Research On Portal Excavation Technology Of Shallowly Buried Weak Surrounding Rock Tunnel

With the rapid development of social economy,more and more highways are constructed in China.Due to complex topography and geological conditions,there is great difficulty in constructing highway tunnels in western mountainous areas in China.Tunnel portal is considered as the throat of a tunnel.Compared with other tunnel sections,the surrounding rocks in tunnel portal are strongly weathered,broken,and shallowly buried.In addition,the construction of highway tunnel requires a large excavation section,and engineering accidents,including collapse and large deformation may occur during tunnel construction.Therefore,it is important to conduct research in entrance technology of the shallow-buried large cross section tunnel with weak surrounding rocks.In this paper,a new two-way six-lane tunnel is used as an example to study the tunnel portal excavation technology using theoretical analysis,numerical simulation,and field monitoring.The contents are as follows:(1)The feature and stability influence factors of the shallow-buried large cross-section weak surrounding rock are analyzed,and advanced support,tunnel excavation method,and support method involved in the tunnel port excavation technology are summarized and analyzed based on the research results at home and abroad.(2)In this paper,tunnel portal is used as the research object.The impact of tunnel portal surrounding rock deformation,surrounding rock stress,primary support structure stress in portal excavation are analyzed using models of four construction methods,including three-bench ring cut method,CD method,CRD method and double side drift method.The numerical simulation results show that after construction using the double side drift method,the ground surface settlement,crown settlement,and periphery convergence are 9.02 mm,22.77 mm,13.51 mm,respectively,which are smaller than the deformation values when other methods are used and are allowed in the specifications.Meanwhile,the surrounding rock and primary support architecture stress are smaller.Therefore,double side drift method is used as the tunnel port excavation solution in combination with pipe-shed advanced support.(3)The project in this paper uses the recommended tunnel excavation solution.The 3D numerical simulation is performed,and crown settlement,periphery convergence,and ground surface settlement are monitored and measured.Then,the simulated values are compared with measured values.Comparison results show that there is deviation between the simulated and measured values,but the surrounding rock deformation rules are consistent,this indicates that the tunnel portal excavation solution recommended in this paper is feasible.