*5.1. Differential Support Scheme*

Taking the intersection of the arc roof and the straight inclined roof of the irregular roadway as the dividing point, as shown in Figure 14, seven anchor cables with a length of 4300 mm and a diameter of 22 mm are installed on the left side of the roadway, with the red ones representing hollow grouting cables and the blue ones representing solid cables; four flexible bolts with a length of 3500 mm and a diameter of 22 mm are installed on the right side of the roadway. Both the anchor cables and flexible rods are matched with the 300 mm × 300 mm × 14 mm arched steel tray, and a reinforced ladder beam as well as a woven mesh are used to strengthen and protect the surface. The initial pre-tightening force of the anchor cable shall not be less than 120 kN, and the initial pre-tightening force of the flexible bolt shall not be less than 80 kN. The grouting material selection

of Huainan Donghuaouke company KWJG-1 type mineral reinforced composite mortar, a product without shrinkage, micro-expansion, high liquidity, and pumping, does not contain chloride. A ZBYSB40/22-7.5 mining hydraulic grouting pump was selected for grouting pumping.

**Figure 14.** New support scheme.

It is required to note the following:<sup>1</sup> a long anchor cable with a size of Φ22 mm × 6500 mm shall also be installed for an arc roof area that is seriously cracked after the initial excavation; <sup>2</sup> for serious floor heave, hollow grouting cables shall be installed at an inclination of 45◦ downward, and the deformations of the floor heave shall be suppressed by grouting coupling as well as the reinforcement of key parts of the floor corner.

#### *5.2. Comparative Analysis of Simulation Effects of New Support*

Figure 15 shows the characteristics and deformation of cracks in the surrounding rocks of the roadway under the new support when the number of operation steps reaches 10.0 × 104. Figure 15a shows that after the differential support is applied the number of cracks in the surrounding rocks of the roadway is significantly reduced, and that the distribution range is significantly narrowed. The above is specifically reflected in the following three aspects: <sup>1</sup> the cracks on the arc roof side are all within the anchorage range, with the maximum evolution depth being reduced to 1.8 m, the cross-boundary support limiting the slipping as well as dislocation of the steeply inclined coal–rock mass, and the integrity of the surrounding rocks being significantly improved; <sup>2</sup> the cracks on the straight inclined roof side are only distributed in the shallow part of the roadway, and the deformation is small; <sup>3</sup> and the number of floor cracks is also appropriately reduced, which is mainly because the 45◦ grouting anchor cable improves the shear capacity of the coal–rock mass in the floor corner, and the surrounding rock on the arc roof side is quite complete, which limits the deformations of the floor coal–rock mass as well as the development of cracks.

Figure 15b shows the evolution law of the displacement of the roadway's surrounding rocks with depths under different supports. The vertical line, EF, in the figure is the center line of the roadway. The distance between the horizontal line, GH, and the horizontal line, PQ, of the roadway floor is 2 m. A total of 40 measuring points are arranged vertically inward 5 m from points E, F, G, and H on the roadway's surface in order to obtain the deformation status of the roadway at different positions. The vertical displacement is monitored at each measuring point on the roof and floor, and the horizontal displacement is monitored at each measuring point on both sides. In addition, 40 measuring points are also arranged on the horizontal line, PQ, of the floor to observe the floor heave status.

**Figure 15.** Maintenance and control effects of the roadway under the new support. (**a**) Distribution characteristics of cracks; (**b**) evolution law of the displacement of the roadway's surrounding rocks with depths under different supports.

It can be seen from the deformation status of the roadway at different positions that the control effects of different support forms are significantly different. It can be concluded from the subsidence data of the roof that deformation mainly occurs in shallow parts less than 1 m above the roof. The displacement of point E under no support, the primary support, and the new support is 205 mm, 98 mm, and 40 mm, respectively. The displacement under the new and primary supports decreases by 80.5% and 52.2%, respectively, compared with that under no support. It can be concluded from the displacement data of the wall on the arc roof side that deformations are mainly concentrated within 2 m of the wall. The displacement of point G under no support, the primary support, and the new support is 165 mm, 85 mm, and 50 mm, respectively. The displacement under the new and primary supports decreases by 69.7% and 48.5%, respectively, compared with that under no support. It can be concluded from the displacement data of the wall on the straight inclined roof side that deformations are mainly concentrated within 0.8 m of the wall. The displacement of point H under no support, the primary support, and the new support are 45 mm, 31 mm, and 20 mm, respectively. The displacements under the new and primary supports decrease by 55.6% and 31.1%, respectively, compared with that under no support. It can be concluded from the displacement data of the floor that deformations are mainly concentrated within 4.2 m. The displacement of point F under no support, the primary support, and the new support is 120 mm, 81 mm, and 52 mm, respectively. The displacement under the new support and primary support decreases by 56.7% and 32.5%, respectively, compared with that under no support. It can be concluded from the displacement data of the horizontal line, PQ, of the floor that the maximum displacement point of the floor is 0.3 m away from point P as the origin, and it shows a single decreasing trend from 0.3 m to 4.0 m. The maximum floor heave under no support, the primary support, and the new support is 220 mm, 178 mm, and 102 mm, respectively. The maximum floor heave under the new support decreases by 53.6% and 19.1%, respectively, compared with that under the primary support and no support. It can be seen that the control effects of the new support at different positions are significantly better than those of the original support. The deformations under the new support decrease by 53.6–80.5% compared with those under no support, while the deformations under the primary support decrease by only 19.1–52.2% compared with those under no support.

#### *5.3. Mine Pressure Monitoring and Analysis*

The working load on anchor cables is monitored for a long time by the dynamometer clam between the tray and the nut, which has the functions of a digital display, historical data memory, and transmission. Figure 16 shows the evolution curve of the working load on anchor cables of the arc roof and the straight inclined roof under the new support. It can be seen from the figure that the evolution characteristics of the working load on anchor cables at different installation positions over time are quite different. It can be seen from Figure 16a that the stability period of the anchor cables is about 40 days, during which the load on the anchor cables is characterized by repeated oscillation up and down, with the maximum load being 168 kN and the minimum load being the pre-tightening force (123 kN), indicating that the coal–rock mass in the shallow part on the arc roof side is prone to damage and failure under compression, which will cause the tray to move and lead to a readjustment of the load on the anchor cables. After repeated mechanical action, balance is finally achieved and the load is stabilized, and the final load is about 161 kN. It can be seen from Figure 16b that the stability period of the flexible bolt is about 13 days, and its working load shows an evolutionary trend of a sharp increase at first, then a slow increase, and finally a stable bearing. The rock mass on this side has good integrity, and the lithology is relatively hard and flexible. There is no repeated oscillation or adjustment for the load on the flexible bolt. The initial pre-tightening force is 83 kN, and is finally stabilized at 165 kN. The characteristics of the load on the bolt–cable at different positions of the roadway also reflect differences in the properties of the rock mass, which confirms that the control methods also need to be differential.

Figure 17 shows the peeping images of the arc roof and straight inclined roof of the roadway. The cracks in the arc roof and the straight inclined roof are distributed within 1.50 m and 1.10 m, respectively. There are seven cracks and two cracked zones on the arc roof side, but only six small cracks on the straight inclined roof side. It can be concluded from the distribution of cracks in the surrounding rocks that the integrity of the rock mass is significantly improved, which is beneficial to the long-term stability of the roadway. Figure 18 is the site photo of the roadway after 150 m of excavation. The surfaces of the arc roof and the straight inclined roof of the roadway are very flat, and there are no deformations and damage similar to those that occur under the primary support.

**Figure 16.** Curve of the working load on bolt/anchor cable under new support. (**a**) Arc roof; (**b**) Straight inclined roof.

**Figure 17.** Peeping results of the surrounding rocks under the new support.

**Figure 18.** Maintenance and control effects of the roadway under the new support.

#### *5.4. Discussion*

5.4.1. Restraining Mechanism of Cross-Boundary Support on Cracks in the Surrounding Rocks

The selection of the roadway support in conditions of large deformations should consist in the common fulfillment of geometric, ventilation, and geomechanical criteria [29]. The new CBAG differential support technology improves the control effect of the irregular roadway. In order to further understand the difference in the control effect of the surrounding rocks under different support forms, Zone A, with the most obvious control effect, is selected for illustration. Figure 19 shows the evolution curves of cracks in Zone A of the irregular roadway under different support forms. It can be seen from Figure 19a that when the operation coefficient reaches 5 × 104, that is, the surface stress of the roadway is just released to zero, the number of shear cracks in Zone A under no support, the primary support, and the new support is 1005, 636, and 337, respectively. The number of shear cracks under the primary support and the new support decreases by 36.7% and 66.5%, respectively, compared to that under no support. At the same time, as the number of operation steps continues to increase, when the operation coefficient reaches 10 × 104 the number of shear cracks under no support, the primary support, and the new support increases by 23.1%, 15.1%, and 0.3%, respectively. It can be seen from Figure 19b that when the operation coefficient reaches 5 × 104 the number of tension cracks in the surrounding rocks of Zone A under no support, the primary support, and the new support is 171, 46, and 16, respectively. The number of tension cracks under the primary support and the new support decreases by 73.1% and 90.6%, respectively, compared with that under no support. When the operation coefficient reaches 10 × 104 the number of shear cracks under no support, the primary support, and the new support increases by 94.2%, 34.8%, and 0, respectively. It can be concluded from the data of shear cracks and tension cracks that as the number of operation steps continues to increase the evolution rate of tension cracks in the surrounding rocks under no support is significantly faster than that of shear cracks. At the same time, the CBAG new technology is used to reconstruct shallow surrounding rocks and bidirectional linkage control of deep and shallow rock masses, thus restraining the evolution of tension cracks and realizing the fundamental control of cracked surrounding rocks of irregular roadways. Therefore, restraining the expansion of tension cracks is the key to controlling the surrounding rocks of irregular roadways.

**Figure 19.** Evolution curves of cracks in Zone A of the roadway under different support forms. (**a**) Shear crack; (**b**) tension crack.

5.4.2. Discussion on CBAG New Support Structures and Grouting Sealing Method

Two new types of support products are adopted for the CBAG differential support, namely hollow grouting cables and flexible bolts, as shown in Figure 20. The hollow

grouting cables, with a specification of 1 × 8–22–2000 MPa, are produced by the fully automatic production line of Shandong Yanxin Company. They are mainly composed of eight high-strength steel wires and stainless steel pipes, which overcome the disadvantages of the traditional welded grouting anchor cable structure, realize the high strength of the anchor cable bolt body, and also change the traditional method of plugging the hole at the tail with a rubber plug. They use a hard pipe to push and squeeze the hose to make the hose expand, and then move the plugging position from shallow inside to the hole of the complete surrounding rocks, that is, from State <sup>1</sup> to State <sup>2</sup> , which overcomes the disadvantage of poor plugging caused by the cracked shallow part. Another product is flexible bolts with a specification of 1 × 7–18–1860 MPa. They are composed of a steel strand bolt body and a tail threaded sleeve. They overcome the disadvantage that the length of the threaded steel bolt is limited by the roadway space and cannot meet requirements of cross-boundary support, and use the nut rotation installation method to realize the rapid installation of the steel strand bolt body.

**Figure 20.** New support products and grouting sealing method.

#### **6. Conclusions**

(1) This work analyzes the asymmetric failure characteristics of the sliding and dislocation deformations of the coal–rock mass on the arc roof side and the loose deformations of the shallow rock mass on the straight inclined roof side of the roadway in a steeply inclined coal seam in Sichuan Province. The differential evolution regularities of cracks in the arc roof and the straight inclined roof during the initial and long-term excavation of the roadway are analyzed. The main reasons for the large deformations of the surrounding rocks are the coordinated deformations of the bolts, the shallow rock mass under the primary support, and the poor reinforcement of long anchor cables on the rock mass within this range.

(2) UDEC discrete element simulation was utilized to analyze the process stress evolution pattern. The result shows that stress release occurs first in the floor strata and then causes the gradual stress release of the surrounding rock on the entire arc roof side. Finally, the roadway shows a typical zonal failure characteristic of "slope-type" deformations for the floor, "dislocation falling" deformations for the arc roof, and micro-deformations for the straight inclined roof.

(3) Based on the non-equilibrium deformation characteristics of the roadway in the steeply inclined coal seam, this work proposes CBAG differential support technology for reinforcing key parts in the floor corner, thick-layer anchorage of the straight inclined roof and the arc roof, and grouting reconstruction. An equivalent thick-layer bearing structure was constructed for the full section of the surrounding rocks of the roadway. Using new hollow grouting cables and flexible bolts, this work anchored shallow grouting reconstructed rock mass into a deep rock stratum, such that a small displacement of deep rock mass can restrain a large displacement of shallow rock mass. In this way, it can adapt to the long-term creep effect of soft surrounding rocks of the roadway in the steeply inclined coal seam. Recommendation: The new grouted cable support should be adopted in all

roadways with similar conditions to the ventilation roadway in Working Face 1961 of the Zhaojiaba Mine.

(4) The analogue calculation shows that the new support greatly narrows the tensile stress zone of the surrounding rock of the roadway, effectively preventing slipping and dislocation deformation of the surrounding rock mass of the arc roof. On-site industrial testing shows that the depth of the maximum crack in the arc roof generally decreases by 61.3%, from 3.88 m to less than 1.50 m, and the development depth of cracks in the straight inclined roof decreases by 47.6%, showing a significant improvement in the surrounding rock control effect.

**Author Contributions:** Funding acquisition, Z.X. (Zhengzheng Xie) and N.Z.; Investigation, Z.X. (Zhengzheng Xie), J.W., N.Z., F.G., Z.H., Z.X. (Zhe Xiang) and C.Z.; Methodology, Z.X. (Zhengzheng Xie), J.W., F.G., Z.H. and Z.X. (Zhe Xiang); Supervision, N.Z. and C.Z.; Writing—Original Draft, Z.X. (Zhengzheng Xie), J.W. and F.G.; Writing—Review and Editing, N.Z., Z.H., Z.X. (Zhe Xiang) and C.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (52104104).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are available from the corresponding author on reasonable request.

**Acknowledgments:** We would also like to thank the anonymous reviewers for their valuable comments and suggestions that lead to a substantially improved manuscript.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.

#### **References**


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