Safety Assessment Of Pipeline In Region Of Stratum Collapse And Soil Liquefaction For Sichuan-east Gas Transportation Project

The pipeline project of Sichuan-east gas transportation cuts through the area where landform and geology condition are complex, the stratum collapse and soil liquefaction has a gigantic threat to construction and work of pipeline, it needs to be resolved in one of the major problems that pipeline is subjected to the special geological disasters, so it has important significance that impact of stratum collapse and soil liquefaction on the pipeline safety is researched.It was researched that impact of collapse and suspending over pipeline safety by announced document adopted the model of Winkler foundation beam, but there isn't a report about adopting other mechanics analysis model. In the paper, it is researched by adopting ideal elastic-plasticity foundation beam model. The results show that elastic-plasticity model is more reasonable and accurate than Winkler model, Winkler model is simpler than elastic-plasticity model, and is easily applied on the project.At the basis of literature presented, formation causes and effects factors are analyzed about stratum collapse by finite element method, and it is researched that stratum collapse and soil liquefaction have made impact on pipeline safety based on the theory of Winkler foundation beam and elastic-plasticity foundation theory. It is built the mechanics analysis model of the pipeline and soil, and is solved the pipeline deformation differential equations, according to pipeline boundary condition, smooth condition and axial displacement condition, these conditions constitute linear and nonlinear equations solved all of unknown element, the built mechanics model is solved by adopting nonlinearity theory, the deformation and stress of pipeline is analyzed. The main conclusions are as follows:1. The plasticity region begins to be formed on hole wall near hole bottom, hole upper cover soil body is acted by oneself gravity load and Vacuum negative pressure, plastic region continuously expands with soil moisture content increasing, When the physics mechanics parameter of the soil body is lowered to certain degree, plastic region is formatted run-through region at the top of the soil hole, at this time soil body is destroyed and pit appears on the ground. The groundwater activity is one of main impact factor of ground collapse; the view of finite element method explains why much the ground collapse occurred during rain.2. The critical length of the stratum collapse region and soil liquefaction region are calculated about four kind mechanics analysis models. The error is about 8% for critical length of the stratum collapse region of considering axis force about the two models, but the error of soil liquefaction is about 3%, the both error increases with the pipeline yield stress increasing.3. Under the length of stratum collapse region and soil liquefaction region is relatively small, the deformation and stress of buried pipeline is analyzed for simplifying project calculation by adopting Winkler model. When the length of stratum collapse region and soil liquefaction region is relatively large, the analysis necessity is especially obvious by adopting ideal elastic-plasticity model.4. The deformation and maximal Mises stress of pipeline are reduced by considering effect of axial force, considering effect of axial force is more reliable and closer to real situation than not considering effect of axial force. When the length of stratum collapse region and soil liquefaction region is relatively small, it can be neglected for simplifying project calculation that axial force impacts on the calculation result of pipeline.5. It can be neglected by adopting elastic-plasticity model that soil spring stiffness impacted on the pipeline calculation results, but Winkler model can't.