Research on the deformation mechanism and ACC control technology of gob‐side roadway in an extra‐thick coal seam with varying thickness

To improve the extraction of coal resources, gob‐side entry driving is gradually being promoted and applied. However, as the thickness of the coal seam increases, the deformation control of the roadway becomes more difficult. Aiming at this problem, a combination of theoretical calculation, numerical simulation, and field test was used to analyze the aspects of the stress environment, surrounding rock properties, and support forms. The phenomenon and mechanization of intensified deformation and failure of roadway with increasing coal seam thickness were revealed. The specific results include: (1) The ratio of principal stress at the excavation position controls the maximum failure depth of roadway, and the direction of principal stress determines the location of the maximum failure depth; (2) As the thickness of the coal seam increases, the properties of the surrounding rock at the excavation position decrease, the principal stress ratio increases, and the deflection angle of the principal stress increases, which lead to an intensification of the deformation and failure of the roadway. Based on the deformation and failure characteristics of the roadway and the shortcomings of the original support form, a control strategy and support scheme based on a new support structure called “Anchor Cable with C‐shaped Tube” is proposed, which has achieved better deformation control effect in on‐site application.


| INTRODUCTION
China is a major coal storage and production country in the world, with the reserves and production of thick and extrathick coal seams accounting for over 40% of the total. 1,2The traditional longwall layered mining method has a large workload, high support cost, and low mining efficiency.The fully mechanized top coal caving technology adopted in the past two decades has improved mining efficiency, but it also faces many challenges, such as large extraction space, wide unloading range, and low resource recovery rate. 3The degree and scope of the impact of mining on the coal seam near the gob (CSNG) have significantly increased.And to improve the recovery rate of coal resources and reduce waste, the method of leaving narrow pillar or no pillar for gob-side enter driving (GED) is gradually being promoted and applied. 4,5However, after the extraction of extra-thick coal seam, the collapse height and rotation angle of the overlying strata are very large, resulting in a more complex stress environment for CSNG.And, due to strong mining and unilateral unloading, the strength and stability of CSNG have also been damaged to varying degrees.Driving roadway in this environment makes it more difficult to control the deformation of the roadway.
To control the deformation of the gob-side roadway, its deformation mechanism should be clarified.Many scholars [6][7][8][9] have conducted research from the perspective of overlying rock structures.Hou et al. proposed a stability principle of large and small structures of the surrounding rock of a fully mechanized gob-side roadway and theoretically analyzed the stress characteristics of the overlying rock structure and its influence on the gob-side roadway. 10Gao et al. used the discrete element method to simulate the movement of the unstable overlying rock structure and discussed the failure mechanism of the gob-side roadway in this process. 11Han et al. also conducted relevant research through physical model experiments. 124][15] Zhang et al. analyzed the relationship between the principal stress difference and roadway deformation through numerical simulation. 16Zheng et al analyzed the distribution of the second invariant of deviatoric stress in CSNG, and studied the crushing mechanism of surrounding rock. 17he majority of the above research is focused on the condition of thick-or medium-thick coal seam, and there is little research on GED in extra-thick coal seam.In contrast, the mining of extra-thick coal seam has a larger impact range and a stronger disturbance to CSNG, resulting in a more complex stress environment when GED.In addition, due to its long occurrence time and the influence of tectonic movements, the extra-thick coal seam has the characteristic of significant thickness fluctuations. 18According to on-site inspections and monitoring, the roadway also exhibits varying degrees of deformation and failure in different coal seam thicknesses, as shown in Section 2 of this paper.And there is still insufficient research on the mechanism of this variation.Therefore, this paper focuses on the deformation mechanism of GED in extra-thick coal seam.Taking Tongxin Mine as an engineering case, based on the deformation and failure of surrounding rock on site, theoretical analysis and numerical simulation methods are used to conduct an analysis of the main influencing factors of roadway deformation and failure, revealing the principle of severe differential deformation in roadway, as shown in Sections 3 and 4 of this paper.
0][21][22] However, due to the large fluctuation of the extra-thick coal seam, it is difficult to choose the most reasonable roadway excavation position for a certain coal seam thickness.In this case, the deformation and failure of the surrounding rock can only be controlled through more effective support.4][25][26] He et al. proposed a joint control technology with an "anchor cable channel steel composite structure" and an "asymmetric anchor cable truss structure" as the core. 27Guo et al. proposed a support scheme using "high-prestress, constant-resistance, and large-deformation anchor cables." 28Xia et al. proposed a support method to strengthen narrow coal pillars with cable-stayed structures at both ends. 29Li and Li et al. have conducted in-depth research on the mechanical properties of materials used for surrounding rock grouting and filling, enriching the control techniques for roadway stability. 30,31he above control methods have all achieved good support effects in their respective projects, but the support is still mainly axial control, with insufficient consideration given to tangential control of the support structure.During the process of deformation and failure of the surrounding rock, lateral displacement and shear expansion of the surrounding rock are very common phenomena, 32,33 resulting in frequent breakage of anchor bolts and cables.5][36] At present, ACC has been successfully applied in roadway support in more than 10 coal mines in China.By utilizing ACC and combining the deformation and failure characteristics and principles of roadway, targeted control strategies and optimized support schemes are proposed.The better support effect has been verified in on-site application, providing reference for similar engineering problems, as shown in Section 5 of this paper.

| Engineering geological conditions
The Tongxin Coal Mine is located in Datong City, Shanxi Province, China, and has an annual output above 16 million tons.The main coal seam is coal seam #3-5 with a burial depth of approximately 450 m.The 8204 working face belongs to the No. 2 panel, the adjacent 8203 working face has been completely mined out, and a 5-m narrow coal pillar is left to protect the 5204 roadway.The average thickness of the coal seam is 17.8 m, which makes it a typical extra-thick coal seam, and the average dip angle of the coal seam is only 2°.The distribution of rock layers on the roof and floor of the coal seam is shown in Table 1.
The 5204 roadway that serves the 8204 working face is excavated along the coal seam floor, and the fully mechanized top coal caving is used for longwall mining.To satisfy the needs of production and transportation, the section size of the roadway is large with a height of 4.0 m, a width of 5.2 m, and a length of approximately 2200 m.However, due to the influence of geological erosion of the coal seam in this area, the coal seam thickness greatly fluctuates, and the coal seam at the excavation position of the roadway is between 6 and 24 m thick.The excavation position of the roadway and the change in thickness of the coal seam exposed by the coal exploration drilling are shown in Figure 1.
The original support scheme of roadway 5204 is very complex, as shown in Figure 2. The roof and two sides are both supported by anchor bolts reinforcement and anchor cables suspension, with an installation spacing of 900 mm × 900 mm.At the top corners of the two sides, anchor cables are used for strengthening, and the installation spacing is 1800 mm.In addition, the anchor cable group consisting of three anchor cables is used to strengthen the roof support, and the installation spacing is 3000 mm × 2500 mm.

| Deformation and failure characteristics
Affected by the strong mining of 8203 working face, the properties of CSNG have decreased.During excavation, the roadway side and roof frequently collapsed, and many support failures had to be repaired, which seriously impacted the safety and efficiency of roadway construction.In addition, the deformation and failure of the surrounding rock in the 5204 roadway varied with the thickness of the coal seam.
Figures 3 and 4 show the typical deformation and failure characteristics of the surrounding rock in the 0-400 and 400-600 m sections of the 5204 roadway, where the thickness of the coal seam is 18-24 and 12-14 m, respectively.The surrounding rock in both sections exhibits significant subsidence of the roof and severe extrusion of the two sides.A large number of supporting components have failed, mainly including steel strip bending, anchor bolt detachment, and anchor cable bending shear dislocation to fracture.In contrast, the deformation of the surrounding rock in the 0-400 m section is more severe, the subsidence range of the roof is wider, and the two sides are more seriously broken.Although reinforcement support is performed, the failure problem of the support components has not been solved.The safety and efficiency of excavation work have been greatly affected.The deformation data of the surrounding rock can more clearly reflect the deformation degree of the surrounding rock.When the 5204 roadway was excavated to 210, 650, 980, and 1100 m, measurement stations #1, #2, #3, and #4 were arranged to monitor the deformation data of the roadway under the original support for a long time.The corresponding coal seam thicknesses at the four stations were 24, 17, 21, and 13 m, respectively.The monitoring results of the four stations are shown in Figure 5.At four stations, the deformation of the roof and the two sides was monitored for 40-60 days.On the 16th, 13th, 15th, and 10th days, the surrounding rock deformation rate at four stations began to gradually slow; on the 40th, 33rd, 38th, and 24th days, the surrounding rock deformation basically stabilized.The deformation of the roof, coal seam side, and coal pillar side were 13.0, 20.8, and 11.1 cm, respectively, at station #1; 11.6, 19.2, and 9.1 cm, respectively, at station #2; 12.6, 20.3, and 10.4 cm, respectively, at station #3; and 10.5, 17.6, and 7.9 cm, respectively, at station #4.
Compared with the four stations, as the thickness of the coal seam increases, the roadway deformation amount and duration show an increasing trend.Among them, subsidence of the roof, convergence of the two sides, and deformation duration of the surrounding rock at station #1 (coal seam thickness is 24 m) are 1.2, 1.3, and 1.7 times those at station #4 (coal seam thickness is 13 m), which further indicates that the change in thickness of the coal seam greatly affects the stability of the surrounding rock of the gob-side roadway.To ensure the stability of the roadway during driving and production, it is necessary to determine the main reasons for this difference and propose more targeted support strategies and schemes.

| THEORETICAL ANALYSIS OF INFLUENCING FACTORS
It is generally believed that the deformation and failure of roadway is directly related to the stress environment and surrounding rock properties at the excavation position, [37][38][39] and the degree depends on the support form used during the excavation process.This section focuses on the above three influencing factors and uses coal seam thickness as the sole variable to conduct theoretical analysis.

| Stress environment
After the completion of mining in the adjacent working face, a mining space is formed, and the overlying rock layers gradually collapse, fracture, and rotate, ultimately forming stable cantilever beam structures and masonry beam structures, 40 as shown in Figure 6.The CSNG is subjected to excavation unloading and compression of overlying rock layers, resulting in stress concentration and deflection of the principal stress direction. 41,42As the thickness of the coal seam increases, the rotation angle of the overlying strata increases, and the stress concentration and deflection angle of the principal stress direction will also change.
According to elasticity, 43 the stress environment is analyzed by using the mechanical model for unequal pressure circular hole as shown in Figure 7.According to the theory of elasticity, the stress state of any point in the surrounding rock of an unequal pressure circular roadway is as follow: where r, θ represent the polar coordinates of any point in the surrounding rock; σ θ ，τ rθ are the radial stress, circumferential stress, and shear stress at any point in the surrounding rock; p 0 is the minimum principal stress; K is the ratio of maximum principal stress to minimum principal stress; α is the deflection angle of the principal stress; t R r = / 2 2 , R is the radius of the circular hole.
Mohr Coulomb criterion is applied as the failure criterion for rocks under a certain stress state, and the stress expression of the Mohr Coulomb criterion in polar coordinates is as follow: where C is cohesion; φ is friction angle.Represent the boundary of the plastic area with R 0 , . By combining Equations ( 1) and ( 2), the implicit equation for the plastic boundary R 0 with respect to θ is obtained: According to Equation (3), the distribution of the plastic area of unequal pressure circular hole is shown in Figure 8, and it can be seen that: (1) When the ratio of principal stress K is different, the surrounding rock exhibits different failure patterns and maximum failure depths, as shown in Figure 8A.
When K = 1, the circular hole is in an isobaric state, and the shape of the plastic area is circular; When K = 1.5, the depth of failure on both sides expands, and the plastic area shows an elliptical shape; As K further increases, the depth of corner failure rapidly expands.When K = 2.25, the plastic area exhibits a significant butterfly shape.(2) As the principal stress direction deviates, the shape of the plastic area also rotates in the same direction, as shown in Figure 8B.When the principal stress deflection angle α = 0°, the maximum failure depth is located at the corner; But when the principal stress rotates counter-clockwise by 30°, the maximum failure depth shifts toward the roof, floor, and two sides.
The above results indicate that the ratio of principal stress K affects the distribution pattern and maximum failure depth of the plastic area, while the principal stress deflection angle α controls the location of the maximum failure depth.In CSNG, horizontal stress unloading and vertical stress concentration result in a larger ratio of principal stress K , making it more prone to elliptical or butterfly shaped failure patterns.And the deflection of the principal stress direction causes the maximum failure depth to transfer to the roof and two sides.As the thickness of the coal seam increases, the deflection angle α increases, the potential height of roof caving increases, and the risk of collapse on both sides intensifies, thus exhibiting the observed failure characteristics and variation on site.

| Surrounding rock properties
By theoretically calculating the width of the limit equilibrium area in CSNG, the state of surrounding rock at the excavation position can be determined.When calculating the limit equilibrium area of the CSNG, the limit equilibrium theory is usually used. 44After the coal seam of the previous working face has been mined, the CSNG gradually undergoes plastic failure from its edge to the inside.The vertical bearing capacity of the CSNG is greatly reduced, and the bearing pressure is continuously transferred to the deep part of the coal seam.When the coal state changes from plasticity to elasticity, the bearing pressure in the coal seam reaches its peak value, as shown in Figure 9.The width of the limit equilibrium area is x 0 , including the crushing section L s and the plastic section L p .
According to the limit equilibrium theory, the expression of the bearing pressure σ z at the top of the coal seam within the limit equilibrium area can be determined: , where M is the thickness of the coal seam, m; λ is the pressure coefficient inside the limit equilibrium zone; φ 0 is the friction angle in the coal seam (°); C 0 is the coal cohesion force, MPa; P x is the horizontal binding force of the gob on the coal seam, MPa.According to the geological production conditions of the 8204 working face and measured results of rock pressure, M = 6-24 m, λ = 0.42, φ 0 = 27°, C 0 = 1.1 MPa, and P x = 0.25 MPa.These parame- ters can be input into Equation ( 4) to obtain the exponential function curve of σ z with respect to x in the limit equilibrium area for different coal seam thickness, as shown in Figure 10.
z zmax (5)   When x = x 0 , σ z reaches the peak value; at this time, where k is the stress concentration factor, which is taken as 2.5; γ is the average weight of the overlying rock, which is taken as 0.025 MN/m 3 ; H is the burial depth of the roadway, which is taken as 450 m.These parameters were brought into Equation ( 5) to calculate σ zmax = 28.12MPa.As can be seen from Figure 10, the width of the limit equilibrium area x 0 and the width of the crushing section L s in the CSNG will expand as the thickness of the coal seam increases.When the coal seam is 6, 9, 12, 18, and 24 m thick, the width of the limit equilibrium area x 0 is 5.9, 8.9, 11.8, 17.7, and 23.7 m, respectively, while the width of the crushing section L s is 3.9, 5.9, 7.8, 11.7, and 15.6 m, respectively.
According to the theoretical calculation results, if a 5-m wide coal pillar is left to excavate a 5.2-m wide roadway along the gob, when M = 6 m, the majority of the roadway excavation site is in an elastic state, and only partially in the limit equilibrium area; When M = 12 m, the excavation site is completely within the limit equilibrium area, and partially enters the crushing section; When M = 18 m, the excavation site has fully entered the range of the crushing section.It can be seen that, with the increase of the thickness of the coal seam, the properties of the surrounding rock at the excavation site gradually decrease, and the collapse of the roof and two sides is more likely to occur during the excavation process.Especially when M ≥ 18 m, the excavation site is located in a large range of damaged surrounding rock, resulting in large overall deformation and poor stability.

| Support form
The deflection of the principal stress and the deterioration of the surrounding rock properties lead to the expansion of the damage range of the roadway roof and two sides, and the stress on the support components is more complex, as shown in Figure 11.The conventional support form is still used, with the following problems: (1) Due to the limited anchoring length and the low prestress that can be applied, anchor bolts have insufficient inhibition on the expansion and development of shallow fractures in surrounding rock.
The frequent falling off of anchor bolts makes it difficult to achieve the reinforcement effect on shallow surrounding rock.(2) The anchor cable can exert a high prestress, but its tangential stiffness is low, and its ability to suppress lateral dislocation of the shallow structural surface of F I G U R E 10 Variation of the width of the limit equilibrium area with the thickness of the coal seam.
surrounding rock is poor.The phenomenon of deflection, dislocation, and tensile shear fracture of the anchor cable reduces its suspension effect.(3) The side and corner areas with concentrated stress have not received sufficient attention to provide stable support for the roof, resulting in low overall stability of the roadway.

| Stress environment and plastic zone distribution of CSNG before GED
According to the theoretical calculation and analysis in Section 3, the stress environment and properties of the surrounding rock of CSNG before GED are important factors affecting the stability of the roadway.By numerically simulating the on-site excavation sequence, the stress distribution and plastic zone expansion of the CSNG before GED can be obtained.After calculating the equilibrium, the stress data of the coal seam floor and the plastic area contour were extracted, as shown in Figure 13.The positions of the coal seam floor and excavation roadway are represented by curves in Figure 13.
According to Figure 13, the range of plastic area in CSNG gradually expands with the increase of coal seam thickness.When the coal seam thickness is 6, 12, and 18 m, the width of plastic area at the top of the coal seam is 5.5, 11.8, and 18.5 m, respectively, which is basically close to the theoretical calculation of the limit    In terms of stress environment, the anisotropic stress rapidly increases and reaches its peak within a range of 10 m from the gob, and maintains a high state within a range of about 20 m.Among them, (1) maximum principal stress σ 1 is always higher than the vertical stress SZZ, while the minimum principal stress σ 3 always below the horizontal stress SXX.It indicates that the main stress direction in the CSNG has deviated and is no longer along the vertical and horizontal directions; (2) the ratio of maximum to minimum principal stress σ 1 /σ 3 is always higher than the ratio of vertical to horizontal stress SZZ/SXX, indicating that the principal stress ratio K increases and the maximum damage depth during excavation will expand; (3) as the thickness of the coal seam increases, the range of areas where the principal stress deflection and principal stress ratio increase gradually expands.
To further study the changes in the stress environment at the excavation position of different coal seam thicknesses, the stress data at the coal seam floor X = −8 m was extracted.After post-processing, the curves of the principal stress ratio and deflection angle at the excavation position with respect to the variation of coal seam thickness were obtained, as shown in Figure 14.
According to Figure 14, as the thickness of the coal seam increases, the variation law of principal stress ratio K and the principal stress deflection angle α at the excavation position is basically consistent, both showing parabolic growth.During the process of increasing the thickness of the coal seam from 6 to 12 m, the principal stress ratio and deflection angle increased significantly from 1.50 to 10°to 1.76 and 37°, respectively; When the thickness of the coal seam increases from 12 to 18 m, the principal stress ratio and deflection angle continue to increase to 1.84 and 43°, respectively; But after the coal seam thickness exceeds 18 m, the variation of the principal stress ratio and deflection angle tend to stabilize.

| Expansion of plastic area
The distribution of plastic area in surrounding rock after GED is shown in Figure 15.Under the combined effect of deterioration of surrounding rock properties and changes in stress environment, a large plastic area has been formed around the roadway.During the process of increasing the thickness of the coal seam from 6 to 12 m and 18 m, due to the increase of the principal stress deviation angle α to principal stress ratio K , the maximum failure depth caused by the stress environment increased and transferred to the roof, floor and two sides.The depth of the plastic area in the coal seam side gradually expanded from 4.16 to 4.68 m and 5.20 m, and the depth of the plastic area in the floor gradually expanded from 3.76 to 4.30 m and 4.83 m.More effective support measures should be adopted to enhance the reinforcement depth and strength of the shallow fractured surrounding rock of the roadway, to suppress the further expansion and development of surrounding rock fractures.

| Deformation law of the surrounding rock
To compare the deformation of surrounding rock with different coal seam thicknesses, the deformation values of the roof, floor, and two sides after GED were extracted, F I G U R E 14 Principal stress ratio and deflection angle at the excavation position.and the displacement change curves of the surrounding rock were plotted as shown in Figure 16.The vertical displacement is positive upwards and negative downwards; the horizontal displacement is positive to the right (the direction of the gob) and negative to the left (the direction of the coal seam).
Figure 16A,B shows the deformation of the roof of the gob-side roadway.As the thickness of the coal seam increases, there are the following laws: (1) the vertical displacement of the roof changes from local subsidence to overall subsidence.At the roof corners on both sides (x = −10.2m and −5 m), the subsidence increases the most, which leads to a significant increase in the generalized span of the roof; (2) the horizontal displacement of the roof gradually changes from squeezing towards the roadway to moving towards the gob; (3) the horizontal displacement difference on both sides of the roof is large, with significant horizontal compression and sliding.
Figure 16C,D shows the deformation of the two sides of the gob-side roadway.As the thickness of the coal seam increases, there are the following laws: (1) the vertical displacement of the two sides shows a gradually increasing trend, but the increase rate gradually decreases.The coal seam side is more affected by the change in coal seam thickness; (2) the vertical displacement of the two sides gradually decreases from the surface to the inside.The vertical displacement difference between different depths is significant and further increases with the increase of coal seam thickness, leading to an increased risk of shear failure; (3) the horizontal displacement of the two sides also shows a gradually increasing trend, and the difference in the shallow part is also significantly increasing, indicating an increase in the fragmentation degree of the shallow part and a significant increase in the risk of collapse.
According to these numerical analysis results of roadway deformation, when GED is performed in an extra-thick coal seam, the thickness of the coal seam will lead to great differences in the deformation of the surrounding rock.Overall, there are two important points: (1) with increasing coal seam thickness, the roadway deformation changes from local deformation to large-scale deformation.The coal seam side and roof corner are the most affected areas with the most obvious deformation increase, and they should be the focus of strengthening support.(2) There is always a large vertical and horizontal displacement difference between different depths in the surrounding rock, which mainly occurs in the shallow part.As a result, separation and irregular dislocation easily occur inside the roof, collapse and shear failure easily occur on two sides.Therefore, the bidirectional control of the shallow surrounding rock in the axial and tangential directions should also be the focus of surrounding rock support.

| STABILITY CONTROL OF GED IN EXTRA-THICK COAL SEAM
According to the deformation and failure law of the gob-side roadway in extra-thick coal seam, and using a new type of support structure called "Anchor Cable with C-shaped Tube (ACC)," the stability control strategies and optimized support schemes are proposed.

| "Anchor cable with c-shaped tube (ACC)" structure and its principle
High strength, high rigidity, and high prestressed anchor cables have been widely used in mine support, greatly enhancing the axial control effect on surrounding rock. 45,46However, according to the shear failure equation of the anchor cable: where N and Q are the tensile force and shear force when the anchor cable is broken, respectively; N f and Q f are the ultimate tensile force and ultimate shear force of the anchor cable, respectively.When the anchor cable is subjected to a combined tensile and shear stress, its shear resistance will decrease with the increase of the pretension of the anchor cable. 47herefore, high prestress will increase the risk of shear fracture of the anchor cable.And anchor cables are usually susceptible to combined tensile and shear forces in the shallow crushing area of surrounding rock.To improve the contradiction between high prestress and low shear resistance of anchor cables in the shallow crushing area of surrounding rock, the research group invented a new support structure called "Anchor Cable with C-shaped Tube (ACC)."The ACC consists of an anchor cable that can exert high prestress and a C-shaped steel tube with adjustable length according to the integrity of the surrounding rock, as shown in Figure 17.
The C-shaped steel tube is nested in the free section of the anchor cable that corresponds to the shallow crushing area of the surrounding rock, which can improve the shear stiffness and strength of the anchor cable in this area.Its effect has been proven by a large number of laboratory tests. 35,36Therefore, this structure has a high bearing capacity and displacement control ability in both axial direction and tangential direction.Shallow crushing surrounding rock can be better reinforced, so that its stress state can be effectively improved, and the overall strength can be enhanced.At present, ACC has been applied in many coal mines in Shanxi and Hebei provinces of China, and has achieved superior support effects.In the appraisal of scientific and technological achievements, it has obtained an internationally leading evaluation.

| Control strategy and support scheme
According to the analysis results of the surrounding rock condition after GED, combined with the ACC support structure, the following three support strategies are proposed.3. Strengthen the control of the coal seam side and roof corner to improve the integrity of the support system. 48,49he coal seam side is the main force to support the roof, and the roof corner is an important connecting part.Strengthening the support of the above two parts can reduce the limit equilibrium area range of the coal seam side and the generalized span of the roof, achieve the synergy of the side, corner, and roof.
Based on the above strategies, a full-section ACCoptimized support scheme is proposed, as shown in Figure 18.

| Optimization effect analysis
To verify the feasibility of the full-section ACC-optimized support scheme, a numerical simulation was conducted to compare it with the original support scheme.The results are as follows:

| Support prestress field
The prestress in the anchor bolts and anchor cables overlap with each other in the surrounding rock to form a stress compression area, restoring the surrounding rock from a two-dimensional stress state to a three-dimensional stress state, thereby achieving reinforcement of the surrounding rock.Therefore, the range, stress value, and superposition of the support prestress field can reflect the improvement effect of the support system on the stress state of the surrounding rock. 50,51From Figure 19A, it can be seen that the support prestress field formed by the original support is | 1927 mainly concentrated within the 1.5 m range of the roadway roof and corner, and there is an extremely low-stress superposition area (less than 0.1 MPa) in the local parts of the two sides and the roof.This will cause local nonlinear deformation of the roadway.In Figure 19B, ACC-optimized support forms a support prestress field with a larger effective range and higher stress values in the surrounding rock.The stress values within the 2 m range of the surrounding rock are all above 0.2 MPa.The two sides, two top corners, and roof are connected to form a large range of stress arch, and the expansion and connection of local cracks can be more effectively suppressed.The integrity of the surrounding rock can be better protected, and its self-bearing capacity can be greatly improved.support is 13.88 cm, the maximum horizontal deformation of the coal seam side is 17.61 cm, and the maximum horizontal deformation of the coal pillar side is 9.64 cm.Under optimized support, the maximum vertical deformation of the roof is 9.28 cm, the maximum horizontal deformation of the coal seam side is 11.21 cm, and the maximum horizontal deformation of the coal pillar side is 6.47 cm, which is reduced by 33.1%, 36.3%, and 32.9% compared to the original support, respectively.

| Engineering application
The engineering application of the full-section ACC support scheme was performed in the 5204 gob-side roadway of the Tongxin Mine, as shown in Figure 22.Station #5 (as shown in Figure 1) was arranged to monitor the deformation of the surrounding rock in the roadway for 40 days.The monitoring results are shown in Figure 23.The deformation rate of the surrounding rock in the full-section ACC support section gradually slowed after approximately 6 days of excavation of the roadway.After approximately 16 days, the deformation of the surrounding rock basically stabilized, and the maximum deformation of the roof, coal seam side, and coal pillar side were 8.7, 10.3, and 7.3 cm, respectively.Compared with stations #1, #2, #3, and #4 in the original support, the deformation of the surrounding rock was more effectively controlled.The deformation of the roof decreased by 33.1%, 31.0%,25.0%, and 17.1%, respectively.The deformation of the coal seam side decreased by 50.5%, 46.4%, 49.3%, and 41.5%, respectively.The deformation of the coal pillar side decreased by 34.2%, 19.8%, 29.8%, and 7.6%, respectively.In addition, the layer separation within 4 m of the roof was only 0.6 cm.The integrity of the shallow surrounding rock of the roof has been greatly enhanced, and the roadway remained safer and more stable.
In terms of cost control, compared with the original support scheme, the full-section ACC support scheme has increased the cost of primary support, with an average increase of about 27,000 yuan per 100 m.But the roadway no longer needs to be repaired several times, about 210,000 yuan can be saved every 100 m.Therefore, the full-section ACC support scheme can save about 183,000 yuan per 100 m, and during the application period in Tongxin Mine, the saved support costs exceeded 50 million yuan.To further reduce the support cost and improve the support accuracy, in the future promotion and application, the length of the C-shaped tube can be adjusted according to the crushing depth of the roadway.And more in-depth research and experiments will be conducted on the matching of parameters of the tube, cable, and hole to fetch the optimal support parameters and establish a support parameter design system under different rock conditions.

| CONCLUSION
This paper mainly studied the stress changes and deformation laws of gob-side roadway in extra-thick coal seam, and propose effective support strategies and schemes to achieve roadway deformation control.The main conclusions are as follows: 1.The CSNG is greatly affected by the mining of the adjacent working face, resulting in a significant increase in principal stress ratio and a deviation in principal stress direction.According to theoretical calculation results, the increase in principal stress ratio leads to the expansion of the maximum failure depth of the surrounding rock, and the deflection of the principal stress direction will cause the maximum failure depth to transfer from the corner to the roof, floor, and two sides.The crushing range and collapse risk of the roof and two sides increase.2. The numerical simulation results indicate that with the increase of coal seam thickness, the principal stress ratio and deflection angle at the excavation position increase in a parabolic manner.The calculation results of the limit equilibrium theory also show that the properties of the surrounding rock at the excavation position will gradually deteriorate with the increase of coal seam thickness.Under the superposition of principal stress ratio increase, principal stress direction deflection, and surrounding rock properties deterioration, the deformation of the roadway changes from local deformation when the coal seam thickness is 6-12 m to overall large deformation when the coal seam thickness is 18-24 m. 3. Based on the deformation and failure laws, theoretical analysis, and numerical simulation of the roadway, it is believed that the increase in failure depth and the deterioration of surrounding rock properties are the main reasons for the severe deformation of the roadway.On this basis, full-section ACC support scheme is proposed, effectively improving the bidirectional control of axial and tangential directions of surrounding rock, and achieving better deformation control effect in the field application.
Although the research in this paper has obtained some useful conclusions, further research is still needed for the transportation law of overlying rock structure and the control mechanism of ACC support.In the subsequent research work, more in-depth theoretical and experimental research will be carried out, and more advanced on-site monitoring technologies will be considered to further clarify the actual working status and support mechanism of ACC support.This will be beneficial for the improvement of support scheme to achieve better support effect and lower support cost.

F
I G U R E Panel layout and coal seam thickness variation.F I G U R E Original roadway supporting scheme.F I G U R E Conditions of the 5204 roadway at the 0-400 m section (M = 18-24 m).(A) Roof.(B) Coal seam side.(C) Coal pillar side.

F I G U R E 4
Conditions of the 5204 roadway at the 400-600 m section (M = 12-14 m).(A) Roof.(B) Coal seam side.(C) Coal pillar side.

F
I G U R E 6 Overburden structure diagram of GED in extra-thick coal seam.GED, gob-side entry driving.F I G U R E 7 Mechanical model for unequal pressure circular hole.

F I G U R E 8
Analytical solution for the distribution of plastic area in unequal pressure circular hole.(A) α = 0°Maximum failure depth extension.(B) K = 2.25 Maximum failure depth deflection.

F I G U R E 9
Mechanical model of limit equilibrium area in CSNG.CSNG, coal seam near the gob.

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NUMERICAL STUDY ON STABILITY OF GED IN EXTRA-THICK COAL SEAM Numerical model generationTo further analyze the impact of gob and coal seam thickness on the stability of GED.According to the geological conditions of the Tongxin Mine, FLAC 3D 5.0 was used for numerical modeling and calculation analysis.The model parameters were obtained through mechanical property testing experiments of on-site rock samples (the reduction coefficient was considered when selecting the parameters).The physical and mechanical parameters of the main rock formations are shown in Table2.The numerical model has a width of 215 m in the X direction, a depth of 10 m in the Y direction, and a height of 150 m in the Z direction.It contains 398,160 zones and 424,053 grids.The horizontal displacement around the model is fixed, and the vertical displacement at the bottom is fixed.According to the in-situ stress test results, the vertical load applied on the top of the model is 8.75 MPa, and the horizontal load applied around the model is 1.6 times the vertical load.All rock formations in the model follow the Mohr-Coulomb yield criterion, and the specific model and formations are shown in Figure12.

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I G U R E 11 Support failure mechanism.T A B L E 2 The physical and mechanical parameters of rock strata.
equilibrium area range.However, the range of plastic area at the expected excavation position of 5204 roadway has decreased compared to the top.When the thickness of the coal seam is 6, 12, and 18 m, respectively, the width of the plastic area at the excavation position is 4, 8, and 11 m, which is basically close to the theoretical calculation of the width of the crushing section.It can be seen that when the thickness of the coal seam is low, most of the coal at the excavation position are still in an elastic state, and the surrounding rock properties are good; As the thickness of the coal seam increases, the coal at the excavation position gradually enters a plastic state, and the properties of the surrounding rock deteriorate.When M = 18 m, the excavation position is already completely in the plastic area.The deterioration of the surrounding rock properties at the excavation position leads to a greater likelihood of loosening and collapse of the roof and two sides, which is one of the main reasons for the intensified deformation and failure of the gob-side roadway in the extra-thick coal seam.

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I G U R E 12 Numerical calculation model.F I G U R E 13 Plastic area and stress distribution in CSNG.(A) M = 6 m. (B) M = 12 m.(C) M = 18 m.CSNG, coal seam near the gob.

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I G U R E 15 Plastic area distribution in surrounding rock after GED.(A) M = 6 m. (B) M = 12 m.(C) M = 18 m.GED, gob-side entry driving.

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I G U R E 16 Surrounding rock deformation after GED.(A) Vertical displacement of roof.(B) Horizontal displacement of roof.(C) Vertical displacement of two sides.(D) Horizontal displacement of two sides.GED, gob-side entry driving.

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Reinforce shallow crushing surrounding rock.Using short ACC instead of anchor bolts increases the anchoring depth and prestress of shallow support, and cooperates with the prestressed field formed by long ACC to suppress axial and tangential deformation of shallow surrounding rock, ultimately forming a safer and more reliable reinforced arch structure in the shallow part of surrounding rock.2. Reasonably arrange deep support.Use long ACC instead of anchor cables and eliminate the form of local combinations.This is conducive to preventing local nonlinear deformation of the top coal, thereby achieving better suspension effect and further improving the stiffness of the shallow reinforcement arch.

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I G U R E 17 Anchor cable with C-shaped tube (ACC).(A) ACC physical photo.(B) ACC support function diagram.SHAN ET AL.

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Figure 20 and Figure 21 show the vertical and horizontal deformation contour of the roadway under two types of support when M = 18 m, respectively.The maximum vertical deformation of the roof under the original

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I G U R E 20 Comparison of roadway vertical deformation contour when M = 18 m.(A) Original support.(B) Optimized support.F I G U R E 21 Comparison of roadway horizontal deformation contour when M = 18 m.(A) Original support.(B) Optimized support.

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I G U R E 22 Engineering application of the full-section ACC support.(A) Field installation of ACC.(B) Deformation control effect of ACC support.ACC, anchor cable with C-shaped tube.F I G U R E 23 Monitoring results of station #3 in the full-section ACC support section.ACC, anchor cable with C-shaped tube.