Analysis On Penetrative Failure Characteristics Of End Soil In Sand Tunnel With Shield Tunneling Based On PFC-CFD Coupling

In view of the water-sands-rich sandstone formation at the end soil,the theoretical knowledge of fluid-structure interaction and elasto-plastic mechanics are used to solve the NS equations,and a PFC-CFD coupled particle flow microscopic model and an indoor similar model are used to simulate the end of the shield tunnel.The influence of the infiltration failure process and infiltration damage on the stability of the end-soil is studied.(1)In view of the special engineering mechanical properties of sand soil,the soil with large dispersion is assumed to be a porous continuous medium,and the fluid-solid coupling mathematical model is established on the basis of considering the dynamic change process of soil permeability.The mesoscale simulation of fluid-structure interaction in porous media was carried out by calculating fluid dynamics and solving the N-S average equation of incompressible fluid in combination with PFC3D particle flow.(2)Since the CFD module does not contain the CFD solver,this article re-develops it based on the python file program and establishes a python program statement to connect CFD and PFC3D to solve the problem of fluid-solid coupling.The accuracy of particle flow numerical simulation is verified by comparison between numerical results and seepage formula and theory.A triaxial shear numerical model of particle flow was established to analyze the influence curve of the mesoscopic parameters of the particles and the macroscopic physical parameters of the soil sample.(3)A PFC-CFD coupled percolation disruption particle flow model was established to analyze the infiltration failure process of sandstone formations at the end of a shield tunnel and the effect of percolation damage on the stability of the end soil mass.During the process of percolation failure,the fluid pressure field,velocity field and soil stress field are in constant change.The average flow velocity,porosity and permeability coefficient in the seepage basin tend to stabilize after a certain number of operations,and the seepage tends to be stable.(4)When the soil at the end of a shield tunnel is subjected to a percolation failure,the contact force between the particles becomes sparse,the soil stability,shear strength decreases,and at the same time the soil slip damage scope,surface subsidence becomes large,and the soil body loses the stability faster.The slip damage surface expands later than when there is no infiltration.When the end soil is damaged by transverse infiltration,the vertical stress field of the soil decreases as a whole,and when the soil is damaged by vertical infiltration,,the vertical stress field of the soil is overall.When it increases,the redistribution range of stress field becomes larger than that of no-penetration,and it has greater influence on lateral permeability.The minimum principal stress is less affected by the penetration,and the maximum principal stress is more affected by the penetration.The major and minor principal stresses of the soil at the end of the infiltration are basically within the envelope of the principal stress of the soil at the end of the infiltration.(5)The effect of different infiltration direction on the overall stability and shear strength of soil:no seepage damage more than the vertical infiltration more than the horizontal infiltration.The main stress reaches the maximum value,and the position where the arch falls is located at the distance from the tunnel entrance as follows:vertical infiltration(0.1m)more than the infiltration(0.2m)more than the horizontal infiltration(0.3m).The direction of fluid flow is the same.Under different fluid pressures,the greater the fluid pressure,the greater the fluid pressure,the smaller the principal stress of the soil,and the more obvious the percolation failure.The larger the instability area,the earlier the loss of particles occurs.The faster the drain rate is.The maximum pressure gradient at the opening at the same fluid pressure and in the direction of the same fluid is affected by the size of the opening.The smaller the opening is,the greater the pressure gradient is and the stronger the permeability is.The amount of subsidence,the maximum subsidence of the earth's surface,and the proportion of disturbed areas in the soil gradually decrease as the ratio of tunnel depth to depth increases.According to the results,the influence of fluid action direction,tunnel depth and depth of penetration and penetration of tunnel are respectively the settlement deformation amount:vertical penetration more than the horizontal penetration more than the waterlessness,depth more than the infiltration;surface Maximum settlement:vertical infiltration more than the ignoring more than the horizontal infiltration,depth more than the infiltration;soil instability region accounted for:horizontal infiltration more than the vertical infiltration more than the water,infiltration more than the buried depth.(6)The large-scale three-dimensional speckle measurement system was used to obtain the whole process of the penetration test failure of the similar model in the indoor end-earth soil,and compared with the case of non-permeability of the tip soil,the failure range and loss of the soil in the presence of the end-ear soil.The stable area is obviously larger than the water-infiltrated area,and the slippage surface of the soil is also much later than the water-free infiltration.The coordinates of the sliding failure surface of the indoor similarity model test and the numerical test were extracted and the curve formula was fitted.From the overall sliding failure surface,the sliding test of the end soil under water penetration was found in the indoor test and numerical test.The similarity of the surface curves is still relatively close.