Model Test Study On The Excavation Effect Of The Branched Loess Tunnel

With the comprehensive development of underground space,a growing number of tunnel construction project appeared,such as urban subway,underground pipe gallery and underground street.The current research on complex tunnel are mainly concentrated in parallel,crossing and branched type of tunnel,research on the crossing section of branched loess tunnel is less,it is mainly reflected in: The design and construction of the crossing tunnel is mainly based on the design and construction of the common tunnel,and there is not a complete set of theories and methods for the characteristics of the branched loess tunnel.Therefore,it is necessary to study the stress and deformation law of the surrounding rock and the initial support steel arch during the excavation of the branched loess tunnel.According to the excavation effect of the tunnel intersection,the thesis studying the change rule of surrounding rock pressure and steel arch strain variation characteristics in model test,and the displacement law of surrounding rock and lining stress distribution characteristics in numerical simulation.Main research contents and results are as follows:(1)The thesis summarizes the research achievements of some domestic and foreign scholars in recent years about complex tunnel type,sucn as parallel tunnel,interchange tunnel,small clear distance tunnel and branched tunnel.A large scale model test system with three-dimensional is designed and manufactured.Under the condition of initial support steel arch,a simulation experiment on crossing section of the branched loess tunnel is conducted to study the stress and deformation laws of surrounding rock and steel arch during the excavation.(2)The stress monitoring data of the surrounding rock obtained by the experiment are reorganized.Changes of surrounding rock stress at different distances from intersection are compared and analyzed in the course of the excavation of the main tunnel and the branched tunnel.The interaction between the excavation of the main tunnel and the branched tunnel is compared.The theoretical stress release rate of the surrounding rock after the end of the excavation is analyzed.The result of the test analysis shows that the influence distance of tunnel face on surrounding rock in the range of 1.75 ~ 2.25D(D as the tunnel span)tunnel face.The influence range of the main tunnel excavation on the deep stress of the surrounding rock is about 1.25 D.The surrounding rock of the main tunnel is affected when the excavation face of branched tunnel is within 1.5D of the intersection.The theoretical stress release rate of the surrounding rock near the intersection of the tunnel is the highest.(3)The basic principle and characteristics of the finite difference program FLAC3 D are expounded.The three-dimensional numerical model of the cross tunnel is established and the construction process is simulated.The deformation law of the surrounding rock in the construction process is analyzed.After the crossing tunnel is through,the settlement pit is produced on the surface above the intersection,and the shape of the settlement pit is irregular.The total settlement of the post excavation section is greater than that of the first excavation section.The displacement direction of the surrounding rock in the tunnel is all toward the direction of the cave.The total settlement value of the vault is maximum in the same section,and the horizontal convergence value of the vertical wall is the highest at the height of 1/4 wall from the top of the vertical wall.In order to reduce the displacement of surrounding rock near the intersection,the excavation step distance can be shortened and timely support.(4)Compared with the results of comparison test and numerical simulation,strain variation characteristics of steel arch and lining stress distribution law is analyzed: The deformation direction of vault,arch shoulder,wall waist and bottom of the steel arch are all to the tunnel.The steel arch frame at the intersection is more severely deformed,and the total strain changes of the vault and bottom are larger in the same section.When there is no invert,the stress concentration of the lining at the bottom of the wall and the bottom of the intersection is the highest,and the tensile stress and compressive stress at the bottom of the lining are striped and interlaced.The stress state is very poor,and the structural damage is the most easy to occur.