Reaearch And Application Of Probabilistic Limit State Design For Tunnels Of Passenger Dedicated Lines

The variability of physical and mechanical parameters of rocks and of performance of lining's material should be dealt with scientifically and reasonably in order to make the lining safe,economical and reliable when designing tunnel lining.The current design code of railway tunnel structure is based on the plastic stage method and the permissible stress method,which apply the single safety factor to lining design all the times even though the surrounding rock condition and lining type are different.This method often leads to material waste and potential risk.The research results of other structures are used without polishing by the study of approximate probabilistic limit state design of railway tunnels.There are some ambiguous concepts,the lack of detailed research on the resistance and mechanical effect of lining and the statistical data the reliability of which is low.In addition,JC method is widely used to calculate the reliability index of tunnel lining,ignoring the uncertainty of the stress state of lining elements,which is not rigorous enough and may make the calculation results deviate.In view of the above,this paper takes the composite lining of double-track and single-track tunnels of 350-km/h and 250-km/h railway line for passenger traffic as the research object,establishes the optimization calculation method of tunnel lining reliability index,defines the concept and calculation method of lining resistance and mechanical effect and study the probabilistic limit state design of high-speed railway tunnel lining which is expressed in terms of partial coefficients.Main works and research results are as follows.(1)The mechanical response of 28 linings of high-speed railway tunnels are calculated with random sample by Monte Carlo stochastic finite element method to obtain the statistical characteristics and distribution types of the axial force,moment and eccentricity of each element.The program of constrained reliability index optimization is compiled using MATLAB.By comparing with JC method,the good optimization performance and practicability of Global Search function are proved.The reliability index of all linings are calculated,showing that there are some linings which are either more conservative or less reliable.The RI of vault element is the smallest,and the stress state is the crack resistance or compression with large eccentricity.(2)The calculation formula of lining resistance are elaborated and deduced with eccentricity as the basic variable from the basic concept of structural resistance.Considering the uncertainty of the lining stress state,the influence of eccentricity on the resistance of plain concrete lining and reinforced concrete lining are analyzed.The statistical parameters of the lining resistance in different stress states are calculated by sampling.The resistance of lining basically conforms to lognormal distribution,and the mean and variation coefficient of different lining varies,which significantly differ from the statistical parameters of resistance of building structural components.(3)The significance of the linings' mechanical effect under the framework of this study is clarified.Sampling calculation shows that the effect of various linings under the action of surrounding rock pressure and self-weight of lining basically conforms to normal distribution,and the variability is small,mostly less than 5%.The mechanical effect ratio of self-weight and surrounding rock pressure is generally small.Generally,the ratio is smaller when the surrounding rock condition is worse.In the range of action-effect ratio,the influence of effect ratio on reliability index is very small.The significance of mean coefficient of mechanical effect is discussed and explained,and the value is determined.(4)The example of partial coefficients' optimization calculation elaborates the meanings of statistical parameters of lining resistance and mechanical effect.According to the practical design function of lining structure and the determination principle of partial coefficients,the partial coefficients are obtained using enumeration optimization method,which are applicable for checking the bearing capacity of composite lining design of double-track and single-track tunnels of 350-km/h and 250-km/h railway line for passenger traffic.The partial coefficients of self-weight and rock pressure are 1.2 and 1.4,respectively.The partial coefficients of resistance vary with rock conditions and lining types.(5)In view of the geometric parameters and geological conditions of the New BaDaLing tunnel,the lining designs under different rock conditions are carried out according to the Code for Tunnel Design of Railway Tunnel and Interim Code for Limit State Design of Railway Tunnel.The safety and reliability levels of three kinds of design schemes are compared and analyzed,and the recommended values of resistance partial coefficients are given according to the analysis results of total probability method.