| With the great development of underground space in China,there are more and more engineering cases of subway tunnel crossing the existing foundation pit due to the limitation of underground space.Shield tunneling is bound to affect the displacement field and stress field of the surrounding soil layer.In addition,the complex load environment around the project is easy to cause diseases such as enclosure structure cracking and surface subsidence during construction,So as to affect the stability and safety of foundation pit.The branch line of Shenzhen Metro Line 6 crosses the deep foundation pit of the reconstruction project of gongchang road in Shenzhen,and the minimum clear distance between the shield and the foundation pit is only 4.65m.Taking this project as the research background,this paper studies the influence law of shield side crossing on the horizontal displacement,bending moment,support axial force and surrounding surface settlement of the foundation pit retaining structure through theoretical research,numerical simulation and automatic monitoring.The main contents and conclusions are as follows:(1)This paper summarizes the characteristics of deep foundation pit engineering near subway,and introduces the deformation mechanism and failure mode of foundation pit under the influence of shield tunneling.Aiming at the additional stress of foundation pit retaining structure caused by shield tunneling,based on Mindlin theory,starting from the tunnel face thrust and shield shell friction,the calculation formula of additional stress at any point in soil under the influence of shield tunneling is deduced.(2)This paper introduces the application scheme of automatic monitoring technology in long and deep foundation pit engineering,and completes the deep foundation pit monitoring of the reconstruction project of the side crossing of Shenzhen gongchang road of Shenzhen Metro Line 6.Through the comparative analysis of numerical simulation results and automatic monitoring results,the effect of shield side penetration on the stress and deformation of foundation pit support structure and the change of surrounding environment is studied.The results show that shield tunneling has a significant impact on the displacement,bending moment,surface settlement and the axial force of the third steel support of the foundation pit near the shield side.Among them,the impact on the retaining pile is mainly reflected in the 7～19m depth of the pile;By setting the bottom plate of foundation pit,the influence of shield on foundation pit can be weakened.At the same time,the deformation calculation formula of the surrounding foundation pit retaining structure under the influence of shield tunneling is derived.(3)Based on the parameter sensitivity analysis of the horizontal distance between shield and foundation pit,the influence of the change of horizontal clear distance on the deformation and stress of foundation pit support structure is studied.The results show that when the horizontal clear distance is only reduced from 8m to 6m,it has a significant impact on the horizontal displacement,bending moment and surrounding surface settlement of the foundation pit retaining pile,and its effect on the deformation and stress of the foundation pit retaining structure on the side adjacent to the shield is more obvious than that on the side away from the shield;The axial force of the third steel support is more sensitive to the change of horizontal clearance than that of the first concrete support and the second steel support.By studying some problems involved in the shield side crossing foundation pit project,this paper obtains the influence law of shield tunneling on the stress and deformation of the supporting structure of the adjacent deep foundation pit,which has certain guiding significance for similar projects in Shenzhen in the future. |
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