| Under the background of large-scale construction in tunnel engineering,the safety or stability is always the most basic and most important content.However,due to the complexity of the tunnel project and the variability of geotechnical parameters,the design and safety assessment of tunnel support structure are still remain in the stage of empirical method or semi empirical method,such as rock mass classification,engineering geological analogy method,making the stability of the tunnel study or evaluation method not mature,and especially the lack of safety factor,thus the support structure cannot be quantitatively analyzed.Actually,the quantitative analysis method plays an important role in the supplement of existing empirical method or semi empirical method and the quantitative analysis of tunnel.So,the core content of this paper is trying to establish the safety factor of support structure of tunnel based on the interaction relation between surrounding rock and support structure,the main research methods include theoretical analysis method,numerical simulation method,reliability method and laboratory model test.The results of the study are verified by numerical examples and engineering examples,respectively,the implementation steps and operation flow of the quantitative analysis method are demonstrated in detail.First of all,the development rules and characteristics of stress of surrounding rock are combed according to the convergence-confinement method which can reflect the interaction characteristics between surrounding rock and support structure.The solving idea and theoretical basis of the safety factor for surrounding rock and support structure of tunnel are studied,and the safety factor is calculated based on analytical method.In order to realize the above process,first,the theoretical analysis model for Ground Reaction Curve(GRC)of tunnel surrounding rock is established based on the Hoek-Brown theory and Duncan-Fama theory;second,the convergence-confinement figure of tunnel is determined combined with the Support Characteristic Curve(SCC.)and Longitudinal Deformation Profile(LDP);third,combining with the definition of safety factor,the quantitative safety evaluation analysis method of surrounding rock and support structure is established according to the equilibrium position of rock deformation and support pressure,and,this analytical method based on the safety factor can be used to describe the safety performance of the support structure quantitatively;fourth,the operation process of safety factor method is shown in detail based on the Liziping tunnel case.According to this part of study,after obtaining the physical and mechanical parameters of rock mass and support,the safety of support design can be evaluated quantitatively using this safety factor method,besides,for the tunnel project that has been put into use,the weak surface of support also can be found in time.Secondly,the reliability method is widely used in safety evaluation and decision making of engineering structures,at the same time,the failure probability of the engineering structure cannot be given and the absolute safety of the structure cannot be guaranteed by the safety factor method.Therefore,it is necessary to study the failure probability of tunnel engineering.However,as for the underground engineering structures,such as the structure of tunnel bolt-shotcrete support,their function are often highly nonlinear,implicit,the above similar characteristics of engineering reliability cannot be solved by the commonly used reliability calculation method.The common methods in the field of reliability such as the First-Order Second-Moment method(FOSM)and the Second-Order Second-Moment method(SOSM)are chosen as the basic method of the paper study.The main contents are as follows:first,the stress controlled reliability calculation function of tunnel is established based on the interaction mechanism between surrounding rock and support structure;second,the approximate calculation formula of the first-order and the second-order and the second order mixed partial derivative are obtained using numerical difference principle,the execution process of the classical FORM and SORM is reformed based on these derived formulae and an expanded FORM and SORM based on numerical difference principle are organized;third,the accuracy,simplicity and practicability of the expanded methods are verified by three numerical examples;fourth,the stability reliability of a highly complex nonlinear form of bolt-shotcrete support system is analyzed by using the expanded method based on an engineering case.Again,the theoretical analytical model is established mainly based on circular excavation section,but the actual tunnel excavation section often presents a non circular section,so the analytical method in use process need a certain transformation.In order to overcome the above assumptions,to make the stability analysis of the tunnel closer to the engineering practice,a quantitative evaluation method based on non analytical model for tunnel structural stability is established by means of numerical simulation method.The main contents are as follows:first,the boundary condition,displacement condition and stress condition of the numerical calculation model are determined according to the stress response principle of surrounding rock during excavation of underground engineering and the current research results of in-situ stress calculation methods;second,the three-dimensional numerical model is established,and the GRC and LDP of the tunnel surrounding rock are solved;third,the initial installation position based on the different construction technology is determined according to the LDP figure,and the convergence-confinement figure of tunnel is established;fourth,the stability evaluation method of support structure and surrounding rock are established according to the mechanical response mechanism between GRC and SCC;fifth,the safety of Luoyixi tunnel case is analyzed,and the operation process is shown in detail.Finally,the assumed conditions of the laboratory model study is less compared with the theoretical analysis method and numerical simulation method,the intuitive experimental data and observation results can be obtained based on the laboratory model study,and results of theoretical analysis and numerical simulation also can be compared and verified.Therefore,it is necessary for the tunnel engineering to be scaled according to the similarity principle,and the study of mechanical properties of model is carried out in the laboratory.The main contents are as follows:first,the cement,fly ash,gypsum,etc.are selected as the raw material,and the mixture ratio design of similar materials based on the physical and mechanical properties of actual rock mass is carried out according to the similarity principle;second,loading test system is designed according to the required surrounding rock level and laboratory site conditions,the system consists of load sharing system,load transf-er system and press control system;third,according to the characteristic that the tunnel model is smaller,in the loading process,the maximum radial displacement of excavated tunnel is difficult to obtain,therefore,the special displacement recording device for tunnel is designed;fourth,the relevant test plan and operation process are established according to the expected test results and data test requirements,and the test model is built and the test study is carried out. |
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