Study On Stability Of Soil At End Of Shield Tunnel Under Seepage

Aiming at two typical strata,sand and clay,the physical and mechanical properties of sand and clay are analyzed by triaxial test in laboratory.The stability of soil at the end of shield tunnel under seepage is studied by combining PFC numerical simulation test with model test in laboratory.?1?Based on the continuity equation of seepage flow,the differential equations of seepage flow in two-dimensional and three-dimensional cases are analyzed according to the isotropy and anisotropy of soil,and three kinds of boundary conditions of differential equations in seepage case are given according to practical engineering.On the basis of Darcy's law,the program of CFD module is edited by Python language,and the fluid-structure coupling of PFC-CFD is realized.?2?The physical and mechanical properties of sand and clay are obtained by triaxial test in laboratory.The triaxial numerical model is established by PFC^3D,and the wall is changed to a flexible wall.The particle parameters are calibrated by adjusting the numerical parameters.It makes the numerical triaxial simulation test more suitable for the mechanical properties of the soil in the laboratory test,so that it can simulate the actual soil better.?3?PFC numerical model is used to analyze the stability of end soil under seepage.Among the three influencing factors of tunnel diameter,groundwater level height and groundwater pressure,the damage range of soil in sandy soil and clay stratum increases with the increase of tunnel diameter,groundwater level height and groundwater pressure.Moreover,due to the influence of cohesion of clay itself,the failure criterion of sandy soil stratum increases.The surrounding area is larger than that of clay stratum,and the destabilization and destruction range of sand stratum and clay stratum end soil mainly occurs within 0.15 meters from the entrance of the tunnel;at the same time,the settlement displacement of the soil body has the same law with the change of the destructive surface of the end soil body under different working conditions.Under the same working conditions,the settlement of sand stratum is larger than that of clay stratum,and the average settlement of sand stratum is larger than that of clay stratum.The stratum is about 0.007 meters.In the study of water pressure on excavation surface of soil,when the diameter of tunnel is small,groundwater can not be discharged in time,so groundwater will gather at the entrance of tunnel and form larger water pressure.With the increase of tunnel diameter,water pressure will gradually decrease.Among the two influencing factors of groundwater level height and groundwater pressure,the water pressure on excavation surface will follow the groundwater pressure.The water level and groundwater pressure increase,and under the same influencing factors,the water pressure at the excavation surface of sand stratum is larger than that at the excavation surface of clay stratum,and the average water pressure in sand stratum is larger than that in clay stratum is about 180 Pa;The critical hydraulic gradient of sand layer is 15 kPa and that of clay layer is 20 kPa,which indicates that clay layer is safer than sand layer under the same construction condition.?4?The damage process of the soil at the end of the laboratory test is recorded by using the three-dimensional digital speckle image acquisition system.Through the acquisition of the image,it can be concluded that the damage range of the soil at the end of the tunnel gradually develops behind the tunnel with the increase of groundwater pressure in sand and clay strata,and the damage range of the sand stratum is larger than that of the clay stratum.The static strain testing system is used to monitor the change of water pressure at the tunnel gate.It can be concluded that the change of water pressure on the excavation surface is the same as that on the end soil failure surface,and the average water pressure in sand layer is about 727 PA higher than that in clay layer.?5?By comparing the numerical simulation test with the indoor test,it can be concluded that the failure surface of the end soil in the indoor test is similar to the numerical simulation,and the fitting equation of the failure surface between the numerical simulation test and the indoor test has a high similarity.The sand layer presents a linear failure mode,and the clay layer presents a circular curve failure mode;the end of the sand and clay layer presents a circular curve failure mode.The variation law of water pressure at excavation surface of soil body is also similar to that in numerical value.