Research On Mechanical Behavior And Cracking Mechanism Of Lining During Large-span Double-arch Tunnel Construction

In order to solve the engineering problem of the first tunnel lining cracking frequently caused by the second tunnel construction of large-span double-arch tunnels,a research method including distributed optical-fibre monitoring,inversion analysis method and finite element simulation that can reflect concrete cracking was proposed in this paper.The Kaidagu large-span double-arch tunnel was excavated in the weak surrounding rock with the method of construction technology without advancing middle drift,an d cracking was observed in the first tunnel lining during the construction of the second tunnel.Based on the engineering case,the mechanical behavior and cracking mechanism of the first tunnel lining during the construction of the second tunnel was syste matically studied.The main research work and results are as follows.1.The deformation and cracking of double-arch tunnel lining were monitored and early warned using distributed optical-fibre monitoring.The fibres were attached to the inner surface of the cracked concrete lining and the inner and outer surfaces of the temporary steel arch support in the defected section of the tunnel.In addition,the fibres were buried into primary lining and installed on the inner and outer surfaces of the steel arches during the subsequent construction of the first tunnel.Based on the strain distribution,the lining cracking was warned in time.The influence range of the second tunnel excavation and the deformation mode of the first tunnel lining were analysed.2.According to the plane curved beam theory,an inversion analysis method based on the fibre-optic strain profile was proposed to determine the deformation,internal force and load distribution of arch structure.This inversion analysis method considered the combined effect of the axial force and bending moment on the deformation of arch structure.The calculation accuracy was verified by finite element simulations.The method was applied to determine the deformat ion and force of the temporary steel arch in defected section of the double-arch tunnel,the steel arches inside the primary lining in the experimental section of the double-arch tunnel and the segments of the Qinghuayuan shield tunnel.The applicability of the method was validated in practical engineering,and the factors causing lining cracking were analysed according to the inversion analysis results.3.In order to verify the feasibility of using the total strain rotating crack constitutive model to reflect the mechanical properties and cracking modes of reinforced concrete components subjected to complex stress and confinement,the finite element simulations based on the constitutive model were c onducted on the loading tests of full-size beams and stirrup-confined concrete short columns.The simulated loading curves and cracking modes were compared with the experimental and the empirical regression model results,and the errors were within an acceptable range.4.A load-structure method–based finite element model was established.The total strain rotating crack constitutive model was used to reflect the concrete cracking in the simulation,and the influence of different load distribution in each construction condition on the deformation and stress of the lining was considered.The deformation mode,crack and force distribution of the first tunnel lining during the construction of the second tunnel were studied.The influence of the parameters on the lining force was analysed,and a lining reinforcement scheme was proposed.After that,a stratum-structure method–based three-dimensional finite element model was established,which considered the nonlinear behavior of the surrounding rock and concrete at the same time.The loosened zone and stress distribution of the surrounding rock,the load,force and deformation of the first tunnel lining,and the longitudinal influence range of the second tunnel excavation during the construction of the second tunnel were analysed.Through the comparative analysis of the results obtained by the two simulation methods,the cracking mechanism of the first tunnel lining was explained.Specifically,construction of the second tunnel resulted in the release of rock pressure and confinement as well as the secondary loading at the top of the middle partition wall in the first tunnel.Under the combined action of these load changes,the first tunnel lining cracked.