Analysis On Construction Timing And Long-term Stability Of Secondary Lining Of Anchor In Soft Rock Tunnel

With the continuous and rapid development of China ’ s transportation industry,the scope of underground engineering construction is expanding,and a large number of weak surrounding rock strata appear.The continuous deformation of soft rock under creep effect over time brings great challenges to the construction and long-term stability of tunnel lining structure,which seriously threatens the safe operation of tunnel engineering.Based on the north bank tunnel anchor project of Yichang Wujiagang Yangtze River Bridge,this paper firstly studies the intelligent optimization algorithm suitable for the displacement back analysis of geotechnical engineering,and then obtains the creep parameters of tunnel anchor multi-layer surrounding rock by numerical simulation inversion.According to the inversion parameters,the reasonable construction time and long-term stability of tunnel anchor secondary lining are systematically studied.The main research work is as follows :(1)According to the nonlinear characteristics of rock creep,the standard differential evolution algorithm which is good at solving nonlinear problems is introduced.In order to enhance its global search ability and avoid falling into local optimum,the idea of probability acceptance degradation in Metropolis criterion is combined with the standard differential evolution algorithm.On this basis,the adaptive differential mutation operator is introduced to obtain the improved differential evolution algorithm.The optimization ability of the improved differential evolution algorithm is tested by high-dimensional complex function.The results show that the improved differential evolution algorithm has the characteristics of fast convergence speed,high calculation accuracy and strong robustness,which is suitable for solving the inverse analysis problem of geotechnical engineering.(2)By using the direct screening method,the Bergs model is selected to describe the creep characteristics of surrounding rock of tunnel anchor.According to the lithology of different strata,the site of tunnel anchor is divided into five strata,and the forward analysis model for numerical calculation of tunnel anchor is established.The improved differential evolution algorithm is used to invert the creep parameters of multi-layer surrounding rock,and the forward calculation results of the inversion parameter numerical model and the field monitoring measurement data are compared.The error between the two is small,which verifies the applicability and accuracy of the improved differential evolution algorithm in inverting the creep parameters of surrounding rock.(3)According to the limit displacement criterion of the secondary lining construction in the specification and the actual construction process of the secondary lining of tunnel anchor,through the regression analysis of numerical model and monitoring data,the stress and deformation state of the secondary lining of tunnel anchor after the construction at different times are compared.It is determined that the secondary lining is applied when the cumulative displacement of all sections of the tunnel anchor saddle section and the front anchor section reaches 80 % of the total displacement,which can make the deformation of the key parts of the secondary lining smaller and the stress more reasonable after the construction,and have certain safety reserves.(4)Based on the numerical model,the variation law of stress and deformation of surrounding rock and secondary lining with time is studied,and it is found that the overall change process presents three stages,so as to analyze the long-term stability of tunnel anchor secondary lining.Various factors affecting the construction timing and long-term stability of tunnel anchor secondary lining are listed and the long-term stress control countermeasures of lining structure under the action of surrounding rock creep are introduced.