Research On Mechanical Behavior And Construction Method Of Large-span Double-arch Highway Tunnel In Soft Strata

The multi-arch structure is a type of tunnel structure which is often used in the construction of highway tunnels in recent decades because of its adavantages such as easy wiring,less land occupied,high utilization rate of underground space,beneficial to environmental protection and adapting to urban development needs,but followed by such as the ceter pillar of water leakage,structural deformation and other diseases.Therefore,a comprehensive understanding of the double-arch tunnel construction process of the mechanical characteristics and deformation of the law has become an urgent need.Based on the Nanshan road double-arch highway tunnel,a research has been carried on about the stability of the center pillar in the construction process,the law of the supporting structure and the construction method through investigation and analysis,numerical simulation,field measurement and other methods in this paper.The main work and results are listed as follows:(1)Based on the previous investigation and analysis,the mechanical condition of the symmetrical center pillar and the asymmetrical center pillar of the surface bias zone are calculated.It is concluded that the asymmetrical center pillar is less than 12.06 percent of symmetrical center pillar in the vertical unbalance force and 61.87 percent in the unbalance moment.Therefore,the asymmetrical center pillar has a better adaptability in the bias zone.The numerical simulation of the portal bias section and the 40m buried depth section are carried out respectively.The internal force monitoring of the representative center pillar section is selected.It is concluded that there is always bias at the top of the center pillar during the whole construction process and it is deflected to the existing hole,and the maximum pressure applied when the up-bench of the following hole is excavated.(2)The FLAC3D software was used to simulate the displacement field and stress field of the initial support of the double-arch tunnel in the soft strata during the construction process.The support parameters were optimized by combining the strata and structure deformation,the supporting structure and the early strength test of the shotcrete.The strata deformation includes surface subsidence,vault subsidence and horizontal convergence.The surface subsidence is mainly affected by excavation of the hole on the same side.The displacement of the initial support is mainly composed of vertical subsidence,and the horizontal convergence is small.The construction of the following hole has a certain impact on the existing hole,but the magnitude is not great.The monitoring of the support structure includes the surrounding rock pressure,contact pressure between the primary lining and secondary lining,steel arch internal force and secondary lining internal force.The construction of the following hole causes the stress redistribution of the existing hole to make the surrounding rock pressure,contact pressure between the primary lining and secondary lining and secondary lining internal force increase,but the steel arch internal force is reduced,and the stress state of the steel arch is improved.(3)Taking Nanshan road double-arch highway tunnel as project background,the construction method of double-arch tunnel is optimized by setting different main hole malposition and excavation step length at different surrounding rock levels.Based on the numerical simulation of the middle-pilot in grade ? surrounding rock and three-pilot in grade V surrounding rock,it is concluded that the reasonable main hole malposition is 2?2.5 times of tunnel diameter,and the step length is determined according to specific situation in grade ? surrounding rock.At the same time,the reasonable main hole malposition is 2.5?3 times of tunnel diameter,and the reasonable step length is about 10m in grade V surrounding rock.