*3.2. Time-Dependent Failure Process of a Roadway in a Steeply Inclined Coal Seam* 3.2.1. Stress Evolution Analysis

Figure 7 shows the nephogram of the maximum principal stress of the four stages in the stress release process of the roadway in the steeply inclined coal seam. It can be seen from the figure that after the excavation of the roadway is started the floor is damaged first, and its stress decreases accordingly. The nephogram of the maximum principal stress is in the shape of an "inverted triangle". The stress of the floor zone on the arc roof side decreases the most, with a relatively wide damage range, and the stress of the wall on this side also decreases in a small range. It is also found that stresses are concentrated in the argillaceous siltstone seam at the lower left of the roadway. With the gradual release of surrounding rock stress the damage degree of the surrounding rocks on the straight inclined roof side is small, and the degree of stress decreasing is not obvious; however, the damage to the surrounding rocks on the arc roof side becomes more and more serious, and the degree of stress decreases becomes increasingly obvious, which further causes the stresses to concentrate in the hard rock stratum at the lower left of the roadway. At this time, the roadway is at risk of being extruded from the lower left.

**Figure 7.** Nephogram of the maximum principal stress of the roadway during the stress release process.

### 3.2.2. Displacement Evolution Analysis

Figures 8 and 9 are the evolution diagrams of the surrounding rock displacement field of the roadway in the steeply inclined coal seam. It can be seen from Figure 8 that when the stress release coefficient, R, is 0.7, the coal–rock mass on the left side of the roadway floor is displaced upward, and the whole floor experiences a typical "slope-type" deformation with a large deformation on the left side and a small deformation on the right side. When the stress release coefficient, R, is 1.0, the coal–rock mass seam of the arc roof slips obviously and experiences a "falling" deformation, and the arc roof surface tends to be flat. It can be further concluded from Figure 9 that when the number of operation steps reaches 10 × 104 the maximum subsidence of the arc roof exceeds 300 mm, and the horizontal displacement at the left shoulder of the roadway is the largest, exceeding 200 mm. At the same time, the maximum floor heave of the roadway is located in the left corner of the roadway, and the deformation also exceeds 200 mm; however, the surrounding rock displacement on the straight inclined roof side is less than 50 mm. Therefore, the surrounding rocks of the roadway are steeply inclined coal seams that experience significant asymmetric differential deformations, the surrounding rocks on the arc roof side are a severely deteriorated zone, and the surrounding rocks on the straight inclined roof side is a slightly damaged zone.

**Figure 8.** Vector diagram of the displacement field of the roadway.

**Figure 9.** Nephogram of the displacement field of the surrounding rocks of the roadway.

3.2.3. Analysis of the Crack Evolution Law

Figure 10 shows the evolution characteristics of cracks in the surrounding rocks of the roadway in the steeply inclined coal seam. It can be seen from Figure 10a that the cracks in the surrounding rock are distributed in different zones, and that the number as well as evolution depths of the cracks in the surrounding rock zone on the arc roof side (Zone A) are significantly higher than those on the straight inclined roof side (Zone B). Furthermore, Figure 10b shows that the cracks in the surrounding rocks are subject to the zonal evolution law. With the gradual release of surrounding rock stress, the cracks in Zone A increase significantly in stages. In particular, when R is 1 the number of cracks increases in a leaping manner; with the increase in calculation steps the number of cracks continues to increase, indicating that the deformation in Zone A has the property of longterm flowing deformation. However, the evolution trend of cracks in Zone B is significantly weaker than in Zone A, and the stability period is also significantly shortened. When the number of operation steps reaches 5 × 104, the number of cracks tends to stabilize. It can be found from the figure that the number of shear cracks in the two zones of the roadway is significantly larger than the number of tension cracks.

**Figure 10.** Differential evolution laws of cracks in surrounding rocks. (**a**) Zonal distribution characteristics of cracks; (**b**) time-dependent evolution curves of cracks.

#### 3.2.4. Analysis of Time-Dependent Failure Mechanism

It can be concluded from the simulation results and field observations that the deformations of the surrounding rocks of the roadway in the steeply inclined coal seam are highly correlated with time and subject to the zonal differential evolution law, which is mainly reflected in the deterioration rate of the surrounding rocks on the arc roof side far exceeding that on the straight inclined roof side. The reasons for the time-dependent failure of the roadway are analyzed from the following two aspects:

(1) Fragmentation of rock mass in the floor corner on the arc roof side of the roadway in the steeply inclined coal seam is the precondition for the time-dependent failure of the surrounding rocks. It can be seen from Figure 8 that, after the excavation of the roadway is begun, the coal–rock mass in the floor corner zone first experiences an upward seam displacement deformation and that the damage degree at this position is the most serious, which provides deformation space for the later slipping failure of the surrounding rock on the arc roof side. The roadway floor also experiences a "slope-type" deformation with a large deformation on the left side and a small deformation on the right side. In order to realize the long-term maintenance and control of the roadway in the steeply inclined coal seam, it is necessary to control key parts of the floor on the arc roof side.

(2) Non-equilibrium deformations of the roadway in the steeply inclined coal seam are caused by surrounding rock stress. The occurrence of the steeply inclined coal–rock seam determines the deflection transfer of surrounding rock stress during the excavation process. Such asymmetric adjustment causes slipping and dislocation deformations of the coal–rock mass on the arc roof side along the seam, and the large-scale linkage of surrounding rocks tends to form a partial stress-bearing arch on the roadway, which intensifies the damage of the surrounding rocks on the peak stress side and structural instability damage. It can be seen from Figure 7 that the maximum principal stress is transferred to the hard rock stratum

deeper in the surrounding rocks, and the damaged as well as deteriorated parts further expand to the depth. The surrounding rock stress and the rock mass deformations have a bidirectional deterioration relationship, which leads to the failure of the bolt anchorage structure as well as the instability and collapse of the severely deteriorated zone; however, there is no large-scale stress concentration in the surrounding rock on the straight inclined roof side, and only a certain degree of damage occurs in the shallow part. Therefore, the floor angle of the roadway in the steeply inclined coal seam shall be timely controlled, and differential control measures shall be taken for surrounding rocks on both sides.

#### **4. CBAG Differential Support Technology**
