The Research Of Surface Subsidence Law And Construction Sequence Of Key Structure By The PBA Method In High Groundwater Level Area

Since the beginning of the new century,the amounts of urban population and private cars have made the transportation system of the city more and more overwhelmed.So,the development of urban rail traffic has become one of the main means to relieve the traffic pressure and facilitate people to travel.But,subway stations are constructed in the central area of the city,and the ground subsidence caused by the construction pose a great threat to the safety and normal use of the surrounding buildings and pipelines under the ground.Based on the project of Wangfujing North subway station,this paper analyzes the ground subsidence law and the construction sequence of the key parts in the construction of the PBA method with separated holes of dewatering well.The results of the study are as follows:(1)The ground cumulative settlement caused by the construction of entering hole stage accounts for more than 70% of the final settlement of the station construction.The curve of surface subsidence presents a “uni-modal” form in the construction stages except the entering tunnels of dewatering well,which has the curve of double peaks.While,the lateral effective affected area is about 1.3 times of the main structure width.(2)The optimal construction sequence of the PBA method with separated tunnels of dewatering well is as follows: firstly the upper holes,secondly the tunnels of dewatering well,finally the lower holes.(3)For the station construction with double-layer and eight holes,the "fourhole excavation simultaneously" is more conducive to control the ground subsidence than the "two-hole excavation simultaneously",which should observe a principle of “staggered excavation,edge-central order,shift for a distance”.(4)For the subway stations with three spans of supporting arch,if the width of the side span larger than the middle span,the supporting arch construction sequence of ‘the middle arch first' is beneficial to reduce the lateral displacement of the middle columns,however,the construction of the side arch needs to be constructed simultaneously.On the other hand,the construction of ‘the side arch first' does not require the synchronous construction of the side supporting arch,which can reduce the difficulty of construction coordination,but measures should be taken to control the lateral displacement of the middle columns.