| With the rapid development of social economy and urbanization,the city population grows rapidly,most large cities in China have been entering the stage of rapid development of urban underground rail transportation.The deep foundation pit of subway station is a systematic project.Due to the complexity of the stratum and the diversity of the retaining structure and environment,it brings a lot of inconvenience to the excavation of the foundation pit of the subway station and increases the construction risk of the subway foundation pit.Based on this,the deformation and stability of the deep foundation pit retaining structure becomes one of the key aspects of metro construction.This has put forward higher requirements for the construction of metro stations,especially the retaining structure during the excavation of deep foundation pits of stations and the impact on the surrounding environment.Therefore,it is necessary to make targeted research to ensure the safety and stability of new metro stations during the construction process.This paper takes Sandaocun West Station,a subway station under construction in the East Extension Project of Changchun Railway Line 2,as the background to study the deformation law of itself and the surrounding environment during the excavation of deep foundation pit of the station and its control method.Firstly,using the numerical analysis method to establish the numerical model of deep foundation pit excavation,study the deformation of the retaining structure and the influence law on the surrounding environment in the process of deep foundation pit excavation.Secondly,by setting up monitoring points around the pit and key locations,analyzing the field monitoring data,further exploring the deformation law of the pit in the actual excavation.And compare and analyze with the monitoring data to verify the accuracy of the model.Finally,analyzing different influencing factors and control measures in the process of deep foundation pit excavation,and optimizing from the perspective of design parameters of foundation pit retaining structure,and proposing foundation pit deformation control methods.The main research contents and conclusions are as follows.(1)Based on the investigation of the engineering profile,hydrogeological conditions and retaining structure of the deep foundation pit of Sandaocun West Station of Changchun Metro Line 2 East Extension Project,a three-dimensional numerical calculation model of the foundation pit was established by using Midas GTS/NX to analyze the influence of the excavation on the retaining structure and the surrounding environment.The horizontal displacement of the retaining structure and the settlement of the surrounding ground surface increased continuously with the continuous excavation,the maximum points were in the middle of the standard section,23.61 mm and 6.18 mm respectively.the settlement of the gas pipeline around the pit was larger than that of the water pipeline,but none of them exceeded the control value,and the settlement deformation of the pipeline adjacent to the terminal section of the pit was smaller.The research results show that the deformation of the retaining structure,the surrounding surface and the settlement of the pipeline in the standard section of this pit is larger than that in the terminal section during excavation,and the deformation in the long side of the terminal section is larger than that in the short side.(2)According to the construction of the station pit,the site monitoring data were collected and analyzed,and it was concluded that during the excavation of the pit,the settlement curve of the surrounding ground surface showed a change pattern of increasing and then decreasing,and the maximum settlement value was at the position about 5～8m from the edge of the pit,and the ground surface beyond 13 m was gradually reduced by the excavation of the pit,and the soil beyond 20 m was less affected.The horizontal displacement of the central position of the foundation pit retaining structure is larger,and the two ends are smaller,in which the horizontal and vertical displacement of the pile top of the retaining structure at the standard section of the foundation pit is larger than that of the end well section.The excavation of the foundation pit has a greater impact on the gas pipeline which is closer to the foundation pit.In each excavation of the foundation pit soil and steel support erection,the pipeline is prone to deformation,and the settlement deformation of the gas pipeline in the adjacent standard section is relatively large at 5.46 mm.(3)Comparative analysis of envelope deformation,surface settlement and pipeline deformation verifies the accuracy of modeling.The deformation conditions of the horizontal displacement of the retaining structure and the surrounding environment during the actual construction are basically within the safe range,indicating that it is effective to use numerical simulation to guide the construction in advance before the construction.Although various unfavorable influencing factors in the actual construction will make the overall monitoring value slightly larger than the simulated value,the overall change trend of the monitoring and simulation results during the excavation of the foundation pit is similar,which can confirm the applicability of the model to this foundation pit project.(4)Based on the numerical model of foundation excavation,the influence law of design parameters of enclosure structure and steel support on foundation deformation is analyzed.This research shows that increasing the steel support rigidity to 1.2 times can effectively reduce the surface settlement by 14.2%.In addition,when the thickness of the retaining structure increases(equivalent to increasing the pile diameter or reducing the pile spacing),the effect of foundation pit deformation control is not obvious,but increasing its elastic modulus to 1.2 times can reduce the surface settlement value and horizontal displacement of the retaining structure by14.72% and 28.32%,respectively.Therefore,the stability of foundation pit excavation can be improved by increasing the reinforcement ratio and concrete grade of the enclosure structure. |
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