Study On The Floating Mechanism Of Shield Tunnel Considering The Time Effect Of Slurry During Construction

Shield construction has the advantages of less personnel demand,faster construction speed and good quality of hole formation.However,tunnels often appear the floating phenomenon during construction.This paper takes the silty clay area of the Xixiangtang Bus Terminal-National University section of Nanning Metro Line 1 as the engineering background.Through the theoretical analysis and finite element simulation method,the floating amount;internal force and its dynamic changes of the segment ring related to the dissipation of the slurry pressure during the construction period of the shield tunnel are studied.(1)Firstly,starting from the floating mechanism of the shield tunnel,the main loads affecting the floating of the tunnel are obtained,and the anti-floating loads that restrains the floating of the tunnel are also obtained.Then,from the perspective of theory and actual engineering,the horizontal and longitudinal calculation models of the tunnel floating condition are obtained.(2)Starting from the dissipation law of slurry pressure,the equilibrium equation is established on the slurry-soil micro-element by elastic mechanics and Darcy's law to obtain the slurry consolidation equation and the slurry pressure reduction function.Then,the slurry is regarded as Newtonian fluid,and the cyclic distribution of the initial slurry pressure is obtained by the laws of elasticity and Newtonian friction.Finally,the floating load F related to the slurry pressure dissipation is integrated,and the effective stratum reaction coefficient Kr and the tunnel longitudinal equivalent stiffness(EcIc)eq are introduced to establish the longitudinal floating calculation model for the tunnel.(3)Using Mathematica to calculate the slurry cake thickness time function x(t)and the floating load function F(x)during the construction of the background engineering section.Based on this result,the analytical solution of the floating amount at each position of the tunnel in the calculation range is obtained by combining the Winkler foundation beam model.The calculation results show that the maximum floating amount is about 90mm,and the stable floating amount is about 46mm.The calculation result is in line with the original measured data.(4)The MIDAS GTS NX was used to establish the upper cross-section model and the three-dimensional load-structure model of the segment ring to study the dynamic changes of the cross-section internal force,the floating amount of the segment ring and the VON-MISES stress.The results of internal force of 12 sets of tunnel cross-section,10 sets of floating amount and 10 sets VON-MISES stress on arch crown,arch bottom and arch lumbar of the segment ring were obtained.The results show that compared with the traditional internal force calculation method,the bending moment,axial force and shear force at various positions of the segment ring in the floating state are increased,and the dangerous section position of the tunnel becomes arch lumbar of the segment ring.The VON-MISES stress maximum is 17900 kN·m-2 at the first ring arch lumbar position behind the shield tail,which can be considered as a dangerous section.Then according to the calculation results of buoyancy and internal force on the tunnel,the anti-floating measures of shield tunnel during construction period are preliminarily put forward.