Research On Disturbance Influence And Deformation Control Technology Of Large Span Double Arch Tunnel Undercrossing Existing Rail Transit

In recent years,with the increasing development and utilization of urban underground space,it is inevitable to encounter new challenges in the construction of dense rail transit lines in the process of tunnel construction,and the disturbance of new tunnel excavation to adjacent rail transit has also become an important hidden danger affecting the safety of tunnel structure and operation stability.The excavation span of multi-arch tunnel is often large,the construction process is complicated,and the surrounding rock disturbance is frequent in the construction process.Especially when the multi-arch tunnel passes through the existing rail transit construction,the disturbance mechanism is complicated,the deformation control is more difficult,and there are large construction safety risks.In view of this,based on the U-shaped channel section of Xi ’an Metro Line 5 under Feng River East Road underpass project,this paper takes the large-span double-arch tunnel as the research object,and studies the disturbance influence and deformation control technology of the existing rail transit construction under the large-span double-arch tunnel by means of field monitoring,model test and numerical simulation.The main research contents and conclusions are as follows:(1)Combined with the field monitoring data and the finite element numerical simulation results,the deformation characteristics of the U-shaped slot structure of the subway under the double-arch tunnel are obtained.The results show that the ground settlement of the double-arch tunnel basically experiences a process of slow increase,sharp increase and gradual stability.The transverse surface settlement curve is symmetrical with respect to the center line of the tunnel after the middle guide tunnel is pierced,and presents a "V-shaped" distribution.After the main tunnel is excavated,the maximum surface settlement value migrates to a certain extent toward the direction of excavation,and is ultimately presented as a "W-shaped" distribution due to the superposition effect.The transverse height difference of the tracks is less than 4mm,and the rail spacing is controlled within the range of-4mm to +6mm,which ensures the safety of the subway operation.(2)A similar model test was carried out to obtain the mechanical characteristics of surrounding rock at different excavation stages in a long-span double-arch tunnel through the existing subway U-shaped groove.The ground disturbance law was explored based on the change curve of the cumulative settlement of the ground surface during the construction of the double-arch tunnel.The model test results show that the vertical stress variation of the arch presents three stages of "stress aggregation,stress release and stable state",and the pressure arch will form in a certain range to bear the upper load.The influence of excavation disturbance on the side wall is greater than that on the vault,and the vertical stress concentration and horizontal stress release are mainly in the range of 0.3 ～ 0.5 times the excavation span of a single hole.The formation disturbance mainly occurs in the excavation stage of the pilot tunnel and the main tunnel,and the formation disturbance deformation of each tunnel excavation has a superimposed effect.The formation disturbance deformation is small in the stage of breaking down the middle door and removing the temporary support,and a small part of the formation disturbance deformation is still generated after the tunnel is pierced due to the soil consolidation.(3)Through the establishment of three-dimensional finite element model under different working conditions,the deformation control technology of long-span double-arch tunnel under the existing rail transit is studied.The influence of important factors such as excavation method,excavation error distance and pre-reinforcement form on the stability of tunnel excavation is discussed.The construction scheme is optimized based on the evaluation of ground settlement,tunnel arch settlement,peripheral convergence,track plate and middle partition wall deformation.After comparison and selection,the tunnel construction scheme adopted is as follows: the tunnel is pre-strengthened by the combination of pipe curtain and rotary jet pile,the main tunnel is separated by 30 m and the double-sidewall heading method is used for excavation.