Research On The Tunneling Parameter Optimization Of Existing Tunnels Over Shield In Composite Strata Of Chengdu Area

With the gradual increase of subway construction,the complex working conditions of different forms of multi-line shield tunnels passing through existing tunnels at close distance have become an unavoidable problem in the construction process of subway,especially in the soft and hard composite strata.It is difficult to control the shield posture,the force on the cutter head is uneven,and the ground response is sensitive.How to ensure the safety of the new and old tunnels during the short-distance shield tunneling is an urgent problem to be solved in shield construction.In the process of shield tunneling,shield excavation is the source of construction disturbance,soil mass is the medium of propagation,existing tunnels are the objects of action,and they influence each other.The optimal control of shield tunneling parameters can reduce the impact of shield tunneling from the source of disturbance,and is the most direct and effective means to maximize the engineering benefits by performing fine control of the upper span construction.Based on the engineering background of Chengdu metro line 8 over existing tunnel line 9,this paper studies the optimization of shield tunneling parameters from the source of construction disturbance.The main research contents are as follows:(1)Based on the analysis of the mechanism and influencing factors of the crossconstruction disturbance on the shield,the main tunneling parameters in upper span are selected,and the sensitivity analysis of tunneling parameters is carried out with the help of fuzzy rough set theory.It is concluded that the influence degree of main tunneling parameters on stratum settlement in the process of shield upper span tunnel is as follows: grouting pressure,barnyard pressure,cutter head torque,shield thrust,propulsion speed,synchronous grouting quantity,and the characteristics of the sand-gravel-mudstone composite formation in the project are combined.According to the characteristics of the compound formation of the sand and gravel mudstone in the project,the characteristics of the shield machine and the mechanism of the construction disturbance analyze the cause of the sensitivity sequencing of the driving parameters and the related parameters control principle.(2)Through three-dimensional numerical simulation combined with the time effect of shield construction disturbance,the subsidence and deformation law of the existing tunnel and the existing tunnel in each stage of the shield tunnel are studied in this paper.Then the influence of the main sensitive factors on the surface settlement is analyzed on the basis of the sensitivity analysis.Based on the principle of simplex search method,the ABAQUS secondary development is carried out by using Python language,and the main sensitive factors are automatically optimized to find out the control range of the main sensitive factors which can safely pass through the existing tunnels below the critical value of early warning.(3)Based on the results of numerical simulation analysis,the actual tunneling parameters were statistically optimized in different zones,and the direction of parameter adjustment and control range were obtained when the shield tunneling was at different positions.Taking the main sensitive factor as the main control index,the dynamic optimization of the whole control flow of tunneling parameters is carried out according to the operation mode and sensitivity analysis results of shield,and the concrete method of parameter control is pointed out for the actual construction.On this basis,the early warning indexes and treatment measures of tunneling parameters are put forward,so as to realize the fine control of span construction on shield.