Research On Segments Floating Control Technology Of Shield Tunneling In Composite Stratum

With the development of urban subway construction,shield tunneling technology is more and more widely used in urban subway construction.Due to the differences in the nature of the stratum and construction technology,when the segment is out of the shield tail,there are always some local or overall floating cases of the segments.The occurrence of such problems would not only cause some potential dangers to the waterproofing in tunnel,but would also affect the quality of the project,resulting in waste of money and resources.Based on the analysis of the segment theory,on-site monitoring and numerical simulation,this paper studies the floating rule and control measures of the segment in the process of construction.The main contents in this paper are described below:(1)The analysis of buoyancy force on segment is divided into two parts-"static buoyancy force" and "dynamic buoyancy force"."Static buoyancy force" is caused by the surrounding water,mud and grout slurry around segments,and the segment in the liquid results in buoyancy."Dynamic buoyancy force" is produced in the process of post-wall grouting,and it is the force resulting in the occurrence of floating pieces,the crossing foundation and damage.The diffusion process can be divided into four stages:filling,infiltration,compaction and splitting grouting.The calculation formula of the minimum casing thickness for the tunnel segment to meet the requirements of anti-floating stability is obtained by considering the influence of grouting pressure on the segment after segment is out of shield tail.(2)After acquiring,organizing,analyzing the data from the on-site construction monitoring survey of a certain shield section in "upper soft and lower hard" strata of an urban rail transit line 1 construction in southeastern coast of China,a floating law in the construction is obtained.According to the mechanical analysis of the segment,the calculation formula of the floating amount of the segment under different soil theology is obtained,after the theoretical deduction.(3)By selecting a certain controlling section of the "soft upper and lower hard"strata in the monitoring interval,a FDM software FLAC 3D is used to simulate the excavation process of the shield tunnel.The influence of grouting parameters,stratum characteristics and soil thickness on the flotation of segment was studied.And the theoretical result is compared to the monitor data in the practice.And finally a control technology measures are put forward to prevent the segment from floating in the course of shield construction.