| Underground space resources can be developed vigorously during the 21st century,at the same time tunnels and underground engineering develop greatly.At present,there is a big imbalance between the technology and theory used in tunnel construction and the rapid development of economy.The state is promoting the degree of mechanization of tunnel construction,which is the trend of the times.In tunnel construction,when the excavation section is large,the conservative sequential excavation method is often adopted,which leads to a narrow work space and difficult entry of large machinery,thus making the construction efficiency low and greatly slowing down the construction progress.This is also a bottleneck restricting the development of new theories and technologies in tunnel construction.Secondly,according to the experience,the construction safety step is selected as the control distance between the primary support or the secondary lining and tunnel face to ensure the safety and stability of the tunnel structure.However,the division method is conservative and rough,besides large mechanical work site requirements conflict is an inevitable problem.This paper aims to solve the above two problems that restrict the tunnel construction technology development,selecting Xinhua tunnel in Zheng Wan railway as cases.Using field monitoring measurement,theoretical analysis and numerical simulation method,research on the feasibility of the full-face excavation method in the large cross section tunnel and the optimization problem of construction safety step.The main research contents are as follows:(1)Field monitoring measurement.In this study,the mileage of Xinhua tunnel exit D1K556+861-D1K556+961 which is selected as the test section,and four monitoring sections were arranged.Using earth pressure box,embedded strain gauge,surface strain gauge,bolt axial force meter and multi-point displacement meter,the data of surrounding rock pressure,concrete stress,steel arch stress,bolt axial force and stratum displacement are monitored to reflect the deformation and stress of surrounding rock and supporting structure.The load-structure stress mode is used to check the safety of the lining structure.According to the monitoring and calculation results,the maximum value of surrounding rock deformation in the tunnel test section is 5.6mm,the maximum value of bolt axial stress is 29.87MPa,the maximum value of steel arch stress is 61.2MPa,stress of shotcrete in some point exceeds the ultimate tensile strength of materials,and the minimum value of safety coefficient of secondary lining structure is 1.90,with a large average value.It is shown that the overall safety of the tunnel construction with full-face excavation method is guaranteed,but the local damage of shotcrete exists.(2)Research on of sequential excavation and full-face excavation.Base on the existing engineering geological data and excavation section of specifications and supporting scheme.Using FLAC3D to research on the deformation and stress of surrounding rock and plastic zone distribution under different excavation method.Results verify the rationality and feasibility of full-face excavation method in Xinhua tunnel.The results show that the radius of the plastic zone is approximately 3.12m-5.04m when the tunnel is excavated in full-face excavation,which is slightly larger than that of the sequential excavation.There is little difference in stress distribution when the tunnel is finally excavated.The maximum arch subsidence of the full-face excavation method is 7.3mm,which of the sequential excavation is 5mm.(3)Research on the safety step.The mechanical model and numerical simulation are used to calculate the stress state and safety degree of the support system and surrounding rock under different construction steps.Determine the ideal safety step between the tunnel face and the inflected arch and the secondary lining by the calculation.According to the research results,the primary support safety step is 60m,and the secondary lining safety step is 80.64m. |
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