| With the structural characteristics of large-span tunnel,multi-arch tunnel,small-spacing tunnel and separated tunnel,the bifurcation tunnel is gradually transformed from double-line large-span tunnel to single-hole separated tunnel.It can be applied to the situation of complex terrain and difficult line layout,and keep the line smooth and easy to connect the bridge and tunnel.It is increasingly used in the construction of railway tunnels in China.Due to the unique layout of the bifurcated tunnel,the structure of the transition between the large-span tunnel,the multi-arch tunnel and the small-spacing tunnel is complex and changeable.The middle wall and the rock column between the tunnel body are thinner than the ordinary situation.In addition,it involves the conversion of various excavation methods.How to adopt a reasonable construction method in each section,carry out the support design of the tunnel transition,and ensure the stability of the rock column between the middle partition wall of the multi-arch section and the small-spacing section has become the key and difficult point in the design and construction of such tunnels.Based on the Baishi No.2 tunnel project in the supporting project of the new Zhongwei-Lanzhou Railway into Lanzhou Hub,this paper studies the construction method and supporting structure of the shallow buried loess bifurcation tunnel by means of theoretical analysis,field test and numerical calculation.The main research work and results are as follows:(1)Consult the relevant literature,understand the research status of the bifurcated tunnel,summarize the design and construction methods of the existing bifurcated tunnel,the tunnel load calculation method and the interaction mechanism between the supporting structure and the surrounding rock,and determine the research content,research methods and related calculation parameters of this topic on this basis.(2)According to the design and construction scheme of the tunnel project,Midas GTS NX software is used to establish a three-dimensional numerical calculation model to simulate the excavation and support process of the large-span section,multi-arch section and small-spacing section of the tunnel under different construction schemes.The stress and deformation law of tunnel surrounding rock and middle partition wall under different construction methods are studied,and the most reasonable construction method of each section of bifurcated tunnel is obtained by comparison.The results show that the bifurcation of the large span-multi arch section of the tunnel is greatly affected by the construction process conversion,and the change trend of vault settlement and horizontal displacement is complex.Compared with the middle guide hole method,the three guide hole method is more favorable to control the horizontal convergence of the multi-arch section of the tunnel,and the influence on the adjacent first tunnel and the large-span section is also relatively small.Affected by the construction sequence of the left and right holes,the middle partition wall is inclined to the first hole to a certain extent.Sandbag backfilling or steel support can be used on both sides to ensure the stability of the middle partition wall.Compared with the bench method and the combined method(bench method + single side heading method),the single side heading method is adopted in the construction of the small-spacing section of the tunnel,which can effectively control the vault settlement and horizontal displacement of the tunnel,and the surrounding rock stress difference between the left and right vaults is smaller.In the construction process of the multi arch-small spacing section,the horizontal displacement of the rock column between the tunnel bodies at the transition is large,and a large plastic strain occurs,which is the weakest area in the construction process.(3)The numerical simulation method is used to analyze the stress law of the supporting structure of the two transition sections of the large span-multi arch section and the multi arch-small spacing section of the tunnel.It is found that near the bifurcation of the large span-multi arch section of the tunnel,stress concentration occurs at key positions such as vault,wall foot and floor.The principle of ’short footage,strong support,fast closure and frequent measurement’ should be adhered to during construction to ensure that the initial support is timely and reliable,and the corresponding plug support structure is matched at the bifurcation to complete the structural closed loop.For the multi arch-small spacing section of the tunnel,the supporting structure has obvious compressive stress concentration at the intersection of the end of the middle partition wall and the middle rock column,and the radial grouting of the surrounding rock should be strengthened.(4)In order to facilitate the construction of the exit end and optimize the tunnel construction process,a reverse construction method without middle wall is proposed for the bifurcation of Baishi No.2 tunnel.Compared with the middle guide hole method and the three guide hole method,the vault settlement value of the single hole in the small clear distance section of the no middle wall method is the smallest,but the vault settlement and the horizontal displacement of the arch foot of the large span section are the largest,and the initial support will bear greater tensile stress.(5)Through the on-site monitoring and measurement of the surrounding rock displacement,pressure and steel arch strain at the bifurcation of the tunnel,it is found that the settlement of the tunnel vault is mainly caused by the excavation of the upper bench.The growth rate is the largest in the first week after excavation,and the growth rate slows down after the support is followed up until it is stable.The stress of steel arch tends to be stable with the successive excavation of each part of the tunnel and the closed loop of the supporting structure,that is,the fast closed support is beneficial to the bearing capacity of the structure.Compared with the numerical calculation results,although there is a certain degree of difference between the measured value and the simulated value,the overall change trend is roughly the same. |
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