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

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 ﬂ oor failure depth of the UCS and to determine the minimum seam spacing of the roof bolting. The maximum ﬂ oor failure depth of the UCS determined through theoretical analysis and numerical simulation is 3.2m and 3.3m, respectively. In general, the anchorage length of rock bolting is less than 2.4 m, so the minimum seam spacing is 5.6m or 5.7m. 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 signi ﬁ cant than 30kN; in addition, the numerical simulation results indicate that the roof-to- ﬂ oor and rib-to-rib convergence are relatively small. Therefore, the LCS entry ’ s minimum roof bolting seam spacing can be determined as 6m. This study could be used to select and design roof bolting under similar close-distance coal seam conditions.


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 halfplane 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 fur-ther. 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.

Mining and Geological Conditions
Chaili Coal Mine is located in Tengzhou City, Shandong Province, China, as shown in Figure 1   L is the distance between the peak front abutment pressure and the panel.

Geofluids
Based on the research of Zhang [9] and Sun [19], the floor failure depth can be calculated by Equations (1)-(4): where φ f is the internal friction angle of the floor, 30°. φ is the internal friction angle of the No.3-1 coal seam, 28°.
Therefore, the maximum floor failure depth of No.3-1 coal seam is as follows:

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 pullout 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.

Test Materials
(1) rock-bolt drawing dynamometer (measuring range is 200kN, and resolution is 1.0kN) (2) jumbolter    Figure 4. 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. As shown in Figure 5, 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 6 indicates that with the increase of drawing force, the displacement of test bolts gradually increases under different seam spacing.

Test Results and Analysis.
(1) As shown in Figure 6( (2) Figures 6(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 Figure 6(e) is that when the drawing force reaches

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 Section 3 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] Table 1.

Simulation Plans.
Where σ v is the vertical stress where the overlying strata apply on gangue.
G b is the shear modulus of the gangue in the gob. K b is the bulk modulus of the gangue in the gob. As shown in Figure 8, the numerical model was solved in this simulation using the following steps: 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  7 Geofluids addition, the Saint Venant theory is used to determine the range of caved zone, which is given by where ε 1 is the maximum principal strain. ε t 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 ε 1 > ε t , 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  9 Geofluids 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 σ v , volume modulus K b , and the shear modulus G b 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 As shown in Equation (7), the volume modulus K b , the shear modulus G b and the vertical stress σ v 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.

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. Figure 10 shows the yield zone distribution around entry for different seam spacing (3-9 m). Figure 10 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) Figures 10(a) Figure 11 shows the deformation of the surrounding rock of the entry for different seam spacing. As shown in Figures 11 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-tofloor 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.
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.

Discussion
In Section 3, 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 Section 4 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.
Section 5 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.

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.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.