Stability Analysis Of Excavation Face Of Special-shaped Shield Tunnel Based On The Upper Bound Theorem Of Limit Analysis

In the process of shield tunneling,it is very important to control the displacement,even collapse of excavation surface for the safety of construction.With the development and successful application of large section special-shaped shield tunneling machine,it is of great significance to study the stability of the excavation surface of special-shaped shield tunnel.This article combines the numerical simulation and analytical method and The evolution process of damaged soil mass in front of excavation face when the instability of face occurs is studied in the cases of elliptic,three-center horseshoe-shaped,five-center horseshoe-shaped shield excavation in the strata with water and without water.Besides,the variation of limited support force of face induced by parameters is studied.The main contents and achievements of this paper are as follows:(1)The finite difference software flac3 D is used to establish the active failure model of excavation face of various special-shaped shield tunnel in homogeneous anhydrous stratum and water-bearing stratum considering fluid-solid coupling stress field.As the results show,decreasing the support force of face can make the face damaged.The slip boundary of the damaged soil can be determined according to the dynamic changes of the combined displacement,plastic zone and plastic shear strain increment.Based on the curve of displacement and support force and nutation criterion,the ultimate support force of face can be determined.(2)Using spatial discretization technology and upper bound theorem of limited analysis,the analytical models of active failure of excavation face of four special-shaped in anhydrous strata are established.As the results show,the stronger the soil parameters,the smaller the support force of face under ultimate state.Under the cases of less depth,ultimate support force of face varies with the depth.When the ratio of depth is larger than 1,the depth has no effect on the limited support force.the increment of limited support force induced by the increase of the area of tunnel section in the cases of tunnels with different shapes.(3)On the basis of considering the results of fluid-solid coupling numerical simulation and with the help of spatial discretization technology,the theoretical models of active failure of four special-shaped shield tunnel face in water-bearing stratum are constructed.MATLAB is used to optimize the mesh and a linear interpolation program is developed to solve the power of soil seepage force in three-dimensional rotating failure mechanism.The results show that with the increase of seepage time,the ultimate support force of the face of the face increases rapidly at first and then tends to be stable.With the rise of groundwater level,the ultimate support force of each shape face changes almost linearly,and the area of failure body is almost unchanged,but the shape changes.(4)The ultimate support force of the tunnel face in water-bearing strata solved by the theoretical method in this paper is compared with the existing research results and numerical simulation results.And the advancement of the theoretical method proposed in this paper is also verified.Finally,the paper also creates a normalized chart to provide reference experience for subsequent projects.