*2.3. Asymmetric Deformation Characteristics of the Roadway*

Figure 3 shows the deformation of the ventilation roadway of Working Face 1961 under the primary support, which is 100 m away from the excavation face. It can be seen from the figure that different positions of the roadway show differential deformation characteristics: <sup>1</sup> The rock mass between the two rows of bolts on the straight inclined roof experiences a "bulge" deformation, with the surface layer being relatively loose and layered shedding occurring locally; <sup>2</sup> the curved roof experiences quite evident deformation, with the coal–rock seam being displaced and deformed, the surface arc shape becoming flat, and the reinforced ladder beams at this position being generally bent and locally broken; and <sup>3</sup> the corner roadway zone on the arc roof side is "extruded" as a whole, while the floor experiences a slope-type deformation (the closer it is to the left side, the greater the floor heave). To sum up, roadway deformations are characterized by asymmetric deformations and failure. The starting point, A, of the asymmetric line is located at the intersection of the arc roof and the straight inclined roof, while the ending point, B, is located at the intersection of 10# coal seam and the mudstone interlayer. The surrounding rock deformation in the zone to the left of the straight line, AB, is significantly greater than in the zone to the right of the straight line, AB. The continued deformation in the left side area caused a continuous reduction in the section of the roadway, with a maximum reduction proportion of 37%. Furthermore, the anchor cables tended to break in some areas, greatly undermining mining safety.

**Figure 3.** Failure status of the roadway under the primary support.

#### *2.4. Evolution Characteristics of Cracks*

In order to further evaluate the maintenance and control effects of the primary support, borehole peeping is carried out on the straight inclined roof and the arc roof at 4 m and 100 m away, respectively, from the excavation face to observe the development depths of initial cracks when the excavation begins, as well as to analyze the evolution of cracks in surrounding rocks after a period of roadway formation, as shown in Figure 4. The equipment used for the observation is a borehole peeping instrument, and the diameter and length of the borehole are 32 mm and 8 m, respectively.

**Figure 4.** Distribution characteristics of cracks in the surrounding rocks of the roadway.

Figure 4a shows the distribution of cracks in the surrounding rocks when the roadway has just been excavated. At this time, the cracks' evolution depths at different roadway positions are obviously different. Cracks in the straight inclined roof are mainly within 0.5 m, and most of them are small annular cracks, with the maximum crack depth being only 0.56 m, while the coal–rock mass on the arc roof is of poor integrity, with the maximum crack depth being 1.76 m. In addition to annular cracks there are also inclined and longitudinal cracks in the hole. Compared with straight inclined roofs there are more cracks in the arc roof, and the cracks' openings are larger, undermining the arc roof's stability.

Figure 4b shows the distribution of cracks in the surrounding rocks 100 m away from the excavation face. Compared with when the excavation had just begun, the cracks on both sides of the surrounding rocks of the roadway expanded significantly at this time. The depths of the cracks in the straight inclined roof expand to 2.10 m, with a growth rate of 73.3%. It is additionally found from the figure that the cracks are mainly distributed within 1.5 m. The maximum development depths of the cracks in the arc roof reach 3.88 m, with an increase of 54.6%. The cracks and cracked zones are generally distributed within 3.0 m. When 100 m away from the excavation face the excavation stress tends to be stable, the cracks in the straight inclined roof expand to the bolt anchorage end area, and the cracks in the arc roof expand obviously beyond the bolt anchorage range, indicating that the shallow rock mass of the roadway as well as the bolt have experienced synchronous deformation and that the reinforcement effect of the bolt on the rock mass is very limited. Moreover, the expanding rate of the anchor cable is generally lower than 3.5%, and continuous rock mass deformation forces the anchor cable to break, such that the primary support can no longer meet the safety maintenance and control requirements of the roadway.

#### **3. Time-Dependent Failure Mechanisms of a Roadway in a Steeply Inclined Coal Seam**

*3.1. Model Establishment and Scheme Design*
