## 3 Creep Instability Mechanism of the Soft Coal Roadway
### Creep Instability Mechanism of the Roadway
Figure 10 shows a cloud diagram of the change in surrounding rock cracks with time under shed cable support. In Figure 10, the red lines represent the number of tensile cracks (represented by T), the green lines represent the number of shear cracks (represented by S), and the crack development area is represented by blue dotted lines. From Figure 10, the following observations can be made:  
1. The development of fractures in the roadway is predominantly characterized by shear fractures. The number of shear cracks consistently exceeds the number of tensile \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Support** & **Diameter** & **Dimension** & **Yield Strength** & **Rupturing Load** & **Pretightening Force** \\
**Material** & **(mm)** & **(mm)** & **(MPa)** & **(kN)** & **(kN)** \\ \hline Bolt & 22 & 2600 & 335 & 167 & 60 \\ Cable & 21.6 & 8000/4000 & — & 607 & 180 \\ Mesh & 4 & 3600 \(\times\) 900 & 235 & 6 & — \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Mechanical parameters of bolt-cable mesh. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Support** & **Model** & **Cross Sectional Area** & **Theoretical Weight** & **Allowable Stress** & **Allowable Bending** \\
**Material** & **Model** & **(cm\({}^{2}\))** & **(kg/m)** & **(MPa)** & **Moment (kN\(\cdot\)m)** \\ \hline U-shaped shed & 36U & 45.7 & 35.8 & 520 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mechanical parameters of a 36U-shaped shed. cracks at each time period. Taking the example of a 15-day excavation, the monitoring area recorded 1714 shear cracks and 192 tensile cracks. As time progresses, the growth rate of shear cracks outpaces that of tensile cracks. 2. The distribution of cracks in the roadway varies with location. After a 15-day excavation, the shear cracks have developed to a depth of 1.3 m, primarily concentrated in the shallow portion of the surrounding rock. In contrast, the tensile cracks are mainly distributed on the surface of the roadway, with a development depth of approximately 0.6 m. As the excavation time progresses, the shallow shear cracks gradually transform into tensile cracks, forming a region of tensile crack distribution. The cracks further propagate into the deeper parts of the surrounding rock. After 120 days of excavation, the range of cracks caused by unsteady creep in the roadway exceeds 5.6 m.
3. The effectiveness of shed-cable support in inhibiting surrounding rock fractures diminishes over time. During the creep instability of the surrounding rock in the soft coal roadway, the distribution of cracks in the mudstone roof is minimal. However, the number of cracks in the coal body increases with time, and the growth rate accelerates. This indicates that the range of both steady and unsteady creeps in the coal body is expanding. Figure 9: Roadway deformation diagram under shed cable support: (**a**) 15 d, (**b**) 30 d, (**c**) 60 d, and (**d**) 120 d.  
Based on the above analysis, the creep instability mechanism of the return airway in the 11,000 working faces can be summarized as follows:  
1. In the early stages of roadway excavation, the shed cable support is unable to effectively inhibit the development of roadway cracks, leading to the rapid expansion of meso-cracks in the floor and ribs of the coal body. The deep coal body experiences steady-state creep dominated by shear cracks, while the surface of the coal body undergoes unsteady creep dominated by tensile cracks. With the increase in excavation time, the meso-cracks continue to propagate and expand into the deeper part of the surrounding rock, resulting in structural failure of the support system. As the shallow shear cracks in the surrounding rock gradually transform into tensile cracks, the number of tensile cracks increases, causing the coal body to transition from a steady-state creep state to an unsteady-state creep state. 2. During the development and expansion of cracks, the primary expansion of tensile cracks occurs from the coal seam and rock joints towards the deeper regions of the surrounding rock.