Continuous-Discrete Coupling Numerical Analysis Of Soil Balance In Shallow Buried Shield Tunneling In Sand Layer

EPB shields are one of the most widely used methods in underground tunnel construction.With the development of underground projects construction,the extreme stratigraphic conditions faced in the construction of shield tunnels are also increasing,especially for weak stratum,due to its poor stability and low soil strength,it is usual to cause unbalanced excavation problems during excavation,leading to a wide range of stratum losses and even forming serious excavation face instability accident.Therefore,it is very important to ensure the balance between the volume of soil entered and out of the chamber during construction,and to maintain the stability of the shield excavation surface.In view of the unfavorable working conditions that shield tunneling is about to face,the influence factors of soil balance in the process of excavation are considered,and the influence of excavation surface stability and stratum disturbance is studied.Meanwhile,in order to make up for the insufficiency of the conventional singlemedium numerical simulation,a refined numerical model is established by innovatively coupling discrete media with continuous media.The main work and conclusions are as follows:(1)Using the method of coupling discrete element media and continuous medium,a coupling numerical model is established.In the large deformation region,discrete medium simulation is used,and the region far from large deformation is simulated by continuum medium,the force and displacement on the boundary of the two regions are transmitted through the coupling surface,which effectively improves the computational efficiency of the discrete element simulation.A refined numerical model of the cutterhead and screw conveyor of the shield machine is established,and the process of soil entering and leaving the soil chamber during shield tunneling is dynamically and completely simulated.(2)The speed of the screw conveyor is directly related to the volume of soil discharged by the shield machine,and the excavation speed directly affects the amount of soil entered the chamber.The two factor decides the pressure of the soil chamber by adjusting the balance between the volume of soil take in and discharged during tunneling,thereby maintaining the face stability.When the screw speed is insufficient and the driving speed is too fast,the pressure of the soil chamber will increase rapidly,forming an extrusion of the soil in front of the shield,and eventually cause passive instability failure;if the screw speed is too fast and the driving speed is insufficient,the soil in front will be unloaded,,resulting in active instability.(3)The influence of the cutterhead speed on the soil balance of the shield is weak,and the soil bin pressure and the stratum deformation in front of the shield are less sensitive to the change of cutterhead rotation speed.However,the high rotation speed of the cutterhead will reduce the support effect of the cutterhead on the soil in front,thus threatening the stability of the excavation surface.(4)The relationship between the ratio S of the soil chamber pressure to the static side pressure of the original stratum and the stratum deformation was studied.The results show that: when the stratum deformation is in a stable state,the corresponding soil chamber pressure ratio S fluctuates in the range of 0.4～0.6;when S > 1.2,the stratum deformation changes abruptly,and the passive instability failure occurs;when S < 0.2,the stratum settlement increases rapidly,and the active instability failure occurs on the excavation face.(5)During the initial stage of the tunneling,many parameters of the shield machine changed suddenly,and the pressure ratio S dropped "cliff-like" from the initial value of 0.95 to about 0.3,it is necessary to prevent the occurrence of such dangerous situations at the initial stage.