Study On The Ground Deformation Induced By Large Diameter Shield Tunnelling Construction

With the acceleration of urbanization process,the development of metropolitan area in urban agglomeration,the construction of the state-level new areas,the implementation of the national marine strategy and the national thoroughfares,China will continue to invest in transport infrastructure and further improve the railway network,highway network and urban rail transit.These have brought unprecedented opportunities for the development of tunnel and underground engineering in China.In the construction of tunnel and underground engineering,shield has become the most important method with its characteristics of fast,safe and environmental protection.In recent years,with the progress of shield technology and the need of operation and development,shield tunneling has been developing towards the direction of large section,large depth and long distance.As the diameter of shield tunneling increases,the area of soil excavated at one time increases,accompanied by the increase of surrounding soil disturbance range and degree.Meanwhile,the geological conditions are more and more complex,and the demand for workers are increasingly higher.The key to the success of large-diameter shield tunnelings is whether the ground surface deformation can be guaranteed within the allowable range.This dissertation is financially supported by the National Key Basic Research Program of China(2015CB057803)——‘Basic Research on Engineering safety of Long and Large Shield Tunnel Across the River and Sea under High Water Pressure'.Take the ground deformation induced by large diameter shield tunnel as the research object,the methods of ground deformation are conducted based on the statistical analysis of field data,theoretical analysis,numerical simulation.The contents and results mainly include the following parts:(1)The study and practices of ground deformation induced by shield are reviewed.Methods for predicting ground deformation due to shield are summarized.The performances of ground deformation caused by shield are determined.The results and problems in previous studies are discussed.The main contents and the research technology route are given.(2)Distribution characteristics of ground surface deformation and the values and influencing factors of empirical parameters in large diameter shield tunnel construction are revealed.Statistical analysis of measured settlement data of large diameter shield tunnels is not yet available.Therefore,measured data of maximal ground settlement from some areas of China are collected.Empirical parameters are obtained by back analysis of Peck formulas.The distribution and influence factors of empirical parameters according to soil conditions are studied.The results from large diameter shield,small diameter shield and shallow tunnel construction are compared.The results show that the range of ground surface settlement,the volume loss and the width parameter of surface settlement trough caused by large diameter shield tunnel is 2.5mm~92.2mm,0.02%~3.36%(average value: 0.53%)and 0.13~0.74(average value: 0.38),respectively.About 91.24% of the volume loss caused by large diameter shield tunnel ranges from 0% to 1.0%.The ground surface settlement,the volume loss and the width parameter of surface settlement trough are proportional to the permeability coefficient of soil,respectively and have no obvious connection with the ratio of shield depth to diameter,respectively.The volum loss and the coefficient of ground settlement width caused by large diameter shield tunnel are smaller than that of small shield tunnel.The volum loss and the coefficient of ground settlement width due to shallow tunnel construction are more discrete than that of shield.(3)Theoretical analytical solutions for predicting the ground surface deformation induced by large diameter shield are proposed considering the effect of excavated area enlargement.The effect of unit weight of slurry on the additional pressure around cutting head,the effect of shield gravity on the non-uniform distribution of softening skin frictions along the shield,the dissipating of grouting pressure along the lining ring and the ground deformation caused by volume loss ratio were considered in the proposed formulas based on Mindlin solutions.The results of example show that the ground deformations which are calculated without considering the above factors will overestimate the heave and settlement value of the ground and underestimate the horizontal deformation of the ground.The ground deformation can be well predicted by the proposed formulas and the development of ground deformation can be reflected.(4)A uniform and non-uniform vertical convergence model,a non-uniform vertical convergence model and a uniform radial convergence model under buoyancy are proposed to demonstrate the ground deformation owing to large diameter shield tunnel.Considering the effect of soil gravity,unloading by excavation and buoyancy,the calculation formulas for ground deformation of the above models are deduced based on virtual image technique.The results of example show that the calculated results of the non-uniform vertical convergence model agree well with the measured data.The maximum horizontal displacement of the stratum is near the depth of tunnel axis.The transverse settlement curve affected by the buoyancy gradually changes from ‘W' to ‘V'.(5)The 3D numerical model of construction process of large-diameter shield tunneling is established.3D finite element numerical model is built considering the non-uniform additional pressure on the shield face and the annular dissipation of grouting pressure at the tail void.The theoretical calculation results and numerical simulation results are compared and analyzed.The numerical simulation results show that the transverse range affected by the construction of large diameter shield tunneling is-3D ~3D.The additional pressure of shield face and the grouting pressure at the tail voil are the major factors causing the ground surface heave.The theoretical calculation results presented in this paper are in good agreement with the numerical simulation results,indicating that the theoretical calculation models proposed in this paper are reasonable.