Study On Soil Deformation Induced By Shield Construction Considering Non-coaxiality And Anisotropy Of Soil

With the rapid development of urbanization in China,the construction scale of urban tunnels is in a historical period of high development.Shield method has become the mainstream construction method of urban tunnel construction in China due to its own construction advantages.The prediction and control of soil deformation caused by shield tunnel construction has always been one of the key tasks of tunnel construction projects.There are widely distributed sandy cobble soil with inhomogeneous soil properties in China.In addition,composite strata are often encountered in the real environment of shield tunnelling,such as extremely uneven strata of sandy pebbles.The anisotropy of soil in heterogeneous strata such as composite strata is significant,and shield tunnelling is bound to induce the principal stress axis rotation.However,the current constitutive models for numerical simulation of shield tunnel engineering are limited to the coaxial assumption under the isotropic framework of soil,which is difficult to fully reflect the complexity of the mechanical response of the soil around the real shield tunnel.The lack of understanding of the true mechanical behavior of soil will make the safety of shield tunnel construction in urban sensitive environment face great challenges.In this paper,the construction of non-coaxial anisotropic constitutive model of soil were taken as the basic starting point,shield tunnelling in sandy cobble soil and clay-sand composite stratum was takeing as the research object,and the Tsinghuayuan Tunnel of BeijingZhangjiakou High-speed Railway was taken as the engineering background.The research methods combining literature survey,numerical simulation,theoretical analysis and model test were all performed to systematically investigate the soil deformation induced by shield tunnel construction in sandy cobble stratum and silty clay-medium coarse sand composite stratum.A two-dimensional non-coaxial anisotropic elastoplastic constitutive model and a threedimensional cross-anisotropic elastoplastic constitutive model were proposed for the anisotropy and non-coaxiality of soil,which are applied to the numerical analysis of shield tunnel excavation through the secondary development of finite element software.Based on the cavity contraction,a closed-form solution for the correlation among the average support force,volume loss rate and surface settlement of tunnel excavation in composite strata was established.The constitutive model,numerical analysis method and theoretical analysis method were verified by field data and centrifugal model test.The main research work and conclusions are as follows:(1)Non-coaxial anisotropic elastoplastic constitutive models suitable for sandy cobble soil and clay-sand composite stratum was proposed.Based on the two-dimensional isotropic Mohr-Coulomb yield criterion,the initial anisotropy of soil was described by ellipticizing the yield surface in the plastic stage combined with the non-coaxial plastic flow rule introduced in the plastic stage,and the two-dimensional non-coaxial anisotropic elastoplastic constitutive model is established.Based on the three-dimensional isotropic Mohr-Coulomb yield criterion,in order to consider the initial stiffness anisotropy of soil,the cross-anisotropy was introduced in the elastic stage,and the three-dimensional cross-anisotropic elastoplastic constitutive model is established.Based on the numerical integration algorithm combined with the modified explicit Euler integration algorithm and the adaptive substepping schemes,the code was written for the proposed non-coaxial anisotropic elastoplastic constitutive model and embedded into the numerical program.Using the finite element numerical simulation of single shear test and triaxial test,the effectiveness and calculation ability of NCAM model and CAM model in ABAQUS software element and multi-element calculation were tested.(2)For the first time,the constitutive model considering non-coaxiality and anisotropy of soil was applied to the practical engineering of shield tunnel construction.Based on the proposed two-dimensional non-coaxial anisotropic elastoplastic constitutive model and threedimensional cross-anisotropic elastoplastic constitutive model,the two-dimensional and threedimensional finite element numerical analysis of soil deformation induced by shield tunnel construction was systematically carried out.The influence mechanism of soil anisotropy and non-coaxiality on soil deformation was proved,and the influence law of anisotropic parameters and non-coaxial parameters on surface settlement was clarified.With the decrease of anisotropic parameter n,the maximum vertical settlement above the tunnel axis increases.The smaller the anisotropic parameter n is,the narrower and steeper the shape of the calculated surface transverse settlement trough is.When the anisotropic shape parameter β=0,the transverse settlement trough becomes “narrow and deep”.Non-coaxiality is likely to affect the shape and maximum value of the settlement trough at the same time.The maximum vertical settlement of the surface increases with the increase of the non-coaxial parameter k.However,the results are not satisfactory due to the small volume loss.In NCAM simulation,when n=0.6and β=0°are considered,the obtained surface transverse settlement trough is almost consistent with the field data.In CAM simulation,when α=0.55,the longitudinal settlement trough obtained by numerical simulation can match well with the measured longitudinal settlement trough.Compared with the existing research,the elastoplastic constitutive model considering the non-coaxiality and anisotropy of soil is used to conduct the numerical simulation of surface settlement induced by shield tunnel construction,and the “narrow and deep” transverse settlement trough that is more matched with the field data can be obtained,which improves the pre-control ability of shield tunnelling in urban sensitive environment.(3)The theoretical prediction method for the correlation among the average support force,volume loss rate and surface settlement of tunnel excavation in composite strata was established.Based on the cavity contraction of single homogeneous soil medium,the closedform solution for the cavity contraction problem in composite strata was derived,and it is applied to tunnel excavation.The theoretical analytical relationship between the average support force of the tunnel in composite strata and the volume loss was obtained.Combined with Loganathan and Poulos soil deformation prediction method and equivalent thickness conversion method proposed by Hirai,the soil deformation calculation method caused by shield tunnel excavation in composite stratum was given,and the influence of various factors on tunnel excavation convergence characteristic curve was clarified.Taking the volume loss as the intermediate variable,the theoretical calculation formula of the correlation among the average support force,volume loss rate and surface settlement of tunnel excavation in composite stratum was established.(4)The centrifugal model test of shield tunnel excavation in silty clay-medium coarse sand composite stratum was carried out,and the transverse settlement trough and ground displacement caused by shield excavation in silty clay-medium coarse sand composite stratum were obtained.The test results show that the elastic modulus of the overburden above the tunnel has a great influence on the soil deformation.With the increase of volume loss rate,the maximum surface settlement increases approximately linearly.According to the prediction and analysis of the surface settlement obtained from the centrifugal model test results,the effectiveness and feasibility of the three surface settlement prediction methods were comprehensively judged,and the NCAM numerical simulation results are relatively good.It further shows that it is necessary to consider the non-coaxiality and anisotropy of soil in the prediction of soil deformation caused by shield tunnel excavation.(5)In order to ensure the safe,efficient and smooth excavation of the shield machine and the normal use of the surrounding existing buildings,based on numerical simulation,theoretical analysis and centrifugal model test,considering the non-coaxiality and anisotropy of the soil,a set of complementary advantages,mutual confirmation,reasonable and effective prediction method of soil deformation induced by shield tunnel construction was formed.