Influence Analysis And Control Method Of Adjacent Underground Construction On Nanjing City Wall

With the continuous development of China’s economy and the intensification of urbanization,the demand for urban transport capacity is increasing.On the other hand,due to the decrease of the total land resources,traffic congestion is becoming more and more significant.In order to alleviate traffic pressure,subway is increasingly becoming a hot spot in the development of major cities.As one of the most intact ancient city walls in China and even in the world,Nanjing City Wall has been eroded by wind and rain for more than 600 years.The construction of subway tunnels in Nanjing for shield construction is an important issue for the influence and protection of city walls.Based on the geological conditions in Nanjing,this paper analyzes and studies the shield construction under the city wall through numerical simulation method.The main work and achievements are as follows :(1)Analysis of influence mechanism of shield construction on existing buildings.This paper introduces the research status and mechanism of surface subsidence caused by shield construction,and focuses on the damage categories of adjacent buildings.(2)Numerical model establishment and calculation method validation.The threedimensional finite element model of soil-tunnel-city wall was established by using the finite element software midas GTS.The finite element software FLAC3 D was used to simulate and analyze the shield tunneling construction process.The rationality of the finite element modeling method was verified by comparing the numerical simulation results with the theoretical results of the empirical formula and the measured data.(3)Analysis of influencing factors of shield construction.The influence of wall stiffness,tunnel depth,tunnel spacing,excavation diameter and other factors on ground settlement,wall deformation and stress during adjacent shield construction are simulated and analyzed.The wall stiffness has a significant constraint on settlement and deformation.The tunnel depth and spacing are negatively correlated with deformation and stress,and the excavation diameter is positively correlated with deformation and stress.It is suggested that the burial depth should be controlled in the silty clay layer between 1.5D and 3D,the spacing should not be less than 2D,and the shield diameter D should not be more than 6.2m.(4)Suggestions for foundation deformation control.By studying the variation law of stress in various parts of the urban wall with the increase of surface settlement,it is concluded that the control of foundation settlement within 20 mm can be used as the basic standard for the control of urban wall influence.(5)Shield construction evaluation method and application.This paper expounds the process of shield construction evaluation of urban wall section,and applies it to Yijiangmen section of Nanjing urban wall.The most unfavorable shield construction parameters calculated are used to model and analyze it,and it is concluded that there is no significant effect on the overall structural safety.According to the deformation and stress,the influence degree of urban wall is divided into zones,in which the wall foot section and wall column section are stress concentration areas,which will have certain adverse effects on the long-term safety of urban wall,and the construction control method is proposed.