CFD-DEM Simulation Study On Seepage Deformation Of Gap-graded Sandy Gravel Soil

Computational Fluid Dynamics-Discrete Element(CFD-DEM)is used as a numerical simulation method to solve practical problems in the field of geotechnical engineering.Compared to laboratory tests,CFD-DEM has the advantages of low energy consumption,ease of operation and low cost.This study uses CFD-DEM’s fluidsolid coupling method to investigate the influencing factors of seepage deformation in sand and pebble strata,and explains the mechanism of seepage deformation in different states from a meso perspective,which is of great significance for the study of accidents such as metro.The main contents and results are as follows.(1)The flow-solid coupling of CFD-DEM is realized by writing the code independently in FISH language,and the benchmark problem is used to verify the water-soil interaction,and the verification results show that the method has a high accuracy in studying the water-soil interaction.(2)A small-scale simulation of gap-graded sandy gravel soil in the local area around the tunnel was conducted to explain the mechanism of seepage deformation from a meso perspective.The erosion process is divided into acceleration,deceleration and stabilisation stages according to the fine-grained erosion characteristics of each stage.The internal force transfer characteristics of particles during seepage are analysed by means of contact force chains.The total number of contacts increases continuously with seepage,the proportion of coarse-grain-to-coarse-grain contacts decreases,the proportion of fine-grain-to-fine-grain contacts increases,the contact force distribution tends to be uniform in all directions,and the number of particles with c connectivity greater than or equal to 3 and the average coordination number increase.(3)The influence of fines content,confining pressure and hydraulic gradient on seepage deformation was studied.The effect of fines content on erosion is concentrated in the initial stage of seepage,where the erosion rate and flow velocity decrease with increasing fines content.The influence of confining pressure on fines erosion is concentrated in the later of seepage,where the erosion rate decreases with increasing confining pressure.Although increasing hydraulic gradient accelerates fines erosion,the final erosion rates of the specimens are similar for all hydraulic gradients.The results of the grey correlation analysis of the correlation between each factor and the erosion rate show that the fines content has the greatest influence on the erosion rate,followed by the surrounding pressure and finally the hydraulic gradient.(4)The results of the orthogonal test to study the seepage deformation law under the coupling of multiple factors show that the relationship between erosion rate and flow velocity and fines content,confining pressure,hydraulic gradient and seepage time can be expressed as exponential functions or power functions.The erosion rate and flow velocity decrease with increasing fines content and confining pressure,and increase with increasing seepage time and hydraulic gradient.On this basis,quantitative relationships between multiple variables and erosion rate and flow velocity were derived using multiple linear regression.The ELM algorithm was used to predict the erosion rate and flow velocity,effectively improving the efficiency of the simulation.This study extends the application of CED-DEM by applying the coupled CFDDEM means to the seepage deformation problem of the metro,and provides a new idea for the meso study of the seepage deformation of sandy gravel layers.