Mechanism And Control Analysis Of Floating Segments In A Shield Section Of Qingdao Metro

As an effective method to improve traffic capacity and alleviate traffic congestion,urban rail transit has received the attention of governments at all levels.As the most important construction method for the construction of subway tunnels,shield method has accumulated rich construction experience,but there are still some problems that need to be solved urgently.For example,the segment that comes out of the shield tail often has local or overall uplift,and is accompanied by quality defects such as misalignment,cracking damage and leakage of the segment,which seriously affects the integrity and durability of the tunnel.Therefore,it is necessary to carry out in-depth research on the mechanism and control of segment floating during shield tunnel construction.This article is based on the phenomenon of segments floating up in a shield section of Qingdao Metro Line 1.First of all,the analysis of the monitoring data of the floating amount in this section and the reasons for the segment floating during construction;Then use the shield hard slurry as the test object,carry out orthogonal test design,determine the flow pattern of the hard slurry,establish a calculation model for the dynamic distribution of the whole buoyancy ring,and use ANSYS software to calculate the internal force of the segment;Finally,the calculation formula for the prediction of the floating amount in the shield tunnel segment is derived,and the targeted floating control measures are proposed based on the prediction formula and orthogonal test results.The specific research contents and results are as follows:(1)The buoyancy volume of the segment rises rapidly within 10 hours after coming out of the shield tail,and the increase of the buoyancy volume slows down within 10 hours to 40 hours.When the plate segment reaches about 40 hours,the upward displacement of the segment tends to be stable.Therefore,the segment floating process can be divided into two stages: the first stage is the upward surge stage before the slurry is condensed;the second stage is the gently rising stage after the slurry condenses to 40 h.The causes of segment floating during the construction of shield tunnels are analyzed.The research focuses on the static buoyancy and dynamic buoyancy generation mechanisms and calculation methods.The dynamic buoyancy is divided into "fan-shaped distribution" and "full ring distribution".(2)An orthogonal test is designed in this article.A total of 16 sets of slurry ratios are set up,which basically include the range of hard slurry ratios commonly used in shield grouting.By drawing the rheological curves of different proportions of the slurry,the hard slurry is Bingham fluid.Based on this,a calculation model for the distribution of the "dynamic buoyancy" in the whole ring is established."Small" distribution.In view of the different forces of the shield tunnel segment in the normal design state and the floating state,the ANSYS software is used to calculate the internal force of the shield respectively.A comparative analysis of the calculation results can be obtained: The maximum bending moment and shear force of the segment in the floating stage increased by 40% compared to the normal stage,and the maximum axial force and minimum axial force of the segment in the floating stage increased by 35% and 40%,respectively,compared to the normal stage.In the upward floating state,the internal force of the segment is greatly increased,which will affect the quality and safety of the segment.Therefore,the buoyancy load should be considered in the segment design,and control measures must be taken for the segment to float during the construction process.(3)According to the designed orthogonal experiment,the coagulation time of 16 groups of slurry was measured.Using MATLAB software to perform range analysis and multiple regression analysis on experimental results to obtain multiple regression equations,A single factor analysis was performed based on the regression equation to investigate the effects of water-cement ratio,rubber-sand ratio,water-swelling ratio,fly-ash ratio,and gel-reduction ratio on the setting time of the slurry.The analysis shows that the influencing factors on the coagulation time of the slurry from primary to secondary are: water-to-binder ratio> powderash ratio> glue reduction ratio> rubber sand ratio> water expansion ratio.When the water-gel ratio is 0.7,the rubber-sand ratio is 0.55,the fly-ash ratio is 1.5,the gel reduction ratio is 0,and the water expansion ratio is 0.2,the slurry setting time is the shortest.(4)Taking the single-ring segment as the research object,considering it as a rigid body,analyzing the force of the segment in the slurry,and considering the time variation of the buoyancy and viscous resistance,the time-varying coefficient of the buoyancy was introduced to derive The formula for predicting the floating capacity of the shield tunnel segment was obtained,and it was in good agreement with the actual measured data of the project.By predicting the floating amount of the segment,the shield attitude can be controlled in advance to restrain the segment from floating.