| With the rapid development of China’s economy,the construction of transportation infrastructure has also entered the stage of rapid growth,especially the urban underground engineering.In the loess area,there are few studies on effect of soil excavation of civil air-defense engineeringon underling metro tunnel,and the construction of the upper foundation pit still has a certain degree of blindness.Therefore,it is of great engineering significance to reasonably predict the deformation characteristics of the underlying shield tunnel under different excavation methods,and to adopt reasonable and effective preventive and protective measures.Based on the background of a super large civil air defense foundation pit in Xi’an Loess Area,this paper analyzes the impact of its construction on the shield tunnel of the underlying subway line 4,optimizes the support and earthwork plan,and mainly studies the following aspects:1.Summarize the current status of research on the impact of foundation pit excavation unloading on the underlying tunnel structure at home and abroad,mainly through numerical analysis research,laboratory test research status,field monitoring research status,and construction technology research status.2.Analyze the mechanism of unloading deformation of the foundation pit,expound the main factors affecting the deformation of the tunnel,and summarize the formula for the uplift of the foundation pit.The program of the finite element software Plaxis and the Hardening-Soil model of soil are introduced.3.Analyze the three projects of similar strata in the loess area,collect the deformation data of the subway tunnel during its excavation and construction,use the Plaxis software,input the soil hardening model of different soil layers,and get the subway tunnel deformation calculated by the software.Comparing the fitted monitoring data with the values calculated by the Plaxis software,the parameters of the soil hardening model for a typical loess layer are given.4.The relative relationship between the super large civil air defense project in Xi’an loess area and the structure of Metro Line 4 is introduced,and the project risk points of this project are summarized.According to the "Technical Specification for Safety Protection of Urban Rail Transit Structure",the impact of the construction of this project on the subway is given Deformation index,and confirmed the protection measures of bench pile + earthwork block excavation in this project.5.Through the three-dimensional numerical simulation of the Plaxis software,the influence of the different excavation methods of the foundation pit on the underlying subway tunnel is analyzed.A reasonable construction plan was determined,that is,first excavate 3m of soil at one time and then excavate the remaining soil in blocks.Suggestions are given for the monitoring plan of this project,and finally the reasonableness and accuracy of the excavation method are verified through the monitoring data of the experimental section. |
