Research On Safe Construction Technology For Shallow Buried Tunnel Section Of Loose Deposit

The thesis takes the Zhengpantai Tunnel of the Chongli Railway as the engineering background.The tunnel has a thick layer of loose gravel soil and poor gradation.The tunnel area is controlled by the strong Mongolian high air pressure in winter,and it is long and cold.After the snow and ice melt in spring,the accumulation body is very easy to collapse and become unstable.In this paper,the physical and mechanical properties of the loose deposit at the entrance of the Zhengpantai tunnel are first tested,and the basic mechanical properties of the loose deposit and the influence of water content are grasped.According to the topography and environment of the tunnel entrance section,taking the reach of the plastic wall of the surrounding rock as the boundary between open and dark excavation,the construction method selection of the tunnel section was studied,and the open excavation method,pile arch method and shallow buried excavation method were selected.The open cut section is constructed by sloping and open digging,and the side slope slope is determined by the strength reduction method.The pile arch method calculated and analyzed the structural stress and deformation characteristics of the slope and pile arch construction process.The maximum horizontal convergence of the cast-in-place pile is at the position of the pile depth of 2m,the maximum horizontal convergence value is 8.58 mm,the maximum arch crown settlement and The bottom bulges were 4.43 mm and 8.45 mm,respectively.At the same time,the construction process of the pile-arch method is introduced,and the advantages and disadvantages of the pile-arch method are analyzed.In the shallow-buried-concealed construction section,three-dimensional finite element analysis was used to analyze the stability of the surrounding rock during the three-step construction method.The most dangerous position of the tunnel is the shallowest buried area,that is,the buried depth is 4m.The maximum surface settlement,vault settlement,bottom uplift,and middle step horizontal convergence at this location are 39.38 mm,22.55 mm,16.08 mm,and 9.06 mm,respectively,while the corresponding field measured data are 45.82 mm,23.80 mm,17.22 mm,9.51 mm,The results are close.The research in this paper further improves the construction technology of loose accumulation tunnels.The research results provide important technical support for the construction of loose accumulations and also provide references for similar projects.