Study On Mechanical Model Of Adjacent Excavation Crossing Existing Shield Tunnel And Application

Nowadays,foundation pits and shield tunnel are two widely existed projects in urban underground engineering.More and more underground projects cross each other appear with the acceleration of urbanization.For instance,the existing shield tunnel crossed over the foundation pit,and the new tunnel pierced up or down through the existing shield tunnel.The excavation of foundation pits and tunnel is bound to affect the surrounding soil and the existing shield tunnel.In severe cases,the existing tunnel will leak,crack,and other accidents,furthermore even affect the normal operation of the existing tunnel.The primary cause of the above accidents is the non-uniform deformation of the existed shield tunnel.It is the key point in urban underground engineering that how to effectively predict and control the deformation of the existing shield tunnel in the adjacent excavation and construction process.In the past,the deformation models of shield tunnel were usually assumed as elastic foundation beam or Timoshenko beam or short beams considering shear effect.The elastic beam can only capture pure bending deformation,but cannot consider the shear dislocation between the shield tunnel lining rings;the Timoshenko beam or short beams considering shear effect can consider the shear deformation between lining rings,but they cannot explain the opening between lining rings.To fill this gap,this paper proposes a deformation model considering the bending-shear effect based on the shield tunnel lining-joint structure.Based on case studies,the analytical solution of the deformation of the existing shield tunnel under the influence of adjacent excavation is derived.And the analytical solution is applied to the new engineering cases.The main work and conclusions are as follows:（1）A shield tunnel deformation model considering the bending-shear effect is proposed,based on the structure of lining-joint of shield tunnel.In this model,the shield tunnel is simplified as a series of short beams connected by shear springs,tension springs and compression springs.Not only can the shear dislocation between the shield tunnel lining rings,but also the opening between the rings be considered.In the model,the shear springs resist the shear deformation between the lining rings of the shield tunnel,the tensile springs and the compression springs jointly bear the opening and bending deformation between the adjacent lining rings;the shear dislocation and opening between the shield tunnel lining rings together determine the final deformation of the tunnel,which is regarded as the"bending-shear effect"of the shield tunnel.（2）Established a mechanical model of the new tunnel construction above-crossing the existing shield tunnel.Based on the structure of the shield tunnel lining-joint,and considering the shield driving effect and excavation unloading of the new tunnel,the analytical solution of the deformation of the existing shield tunnel is derived.The existing shield tunnel is regarded as a series of short beams connected by tension springs,compression springs and shear springs on Winkler foundation.Considering the influence of shield thrust,shield shell friction,grouting pressure and excavation unloading,the additional stress at the axis of the existing tunnel is solved by Mindlin formula,and then the energy variation equation is established based on the principle of minimum potential energy to solve the deformation of the existing tunnel.The field measurement data verify the correctness of the theoretical method in this paper.（3）The mechanical model of the existing shield tunnel under the foundation pit is established,the relationship between the opening between adjacent lining rings and the longitudinal deformation of the tunnel is proposed,and then the analytical solution of the deformation of the existing shield tunnel is derived.The law of the shear dislocation and opening between existing tunnel adjacent lining rings under the unloading of foundation pit excavation is obtained.The theoretical method is applied and analyzed based on four engineering examples.Euler-Bernoulli beam method,Timoshenko beam method,shear effect method and the method in this paper are used to solve the deformation of the existing tunnel under unloading of the foundation pit.The comparison of measured data shows that the method in this paper can well predict the deformation of existing shield tunnel caused by foundation pit excavation.（4）The mechanical model of the new tunnel passing through the existing shield tunnel is established.Based on the Pasternak two-parameter foundation model,the analytical solution of the deformation of the existing shield tunnel is derived.The existing shield tunnel is regarded as a series of short beams connected by tension springs,compression springs and shear springs resting on the Pasternak foundation.The deformation of the existing tunnel is solved and compared with the calculation results of the Winkler foundation model.Combined with three engineering cases,the Euler-Bernoulli beam method,the shear effect method and the method in this paper are used to calculate the deformation of the existing tunnel.The results show that the calculation results of the method in this paper are most consistent with the field measured data.（5）Through FLAC^3D three-dimensional numerical calculation and similar model test,the influence of adjacent excavation and construction on the existing shield tunnel are studied.The model of the lining ring and the bolt joints were constructed,and the numerical calculation,model test and theoretical calculation results were compared and analyzed.The results showed that the theoretical calculation values are in good agreement with the model test data and numerical calculation results.The model test shows that the main influence range of the new tunnel excavation on the existing tunnel is 2.0 times the tunnel diameter on the left and right sides of the two tunnels intersection respectively.