# Study of the Failure Mechanism of Soft Rock Mining Roadways Based on Limit Analysis Theory
## 2 Analysis of the Deformation and Failure of a Soft Rock Mining Roadway
### Physical Similarity Simulation Experiment Verification
#### 3.2.3 Results and Discussion  
The failure characteristics of the rock surrounding the roadway under different confining pressures during the loading process are shown in Figures 7-9. The failure evolution process of the model can be divided into three stages: the first stage is the crack initiation stage (step 1-step 3); the second stage is the development stage of surrounding rock deformation (step 4-step 6); and the third stage is the violent development stage of surrounding rock deformation (step 7-step 9). From Figure 7, it can be seen that when the overburden load \(\sigma_{\rm s}\) = 3.0 MPa (step 2), the shear failure of the weak coal on the roadway side is first caused by the stress concentration on the roadway side. Fissures are first generated at the bottom corner of the roadway and expand with increasing surrounding rock pressure to the blind support area (area C) above the syncline. Finally, inclined shear cracks form inside the roadway side, and the fracture development area is distributed in an \"arc\" shape. This is basically consistent with the theoretical calculation of the velocity discontinuity line shape. Due to the large damage depth of the middle and upper coal on the two sides of the roadway, the middle and upper bolts are in a failure state, and the maximum damage depth of the roadway side is 2.8 m. There is no obvious deformation and no crack propagation in the roof and floor. With the increase in surrounding rock pressure, when the overburden load \(\sigma_{\rm s}\) = 4.5 MPa (step 3), the cracks in the roadway side continue to extend to the blind support area in the direct roof (area C) and the low-strength support area (area B) above the slope. The mechanical properties of the direct roof and coal are the same; thus, they are both weak rock masses. As a result, there is no change in the failure mode of the direct roof, the morphology of the fracture development zone, or the roadway side. The failure mode is shear slip failure, and the morphology of the fracture development zone is an \"arc\" distribution. However, the scope and damage degree are significantly higher in the direct \begin{table}
\begin{tabular}{c c c} \hline \hline
**Loading Sequence** & **Stress in the Surrounding Rock (MPa)** & **Applied Vertical (kN)** \\ \hline Step 1 & 1.5 & 9.38 \\ Step 2 & 3.0 & 18.75 \\ Step 3 & 4.5 & 28.13 \\ Step 4 & 6.0 & 37.50 \\ Step 5 & 7.5 & 46.88 \\ Step 6 & 9.0 & 56.25 \\ Step 7 & 10.5 (K \(\simeq\) 1.17) & 65.63 \\ Step 8 & 12.0 (K = 1.33) & 75.00 \\ Step 9 & 13.5 (K = 1.50) & 84.38 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Stepwise loading table of the experimental model. roof than in the roadway side. The anchor cable anchorage section of the two shoulder angles of the roadway is located in the weak direct roof (B area); thus, the support strength is low. The expansion of the fissure leads to the failure of the anchor cable at the shoulder angle of the roadway, which increases the direct roof failure range. The maximum damage range of the direct roof is 15.8 m. The theoretically calculated value of the roof failure height is the same as the simulation results, both of which are approximately 7.27 m. For the high strength support area (area A), because the anchoring section of the anchor cable in area A is mostly in the hard basic roof, the direct roof anchored by the anchor cable in area A has no deformation and no crack expansion. The floor has no obvious deformation and no crack propagation. Through the control imposed by the bolt and anchor cable system in the surrounding rock and the strength of the surrounding rock itself, the roadway remains stable, as shown in Figure 8.  
Figure 8: Failure characteristics of roadway-adjacent rock (\(\sigma_{\text{s}}\) = 4.5 MPa). (**a**) Experiment photograph; (**b**) sketch. Figure 7: Failure characteristics of roadway-adjacent rock (\(\sigma_{\text{s}}\) = 3.0 MPa). (**a**) Experiment photograph; (**b**) sketch.