Study On The Squeezing Mud Under Submerged Tubes

In recent years,with more and more intensive traffic infrastructure construction,underwater tunnels are gradually widely used in southeast coastal areas of China.But in coastal areas,the soils is mostly silt or muddy soil,which has low strength,high compressibility,poor permeability and obvious rheological properties.This will easily cause great security risks in the process of settlement of immersed tunnel,therefore,controlling the thickness of silt interlayer between foundation trench and immersed tube becomes the key problem to control the safety of immersed tube.However,whether or to what extent sludge dredging should be carried out in the construction to meet the safety of the project has not been verified by the calculation method at the present stage.Relying completely on the dredging standard which is determined by experiences will cause a lot of unnecessary dredging costs.Therefore,it is of great significance to formulate reasonable dredging standards to ensure the smooth construction of immersed tunnel.Based on the research background of immersed tunnel engineering,this paper studies the relationship between silt density and dredging thickness under engineering load.Firstly,the influence of silt deposition and rheological characteristics on the actual effect of silt squeezing is analyzed,and then the small-scale indoor model tests and numerical analyses by Coupled Eulerian-Lagrangian(CEL)finite element method are used to study the thickness of silt squeezing under different silt density(1)The sedimentation law of silt in still water.When the content of fine particles in the sludge is high and the deposition time is short,the density,water content and particle content of the sludge will not change significantly with the increase of deposition depth.It shows that the influence of the physical properties of the silt caused by the silt deposition on the back silting can be ignored in the process of studying the silt squeezing by immersed tube.(2)Laboratory rheological test of silt.During the test,the silt presents a pseudoplastic fluid state,and the shear stress decreases with the increase of water content.Herschel-Bulkley model is used to fit the rheological parameters of the sludge.Initial yield stress,initial viscosity and consistency coefficient decrease with the increase of silt moisture content.When the moisture content of silt is in the range of 1.3wl ～ 5.5wl,the model can describe perfectly the rheological behavior of silt.It provides reasonable parameters for the rheological constitutive model of silt in the later finite element calculation.(3)The model test of immersed tube extrusion siltation.Silts with different water content(density)were prepared from marine soft soil in Shenzhen to simulate the process of squeezing silt.The results show that the change of settlement of the immersed tube with time is in the shape of "L".Under the same load,the larger the silt moisture content is,the thinner the thickness of the silt layer is.When the thickness of back silting is 5cm,the water content of silt is more than 150%,and there will be no silt layer remained at the bottom of immersed tube.Moreover,the change of silt moisture content will not affect the content of particle size distribution of the silt layer.The model test results can be used to verify the rationality and accuracy of CEL numerical method in the following paper.(4)The finite element numerical calculation of the sinking pipe compaction model.CEL finite element method is applied to analyze silt squeezing in immersed tube.The reliability of CEL method is verified by simulated model test.Taking the actual immersed tube project as an example,CEL method is applied to calculate the dredging thickness corresponding to different sludge bulk density.Finally,the relationship curve between the sludge bulk density and its corresponding dredging thickness is obtained,which provides a reference for determining the dredging standard of the actual project.