Experimental Study On Optimization Of Four Bar Lattice Girder With "8" Diagonal Steel Bars In Tunnel

With the popularity of the New Austrian Method,Lattice girder with its unique advantages in the tunnel and underground engineering has been used widely.Based on the tunnels of constructing Menghua railway,a total of 10 model tests of 7 field tests with a total of 90 m length in 3 sections were carried out for single line H150 and double line H180.Based on the scientific theoretical analysis of deformation law,failure mode and ultimate bearing capacity of the structural test structures in full-size tests,three kinds of field tests were carried out.The following conclusions are drawn on the basis of force condition of lattice girder,the deformation of the surrounding rock and the contact pressure between the surrounding rock and the primary support of field test and model tests.1.The diameter of diagonal bar of lattice girder can be optimized to 10mm,which can ensure the carrying capacity and control the economic costs.2.The restraining effect of surrounding rock is very effective to improve the bearing capacity of the initial support,which suggests guarantee the primary support should be closely attached to surrounding rock in the actual construction.3.In the model test,it is proposed that the optimal load condition of the single-track tunnel is the arch load in the case of the surrounding rock and the single-track tunnel is not suitable for the design of the surrounding rock with the lateral load as the active load.4.In the field test,the standard design,the optimal design of the diagonal bar and the optimized design of the diagonal bar and main steel can form the effective primary support in the V grade loess surrounding rock with the shotcrete to maintain the stability of the surrounding rock.5.Considering the stress of the lattice girder,the deformation of the surrounding rock and the contact pressure between the surrounding rock and the primary support,and the economic,human and time cost,the optimized design of the diagonal bar and main steel is the best.6.The failure of the primary support is a compression and shear failure as the small eccentric compression controlled by ultimate strength of concrete.Lattice girder only provide the residual bearing capacity of primary support,ensuring the structure integrity and ductility after the concrete is out of work.7.The concrete and main steel of the standard design,the optimal design of the diagonal bar and the optimized design of the diagonal bar and main steel of testing primary support consist of lattice girder still have sufficient bearing capacity.It is worth considering a further optimization design of main steel.