Influence Of Deep Foundation Pit Excavation On Subway Stations And Optimization Of Support Parameters

In order to achieve efficient utilization and long-term planning of urban land resources,the number of horizontal and vertical expansion projects for urban subway stations is gradually increasing.When the subway station is fully unloaded,the deep foundation pit’s partial enclosure structure is located on the top arch of the underground subway station,which cannot provide effective support for the foundation pit structure and increases the difficulty of deformation control of the underground subway station.At present,there is relatively little research on the vertical increase of subway stations at home and abroad,so it is urgent to study the mechanical response law between the underground subway station and the foundation pit.Based on this,this article relies on the open and concealed excavation and construction project of Pingguoyuan Station on Beijing Metro Line 6,and combines case statistics,theoretical analysis,monitoring data,and finite element simulation to conduct simulation research on the fully unloaded underground subway station.The main research content of this article is as follows:（1）Analyze the deformation mechanism of existing underground structure foundation pit construction,summarize and summarize 25 similar projects in China using theoretical and case statistics methods.In the above projects,over 84%of the maximum deformation of underground structures can be stabilized within a control value of 20mm through control measures,clarifying the possibility of unloading and adding layers for shallow buried and concealed excavation of large cross-section subway stations;Subsequently,the unloading area S_effective and the shape coefficient of the foundation pit were introducedαBy fitting and analyzing the main factors affecting the deformation of the underlying structure caused by foundation pit construction,the inherent law of unloading strain of the underlying structure under the geological conditions of silt,sand,and mud was obtained.（2）A three-dimensional numerical model of"subway station deep foundation pit"was established using MIDAS GTS NX finite element software based on the key and difficult points of the project and the actual construction situation.The entire construction process of the underground subway station deep foundation pit was simulated,revealing the corresponding mechanical laws between the subway station,support structure,and strata during construction.Through the analysis of monitoring schemes and data in practical engineering,the deformation patterns between subway stations,support structures,and strata during the construction of deep foundation pits in underground subway stations were obtained.The numerical simulation results were verified,and the maximum deformation of the station basically conforms to the deformation pattern of uplift in sandy clay areas.（3）Based on the above research,combined with common deformation control measures in similar projects,the support parameters of the deep foundation pit of the underground subway station were optimized.A three-dimensional analysis model was established for different internal support channels,enclosure structure thickness,and grouting area.The influence of different support parameters on the stress and strain of the underground subway station and enclosure structure was obtained.After comprehensive comparison of its improvement effect,the optimal solution was determined to be the three internal supports;It is recommended to use a manual excavation pile layout plan with an equivalent thickness of0.6～0.8m;Moreover,the roof grouting has a relatively strong inhibitory effect on station uplift,and the grouting located on both sides of the station has a better control effect on ground deformation.