Risk Analysis Of Buried Pipe Subjected To The Rain-induced Landslide In Mountain City

As city’s infrastructures, the sewage pipelines are closely related to the developme-nt of the city and the life of citizens. However, in mountain city, strong rainfall andgeological hazards caused by it always threaten normal operation of urban sewagesystem. On the background of Chongqing downtown sewage system’s A pipelineproject, the risk of the buried box culvert to landslide was analyzed. The main work andconclusions were present as follows:Firstly, rainfall data of thirty years provided by Chongqing Meteorology Bureauwere statistically, and the rainfall probability of different grades was calculated. Thepaper gave the rainfall intensity and the duration of rainfall when the rainfall was morethan10millimetrers from2003to2009.Secondly, the paper focused on the the relationship of the rainfall and water content.According to the law of conservation of mass and Darcy law, the flow equation ofunsaturated soil was derived. And combined with boundary conditions of rainfallinfiltration, the paper gave the numerical solution of the equation. The expression ofwater content-cohesion and water content-angle of internal friction was given by fittingthe experimental data. Analysis showed that the rainfall raised water content, decreasedthe conhesion and the angle of internal friction, and decreased the strength of soil. Theresult was that the landslide occurred more easily.Then, taking conhesion and the angle of internal friction as uncertain factors, thepaper calculated the stability factor and failure probability based on the simple Bishopmethod and transfer coefficient method, and calculated the reliability index based onfirst-order second-momemt method. The results were compared using Monte-Carlosimulation method. The example showed that the possibility of landslide was smallunder the normal case. But the rainfall changed the soil parameters, reduced the stabilityfactor and reliability index, and increased the possibility of landslide. The instabilitypossibility increased with the increase of water content.And then, the paper calculated the landslide thrust under different rainfall. Theburied box culvert was modeled through a nonlinear finite-element software ABAQUS,and the damaged load of pipeline under landslide was got. Then the basic function wascreated by using the total landslide thrust and the damaged load, and the probability offailure was calculated. The result showed that when a landslide happened, the failure probability of the pipeline was larger than50%.Finally, according to the failure probability of pipeline after rainfall, the papersuggested the consequence divide into four grades. If considering the rainfall probability,the paper suggested the risk of slope divide into three different grades, for guiding thesafety operation of sewage system.