Study On Support's Mechanical Characteristics And Construction Method Of Extra-large Cross-section Tunnel With Small Clearance

With the increasing traffic volume,limited by the terrain,the overall line planning and other issues,the construction of tunnels with extra-large span and small clearance will become more and more.This kind of tunnel is represented by two-way eight-lane highway tunnel.The span of it can reach 20?25m and the height is about 14m.It has the characteristics of large cross-section,small net clearance and low flatness.Tunnel excavation is full of risk under the mutual influence between the double hole and the complex and changeable condition of structural force and deformation.At present,the number of bi-directional eight-lane highway tunnels built in China is very few,and the experience of reference projects is very limited.There is a lack of a set of systematic and feasible methods of design and construction and corresponding supporting technologies.Also,the current highway tunnel design and construction Norms did not involve two-way eight lane tunnel with small clearance.Therefore,it is of great significance to carry on the systematic study of tunnels with extra-large cross-section and small-clearance.Taking Qichongcun 1#two-way eight-lane highway tunnel built on Guiyang 1.5 ring as project basis,using a combination of theoretical analysis,field tests and numerical simulation research tools,the following aspects were studied:(1)By summarizing the derivation ideas and shortcomings of the theory of surrounding rock pressure in the current norms,the failure mode of the surrounding rock is reestablished,and the calculation formula of the surrounding rock pressure applied to tunnels with extra-large cross-section and small-clearance is deduced.The Qichongcun 1#Tunnel is taken as a project example to verify the formula.(2)Using numerical calculation method to simulate the excavation method which is suitable for tunnels with extra-large cross-section and small-clearance under different surrounding rock conditions,and the optimal excavation method under different surrounding rock grades is determined.Combined with engineering practice,the double vertical wall guide method is proposed to optimize the double guide hole excavation method,and its feasibility is verified by simulation.The reasonable standard of double-line clearance is determined,and the reasonable spacing suitable for tunnels with extra-large cross-section and small-clearance under different surrounding rock conditions is studied.(3)Based on the Qichongcun 1#Tunnel,the mechanical properties of the support during the construction of the tunnel with extra-large cross-section and small-clearance are studied.Through the on-site monitoring of the typical section's support condition,which includes the contact pressure of the two linings,the stress of the shot concrete,the stress of the steel arch,the axial force of the bolt and the stress of the secondary lining,the force law of the tunnel's supporting structure is acquired.At the same time,the feasibility of the lining design scheme is verified.(4)Specific to the opening area of the large cross-section,the landslide's influencing factors,classification methods and the basic disposal principles are summarized.Through the investigation and summary of the cases of collapse in the opening area of the large cross-section tunnels in the country,the key points of the treatment measure are summarized.Aiming at the collapse occurring at the Qichongcun 1#Tunnel's opening area with shallow-depth and unsymmetrical loading,the mechanical mechanism of collapse in the tunnel with extra-large cross-section is studied and the treatment scheme that can be used for reference is given.