Study On The Interaction Between Tunnel Support And Surrounding Rock In Red Sandstone Strata

The Cretaceous red sandstone is a typical stratum in the Yangcheng tunnel site of Haoji Railway,which has dilatancy and softening characteristics under unloading.However,the existing elastoplastic theory of tunnel surrounding rock is still not perfect in describing its dilatancy and softening properties,which leads to the difficulty in solving relevant analytical solutions to accurately reflect the deformation of surrounding rock in actual engineering.In addition,in the process of tunnel construction,there are some problems,such as shotcrete cracking,bending of steel arch,etc.,which put forward higher requirements for the flexibility of support structure system.It is a good idea to adopt the multi-stage deformation control based on the"flexible support control concept",but there is no theoretical method of support design that utilizes the characteristics of multi-stage application.In order to solve the above key problems,based on the red sandstone section of Yangcheng tunnel,this paper carries out in-depth and systematic research from three aspects:elasto-plastic analysis of tunnel surrounding rock,analysis of the whole construction process of primary support and surrounding rock interaction considering the characteristics of multi-stage construction,and analysis of the rationality of design parameters of flexible support.The main conclusions are as follows:（1）By matching the D-P criterion with the improved unified strength criterion,the expression of the modified strength criterion and the calculation formula of the middle principal stress are established,which can reasonably consider the dilatancy effect of rock mass;Based on the modified strength criterion and nonlinear attenuation model of strength parameters,combined with the elastic-plastic theory,a new elastic-plastic analytical formula for surrounding rock is obtained which can reflect the mechanical properties of red sandstone strata;Compared with the new type,the traditional type underestimates the strength condition of surrounding rock,which easily leads to conservative support design;The distribution law of the surrounding rock principal stress along the diameter depth is significantly affected by the influence factor of the middle principal stress b,and the calculated value ofσ_z is significantly different from that of（σ_θ+σ_r）2,which distorts the relevant hypothesis in the study;The surrounding rock characteristic curves of tunnel in red sandstone strata under different b can be well fitted by the function ofy=Ae^-x/t+y ₀.（2）The supporting characteristic equation considering the multi-stage operation is put forward,including the bearing stage of steel arch alone,the bearing stage of steel arch and shotcrete mixture together;Six basic combination states of surrounding rock and primary support are defined from the two aspects of surrounding rock elastoplastic and support application in different stages;Combined with the elastic-plastic theory,the interaction mechanical analytical model of tunnel primary support and surrounding rock based on six basic combination states was established,and the corresponding numerical solution process was programmed by MATLAB;Through the analysis of the example,it is found that the initial position of steel arch and shotcrete as well as the supporting stiffness have obvious influence on the deformation control effect of surrounding rock and the stress of supporting structure.Compared with the traditional support design with excavation and support,the support design with the characteristics of multi-stage application can effectively enhance the ultimate bearing capacity of the support system,improve its stress safety state,and provide the possibility to reduce the thickness of spray mixing and save the project cost.（3）Based on the process of determining the parameters of support design by stages,and considering the deformation control of surrounding rock and the safety of support stress,the order of sensitivity of support design parameters is determined as follows:Length of unsupported section n≈Separate bearing section length of steel arch frame d>Stiffness of steel arch k_{se t}>Shotcrete stiffness k_shot;The reasonable value range of each parameter is as follows:0～1.33m（n）、0～0.6m（d）、30～50MPa/m(k_{se t})、170～230MPa/m(k_shot).The above conclusions can be used to guide the surrounding rock deformation prediction and the support design using the characteristics of multi-stage application of the combined support tunnel in red sandstone strata.It also has reference significance for other strata with strain softening property.