Study On Construction Disturbance Characteristics And Pre-reinforcement Technology Of Shield Underpass Existing Section Tunnel

In recent years,with the rapid development of urban rail transit in China,gradually formed a staggered underground transportation network.At the same time facing many complex new construction problems.In order to reduce the risk of new tunnel underpenetration,this paper relies on a Xi’an subway shield section underpass existing metro section tunnel project.Through the combination of model test and finite element analysis to study the disturbance characteristics of shield underpass construction on the existing tunnel structure and surrounding strata,and explore reasonable and effective pre-reinforcement technology for close construction.The research results are as follows:1.The shield machine test device is designed according to the similarity principle,and the existing tunnel lining structure strain monitoring system,surrounding rock stress monitoring system and surface settlement monitoring system are established.The model test results show that: The width of the surface settlement trough increased from 6.7D when the tunnel was first completed to 8.3D(D is the tunnel diameter).The maximum surface settlement was located between the middle line of the tunnel and the middle line of the existing tunnel,and the maximum settlement point gradually shifted to the side of the middle line of the existing tunnel with the increase of the net distance.The shield tunneling has a settlement response to the surface within 1D～1.5D in front of the face,and the strata at about 3D behind the face are gradually consolidated and stabilized.The surrounding rock at the bottom of the existing tunnel takes on the unloading action near the Central Line of new tunnel,and bears additional earth pressure about 1.4D away from the Central Line of the new tunnel.The variation range of the arch and arch bottom strain,longitudinal additional bending moment,circumferential additional bending moment and circumferential additional axial force of the existing tunnel decreases with the increase of the net distance and the decrease of the buried depth.2.According to the model test condition,ABAQUS software was used to establish a three-dimensional nonlinear finite element model.The numerical calculation results revealed that the underpass construction would cause stress release in the existing tunnel vault,left and right waist and surrounding rock at the bottom of the arch in the unloading zone above the arch of the new tunnel,and the stress release in the surrounding rock at the arch bottom was the largest.This results in the vertical deflection of the existing tunnel structure along the longitudinal direction,and the vertical deformation curves of the tunnel lining are "U" shape and "W" shape,respectively.When the face of the tunnel is located in the range of about 1D before and 3D after the underpass,the existing tunnel structure at the overlapping position is the most severely affected by the underpass construction.The shape of the lining ring of the existing tunnel in the overlapping section gradually "ellipses",and torsion deformation occurs with the process of tunneling construction,showing obvious characteristics of aging deformation.3.By comparing the analysis results of surface settlement and internal forces of existing tunnel structures under model tests and numerical simulations,it can be seen that there are certain differences in data between the two research methods,but the laws reflected by each method are consistent.Aiming at the most unfavorable undercrossing construction conditions with a clear distance of 2.2m and a buried depth of 12 m,by comparing the reinforcement effects of five types of stratum pre grouting reinforcement ranges,it is clear that the longitudinal grouting reinforcement area of the new tunnel should be expanded by 1.5D along the left and right arch waists of the existing tunnel,to achieve good grouting reinforcement effects.At this time,the maximum vertical deformation of the arch bottom lining of the existing tunnel can be reduced from 27.25 mm when not reinforced to 8.88 mm,Comply with the control standards for deformation of existing structures in the specifications.