Research On Construction Method Optimization And Support Countermeasures Of Large Span Variable Section Bifurcation Tunnel

The excavation cross-section of highway tunnels is becoming larger and more complex,and the development trend of large-span variable cross-section bifurcated tunnels is highway tunnels.At present,there is a lack of corresponding design and construction specifications for large-span variable cross-section bifurcated tunnels in China,and the number of completed large-span variable cross-section tunnels in China is relatively small,lacking reference experience for similar projects.Therefore,it is of great significance and application value to study the long-span bifurcated tunnel with variable cross-section and obtain a set of construction methods that comprehensively coordinate construction safety and Engineering economics to guide the construction.This article takes a tunnel in Xiamen,Fujian Province as a supporting project and optimizes the construction methods and support strategies for largespan variable cross-section bifurcated tunnels through on-site monitoring,numerical simulation,and other methods.The mechanical changes during the entire construction process are analyzed.The main research work and achievements are as follows:(1)The FLAC3 D software was used to optimize and explore the construction methods and support methods of variable cross-section tunnels.Combined with the surrounding rock conditions of the tunnel,a construction method such as the double wall ten step method was proposed to ensure tunnel safety,save project budget,and accelerate construction speed.Compared to the initial design and construction method,the optimized method can save 72 days of construction time and reduce the use of temporary support by 56.1%.(2)We optimized and compared the construction methods and contents of the ultra small clear distance bifurcation tunnel section,and analyzed the displacement and stress transfer laws of the tunnel and middle rock column under typical working conditions.The deformation and stress changes of the middle rock pillar mainly occur during excavation of the heading near the side of the middle rock pillar.Reducing the excavation footage and length,and timely implementation of initial support can effectively reduce the disturbance of construction to the middle rock pillar.(3)A comprehensive mechanical model of the surrounding rock and support structure was established,and the mechanical characteristics of the entire construction process of the optimized large-span variable cross-section bifurcation tunnel were studied.The displacement distribution characteristics of the surrounding rock and the redistribution state of the stress field were analyzed.Through analysis,it can be concluded that the displacement deformation of tunnel surrounding rock mainly consists of two parts: the rapid deformation stage and the relatively stable stage.The displacement deformation is proportional to the size of the crosssection,and reaches its maximum value in the middle of section FC5.The arch settlement and horizontal convergence displacement are 26.08 mm and 9.89 mm respectively.The vertical and horizontal displacement of the rock column in the bifurcation tunnel at ZK3+860 have decreased by 34.91% and 34.55% compared to ZK3+900.The depth of failure in the plastic zone is directly proportional to the size of the cross-section,and the plastic zone of the surrounding rock at the intersection of each variable cross-section and bifurcation tunnel is in a continuous state,which affects the construction safety of the tunnel.(4)According to the monitoring purpose,the monitoring content was set,and the accuracy of the calculation results was demonstrated by comparing on-site monitoring data with numerical simulation results.Based on the Bayesian update method,the displacement and deformation values of the surrounding rock using the empirical model method are updated multiple times.By adding uncertainty factors such as model deviation factors and measurement errors,and using a few actual monitoring data to update the previous prediction,the accuracy is improved and the uncertainty of the prediction is reduced.After six data updates,the predicted values are basically close to the actual measurement values.