| In recent years,with the rapid development of highway construction of China,more and more highways need to build mountain tunnels when crossing mountains.It is very difficult to build tunnels in the mountain area because of its complicated geological conditions.The tunnel portal section is often treated as the main risk control section of tunnel engineering.Compared with other engineering sections,the surrounding rock of the portal section is broken and weathered seriously,which increases the difficulty of tunnel design and construction.If it was not handled properly in the design and construction process of tunnel portal section,it would be very easy to cause tunnel collapse and other accidents,which would seriously affect the progress and safety of the project.Therefore,it is very important to study the deformation characteristics and stability of surrounding rock in tunnel portal section.This paper takes a highway tunnel as the engineering background to study the deformation characteristics and support design of tunnel portal section by theoretical analysis,field monitoring,and numerical simulation,etc.The main research findings are as follows:(1)Based on the domestic and international relevant literature and 20 engineering examples,the influencing factors of weak broken rock deformation and the characteristics of surrounding rock deformation under various factors are summarized,and the action mechanism of the main supporting structure at the tunnel portal section is analyzed.This paper introduces the main construction methods of the tunnel portal section,and compares the advantages and disadvantages of each excavation method and the applicability by investigating the domestic and international engineering examples.(2)According to the construction site of a tunnel,the monitoring scheme is formulated,and the deformation of surrounding rock is divided into three periods combined with field monitoring data,namely,the fast deformation period,the slow deformation period and the deformation stable period,respectively.By establishing grey model to predict the deformation of surrounding rock,it is found that the Verhulst model and the DGM(2,1)model have higher fitting accuracy and can be used to predict the deformation of surrounding rock for the tunnel.The Verhulst model is used to predict the final deformation of surrounding rock and the unstable time of surrounding rock,and the prediction results are good.(3)According to the actual situation of tunnel engineering,the numerical model is established by using Midas/GTS finite element software.By comparing the simulation results of ground settlement,vault settlement and cave circumference convergence with field monitoring data,this paper determines the actual parameters of rock and soil.Through numerical simulation,the information of surrounding rock deformation,stress and plastic zone distribution is obtained.Combined with the actual situation of engineering,the characteristics of surrounding rock deformation,stress and plastic zone are studied,and the stability of the surrounding rock and the cause of collapse are analyzed.(4)Aiming at the vault collapse occurred in the construction of portal section in the original design scheme,the analysis working conditions are established from aspects of the advance support and the excavation method.By using simulation to verify the feasibility of various working conditions,the original scheme of design and construction is optimized,and a more reasonable scheme of excavation and support is finally obtained,which provides guidance for the tunnel portal section design and construction. |
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