Microscopic Numerical Simulation Study On Clogging Characteristics Of Prefabricated Vertical Drain In Dredged Slurry Under Vacuum Loading

Vacuum preloading method is widely applied to treatment of dredged slurry.Particles of the dreged slurry of high water content,high compressibility and no stable skeleton can easily be migriated,accumulated and compacted at surface of the prefabricated vertical dtrain(PVD),when subjected to vacuum load.The migrated and compacted soil particles will form a so called “soil column” around the PVD membrane,which will reduce the drainage and limit the strength improvement of the ground,especillay for deep soil layers.The presented researches mainly reveal the clogging characteristics of PVD in dredged slurry by macroscopic indexes,such as dissipation value of pore pressure,degree of consolidation and foundation strength,etc.Study on the microcosmic movement characteristics of the slurry particles and the macroscopic effects,such as the enlargement of soil column range and the reduction of permeability coefficient,is less in and abroad.In this paper,the PVD membrane and soil particles are described using distinct element method(DEM),the pore water is described by computational fluid dynamics(CFD).By establishing the micro-numerical simulation model of consolidation of dreged slurry,the fluid-particle interaction under vacuum negative pressure was analyzed.The microscopic mechanisms of the PVD clogging were revealed,involving the movements and contacts of the slurry particles,the retention of slurry particles in the PVD membranes and the evolution of the pore geometries.The main research contents and results are as follows.(1)The clogging problem of PVD in foundation drainage consolidation is summarized.The research status and deficiency of soil particle microscopic numerical simulation are discussed.The basic principles of computational fluid dynamics(CFD)and distinct element method(DEM)are summarized,and the CFD-DEM coupling principle,interaction force and related software(SediFOAM)are introduced.(2)The model of single particle free settlement in still water and the onedimensional consolidation model are established.The calculation validity of the CFDDEM slover SediFOAM is verified by comparing the numerical simulation solution with the analytical solution.(3)The clogging model of PVD is established by DEM modeling soil particle and membrane and CFD modeling fluid.Hexagonal packing structure is used to simulate the PVD membrane.The process of soil particle migration and membrane clogging is reproduced under different differential pressure.The movement characteristics of soil particles in the pore openings and the changes of pore ratio and permeability coefficient in the surface area of the membrane are statistically analyzed.The results show the main reason of the clogging of membrane is the compact soil layer(i.e.filter)formed on the surface of membrane.The results show that the permeability and void ratio at the filter layer decrease quickly to stable values when the clogging evolves,which is independent of the applied vacuum pressure difference,but related to the particle size distribution.The linear relationship in the lg-coordinates between permeability and void ratio of the filter.(4)To simulate the formation process of the filter on the surface of the PVD,the particle motion model is established with DEM for modeling soil particle and membrane,and with CFD for describing the pore-water movements.By statistical studying of the particles with different heights and diameters,it is found that in the formation process of the filter,the larger particles reach a stable state before the smaller ones,which form the main structure of the filter.After that,the smaller ones continue to move in the pores of the filter,which will make the filter denser.