Research On Fuzzy Evaluation Of Shield Tunnel Safety Degree And Optimization Of Adjacent Foundation Pit Support

With the rapid development of subway construction and urban underground space development,subway shield tunnels in service will inevitably be affected by the excavation of adjacent foundation pits.According to the research of related literature,the existing foundation pit support design often rarely takes into account the current operating status of the existing subway tunnel,including the existing deformation of the tunnel,tunnel diseases,and so on.In fact,subway tunnels in different operating conditions are affected to different degrees by foundation pit excavation.Therefore,this paper proposes a safety fuzzy evaluation system that considers the current status of tunnel operation,and based on this evaluation system,conducts related research on the optimization of foundation pit support.The main work is as follows:(1)Consult the relevant literature and discuss the current research status at home and abroad from three aspects: the influence of foundation pit excavation on tunnels,fuzzy evaluation application,and foundation pit support optimization.At present,when evaluating the impact of excavation of foundation pits on tunnels,the deformation control value specified in the specification is the main focus,and the current status of subway operation is less considered,and there is a "hard boundary" problem.However,the existing deformation and disease of the tunnel have a greater impact on the stress state and deformation of the tunnel and cannot be ignored.In the fuzzy evaluation of subway tunnel safety,there is little research on its application,and further research is needed.(2)Based on the shortcomings,considering the current status of the subway shield tunnel operation,a fuzzy safety evaluation system including multiple indicators such as the existing displacement and radial convergence value of the tunnel,and tunnel water leakage should be established.According to the survey of subway operation status,the safety status of the tunnel section before the excavation of the foundation pit is evaluated,and the fuzzy evaluation level of the tunnel safety is determined.The safety level of the tunnel before the excavation is the second level,and the tunnel safety level is relatively high.(3)Through the establishment of a three-dimensional finite element analysis model,the influence of the excavation of the foundation pit on the subway shield tunnel is analyzed.The calculation results show that the tunnel displacement and radial convergence value meet the requirements of relevant regulations.However,according to the results of the fuzzy evaluation and considering the current status of subway operation,the safety evaluation level of the tunnel is reduced to the fifth level,and the tunnel safety level is relatively low.Under the original support plan,the tunnel safety is insufficient after the excavation of the foundation pit,and the ability to withstand external influences is reduced.(4)According to the change of the security level membership before and after the excavation of the foundation pit,with the five-level security level membership of the tunnel0.1 as the control threshold,combined with the finite element calculation results,the allowable residual deformation envelope diagram of the tunnel displacement and radial convergence value is obtained.Taking this as the optimization goal,the support optimization ideas are proposed,and the orthogonal analysis is used to consider the economy,and the support optimization plan is determined.Finally,according to the measured data after the excavation of the site foundation pit is completed,the tunnel has a maximum displacement of8.8mm and a maximum radial convergent deformation of 6.9mm.According to the results of the fuzzy safety evaluation,the tunnel security level belongs to the third level,and the membership degree of the fifth-level tunnel security level is less than 0.1,and the tunnel meets the established safety goals.The effectiveness of the optimization method for foundation pit support is verified under the control threshold of the shield tunnel security grade membership.It is proved that this method can more comprehensively and scientifically consider the influence of the current status of subway shield tunnel operation,and optimize the design of foundation pit support.