left\{\begin{array}{l}{P}_a=={\displaystyle \frac{\tan (\beta -{\varphi }_6)}{2}}\times \left[{\displaystyle \frac{h_m\left[2{\gamma }_1\left(h_s+H_m-h_m\right)+{\gamma }_2{h}_m\right]}{\tan {\varphi }_6}}+{\displaystyle \frac{10.39\times (K_g-1){\sigma }_c^{1.042}\sin {\varphi }_2}{K_g^{7.7}}}\times Q\right]\\ Q=\left[\begin{array}{l}{\displaystyle \frac{\left(l_s-l_{s1}\right)\left[\left(l_s+l_{s1}\right)(K_g-1)\left(h_c+h_s\right)-2l_s{l}_{s1}{K}_g\sin {\varphi }_2\right]}{2\left[(K_g-1)\left(h_c+h_s\right)-K_g{l}_{s1}\sin {\varphi }_2\right]\left[(K_g-1)\left(h_c+h_s\right)-K_g{l}_s\sin {\varphi }_2\right]}}\\ +{\displaystyle \frac{h_s\cos ({\varphi }_2-{\varphi }_3)}{\sin {\varphi }_3}}\left({\displaystyle \frac{\left(2l_{s1}-h_s\tan {\varphi }_2\right)(K_g-1)+2K_g{l}_{s1}\left(h_s\tan {\varphi }_2-l_{s1}\right)\sin {\varphi }_2}{\left[(K_g-1)-K_g{l}_{s1}\sin {\varphi }_2\right]\left[(K_g-1)-K_g\left(l_{s1}-h_s\tan {\varphi }_2\right)\sin {\varphi }_2\right]}}\right)\end{array}\right]\end{array}\right.\\ \end{array}$$

(18)

Based on the single influencing factor control variable method, with other parameters held constant, the impact of the roadway-side support structure’s lateral compression of the gangue pile on the lateral resistance is analyzed. In the gangue compaction phase during the roof stability stage, assume that the gangue’s unit weight, γ₁, in the loose state is 18.5 kN/m³, and the unit weight γ₂ in the compacted state is 22.2 kN/m³. Since the height of the collapse zone is 3–8 times the mining height, take h_k = 5h_m. By substituting the appropriate parameters into Equation (18), the distribution characteristics of the lateral load on the roadway-side support structure under various influencing factors are obtained, as shown in Figure 8a–f. In the figure, the horizontal compressive stress toward the roadway-side is indicated as “+”.

In Figure 8, it can be seen that the distribution characteristics of lateral load on the roadway-side support structure under the influence of various factors are as follows:

(1)

As the internal friction angle of the gangue increases, the efficiency of the vertical stress transitions into compressive stress after the gangue compaction improves, and the lateral pressure on the support structure increases gradually.

(2)

As the wedge angle increases, the vertical support resistance from the gangue beneath the wedge-shaped triangular block ABC increases, thereby reducing the lateral resistance on the roadway-side support structure.

(3)

As the mining height, the thickness of the roof beam, the arc of roof rotation, and the length of key block B increase, the required gangue support force for stabilizing the basic roof “given deformation” stage also increases. The compression and compaction degree of the collapsed gangue pile increases, resulting in a higher lateral pressure on the roadway-side support structure.

2.3. Stability of Lateral Sliding of Supporting Body Beside Gob-Side Entry

From the perspective of the overall stability of the side support of the gob-side entry, during the entire process of “given deformation” and compaction bearing in the interaction between the key block and the gangue in the gob, the roadway-side support exhibits a tendency to rotate and slide inward due to the effects of roof rotation and lateral compression from the gangue pile [32]. Based on the analysis of the ultimate equilibrium conditions of the support structure, a self-stability model for the coal gangue roadway-side support can be obtained, as shown in Figure 9.

From Figure 9, it can be seen that assuming the unit length of the roadway-side filling wall is L_m, the external forces acting on it are uniformly distributed and applied at L_m/2. When the roadway-side support is in a critical sliding state laterally, the lateral thrust exerted via the filling area on the filling wall must be less than or equal to the sum of the frictional force at the interface between the roof, floor, and the wall and surrounding rock, as well as the internal shear strength of the wall, that is,

$$F_z\sin {\varphi }_2+P_a\le f_1+f_2+2F_{Jb}+F_{Jx}\cos {\varphi }_7$$

(19)

where ϕ₇ is the angle between the slip surface inside the roadway-side support and the horizontal, F_Jy is the compressive resistance of the internal slip surface, G_c is the self-weight of the roadway-side support per unit length, F_z is the deformation resistance of the overlying rock layer, and F_Jb is the shear resistance of the internal wall surface of the roadway-side support per unit length. f₁ and f₂, are the static frictional forces generated via the resistance to lateral pressure at the upper and lower ends.

According to Mohr’s circle theory, when the angle of the slip surface φ = 45° + ϕ₇/2, the shear resistance is maximized. Therefore, the following condition holds:

$$\left\{\begin{array}{l}{f}_1={\mu }_1\left(F_z\cos {\varphi }_2\right)\\ f_2={\mu }_2\left(F_z\cos {\varphi }_2+G_c\right)\\ G_c={\gamma }_3{L}_m{L}_n{h}_m\\ F_{Jb}=c_w{L}_n{h}_m\\ F_{Jx}={\displaystyle \frac{L_m{L}_n}{\cos {\varphi }_7}}\left(c_w+{\sigma }_{Jy}\tan {\varphi }_7\right)\end{array}\right.$$

(20)

In the equation, μ₁ and μ₂ represent the friction coefficients between the filling wall and the roof and floor, respectively. γ₃ is the unit weight of the support structure. c_w is the cohesion of the roadway-side support structure, and ϕ_w is the internal friction angle of the slip surface.

By solving the system of Equations (19) and (20), the following can be obtained:

$$P_a\le \left({\mu }_1+{\mu }_2\right)F_z\cos {\varphi }_2+\left({\mu }_2+\tan {\varphi }_7\right){\gamma }_3{L}_m{L}_n{h}_m+\left(2c_w{h}_m+L_m{c}_w\right)L_n$$

(21)

From Equation (21), it can be seen that the lateral sliding stability of the roadway-side support wall is influenced by factors such as the mining height, h_m, and the support structure width, L_n, as shown in the figure. Overall, the degree of lateral loading on the roadway-side support is positively correlated with h_m and L_n. When h_m or L_n is smaller, the lateral bearing capacity of the roadway-side support is weaker, making it more prone to sliding failure. Conversely, when h_m or L_n is larger, the lateral bearing capacity of the roadway-side support increases, making it less likely to experience sliding failure and thus making the roadway-side support more stable. At the same time, according to Equation (21), the relationship between lateral load and mining height and supporting width of roadway-side supporting body is drawn, and the results are shown in Figure 10.

A.A. Ashimova [33] improved the shear strength of rock by loading a hardening composition into rocks with an array of cracks to achieve hardening in the weakened areas. This result significantly enhanced the shear resistance of the rock. Generally, most roadway-side supports focus on their compressive strength, with little research on improving their shear strength. This provides new research directions for the stability study of shear slip in roadway-side support systems and offers new ideas for the design of support materials and the construction of support systems in the future.

3. Gangue-Side Lateral Pressure-Control Technology

By analyzing the lateral loading characteristics of the roadway-side support, including the overlying roof and gangue in the gob during the coal seam recovery process, it is found that the lateral loading on the roadway-side support is related not only to the mechanical parameters of the surrounding rock but also to the movement state of the overlying roof and the bearing width of the roadway-side support itself. Based on the above research findings, a combined control scheme of “roof cutting + rational width design of roadway-side support” is proposed for the stability control of the surrounding rock in the gob-side entry retaining roadway at the 15,150 working face of a certain coal mine.

3.1. Engineering Geological Conditions

Based on the geological conditions of the NO.4 coal seam 15,150 working face in a coal mine in Luoyang, the coal seam has a thickness of 3.0 m and is nearly horizontal. The coal seam has a Pomeroy hardness coefficient of 2.9, with one to two layers of interbedded gangue locally. The roadway is driven along the roof, with a rectangular cross-section, and the clear width × clear height is 5600 × 3000 mm. The schematic diagram of the coal seam column in the working face is shown in Figure 11.

3.2. Construction of the Numerical Simulation Model

3.2.1. Selection of Model Parameters

To more intuitively display the lateral bearing characteristics of the gob-side entry retaining roadway-side support, FLAC3D V6.0 numerical calculation software was used to simulate the initial state of the coal and rock layers, the compaction of the gob, and the mechanical characteristics of the roadway-side support using constitutive models such as Mohr–Coulomb, double-yield, and strain-softening [34,35]. The comparison of the stress–strain relationships for the gob materials in the strain-softening and double-yield models is shown in Figure 12 and Figure 13, while the main mechanical parameters of the collapsed gangue and the roadway-side support are provided in

Table 2. Parameters of double-yield model for bearing characteristics of gangue in goaf.

Category	Density/kg m−3	Bulk Modulus /GPa	Shear Modulus/GPa	Internal Friction Angle/°	Shear Dilatancy Angle/°
Value	1800	17.34	5.23	15	8

,

Table 3. The stress–strain relationship of goaf material in double-yield model.

Strain	Stress/MPa	Strain	Stress/MPa	Strain	Stress/MPa
0	0	0.05	1.096	0.10	6.986
0.01	0.141	0.06	1.524	0.11	13.665
0.02	0.310	0.07	2.114	0.12	67.210
0.03	0.516	0.08	2.980	0.121	104.4
0.04	0.770	0.09	4.373	0.122	229.1

,

Table 4. Strain-softening model parameters of bearing characteristics of roadside filling body.

Category	Value/kg m−3	Bulk Modulus/GPa	Shear Modulus/GPa
Value	1100	0.06	0.0759

and

Table 5. Stress–strain relationship in strain-softening model.

Plasticity Parameters	Cohesion/MPa	Internal Friction Angle/°	Plasticity Parameters	Cohesion/MPa	Internal Friction Angle/°
0	2.9	30	0.2	1.8	22
0.06	2.4	28	0.25	1.7	19
0.1	2.2	26	0.3	1.5	16
0.15	1.8	24	1	1.5	16

.

3.2.2. Construction of the Numerical Model

The model is subdivided and refined to improve computational efficiency while ensuring calculation accuracy. The model dimensions are set to 150 m × 82.18 m × 100 m, with the roadway left along the roof. Displacement boundary conditions are applied to the sides and bottom of the model. The sides are constrained in the normal direction, while the bottom is fully constrained in all directions. An equivalent load of 1.79 MPa is applied to the top, and the lateral pressure coefficient is set to 1. The numerical simulation model for the gob-side entry retaining roadway is shown in Figure 14. The physical and mechanical parameters of coal and rock strata are shown in

Table 6. Physical and mechanical parameters of coal and rock formations.

Serial Number	Appellation	Density/kg m−3	Bulk Modulus/GPa	Shear Modulus/GPa	Cohesion/MPa	Internal Friction Angle/°
1	Fine sandstone	2600	2.7	1.6	2.0	35
2	Grit stone	2300	4.2	2.9	5.0	34
3	Fine sandstone	2550	2.7	1.6	2.0	35
4	Siltstone	2630	5.0	3.8	6.0	35
5	Medium-grained sandstone	2821	3.3	2.5	4.0	37
6	Siltstone	2630	5.0	3.8	6.0	35
7	Medium-grained quartz sandstone	2330	12.22	10.79	2.5	42
8	Mudstone	2940	9.97	7.35	1.2	32
9	Coal	1450	5.3	4.91	1.25	32
10	Sandy mudstone	2785	5.12	4.73	2.45	40
11	Siliceous mudstone.	2699	8.33	5.00	8.9	35

.

3.2.3. Verification of the Numerical Model Feasibility

Firstly, the Mohr–Coulomb constitutive is given to the whole large model. When the mining operation is carried out to the coal seam mining-the establishment of the roadside support body, the double-yield constitutive model is first given to the goaf, and the roadside support body model based on the strain-softening constitutive model is established at a certain distance from the working face. The purpose of this approach is to restore the sequential steps of mining operations as much as possible in order to ensure that the simulation results are similar to the actual results.

To validate the reliability of the gob model [36,37], the vertical stress simulation results of the gob in the 15,150 working face were analyzed. The results are shown in Figure 15 and Figure 16. The vertical stress in the gob gradually increases from 0.4 MPa at the edge to 3.12 MPa at the center of the gob, approximately 80 m from the edge. The vertical displacement increases gradually from 0.025 m at the edge of the gob to 0.58 m at the center. In the vertical direction, the compression displacement of the gangue in the gob decreases from the top to the bottom, and the displacement of the overlying roof layer changes continuously, in line with the roof migration law and the actual field conditions. A comprehensive analysis shows that using the double-yield model to simulate the bearing characteristics of the gob gangue is feasible.

3.3. Design of the Numerical Model for the Cutting Roof Scheme

3.3.1. Analysis of the Impact of Cutting

As analyzed previously, the lateral loading strength of the roadway-side support in the gob-side entry retaining system is not only related to the mechanical properties of the surrounding rock (such as E₀, h_j, β, ϕ₆, etc.) but also strongly influenced by the movement state of the overlying strata during mining (such as φ₁, l_s, etc.), mining height, and the bearing capacity of the gangue in the gob. To effectively reduce the lateral loading on the roadway-side support during the mining process, the pre-splitting top-cutting technique can be employed to mitigate the impact of the roof rotation during mining [38]. By severing the lateral basic roof of the roadway, the cantilever length of the roof is reduced, and the roof strata outside the cut line in the gob can collapse and fill the gob in time, forming a cushion layer. This reduces the rotational displacement space of the upper basic roof, decreasing its rotational angle, and it allows the roof strata to quickly transfer the load to the gob, thus forming a stable structure. As shown in Figure 17, by severing the stress transmission between the overlying strata in the gob and the roof of the entry, when the hydraulic supports are pulled forward, the roof loses its support and rapidly collapses into the gob.

To verify the effectiveness of the top-cutting technique, the simulation results of the vertical stress in the gob-side entry retaining surrounding rock and the vertical displacement of the roof during its movement were analyzed. A FLAC3D numerical model was constructed to simulate the pre-splitting top-cutting interface, for which the interface parameters were adjusted to increase their strength and stiffness so that they are significantly higher than the surrounding rock in the roadway. This simulates the effect of blocking stress transmission along the structural plane. Additionally, the “update” parameter for the interface is set to “off” [39], preventing the surface from moving after the structure is disturbed and avoiding the search for new contact points. This simulates the separation of nodes and discontinuous deformations such as the sliding and opening of the roof structure after the top-cutting process. The results are shown in Figure 18, Figure 19 and Figure 20.

The curve of the roof and floor displacement before and after top-cutting is shown in Figure 18. In the case without top-cutting, the roof displacement remains almost unchanged at a distance of 35 m in front of the working face. However, behind the working face, at distances of 0–40 m, the displacement begins to increase slowly; at distances of 40–80 m behind the working face, the displacement increases rapidly, with the maximum displacement reaching 455 mm. When the top-cutting gob-side entry retaining technology is used, the maximum displacement of the roof and floor stabilizes at 282 mm, significantly reducing the maximum displacement by 38.02%.

Figure 19 and Figure 20 show the lateral horizontal stress distribution in the gob-side entry retaining support body and the internal elastic–plastic region changes before and after top-cutting. The fallen gangue, under the influence of the self-weight of the overlying rock layers, generates lateral horizontal stress along the working face’s direction during the crushing and compaction process, which exerts compressive forces on the roadway-side support. The lateral load distribution is generally “saddle-shaped”. In the case, without top-cutting, the lateral loading on the top and bottom of the roadway-side support is relatively high. When the height is less than 0.85 m, the load gradually decreases from 2.91 MPa to 0.95 MPa; at the middle position, the stress gradually increases to 1.47 MPa, and when the height reaches 2.4 m, the lateral load on the support decreases to 0.88 MPa. As the height continues to increase, the stress rises to 2.31 MPa, and at this point, the support body is almost completely damaged.

In the case with top-cutting, the lateral load change trend is roughly the same, but the top-cutting blocks the stress transfer between the overlying rock layer of the gob and the retaining top plate of the entry. As a result, the fallen gangue heap only bears the self-weight of the overlying rock, reducing the deformation degree. The lateral stress generated on the roadway-side support is overall significantly lower than in the case without top-cutting, with the maximum lateral stress being 2.84 MPa and the minimum being 0.46 MPa. The internal elastic region of the support body remains intact.

The above analysis indicates that the top-cutting gob-side entry retaining technology has a significantly better surrounding rock control effect compared to the traditional gob-side entry retaining technology.

3.3.2. The Calculation of the Cutting Height

The top-cutting height of the gob-side entry retaining must match the burial conditions of the overlying rock layers to ensure the precise collapse of the target rock layer and protect the integrity of the key stratum structure. The design formula for the pre-cracking top-cutting height [40] is as follows:

$$H_F=\left(H_M-\Delta H_1-\Delta H_2\right)/\left(K-1\right)$$

(22)

In the above equation, H_F is the minimum top-cutting height. H_M is the coal seam mining height. ∆H₁ is the bending and subsidence amount of the working face top. ∆H₂ is the floor heave caused by the pressure relief due to coal seam mining. K is the swelling coefficient of the overlying rock layer in the gob.

According to the production geological conditions of the 15,150 working face, the overlying layers consist of mudstone (2.2 m) and medium-grained quartz sandstone (19.2 m), with the swelling coefficient K taken as 1.2. To ensure the smooth collapse of the top layer within the top-cutting height range, the cutting height is increased to 21.4 m.

3.3.3. The Calculation of the Cutting Angle

According to the specific engineering geological conditions of the 15,150 working face, the deformation characteristics of the gob-side entry retaining surrounding rock were simulated for top-cutting angles of 0°, 10°, 20°, 30°, 40°, and 45°. The changes in the elastic–plastic zone were observed, with the results shown in Figure 21.

The lateral load distribution on the roadway-side support at different top-cutting angles is shown in Figure 21. As the top-cutting angle increases from 0° to 40°, the lateral load on the roadway-side support is primarily concentrated at the lower corner near the gob side, with the load at the middle of the support being smaller. The values are 0.46 MPa, 0.53 MPa, 0.49 MPa, 0.37 MPa, 0.39 MPa, and 0.32 MPa, respectively. The load at the top corner is slightly higher. As the cutting angle increases, the deformation of the support side corner, influenced by the torque from the roof rotation, initially decreases and then increases, with the peak horizontal stress within the support gradually decreasing. When the cutting angle reaches 20°, the plastic failure zone of the roadway-side support transitions from primarily compressive failure to shear failure. The support’s internal integrity remains with a relatively complete elastic region. However, when the cutting angle reaches 40°, the support is completely destroyed, and its bearing capacity is significantly reduced. Therefore, the top-cutting angle should not exceed 20°.

Considering the on-site conditions of the gob-side entry retaining and the use of drilling equipment, if the top-cutting angle is too small, the operational workload will increase. After comprehensive consideration, a cutting angle of 20° is recommended.

3.4. The Reasonable Width Design of Roadway-Side Support

Section 2.3 simplifies the mechanical equation of the slip surface when the roadside support body slips. By simplifying the parameters in the equation, the relationship between stability (i.e., the force on the roadside support body) and the coal seam mining height and the width of the support body is calculated, that is, Equation (19), which is similar to a binary relationship. If the sum of the two (coal seam mining height and support body width) is less than the maximum load that the support body can bear, the support body will slip and fail, indicating that its lateral bearing performance is insufficient. Therefore, when the coal seam mining height is determined, it is necessary to control the width of the roadside support body so that the load strength of the roadside support body does not exceed the slip stability characteristics under the influence of the width factor, so as to ensure the stability of the roadside support body. The mining height parameter in Figure 10 is set to 3.0 m. By setting the vertical section, the intersection line of the surface and the section is mapped to obtain Figure 22. Figure 22 shows the relationship between the width of roadway-side support and its lateral bearing strength.

A total of six different support body width schemes were set within the range of 1.0 m to 2.0 m, and the variation laws of the surrounding rock stress in the gob-side entry were obtained under different conditions [41,42]. Figure 23 and Figure 24 show the simulation results of lateral stress and plastic zone distribution of the gob-side entry support body under different width conditions. It is worth noting that the cutting angle refers to the angle between the roof cutting seam and the vertical plane. As the width of the support body increases, its lateral load also increases, and the lateral stress distribution overall presents a trend of initially increasing and then gradually stabilizing, enhancing the overall bearing capacity of the support body. The collapsed gangue, affected by the self-weight load of the overlying strata, generates lateral horizontal stress along the working face direction during the crushing and compaction process, exerting compressive force on the roadway-side support. The lateral load rapidly decays in the 0–0.3 m section from the base of the siltstone. When the width is 2 m, the maximum lateral stress reaches 2.12 MPa, and when the width is 1 m, the stress is 1.03 MPa. The lateral stress reaches a minimum at 0.91 m, with the minimum stress being 0.07 MPa when the width is 1 m. As the height increases, the stress gradually increases, but it remains below 1.5 MPa. As the support body width increases, the lateral horizontal thrust of the gangue pile decreases gradually.

When the width of the roadway-side support body is less than 1.6 m, the variation in lateral load of the support body becomes more pronounced as the width increases, and a more stable elastic region gradually appears inside the support body. The overall bearing capacity of the support body increases. When the width of the support body exceeds 1.6 m, the variation in lateral load tends to stabilize as the width increases, and the elastic region remains essentially unchanged. This indicates the presence of an optimal threshold. By adjusting the width of the roadway-side support body, the strength of the support can be made to match its lateral load, thereby maintaining the stability of the entire support and achieving the goal of lateral load reduction. The overall bearing capacity of the support body increases, and the optimal width is ultimately determined to be 1.6 m.

4. Industrial Experiment

4.1. Wall Filling Technology and Deformation Control Scheme for Gob-Side Entry Retention

The basic construction process of the roadway-side support body is shown in Figure 25.

The schematic diagram of the strengthened support for the roadway-side support body is shown in Figure 26. Two φ22 mm × 1600 mm threaded steel tie rods are applied 500 mm from the roof, with a spacing of 900 mm and a row spacing of 800 mm. The tie rods are connected with a φ14 mm trapezoidal beam. The tail end is equipped with a 120 mm × 120 mm, 10 mm thick disk-shaped tray. The lower part of the roadway-side support body, with a height of 1200 mm, is supported using a mesh of anchor beams. At 500 mm from the floor, anchor rods with a 10° inclination are set at the same specifications, with a row spacing of 800 mm. The steel mesh is required to be made of steel bars with a diameter of 6.5 mm, using 12# iron wire for double-strand networking. The overlap of the steel mesh should not be less than 100 mm.

Based on the production geological conditions and equipment setup of the 15,150 working face, the roadway-side support frame is mainly used to reinforce the roof of the roadway within 100 m behind the gob-side entry. The support frame, together with the filling wall, provides effective support to the roadway roof. On the one hand, it prevents the rapid subsidence of the roadway roof; on the other hand, it helps the filling wall achieve sufficient solidification strength, ensuring the stability of the roadway in the later stages. The support system in this area should avoid applying excessive pressure to the roadway roof, which could cause roof damage, and it must also avoid damaging the roadway roof’s anchor bolts and cables. The diagram of roadway-side support reinforcement is shown in Figure 27.

4.2. The Control Effect of the Surrounding Rock in the Gob-Side Entry

Five surface displacement monitoring stations are arranged along the gob-side entry section of the 15,150 working face (already left). A new station is installed every 40 m as the entry advances. The overall layout of the stations is shown in Figure 28.

(1)

Roadway surrounding rock displacement observation.

Displacement monitoring of the gob-side entry surrounding rock was carried out at monitoring stations NO.1, NO.2, NO.3, and NO.4, as shown in Figure 29. The surrounding rock of the 15,150 working face gob-side entry deforms to a certain extent due to mining disturbance, but compared to the large cross-sectional roadway, the deformation is relatively small, and the stability of the roadway-side support is high. During the advancement of the working face, the changes in the horizontal displacement of the roadway roof and the gob-side support body are basically consistent. Displacement increases began to appear in the surrounding rock when the working face was 80–90 m ahead. The maximum displacement of the roof at the NO.1 station was 27 mm, and at the NO.2 station, it was 26.3 mm. The maximum displacement of the side of the roadway-side support at the NO.1 station was 32.44 mm, and at the NO.4 station, it was 31.29 mm. When the distance to the working face reached 150 m, the surrounding rock deformation gradually stabilized.

(2)

Anchor bolt and anchor cable load observation.

The force monitoring results of the pull-back anchor bolts for the gob-side support at station NO.5 are shown in Figure 30. The measuring tool chosen is the bolt stress meter. When stress changes occur within the measured structure, the bolt stress meter is subjected to tension or compression. By reading the data from the dial, the force acting on the bolt at that moment can be determined. The force on the pull-back anchor bolts in the gob-side entry of the 15,150 working face gradually stabilized around the 13th day. The anchor bolt force initially decreased and then gradually increased, eventually maintaining around 115 kN. The maximum yield strength of the same model threaded steel pull bolts is 60.5% of the maximum yield load, which is 189.97 kN. During the axial force observation period, there were no instances of anchor bolt breakage or failure, indicating that the support design meets safety requirements and ensures the production safety of the working face.

No bolt breakage or failure occurred during the axial force observation period, indicating that the support design meets the safety design requirements and ensures the production safety of the working face.

(3)

Deep roof surrounding rock displacement monitoring.

The displacement of the deep roof surrounding rock at stations NO.1 and NO.3 was monitored, and the results are shown in Figure 31.

As the working face advanced, the deformation of the tunnel roof surrounding rock gradually increased. When the monitoring station was located 100 m away, the deep surrounding rock displacement remained consistent. At 80 m from the measurement station, the deformation rate began to slowly increase. After the working face advanced to 40 m from the measurement station, the deformation rate of the transport tunnel slowed down. The displacement of the 4 m baseline point at station No.1 increased from 0.001 mm to 16.2 mm, and the 10 m deep baseline displacement was 12.6 mm. At station No.3, the displacement of the 4 m and 5 m deep baseline points were both 13.8 mm, indicating that no delamination occurred at the roof at this time.

Based on the above observations, it can be concluded that, after the implementation of the load-reduction control plan, the control effect on the surrounding rock of the gob-side entry retaining is significant. The surrounding rock displacement is small, which effectively improves the lateral bearing capacity of the roadway-side support and ensures safe production operations.

Due to the presence of hard rock strata above the coal seam in this study, the characteristics of mine pressure are more pronounced, and pressure-relief methods often adopt principles such as roof cutting and strong support systems with bolts and cables. However, for soft rock roadways, the analysis of surrounding rock stress and the construction of support systems often require different approaches. For example, based on the parting behavior during coal seam mining [43], the study analyzes the weakening and stable-state regions of parting rocks by studying the state of a parting within the seams. Evaluating the stress–strain state (SSS) of parting rocks, including vertical, inclined, and lateral pressures, ensures that the operations in the lower seam do not adversely affect the state of the main upper working network. Furthermore, the boundary of the influence zone caused by mining activities is determined through boundary angles in the inclination and strike cross-sections of the mining face, allowing for the identification of surface deformation induced via coal mining [44]. These findings enrich the theory of rock movement and provide alternative methods for managing roadway deformation during mining operations.

5. Conclusions

This paper has focused on the lateral loading mechanism of the gob-side entry retaining support. A comprehensive approach was used, including laboratory tests, a theoretical analysis, numerical simulations, and field engineering practice. The study separately investigated the lateral loading characteristics of the gob-side entry retaining support under the rotation of the roof and the compressive deformation of the gob material. An effective lateral load-reduction control plan for the gob-side entry retaining surrounding rock was proposed. Based on specific production conditions, field practice was carried out, and the main results obtained are as follows:

(1)

Based on the failure characteristics of the roof, a simply supported beam hinge structure model during the rotation of the key block was constructed to indirectly derive the lateral loading expression of the gob-side entry retaining support.

(2)

Based on the Coulomb soil mechanics model, the lateral loading characteristics of the gob-side entry retaining support under the compaction of gangue in the goaf were obtained, determining the relationship between resistance and factors such as the mining height and swelling coefficient. By combining the analysis of roof rotation, the stability of the lateral load on the retaining wall was examined in relation to support height, width, and the dynamic friction coefficient of the roof and floor. Numerical simulations were used to obtain the lateral loading curve of the retaining wall under the combined effect of roof rotation and gangue compaction.

(3)

Based on the above research, a combined lateral load-reduction control technology for gob-side entry retaining support, termed “Cutting roof and Enhancing support strength,” is proposed. The effect of cutting on the lateral load reduction in the retaining support was simulated, and the relationship between the cutting angle, support width, and lateral load-reduction characteristics was discussed. Based on the geological conditions of the coal seam in the 15,150 working face, suitable parameters were determined: a cutting height of 21.4 m, a cutting angle of 20°, and a retaining support width of 1.6 m. Monitoring through the field installation of measuring points showed that the lateral deformation of the retaining support was effectively controlled, demonstrating the feasibility of the lateral load-reduction control technology.

This paper has preliminarily explored the lateral response characteristics of the roadway-side support under the collapse of overlying strata and the compaction of gangue in the gob. Further research is needed to expand on these findings. For example, the influence of geological structures, as well as the coupling characteristics between the roof, floor, and roadway-side support, on the lateral loading of the support could be considered. Additionally, the lateral movement control technology of the surrounding rock for roadway-side support proposed in this paper is primarily based on roof control techniques, with limited research on direct control techniques for gangue in the gob. The results show that the lateral deformation of the roadside support body is effectively controlled, demonstrating the feasibility of the lateral load-reduction control technology.