Study And Application Of Space Effect Of Shallow Buried Tunnel With Large Section

With the acceleration of urbanization in China,China’s traffic construction is developing towards a more efficient traffic network,and more and more large section tunnel projects have been built.At present,the tunnel with large cross-section is mostly excavated by sections.The large cross-section is transformed into a small cross-section,and the whole cross-section is followed by two lining.However,in the actual construction,more consideration is given to the construction excavation method and support structure,and only the second lining is required to follow closely,and the specific requirements of the second lining follow-up distance are not clear.Therefore,it is of great practical significance for the design and construction of actual engineering projects to study the influence of space effect under different excavation methods and the follow-up distance of secondary lining.In this paper,the influencing factors of space effect are analyzed,and a method to evaluate space effect is proposed based on the proximity degree of stress deformation or failure at the same location.Relying on Yubei District fire phoenix mountain no.2 tunnel project in chongqing,with its bifurcation of the left line of large span section as the research object,using FLAC^3D finite difference software tunnel three-dimensional numerical model is established,combined with the actual monitoring data,by the methods of CD double side wall and back wall arch first three different construction ways of shallow buried large section tunnel excavation,the large span period of ground surface subsidence The deformation and stress redistribution of surrounding rock are calculated,and the influence of the subsequent change of secondary lining on the deformation and stress redistribution of surrounding rock is analyzed.The stress distribution and surface deformation characteristics of tunnel under different excavation methods are studied,and the influence of space effect and the significance and function of secondary lining follow-up are analyzed and evaluated.The advantages and disadvantages of the three excavation methods are compared,and some useful construction suggestions are put forward.The main work and conclusions of this paper are as follows:（1）This paper summarizes the mechanical characteristics of the excavation method and the deformation characteristics of surrounding rock and its influencing factors,analyzes the influencing factors of the space effect,and puts forward a method to evaluate the space effect based on the proximity degree of stress deformation or failure at the same position.（2）In order to make the calculation accord with the field practice,the three-dimensional numerical model of the similar strata structure in the long span section was established,and the elastic modulus of the deformation parameters of the surrounding rock in the long span section was calculated by the displacement back analysis method,and the calculated parameters of the elastic modulus of the long span section were obtained by comparing with the actual monitoring data.（3）The three-dimensional numerical models under two different construction methods are established.The FLAC^3D finite difference software is used to carry out the dynamic numerical simulation of the whole process of the three-dimensional model construction.Combined with the actual monitoring data,the influence of the construction method（excavation span）and the secondary lining following on the space effect of the shallow buried tunnel with large section is studied.The study shows that the deformation and stress of the surrounding rock can be obviously reduced in the long span construction process by using the double-wall guide method and the two-liner following scheme.The follow-up of secondary lining and the reduction of primary excavation span can obviously improve the constraint effect of space effect.（4）Based on the numerical results and the failure proximity analysis,the application of space effect in the long span construction with arch first and back wall method is studied.The study shows that the method of arch before wall can make full use of the space effect,and the deformation and stress of surrounding rock can be effectively controlled.