Study On Mechanism Of Segment Floating Of Large Diameter Slurry Balance Shield Tunnel

Shield method is widely used in tunnel construction for its advantages of low disturbance,high safety and high efficiency.During the construction process of shield method,as the excavation diameter of shield machine is larger than the outer diameter of segment,after the segment is disengaged from the shield,synchronous grouting is needed to reduce the surface settlement in the void between the segment and soil layer.In the process of synchronous grouting,the segment will be affected by the grout pressure and other factors,resulting in local or overall upward displacement,which will cause the segment to deviate from the original design axis,especially for large diameter shield tunnel,the void at the tail of the shield will be larger,and the floating problem will be more prominent.It is one of the key issues in the construction process of large diameter shield tunnel to develop reasonable control measures to ensure that the axial direction of the tunnel meets the design requirements.Numerical simulation is one of the common methods to study the floating problem of segments.but the existing research usually simplifies the model,such as assuming that the dissipation process of the buoyancy force on the segments is equal difference decline,and not considering the grout time-variability,which leads to the deviation of the model calculation results from the actual situation.In view of the above problems,this paper improves and optimizes the segment floating simulation model in the process of shield tunneling,establishes the segment floating simulation analysis model in the process of shield tunneling considering the dissipation process of segment floating force and the time-varying grout,discusses the influence of different factors on segment floating,proposes the control measures of the segment floating,verifies the control measures by combining the field monitoring data of the shield tunnel project of “Yuji” Intercity Railway.The main work is as follows:(1)Based on the theory of computational fluid dynamics,the grouting filling process of shield tail void is numerically simulated to study the law of grout pressure distribution.The results show that under the action of gravity and grouting pressure,the pressure of grout in the void at the shield tail shows the distribution law of low at the top and high at the bottom;with the increase of grouting pressure,the pressure of grout in the shield tail void increases continuously,and at the same time,the grout pressure on the segment and surrounding soil also increases.(2)The segment floating simulation analysis model of shield tunneling process considering the dissipation process of segment buoyancy and the time-varying grout is established.The accuracy of the simulation analysis is verified by comparing the simulation analysis and calculation results with the measured segment floating data.On this basis,the influence of synchronous grouting pressure and initial setting time of grout on segment floating is studied.The results show that the segment floating amount increases with the increase of grouting pressure;The floating amount of segment is greatly affected by the solidification time of grout,the longer the grout initial solidification time,the larger the floating amount of segment.(3)Aiming at the segment floating problem of a large-diameter slurry balance shield tunnel in Henan Province,the segment floating control measures to reduce the initial setting time of grout and control the grouting pressure are proposed,and the control effect is verified through simulation analysis and measured data.The results show that the overall floating amount of segment can be effectively controlled by using the grouting grout with optimized initial setting time of 6 hours and grouting pressure of 0.25 MPa in a large-diameter slurry balance shield tunnel in Henan Province,After anti-floating measures are taken,the average floating amount of segments is reduced by 50% compared with the average floating amount of segments without anti-floating measures.(4)Optimize the control measures of segment floating,and adjust the grouting pressure at the upper and lower grouting openings of synchronous grouting to be large at the top and small at the bottom.Through numerical simulation calculation,it is obtained that when the initial setting slurry is used for 6 hours,compared with the condition that the grouting pressure of the upper grouting port is 0.35 MPa and the grouting pressure of the lower grouting port is 0.25 MPa,the maximum floating amount of the segment is reduced by 40% when the grouting pressure of the upper and lower grouting ports is 0.35 MPa.