# Roof Bolting Anchoring Performance Research on the Entry under the Gob of Close-Distance Coal Seam

Authors: Yongjie Yang; Gang Huang; Haiyu Ji
Journal: Geofluids
 (2022)

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## Abstract

Seam spacing plays a crucial role in selecting roof bolting of the close-distance coal seam. This work utilized three methods to determine the minimum roof bolting seam spacing of the lower coal seam (LCS) entry after the upper coal seam (UCS) mining. Based on the entry of the No.3-2 coal seam (LCS) in Chaili Coal Mine in China, theoretical analysis, pull-out bolt test, and numerical simulation were performed to calculate the maximum floor failure depth of the UCS and to determine the minimum seam spacing of the roof bolting. The maximum floor failure depth of the UCS determined through theoretical analysis and numerical simulation is 3.2 m and 3.3 m, respectively. In general, the anchorage length of rock bolting is less than 2.4 m, so the minimum seam spacing is 5.6 m or 5.7 m. To further determine the anchorage performance of the roof, the pull-out test was employed on the entry roof of the LCS. When the seam spacing is no less than 6 m, the test results show that the pull-out force of the bolt is more significant than 30kN; in addition, the numerical simulation results indicate that the roof-to-floor and rib-to-rib convergence are relatively small. Therefore, the LCS entry’s minimum roof bolting seam spacing can be determined as 6 m. This study could be used to select and design roof bolting under similar close-distance coal seam conditions.


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## Body

## 1. Introduction

The close-distance coal seam (CDCS) reserves are abundant in China [1]. Affected by mining and geological conditions, the mining method of the CDCS is primarily based on experience. Descending mining, which is widely used in the CDCS, first retreats the upper coal seam (UCS) and then develops the entry in the lower coal seam (LCS) until the overburden is stable. The entry in the LCS is mostly located under the gob or chain pillar. Due to the small seam spacing, the mining activity in the LCS will contribute to the destruction of the overburden time after time, which challenges the surrounding rock control of the entry. Therefore, seam spacing is a crucial factor in the entry’s support design, especially under the gob in the LCS. Due to the UCS’s mining, the LCS roof is badly damaged when the seam spacing is minimal, and the rock bolts anchor in the loose and broken surrounding rock, leading to poor roof anchoring performance. In addition, it is hard to obtain a good support effect with rock bolting. At this time, passive support such as steel arch support is supposed to be taken into account to ensure the surrounding rock stability of the LCS entry. Conversely, when the seam spacing is large, the roof of the LCS seam is relatively complete, and the rock bolt can be anchored in the stable surrounding rock. At this time, the rock bolting could reach well technical and economic effects.The fracture and development law of the overburden of UCS in CDCS has received a great deal of research attention. Li et al. obtained the development characteristics of the overburden using the geological penetration radar [2, 3]. With a physical similarity simulation (PSS), Li et al. [4] determined the first and the periodic weighting interval of the main roof. Cui et al. [5, 6] used 3DEC to analyze the development law of the overlying strata’s cracks in western China. Ning et al. [7] evaluated the height of fractured zones of the CDCS. Zhang et al. [8] researched the overburden’s bearing structure and stability characteristics employing numerical simulation, PSS, and theoretical analyses.Several recent studies have focused on floor failure depth and stress distribution during UCS mining. Zhang et al. [9] built a floor failure mechanical model of the UCS. Sun et al. [10] proposed a mechanical model based on half-plane theory to obtain the evolution law of the floor. Tan et al. [11] utilized micro seismic techniques, numerical simulations, and borehole inspection to explore the distributions of strata failure. By using UDEC, Liu et al. [12] analyzed the stress distribution of LCS.Recent studies have explored the LCS entry’s optimal layout and surrounding rock stability control in CDCS. Wang et al. [13] suggested that the creep characteristics of surrounding rock are the most critical factor in selecting for the entry’s layout of the LCS in deep mines. Wu et al. [14] built a mechanical model to research the stress distribution under chain pillars and adopted numerical simulation to select the optimal entry layout in ultra-close coal seams. Xu et al. [15] focused on the different excavation schemes in the CDCS mining and determined to leave a small coal pillar for excavation. Zhang et al. [16] established a layout of the entry using numerical simulation to minimize the risk of disasters. Zhang et al. [17] put forward the grouting reinforcement method of local fractured zones under a super thick hard roof in an ultra-close-distance coal seam. Zhang et al. [18] focused on surrounding rock stability control and technical parameter design for the gob-side entry retained under the gob.In conclusion, a large body of literature has focused on the fracture and development law of the overburden of UCS, floor failure depth and stress distribution during UCS mining, and the optimal layout and the surrounding rock stability control of LCS entry in CDCS. As the decisive factor of surrounding rock stability control of the LCS entry, it is necessary to deepen the research on seam spacing further. Due to the complexity of mining and geological conditions, rock bolting or steel arch support is widely used in LCS entry. However, quantitative research on roof bolting technology and support parameters according to the seam spacing size has not yet been conducted. In this paper, taking the tailgate of the No.3-2 coal seam (LCS) in Chaili Coal Mine as an example, the performance of roof bolting is comprehensively studied by three methods. First, the theoretical analysis was used to calculate the maximum floor failure depth. Then, the pull-out test was carried out to determine the minimal seam spacing for roof bolting. With the thought that the distribution depth of yield zone of the floor can represent the maximum floor failure depth, the numerical model was established by FLAC3D. In addition, because the rock bolts are supposed to anchor in relatively intact rock mass, the bolts should not reach the edge of the yield zone in the numerical simulation while determining the minimal seam spacing. This study could be used to select and design roof bolting under similar CDCS conditions.


## 2. Mining and Geological Conditions

Chaili Coal Mine is located in Tengzhou City, Shandong Province, China, as shown in Figure1. The current mining seam is the No.3 coal seam. No.3 coal seam is divided into No.3-1 coal seam and No.3-2 coal seam, and the thickness of these two coal seams is 4.8 m and 3.2 m, respectively. The seam spacing between No.3-1 coal seam and No.3-2 coal seam varies from 2 m to 9 m. Figure 2 shows the generalized stratigraphy column.Figure 1
The position of Chaili Coal Mine.Figure 2
Generalized stratigraphy column.The panel layout for the No.3 coal seam is 150 m wide by 1766 m long, with a 5 m wide chain pillar between the panels. The tailgate in No.3-2 coal seam under the gob is developed with the dimension of 4 m long ×3.2 m wide, located directly below the tailgate of No.3-1 coal seam. Furthermore, the cross-section of the tailgate is supported by steel arches.


## 3. Calculation of the Floor Failure Depth of No.3-1 Coal Seam

According to Zhang [9], the floor failure mechanical model of No.3-1 coal seam (UCS) can be established as shown in Figure 3. Plastic deformation of the floor of the UCS is forming (Area I) when reaching the limit of elasticity. Up to the peak value, the plastic failure zones within the abutment pressure range are linked together, leading to the floor heave of the gob and the plastic deformation transferring from Area II to Area III, forming a continuous sliding surface. At this time, the maximum floor failure depth hmax is reached.Figure 3
Schematic diagram of the failure depth of No.3-1 coal seam floor.WhereS is the seam spacing between No.3-1 coal seam and No.3-2 coal seam ranging from 2 m to 9 m.hmax is the maximum floor failure depth of No.3-1 coal seam.L is the distance between the peak front abutment pressure and the panel.Based on the research of Zhang [9] and Sun [19], the floor failure depth can be calculated by Equations (1)–(4):
(1)hmax=L·eαtanφf·sinθ,(2)L=M2ζflnKγH+c·cotφζ·c·cotφ,(3)α=φf−φ2+π4,(4)θ=φf2+π4,where φf is the internal friction angle of the floor, 30°.φ is the internal friction angle of the No.3-1 coal seam, 28°.M is the height of the gob, 10.6 m.K is the stress concentration coefficient, 1.4.γ is the average bulk density of the overburden, 25 kN/m3.H is the average buried depth of the No.3-1 coal seam, 300 m.c is the cohesion of the No.3-1 coal seam, 1.45 MPa.f is the friction coefficient of the interface between the No.3-1 coal seam and the floor (f=tanφ).ζ is the triaxial stress coefficient, ζ=1+sinφ/1−sinφ.Therefore, the maximum floor failure depth of No.3-1 coal seam is as follows:(5)hmax=3.2m.


## 4. Roof Bolt Pull-Out Test of No.3-2 Coal Seam under the Gob of No.3-1 Coal Seam

To verify the anchorage performance of the roof of the LCS (No.3-2 coal seam) in the CDCS, the bolt pull-out test was carried out on the roof of No.3-2 coal seam to determine the minimum seam spacing of the roof bolting. If the pull-out force of the bolt is more significant than 30kN, it is considered that the roof is in good anchorage condition [20]. Otherwise, it is considered that the roof is not suitable for rock bolting.


### 4.1. Test Materials

(1)
rock-bolt drawing dynamometer (measuring range is 200kN, and resolution is 1.0kN)(2)
jumbolter(3)
rock bolt (Φ20mm × L2000mm), and resin cartridge (Φ18mm × L600mm)


### 4.2. Test Method

The test site is shown in Figure4. Eight test points were arranged at a seam spacing of 2-9 m. The jumbolter was used for the roof bolt installation within two days of headgate excavation, and the bolt pull-out test was carried out within 4 hours.Figure 4
Arrangement of measuring points for bolt pull-out test.As shown in Figure5, three test rock bolts were installed on the entry roof at every test point, the side bolts were 500 mm away from the entry’s rib, and the other was in the middle. During bolt installation, the bearing plate must be in close contact with the surface of the surrounding rock of the entry and the installation torque of tension nut was supposed to be relatively large (100~185 Nm) [20, 21]. Therefore, the installation torque of the tension nut was made 150 Nm. The drawing dynamometer should be coaxial with the bolt axis. Meanwhile, ensure that the bolt body does not contact the borehole wall. After installing the test bolt, apply bolt load slowly and continuously, and record bolt displacement every 5kN. Stop the pressure application and record the value of the hand pump until the test bolt fails.Figure 5
Installation position of test rock bolt of the entry.


### 4.3. Test Results and Analysis

Figure6 indicates that with the increase of drawing force, the displacement of test bolts gradually increases under different seam spacing.
(1)
As shown in Figure6(a), when the initial drawing force reaches 5kN, the displacement of test bolts reaches 170 mm. With the increase of drawing force, the test displacement increases, and the displacement is 250 mm when the drawing force is 20kN. When the drawing force is 21kN (<30kN), all three test bolts fail, indicating that the roof bolt does not have anchor performance under the 2 m seam spacing(2)
Figures6(b), 6(c), and 6(d) also present that when the drawing force is less than 30kN, all three bolts are pulled out at a seam spacing of 3-5 m(3)
At the 6 m seam spacing, what is striking in Figure6(e) is that when the drawing force reaches 30kN, the entire test bolts do not fail. Meanwhile, the displacement of the bolts is small(4)
Figures6(f), 6(g), and 6(h) show the test results at a seam spacing of 7-9 m. When the drawing force reaches 30kN, the displacement is also small. The maximum drawing force that test bolts fail is 50kN, 60kN, and 65kN, respectivelyFigure 6
Drawing force-displacement curve of test rock bolt.
(a)
2 m seam spacing(b)
3 m seam spacing(c)
4 m seam spacing(d)
5 m seam spacing(e)
6 m seam spacing(f)
7 m seam spacing(g)
8 m seam spacing(h)
9 m seam spacingConsequently, when the seam spacing between No.3-1 coal seam and No.3-2 coal seam is no less than 6 m, the drawing force of test bolts can reach 30kN, the roof of the entry in No.3-2 coal seam is in good anchoring condition.


## 4.1. Test Materials

(1)
rock-bolt drawing dynamometer (measuring range is 200kN, and resolution is 1.0kN)(2)
jumbolter(3)
rock bolt (Φ20mm × L2000mm), and resin cartridge (Φ18mm × L600mm)


## 4.2. Test Method

The test site is shown in Figure4. Eight test points were arranged at a seam spacing of 2-9 m. The jumbolter was used for the roof bolt installation within two days of headgate excavation, and the bolt pull-out test was carried out within 4 hours.Figure 4
Arrangement of measuring points for bolt pull-out test.As shown in Figure5, three test rock bolts were installed on the entry roof at every test point, the side bolts were 500 mm away from the entry’s rib, and the other was in the middle. During bolt installation, the bearing plate must be in close contact with the surface of the surrounding rock of the entry and the installation torque of tension nut was supposed to be relatively large (100~185 Nm) [20, 21]. Therefore, the installation torque of the tension nut was made 150 Nm. The drawing dynamometer should be coaxial with the bolt axis. Meanwhile, ensure that the bolt body does not contact the borehole wall. After installing the test bolt, apply bolt load slowly and continuously, and record bolt displacement every 5kN. Stop the pressure application and record the value of the hand pump until the test bolt fails.Figure 5
Installation position of test rock bolt of the entry.


## 4.3. Test Results and Analysis

Figure6 indicates that with the increase of drawing force, the displacement of test bolts gradually increases under different seam spacing.
(1)
As shown in Figure6(a), when the initial drawing force reaches 5kN, the displacement of test bolts reaches 170 mm. With the increase of drawing force, the test displacement increases, and the displacement is 250 mm when the drawing force is 20kN. When the drawing force is 21kN (<30kN), all three test bolts fail, indicating that the roof bolt does not have anchor performance under the 2 m seam spacing(2)
Figures6(b), 6(c), and 6(d) also present that when the drawing force is less than 30kN, all three bolts are pulled out at a seam spacing of 3-5 m(3)
At the 6 m seam spacing, what is striking in Figure6(e) is that when the drawing force reaches 30kN, the entire test bolts do not fail. Meanwhile, the displacement of the bolts is small(4)
Figures6(f), 6(g), and 6(h) show the test results at a seam spacing of 7-9 m. When the drawing force reaches 30kN, the displacement is also small. The maximum drawing force that test bolts fail is 50kN, 60kN, and 65kN, respectivelyFigure 6
Drawing force-displacement curve of test rock bolt.
(a)
2 m seam spacing(b)
3 m seam spacing(c)
4 m seam spacing(d)
5 m seam spacing(e)
6 m seam spacing(f)
7 m seam spacing(g)
8 m seam spacing(h)
9 m seam spacingConsequently, when the seam spacing between No.3-1 coal seam and No.3-2 coal seam is no less than 6 m, the drawing force of test bolts can reach 30kN, the roof of the entry in No.3-2 coal seam is in good anchoring condition.


## 5. Numerical Modeling

The floor failure depth of No.3-1 coal seam (UCS) and seam spacing significantly influence the coal entry support in the No.3-2 coal seam (LCS). According to the calculation results of the floor failure depth of No.3-1 coal seam in Section3 and the test results in Section 4, determining the minimum seam spacing of the rock bolts may cause safety problems. As an effective tool for studying rock mechanics behaviors in underground spaces [22, 23], this section further studies the floor failure depth of the No.3-1 coal seam after mining and the bolt anchoring performance under different seam spacing by numerical simulation.


### 5.1. Numerical Configuration

A numerical model (Figure7) using FLAC3D software was established to explore the floor failure depth of No.3-2 coal seam (LCS) and the anchoring performance of roof bolting. The dimensions of the model were 380 m(length) ×150 m(width) ×80 m(height). The panel layout for No.3-1 coal seam (UCS) was 150 m wide by 150 m long with a 5 m wide chain pillar between the panels. The tailgate in No.3-2 coal seam under the gob was excavated with the dimension of 4 m long ×3.2 m wide, located directly below the tailgate of No.3-1 coal seam. A vertical load of 6 MPa was applied to the upper boundary to simulate an overburden pressure by assuming the overlying unit weight is 25kN/m3. The final scaled rock mass properties are listed in Table 1.Figure 7
Configuration of the model performed using FLAC3D.Table 1
Rock strata properties used in the numerical model.
No.LithologyThickness (m)D (kg/m3)K (MPa)G (MPa)T (MPa)c (MPa)f (deg)1Medium-fine sandstone412535400015003.003.50352Mudstone102526250012006.501.38303Sandy mudstone32731334032801.321.48334No.3-1 coal seam4.81462150010204.301.04315Mudstone3-92526250012006.501.38306No.3-2 mudstone3.21462150010204.301.04317Sandy mudstone42738180011002.002.50288Medium-fine sandstone42731634092804.233.48339Sandy mudstone72731334032801.321.4833WhereD is the average density of the rock strata, kg/m3. K is the bulk modulus of the rock strata, MPa. G is the shear modulus of the rock strata, MPa. T is the tensile strength of the rock strata, MPa. c is the cohesion of the rock strata, MPa. f is the friction angle of the rock strata, deg.


### 5.2. Simulation Plans

Whereσv is the vertical stress where the overlying strata apply on gangue.Gb is the shear modulus of the gangue in the gob.Kb is the bulk modulus of the gangue in the gob.As shown in Figure8, the numerical model was solved in this simulation using the following steps:
(1)
the retreat of the two panels of No.3-1 coal seam (UCS) after initial balance, respectively(2)
the development of the entry of No.3-2 coal seam (LCS) with seven different seam spacing (3-9 m)Figure 8
Numerical simulation process by FLAC3D.The caved materials are then compacted and consolidated after panel advances far enough in longwall mining [24, 25]. Consequently, the entry of the LCS in CDCS is supposed to develop until the gob is generally consolidated. The constitutive model applied in the caved zone is Double-Yield to simulate this actual situation. In addition, the Saint Venant theory is used to determine the range of caved zone, which is given by
(6)ε1>εt,where ε1 is the maximum principal strain.εt is the critical value of tensile strain.Precisely, the two panels retreated in the No.3-1 coal seam (UCS) after several timesteps. According to Equation (6), when ε1 > εt, it is considered that the overburden of the UCS has collapsed. Therefore, customize the function in fish to determine the range of caved zone, and compare the results with those calculated by empirical formula, as shown in Figure 9. If the range of caved zone is consistent with the result of empirical formula [26], then convert the constitutive model of caved zone from Mohr-Coulomb to Double-Yield. At the same time, the mechanical parameters in the yield zone are reduced to simulate the fractured zone, the vertical stress σv, volume modulus Kb, and the shear modulus Gb of the gangue in the gob are updated at regular intervals until the balance is calculated. Based on the research of Bai et al. [27], the properties of double-yield constitutive model can be given by
(7)Kb=4Gb3=σv2ε.Figure 9
The stress-strain fitting curve.As shown in Equation (7), the volume modulus Kb, the shear modulus Gb and the vertical stress σv can be expressed as a function of the vertical strain ε. Consequently, after several intervals, such as 50 timesteps, the vertical strain of the gob material is recorded in numerical simulation, and then changes these properties according to Equation (7). Then, the LCS entry is developed once every 5 m and rock bolting until the end of the development.


### 5.3. Yield Zone Distribution around Entry for Different Seam Spacing

This part is set out to seek the proper seam spacing for roof bolting of the entry of No.3-2 coal seam (LCS). The floor failure depth is reflected by the yield zone of the floor of No.3-1 coal seam (UCS). According to suspension theory, rock bolts are supposed to anchor in relatively intact rock strata. Therefore, the end of the rock bolt had better anchor out of the yield zone. Figure10 shows the yield zone distribution around entry for different seam spacing (3-9 m).Figure 10
Yield zone distributions around entry for different seam spacing.Figure10 shows that with the increase of seam spacing, the floor failure depth (ranging from 3.0 to 3.3 m) of No.3-1 coal seam (UCS) has no noticeable change, but the yield zone around the entry, particularly the roof, gradually decreases.
(1)
Figures10(a), 10(b), and 10(c) show the yield zone distribution at a seam spacing of 3-5 m. It can thus be seen, the yield zone around entry is large, and the rock bolt cannot anchor in relatively intact rock strata(2)
At the 6 m seam spacing, what stands out in Figure10(d) is that the end of the rock bolt exactly anchors out of the yield zone. Meanwhile, the distribution of yield zone around entry is small(3)
Figures10(e), 10(f), and 10(g) also present the distribution of yield zone around entry is not large, and the rock bolt can anchor in the relatively intact rock strata(4)
Thus, when the seam spacing between No.3-1 coal seam and No.3-2 coal seam is no less than 6 m, the distribution of yield zone is not large, and the rock bolt can anchor in relatively intact rock strata


### 5.4. Deformation of Surrounding Rock of the Entry for Different Seam Spacing

It is one of the purposes of rock bolting to control the deformation of surrounding rock. Figure11 shows the deformation of the surrounding rock of the entry for different seam spacing.Figure 11
Displacement distribution around entry for different seam spacing.
(a)
3 m seam spacing(b)
4 m seam spacing(c)
5 m seam spacing(d)
6 m seam spacing(e)
7 m seam spacing(f)
8 m seam spacing(g)
9 m seam spacingAs shown in Figures11 and 12, with the increase of seam spacing, the convergence of roof-to-floor and rib-to-rib gradually decreases. In addition, the convergence of the right rib is greater than the left rib.
(1)
At 5 m seam spacing, the convergence of roof-to-floor and rib-to-rib is 801 mm and 1261 mm, respectively. The surrounding rock deformation of the entry is quite large, indicating that the rock bolting performance is terrible.(2)
When the seam spacing is no less than 6 m, the convergence of roof-to-floor and rib-to-rib is relatively small. At 6 m seam spacing, the convergence of roof-to-floor and rib-to-rib is 600 mm and 990 mm, respectively. At 9 m seam spacing, the convergence of roof-to-floor and rib-to-rib is 449 mm and 436 mm, respectively. Compared with the seam spacing of 3-5 m, the surrounding rock of the entry is well controlled.Figure 12
Simulation displacement of surrounding rock of the entry for different seam spacing.Consequently, when the seam spacing is no less than 6 m, the rock deformation of the entry is relatively small, the yield zone around the entry is under control, and the rock bolting performance is relatively good.


## 5.1. Numerical Configuration

A numerical model (Figure7) using FLAC3D software was established to explore the floor failure depth of No.3-2 coal seam (LCS) and the anchoring performance of roof bolting. The dimensions of the model were 380 m(length) ×150 m(width) ×80 m(height). The panel layout for No.3-1 coal seam (UCS) was 150 m wide by 150 m long with a 5 m wide chain pillar between the panels. The tailgate in No.3-2 coal seam under the gob was excavated with the dimension of 4 m long ×3.2 m wide, located directly below the tailgate of No.3-1 coal seam. A vertical load of 6 MPa was applied to the upper boundary to simulate an overburden pressure by assuming the overlying unit weight is 25kN/m3. The final scaled rock mass properties are listed in Table 1.Figure 7
Configuration of the model performed using FLAC3D.Table 1
Rock strata properties used in the numerical model.
No.LithologyThickness (m)D (kg/m3)K (MPa)G (MPa)T (MPa)c (MPa)f (deg)1Medium-fine sandstone412535400015003.003.50352Mudstone102526250012006.501.38303Sandy mudstone32731334032801.321.48334No.3-1 coal seam4.81462150010204.301.04315Mudstone3-92526250012006.501.38306No.3-2 mudstone3.21462150010204.301.04317Sandy mudstone42738180011002.002.50288Medium-fine sandstone42731634092804.233.48339Sandy mudstone72731334032801.321.4833WhereD is the average density of the rock strata, kg/m3. K is the bulk modulus of the rock strata, MPa. G is the shear modulus of the rock strata, MPa. T is the tensile strength of the rock strata, MPa. c is the cohesion of the rock strata, MPa. f is the friction angle of the rock strata, deg.


## 5.2. Simulation Plans

Whereσv is the vertical stress where the overlying strata apply on gangue.Gb is the shear modulus of the gangue in the gob.Kb is the bulk modulus of the gangue in the gob.As shown in Figure8, the numerical model was solved in this simulation using the following steps:
(1)
the retreat of the two panels of No.3-1 coal seam (UCS) after initial balance, respectively(2)
the development of the entry of No.3-2 coal seam (LCS) with seven different seam spacing (3-9 m)Figure 8
Numerical simulation process by FLAC3D.The caved materials are then compacted and consolidated after panel advances far enough in longwall mining [24, 25]. Consequently, the entry of the LCS in CDCS is supposed to develop until the gob is generally consolidated. The constitutive model applied in the caved zone is Double-Yield to simulate this actual situation. In addition, the Saint Venant theory is used to determine the range of caved zone, which is given by
(6)ε1>εt,where ε1 is the maximum principal strain.εt is the critical value of tensile strain.Precisely, the two panels retreated in the No.3-1 coal seam (UCS) after several timesteps. According to Equation (6), when ε1 > εt, it is considered that the overburden of the UCS has collapsed. Therefore, customize the function in fish to determine the range of caved zone, and compare the results with those calculated by empirical formula, as shown in Figure 9. If the range of caved zone is consistent with the result of empirical formula [26], then convert the constitutive model of caved zone from Mohr-Coulomb to Double-Yield. At the same time, the mechanical parameters in the yield zone are reduced to simulate the fractured zone, the vertical stress σv, volume modulus Kb, and the shear modulus Gb of the gangue in the gob are updated at regular intervals until the balance is calculated. Based on the research of Bai et al. [27], the properties of double-yield constitutive model can be given by
(7)Kb=4Gb3=σv2ε.Figure 9
The stress-strain fitting curve.As shown in Equation (7), the volume modulus Kb, the shear modulus Gb and the vertical stress σv can be expressed as a function of the vertical strain ε. Consequently, after several intervals, such as 50 timesteps, the vertical strain of the gob material is recorded in numerical simulation, and then changes these properties according to Equation (7). Then, the LCS entry is developed once every 5 m and rock bolting until the end of the development.


## 5.3. Yield Zone Distribution around Entry for Different Seam Spacing

This part is set out to seek the proper seam spacing for roof bolting of the entry of No.3-2 coal seam (LCS). The floor failure depth is reflected by the yield zone of the floor of No.3-1 coal seam (UCS). According to suspension theory, rock bolts are supposed to anchor in relatively intact rock strata. Therefore, the end of the rock bolt had better anchor out of the yield zone. Figure10 shows the yield zone distribution around entry for different seam spacing (3-9 m).Figure 10
Yield zone distributions around entry for different seam spacing.Figure10 shows that with the increase of seam spacing, the floor failure depth (ranging from 3.0 to 3.3 m) of No.3-1 coal seam (UCS) has no noticeable change, but the yield zone around the entry, particularly the roof, gradually decreases.
(1)
Figures10(a), 10(b), and 10(c) show the yield zone distribution at a seam spacing of 3-5 m. It can thus be seen, the yield zone around entry is large, and the rock bolt cannot anchor in relatively intact rock strata(2)
At the 6 m seam spacing, what stands out in Figure10(d) is that the end of the rock bolt exactly anchors out of the yield zone. Meanwhile, the distribution of yield zone around entry is small(3)
Figures10(e), 10(f), and 10(g) also present the distribution of yield zone around entry is not large, and the rock bolt can anchor in the relatively intact rock strata(4)
Thus, when the seam spacing between No.3-1 coal seam and No.3-2 coal seam is no less than 6 m, the distribution of yield zone is not large, and the rock bolt can anchor in relatively intact rock strata


## 5.4. Deformation of Surrounding Rock of the Entry for Different Seam Spacing

It is one of the purposes of rock bolting to control the deformation of surrounding rock. Figure11 shows the deformation of the surrounding rock of the entry for different seam spacing.Figure 11
Displacement distribution around entry for different seam spacing.
(a)
3 m seam spacing(b)
4 m seam spacing(c)
5 m seam spacing(d)
6 m seam spacing(e)
7 m seam spacing(f)
8 m seam spacing(g)
9 m seam spacingAs shown in Figures11 and 12, with the increase of seam spacing, the convergence of roof-to-floor and rib-to-rib gradually decreases. In addition, the convergence of the right rib is greater than the left rib.
(1)
At 5 m seam spacing, the convergence of roof-to-floor and rib-to-rib is 801 mm and 1261 mm, respectively. The surrounding rock deformation of the entry is quite large, indicating that the rock bolting performance is terrible.(2)
When the seam spacing is no less than 6 m, the convergence of roof-to-floor and rib-to-rib is relatively small. At 6 m seam spacing, the convergence of roof-to-floor and rib-to-rib is 600 mm and 990 mm, respectively. At 9 m seam spacing, the convergence of roof-to-floor and rib-to-rib is 449 mm and 436 mm, respectively. Compared with the seam spacing of 3-5 m, the surrounding rock of the entry is well controlled.Figure 12
Simulation displacement of surrounding rock of the entry for different seam spacing.Consequently, when the seam spacing is no less than 6 m, the rock deformation of the entry is relatively small, the yield zone around the entry is under control, and the rock bolting performance is relatively good.


## 6. Discussion

In Section3, through theoretical analysis, it was found that the peak value of front abutment pressure is 2.5 m in front of the panel, and then the floor failure depth of the No.3-1 coal seam (UCS) was 3.2 m. Compared to the previous calculation results, the value of L is minor, mainly because the buried depth H of the No.3-1 coal seam (UCS) is small, and L is closely related to H (see Equation (2)).The result of the pull-out test of roof bolting in Section4 showed that the pull-out force of bolt can reach 30kN at the seam spacing of more than 6 m, suggesting that the rock bolts can be anchored in the relatively intact rock strata, which was consistent with the suspension theory.Section5 determined that the maximum failure depth of the No.3-1 coal seam (UCS) is 3.0-3.3 m by numerical simulation, which was consistent with the theoretical analysis results in Section 3. Furthermore, the simulation results showed that the rock bolting performance is good when the seam spacing exceeds 6 m, which coincided with the pull-out test in Section 4.What is noteworthy is that this work was based on Chaili Coal Mine. Due to different mining and geological conditions, the applicability of presented theoretical analysis, pull-out test, and numerical simulation methods on other coal mines should be further studied.


## 7. Conclusions

(1)
The maximum floor failure depth calculated by the theoretical analysis was 3.2 m. Generally, the anchorage length of the rock bolt is less than 2.4 m. Thus, the roof bolting seam spacing is at least 5.6 m.(2)
The pull-out bolt test was carried out on the lower coal seam’s entry roof. The results show that when the seam spacing is no less than 6 m, the drawing force is more significant than 30kN. Therefore, the minimum seam spacing of roof bolting is taken as 6 m.(3)
Numerical simulation was set out to seek the floor failure depth and the performance of roof bolting. Simulation results show that the floor failure depth of the upper coal seam is 3.0-3.3 m. Compared with 3-5 m seam spacing, when the seam spacing is no less than 6 m, the yield zone of the lower coal seam’s roof is relatively not large. The surrounding rock deformation of the entry significantly decreases, indicating that the performance of roof bolting is good.In conclusion, through theoretical analysis, pull-out bolt test, and numerical simulation, this paper determines the minimum seam spacing of roof bolting is 6 m, where the entry under the gob of the close-distance coal seam. This study quantitatively studies the influence of seam spacing on selecting support methods, providing a reference for roof bolting design of coal entry under similar conditions.



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*Source: 8991574-2022-05-02.xml*