Study On Construction Deformation And Structural Dynamic Response Of Tunnel Under Existing Line

In recent years,in order to meet the convenient travel needs of the people,the construction scale of urban underground traffic projects is constantly expanding.In the face of the complexity of the existing lines on the surface,it is inevitable that there is a difficult construction problem of double-line tunnels passing through the existing lines.In the process of tunnel construction,the empty surface leads to the movement of the surrounding rock and soil like the inside of the tunnel,which disturbs the overlying stratum and induces deformation,which leads to the deformation of the roadbed structure and affects the safe operation of the line.At the same time,the surrounding soil layer vibrates under the exciting force of the train,which will further cause the resonance of the tunnel structure,which may reduce the mechanical performance of the tunnel structure and affect the normal use.Therefore,it is necessary to study the dynamic response of the tunnel structure under the action of dynamic load and the construction deformation of the tunnel under the operation line.Taking the subgrade of Beijing-Zhangzhang high-speed railway under the south underground tunnel of Changping line of Beijing Metro Line 27 as the engineering background,by means of Flac3 D finite difference numerical simulation,through the analysis of the construction deformation law of the tunnel under the existing line and the dynamic response law of the tunnel structure under the train load,the main conclusions are drawn as follows:(1)With the progress of construction,the settlement values of high-speed railway roadbed,surface and vault gradually increase at first,and finally tend to be stable.Comprehensively considering the control requirements of roadbed deformation value,in the upper and lower steps reserved core soil plus temporary inverted arch,CD method and three-step method,only the settlement value caused by reserved core soil on upper and lower steps plus temporary inverted arch meets the requirements of deformation control.This method is recommended for excavation,and reasonable construction parameters are selected to ensure the safety and stability in the process of tunnel construction.(2)With the increase of tunnel depth,the displacement of existing roadbed,surface and vault decreases gradually caused by tunnel excavation.After the tunnel is completed,there will be different degrees of positive and negative shear stress at the four corners of the tunnel section,the positive shear stress appears at the 45 °and225 °tunnel sidewall position,and the negative shear stress appears at the tunnel sidewall position perpendicular to the positive shear stress.(3)The overall trend of the vibration response of the displacement,acceleration and velocity time-history curve of the tunnel monitoring point under the train load is the same,and the peak value appears in the vault position,which has little influence on the arch waist and arch foot position.(4)Based on the three-dimensional numerical finite difference method,the artificial excitation function is used to simulate the train vibration load,and the displacement,acceleration and dynamic stress of the tunnel structure are selected as the evaluation index of dynamic response analysis.the dynamic response law of tunnel structure under train load vibration under different working conditions(different speed,different axle load,different buried depth,different crossing angle)is studied,and the results are as follows:(1)The displacement,acceleration,maximum principal stress and minimum principal stress at the most dangerous monitoring point under train load are 0.855 mm,346 mm/s2,-0.202 1 MPa and-0.694 8 MPa,respectively,which are lower than the structural vibration safety control limit,which meets the safety requirements and will not cause damage to the tunnel structure.(2)The change of train speed has a great influence on the displacement and acceleration of the tunnel structure,but has little influence on the dynamic stress.When the buried depth of the tunnel is shallow,the dynamic response of the tunnel structure increases gradually with the increase of the train speed,and when the buried depth is less than 10 m,the tunnel structure is greatly affected by the train impact load.on the other hand,the influence of train speed on the dynamic stress response of tunnel structure is relatively small,and it has a great influence on displacement and acceleration.(3)When the train axle load changes,the change of the displacement of the tunnel structure is greater than that of the acceleration response,and the change range of dynamic stress is basically small.(4)The greater the intersection angle between the new tunnel and the existing roadbed,the weaker the vertical displacement and dynamic response under the influence of dynamic load,in which the dynamic response is the smallest at90 °intersection,but the change of peak acceleration has no general rule to follow.When the new tunnel is orthogonal to the existing roadbed,the dynamic response strength of the tunnel will be weakened.