Study On Deformation Control And Surrounding Rock Stability Of Shallow Buried Bifurcated Tunnel In Loess

In recent years,China ’s highways and railway networks have been rolled out nationwide.With the rapid development of transportation infrastructure in the western region,railway tunnel construction has entered a new era of leapfrog development.In order to solve the problem of difficult line design under complex terrain and poor geological conditions in mountainous areas,bifurcated tunnels have been gradually applied.Due to the complicated construction process,difficult construction support and poor stability of the bifurcated tunnel,and the influence of the special engineering properties of the loess in the northwest region,the excavation construction of the shallow buried bifurcated tunnel becomes particularly difficult.Based on the current situation,this paper relies on the Baishi No.2 tunnel of the new Zhonglan Railway introduced into the Lanzhou Hub Project.Through theoretical analysis,numerical calculation and field monitoring,the deformation control of the shallow buried loess bifurcation tunnel and the stability control of the surrounding rock in the construction stage are studied.The main research work and results are as follows :(1)Through the investigation of domestic and foreign literature on shallow buried bifurcation tunnel,the classification criteria of tunnel depth and section type are clarified,the protection principle and application scope of different supporting structures of tunnel are familiar with,and the different methods of tunnel stability analysis are summarized.(2)Based on the actual engineering project,using Midas-GTS NX finite element software,a three-dimensional numerical analysis model of large-span section and multi-arch section of shallow buried loess bifurcation tunnel under different advanced support is established.The deformation and stress of surrounding rock,lining stress and internal force of advanced support are analyzed.Taking the large-span section as an example,in terms of surrounding rock deformation,compared with no advance support,the surface settlement decreases by22.1 % and the vault settlement decreases by 42.3 % under the advance pipe roof support.Under the support of advanced small pipe,the surface settlement decreased by 11.7 %,and the vault settlement decreased by 27.4 %.In terms of the force of the advanced support,the maximum axial forces of the advanced small pipe and the pipe shed are 63.5 k N and 182.1 k N,respectively,indicating that the pipe shed can withstand more surrounding rock pressure and reduce the pressure shared by the initial support and the secondary lining.Under the pipe shed support,the distribution range and radial depth of the plastic zone are smaller than those of the advanced small pipe support.The above research results show that the advanced pipe shed is more efficient than the advanced small pipe support in terms of spatial effect and mechanical characteristics.In the specific construction process,it can be more conducive to preventing the risk of roof fall and collapse of surrounding rock and improving the overall stability of the tunnel.(3)The deformation,stress,plastic zone and stability of surrounding rock in the large-span-multi-arch transition section during tunnel excavation were analyzed by numerical simulation.The results show that the area close to the bifurcation interface is greatly disturbed by excavation,and the stress concentration at the interface is easy to destabilize.The floor of the large-span section rises up with the construction.During the construction,the tunnel face should be closed at the interface between the single hole and the double hole,and the plug wall should be applied and the inverted arch should be applied.Under the design and construction method of combining the double side drift method with the middle drift method,the tunnel stability and structural safety can meet the needs of safe construction.(4)In view of the complexity of the stress mechanism of the shallow buried loess bifurcation tunnel,according to the deformation characteristics of the tunnel,the strength reduction method is used to evaluate the stability of the surrounding rock based on three aspects : whether the plastic zone is connected,whether the feature point has displacement mutation and whether the calculation converges.The optimal cyclic footage and the spacing between adjacent working faces in the construction process of the multi-arch section of the bifurcated tunnel are emphatically studied.The results show that under different cyclic footage,the vault settlement and the surface midline point settlement increase with the increase of the strength reduction coefficient,and the plastic zone becomes more complex.From the economic and safety performance analysis,the reasonable driving footage is 1.0 m;when the distance between the left and right tunnel face is 16m(2 times the diameter of the tunnel),the settlement deformation of the first tunnel vault is the smallest,the disturbance to the middle partition wall is low,and the lining support effect is the best,which is most conducive to the stable construction of the tunnel.(5)The data of vault settlement,horizontal convergence and surrounding rock pressure of the target section of the bifurcation tunnel monitored on site are integrated,and the relationship between them and the excavation progress is analyzed.The results of field monitoring and numerical calculation are compared to verify the reliability of the numerical model.