| With the rapid development of the urbanization in China,the operating mileage of underground rail transit is increasing and more and more subway tunnels are built in the water-rich sandy stratum.Shield tunnels have a large number of joints,bolt holes,and grouting holes.And cracks,damage and other aging problems gradually appear on the concrete shield tunnels with the serving time,which leads to the expansion of the joints,and causes water and sands intrude into the tunnels.Such problems can seriously damage the people’s lives and property.With the water and sands pouring into the tunnel,the tunnel structure may be destroyed.The mechanism of the intrusion of groundwater and sands into tunnels involves the movement of sands carried by the undergournd water,the interaction between the fluid and the structure,and the interaction between the particles and the tunnel structure.This cannot be fully revealed with the physical experiments and macro-scale numerical simulation methods.The discrete element method(DEM)models soils at the particle scale,and can provide profounding infromation of the problem.Based on the above analysis,this study develops a numerical method to achieve three-way coupling among the fluid,the particles,and the structure.In the method,DEM is adopted to model the particle phase,and the computational fluid dynamics method(CFD)and computational structural dynamics method(CSD)are applied to capture the state of the fluid and the structure,respectively.The proposed CFD-DEM-CFD coupling method is verified by the some classic numerical cases and laboratory experiment.Then the coupling method is applied to simulate the process of the tunnel intrusion of groundwater and sands,and to study the movement of the soils,and the mechanical state of the tunnel structure.The main work and conclusions of the paper are as follows:(1)The two-way coupling between the particles and the fluid are achieved on the fluid grid scale based on the local average theory,and the influence of the movement of the fluid boundary is taken into accout with the ALE method.The two-way coupling between the fluid and the structure is carried out with the Dirichlet–Neumann domain decomposition method and the dynamic relaxation method is applied to relax the fluidstructure interface.Based on the stress calculation method on the boundary of the discrete domain,the particle-structure interaction force is taken into account.To deal with the interaction between the partilces and the structure,the representive particle method is proposed.Based on the above methods,the couplings method among the fluid,the particles and the structures are realized.And a CFD-DEM-CSD coupling solver,cfdem FSI,is developed based on two open-source frameworks,LIGGGHTS and Open FOAM.(2)The accuracy of the fluid-structure interaction of the cfdem FSI solver is calibrated by the elastic plate behind a rigid cylinder problem(FSI3).And the interaction between the fluid and massive particles are examined by modelling the upward seepage in sands.In order to validate the accuracy of the CFD-DEM-CSD coupling model,an experiment validation where water and particles leak through orifice affected by immerged elastic plate is carried out.(3)The CFD-DEM-CSD method and the conventional CFD-DEM coupling method are both used to simulate the problem of water and sand intrusing into tunnel.It is revealed that the drop speed of the pore water pressure of the new method is fast than the conventional method,and the area of soil deteriorate of CFD-DEM-CSD model is also larger than the CFD-DEM model.The newly proposed method can also capture the soil loosening of the tunnel sublayer,which can cause serve damage to the tunnel.(4)The mechanism of the soil movement,the dissipation of the pore water pressure,the development of the soil stress,and the tunnel stress are studied.Although small amount of the soil leaks through the crack,large area of soil loosening is observed.The leakage of water and sand leads to the ground stress degradation around the crack,but also forms some stress concentration regions on both sides of the crack.The stress concentration is also found on the tunnel structure.The tunnel is found to move to the direction of the crack,which may lead to soil cavities on the opposite side of the crack.(5)The process of ground movement and tunnel deformation after water and sand leakage affected by various factors are studied.The area of soil lossening is enlarged with the increase of the crack width.When water and sand leakage occur above the haunch,the soil disturbance region also concentrate on the area above the haunch.When water and sand leakage occur on the haunch,the region of the soil disturbance and the stress concentration on the tunnel are enlarged.When water and sand leakage occur below the haunch,the tunnel is not able to stay stable,and undergoes significant sinking and lateral movement.The soil disturbance region is also sharply expanded,and the stratum is extremely unstable.Besides,the stress concentration of the tunnel is also enhanced,and the tunnel is more likely to be damaged.The increase of the water level will increase the soil disturbance region.The increase of the tunnel buried depth will simultaneously increase the water and soil stress near the crack,which increase the speed of the particle leakage,enlarge the soil disturbance region,and also deteriorate the tunnel stress state. |
