| Drilling and blasting method is the main construction method of mountain tunnels at present,and the commonly used lining form is composite lining.Due to the construction technology problems,composite lining often has problems such as secondary lining cracking,insufficient thickness,Void behind secondary lining,falling blocks,which affect the safe operation of railway tunnels.Prefabricated assembled lining has the advantages of high construction efficiency,pipeline production of prefabricated blocks,and easy quality assurance,which can well solve the problem of lining quality in the construction process of railway tunnels.This thesis studies the mechanical behavior of the“step”joint composed of the arch precast block and the formwork side wall.Based on a tunnel project.The mechanical model of ’step’ joint is established on the basis of elastic-plastic mechanics.The analytical process of joint stiffness is derived by using the model,and the parameter sensitivity analysis is carried out.Combined with field monitoring data,the proportion of surrounding rock pressure shared by secondary lining is analyzed.The mechanical properties of ’step’ joints were studied by numerical simulation.In addition,according to the minimum bending moment method,active adjustment method and the minimum displacement method proposed in this thesis,the precast block scheme of railway tunnel arch is studied.The research contents and conclusions are as follows:(1)The mechanical model of ’step’ joint based on arch prefabrication was established,and the analytical method of the joint stiffness was analyzed.According to the analytical method,the sensitivity of joint parameters was analyzed,and it was concluded that with the increase of the thickness of precast block,the stiffness value of step joint was significantly increased,and the stiffness of step joint was highly sensitive to the thickness of precast block.The initial stiffness of step joint is sensitive to the change of friction coefficient,but in general,its sensitivity is less than that affected by the change of precast block thickness.Considering that the friction coefficient value is greatly affected by the construction and environment,and it is difficult to control in practical application,from the perspective of safety,the friction effect of the joint surface can not be considered in the design work.(2)A three-dimensional finite element model of the ’step’ joint was established to study the mechanical properties of the precast block joint of the arch,and the mechanical properties of the joint under different load combinations were obtained.The analysis results show that the values of the maximum opening,vertical displacement and rotation angle of the ’step’ joint increase with the increase of bending moment,and the corresponding relationship curves can be roughly divided into two parts:the linear section and the curve section,corresponding to the elastic stage and plastic stage of the concrete joint.The bending stiffness curve of the ’step’ type joint can be mainly divided into two sections.When it is in the elastic stage,the bending stiffness of the joint is almost constant.When it enters the plastic stage,the bearing capacity of the ’step’ type joint is rapidly weakened,and the stiffness decreases rapidly.In addition,the stiffness of the joint in the elastic stage has little relationship with the axial force,but the continuous interval of the elastic stage is positively correlated with the axial force.(3)According to the on-site monitoring results of surrounding rock pressure during the construction of the relying tunnel project,and based on the relationship between the contact pressure between the circular composite lining layers,the on-site measured contact pressure between the lining structure layers is analyzed.Referring to the recommended value of the domestic highway tunnel specification,the proportion of surrounding rock pressure shared by the secondary lining of the relying tunnel project is 33%.Through the axial force and bending moment values measured by field monitoring data,the monitoring data are applied to the ’load-structure’ model,and compared with the field measured data to verify the applicability of the stiffness parameters obtained by the analytical method.(4)According to the sharing ratio of the surrounding rock pressure of the secondary lining obtained by the geological conditions of the tunnel and the field monitoring data in this thesis,the internal forces of the secondary lining under the conditions of grade Ⅲ,gradeⅣ and grade Ⅴ surrounding rock are calculated by using the ’load-structure’ finite element model.The location selection of the ’step’ type joints is studied by using the minimum bending moment method,the active adjustment method and the minimum displacement method.Based on the application of the ’step’ type joints,six block modes are given,and the scheme comparison and analysis are carried out. |
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