Study On The Structure And Supporting Measures Of Shallow Tunnel Under Unsymmetrical Pressure Under Complex Urban Conditions

The entrance and exit section of urban tunnel is often affected by factors such as shallow burial bias,dense surrounding buildings and complex geological conditions,which makes it difficult and risky to build.Taking the south main line entrance section of jialing river bridge tunnel project of zengjiayan in chongqing city as the research object,this paper proposes reasonable tunnel structure type and support measures by combining theoretical calculation,numerical simulation and site monitoring,which can provide reference or experience for similar projects.The main contents and conclusions of this paper are as follows:1.depth of buried tunnel defined systematically,and the scope of application,and analyzes the structure section of shallow bias multi-arch tunnel type,put forward the asymmetric arch span and shape with the rectangle multi-arch tunnel profile for half bright and half dark vary traffic bias tunnel type,provide the basis for the following selection and extension of research on asymmetric structure.Based on the theoretical derivation and mechanical analysis of the biased tunnel,the influence of surface inclination,poisson’s ratio and excavation distance on the stress distribution of surrounding rock is obtained.The common disasters of shallow buried bias tunnel and the function of supporting measures are analyzed in detail.2.Based on the structural analysis of excavated road cutting and asymmetric arch and double arch tunnel,through the analysis and co MParison of co MPrehensive factors such as geological conditions,safety,economy and environmental protection,it is concluded that the excavated road cutting will greatly damage the surrounding environment and adversely affect the later safe operation.On this basis,MIDAS/GTS NX was used to establish a two-dimensional asymmetric arched multi-arch tunnel model and analyze its structural mechanical properties.It was concluded that stress concentration was prone to occur in the wall foot,arch foot and invert,and the horizontal displacement of anti-slide pile always increased during backfilling excavation,especially in the excavation of the left hole.By simulating and calculating the deformation law of surrounding rock under different load release coefficients,it shows that the earlier the support is,the larger the stress of lining is,and the smaller the stress of surrounding rock is.Therefore,reasonable support time and parameters can be selected to control the deformation of the weak part of the tunnel.The safety of the shallow-buried bias asymmetric arch five-lane double-arch tunnel structure is verified.3.Study the load distribution and theoretical calculation of surrounding rock pressure in shallow buried bias rectangular and arched multi-arch tunnels,establish a load-structure model,and check the safety coefficient and crack width of the whole structure.Under the same conditions,the comparison between asymmetric arch and rectangle arch tunnels(axial force,bending moment,safety factor and crack width)shows that the structure of asymmetric arch arch has better and safer forces,and the use of arch structure can improve the structural forces.However,the construction technology of the rectangular and arched multi-arch tunnels is easier and the section utilization rate is higher,which also meets the safety requirements of the specifications.Therefore,the structural type of the rectangular and arched five-lane multi-arch tunnels is chosen.The stress and deformation rule of this special tunnel are discussed preliminarily,and it is concluded that the stress concentration of the deep buried wall foot,the right end of the frame beam and the contact of anti-slide pile are affected by the bias pressure,so they are the weak parts in the design and construction,so the construction quality and safety must be guaranteed.4.In the field engineering,dynamic monitoring and measurement are carried out on the vault,slope,beam and anti-slide pile,and the surrounding rock displacement change rules are obtained,and the later development of the tunnel is predicted through functional regression analysis.The comparison between the calculated results and the measured values shows that the structure scheme of this project is reliable,and the type of asymmetric structure of shallow buried bias tunnel is expanded,which can provide theoretical basis and accumulate valuable experience for similar practical projects.