Microscopic Mechanism Of Erosion Disaster In Shanghai Sandy Silt Caused By Pipeline Leakage

With the increase of service life and the effect of external loads,the buried drainage pipelines gradually appear aging and damage.Shanghai is located in the southeast coast,with abundant rainwater and high groundwater level.Under the action of groundwater,soil erosion or migration occurs,and it is finally discharged through broken drainage pipes.As time goes by,the drainage pipeline forms a void around it.Once the ultimate bearing capacity is exceeded,it will collapse and cause huge losses to people’s lives and property.Therefore,it is very urgent to conduct research on soil seepage caused by pipeline leakage in Shanghai.Because the soil erosion process is difficult to observe in field tests and indoor tests,and the interaction between water and soil is involved in this process,this paper uses the PFC-Fluent joint calculation method to analyze the pipeline in the sandy silt layer in Shanghai.Research on the meso-mechanism of leakage and erosion.The main work content and results are as follows:（1）Use PFC^3D to build a soil model,Fluent builds a flow field,and realizes the coupling calculation of the two through the built-in CFD module of PFC.This method is used to build a pipeline leakage model of sandy silt layer,and the simulation results are compared with the test results to verify the correctness of the model.By analyzing the laws of stratum subsidence,stratum loss,movement trajectory of fine particles,loss of fine particles,and changes in porosity of the stratum,we can study the effect of fine particle size,damage opening size,groundwater level,and pipeline depth on pipe leakage from a meso scale.Causes the impact of soil erosion disasters.（2）Analysis and simulation results found that when the pipeline leaks in the sandy silt layer,fine particles migrate along the pores of the framework particles under the action of water flow.If the width of the damaged opening is twice the particle size of the skeleton particles,the skeleton particles will block the damaged opening,but the fine particles can still be washed into the pipeline.At this time,it is mainly reflected in the fine particle siltation inside the pipeline,which has little effect on the surface settlement;When the width is greater than one time of the particle size of the skeleton particles,the skeleton particles will be washed into the pipeline and eventually cause significant surface settlement.Therefore,restraining the expansion of the damaged opening is very important for the management of pipeline leakage.The higher the groundwater level,the more serious the soil loss,and the more obvious the migration of fine particles.In the seasons when the groundwater level is higher,the surface subsidence and pipeline siltation caused by soil seepage are more significant.The smaller the buried depth of the pipeline,the greater the surface settlement,so the impact of the leakage of shallow buried pipelines is greater.（3）Sandy silt in the northern part of Shanghai is often covered with a silty clay layer,so this paper simulates this type of formation.The simulation results show that the two sides of the settlement trough in the overlying silty clay layer of sandy silt are relatively steep,and there are tension cracks in the overlying clay layer,which is quite different from the settlement trough of the sandy silt layer.And because the overlying clay has a certain degree of self-stability,the surface settlement value of this stratum is smaller than that of the sandy silt layer without overlying clay.（4）The influence of factors such as the size of the damage opening,the groundwater level and the thickness of the clay layer on the leakage of a pipe in a silty clay layer overlying sandy silt is analyzed.The larger the width of the damage opening,the more obvious the seepage phenomenon.The loss of skeleton particles is the main factor that causes the surface settlement;the higher the groundwater level,the more serious the loss of fine particles,and the soil compactness decreases and the permeability increases during the seepage process.The erosion effect of the silty clay is more obvious;because the clay has a certain"self-stabilization"ability,the thicker the clay layer,the smaller the final settlement value and damage range of the ground surface.When the silty clay collapses to the vicinity of the damage opening,it will cause siltation of the damage opening.Blocking,inhibit the development of pipeline leakage.