# 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
When the overburden load \(\sigma_{\rm s}=7.5\) MPa (step 5), there is no obvious change in the shear failure range of the roadway side. However, due to the subsidence of the roof, the coal on the roadway side is squeezed along the slip line, increasing the opening degree of the macroscopic cracks on both sides of the roadway, resulting in a significant increase in the displacement of the roadway side. The cracks in the blind support area (area C) and the low-strength support area (area B) continue to extend deeper, and the cracks exist outside the high-strength anchorage zone of the anchor cable. When the fracture penetrates the interface between the direct roof and the basic roof, the fracture extension direction changes significantly. The main reason for this is that the mechanical properties of the basic roof rock are different than those of the direct roof, and the failure mode of the fracture changes from slip shear failure to tensile failure. The fracture development zone (zone E) in the basic roof is roughly symmetrically distributed about the D zone. Under the influence of the surrounding rock stress, the roadway floor bends and deforms toward the free face, and tensile cracks are generated in the floor surface. Due to the different deflections of each layer, separation cracks are generated between the layers, and the floor crushing range gradually expands downward, with a maximum failure depth of 1.25 m, as shown in Figure 9.  
With increasing surrounding rock pressure, the original microcracks extend and connect with each other and finally transform into macroscopic shear slip surfaces. Multiple slip surfaces combine to form a shear slip band (zone E). Under the action of self-weight and roof pressure, the status of the rock mass in the shear slip zone (zone E) and its inner anchored rock mass (zone D) changes from static to dynamic. Gradual shear slip movement occurs at the free surface inside the roadway. On the roadway side, the more complete coal exhibits shear along the slip surfaces, resulting in the slip of the anchor body on the roadway side. The three bolts in the upper part of the roadway are separated from the anchor body; thus, the movement of the upper and lower sides is inconsistent, resulting in the coal protruding from the roadway side. Once the side block of the roadway slips, the roof slip block sinks as a whole. With an increase in confining pressure, the width and length of the shear slip zone show increasing trends. At this time, the crack has extended to the anchor cable anchorage zone (zone D). The anchor cable anchorage zone is separated from the basic roof rock layer as a whole, resulting in the risk of failure of the anchorage system. At this time, the anchor cable plays the role of only suspending the anchor bolt anchorage zone, and the bearing capacity is small. In addition, the density of vertical cracks in the surrounding rock is further increased, and there is a risk of overall collapse. Figure 9: Failure characteristics of roadway-adjacent rock (\(\sigma_{\rm s}=7.5\) MPa). (**a**) Experiment photograph; (**b**) sketch. The roof strata of the roadway have a large area of subsidence along the shear slip zone (zone E), and this type of failure of the roof is categorized as an overall subsidence type. The deformation and failure of the floor are severe, the floor heave increases, and the area of the roadway section gradually decreases. When the overburden load \(\sigma_{\rm s}\) = 13.5 MPa (step 9), the distribution range of cracks reaches the maximum. The distribution pattern of the velocity discontinuity line obtained by a similar simulation is basically consistent with the distribution pattern of the theoretical calculation, as shown in Figure 10.