## 2 Pipe-Soil Collapse Analysis Model
### Calculation Results and Analysis  
Comparing the settlement displacement calculated by the pipe-soil finite element model established in this article with the settlement displacement recorded by the physical model test, the error is within 10%. The settlement displacement of the buried pipeline calculated using the engineering calculation model test formula 8 is compared with the finite element model results, and the error is within 5%. This error value shows that the results of this model are reliable. As shown in Figure 4.  
It can be seen from Figure 4 that as the collapse width increases, the settlement displacement continues to increase. The collapse width is between 0.09 m and 0.27 m, and the settlement displacement increases rapidly. The collapse width is between 0.27 m and 0.54 m, and the settlement displacement increases slowly and levels off. This is due to the weakening of the pipe-soil interaction as the collapse width increases. The mechanical analysis of the dynamic evolution process of buried pipeline collapse is shown in Figure 5. (Note: In Figure 5 the color of the pipe represents the contact state of the pipe and soil; the red represents the contact, and the yellow represents the separation).  
As the collapse process progresses, the Von-Mises stress and settlement displacement of the buried pipeline increase exponentially and have not reached the failure stress of the buried pipeline. As the collapse process progresses, the entire process, from the initial deformation of the pipe-soil structure to the tensile failure of the buried pipeline, can be divided into three stages: the top compression stage, the transition stage, and the bottom tension stage. Before the collapse depth reaches 80 mm, it is the top compression stage; between 80 mm and 160 mm, it is the transition stage; and after the collapse depth reaches 160 mm, it is the bottom tension stage. As the collapse process progresses, the distance difference between the settlement displacement of the soil and the settlement  
Figure 4: Verification of the finite element model.  
Figure 3: (**a**) Boundary condition; (**b**) Simulation of collapse.  
displacement of the buried pipeline gradually increases from zero. This shows that the pipe-soil deformation is coordinated in the early stage and non-coordinated in the later stage. When the collapse depth reaches 48 mm, \"pipe and soil separate.\" This is because at this stage, the pipe initially sinks together with the soil, but the properties of the pipe and soil are different. The stiffness of the pipe is much greater than that of the soil. After a period of time, the pipe no longer moves together with the soil; the two movements appear to be out of sync. The pipeline begins to separate from the soil, and the suspended length continues to lengthen over time. However, the soil does not completely separate from the pipeline during this process, and the pipeline bears all the loads given by the soil. By comparing the section stresses at different locations of the buried pipeline, it can be concluded that the most dangerous section is the mid-span section. Through shear force and axial force, it can be concluded that axial stress is the control stress. By observing the stress levels at different locations in the same section, it can be concluded that the bottom of the pipe is the control point, and excessive tensile stress is the main cause of the failure of buried pipelines.