Study On The Influence Of Deep Foundation Pit Excavation On Subway Station With Common Diaphragm Walls

After decades of subway construction,the subway network of large cities in China has seen its initial scale.It is inevitable that the foundation pit projects near the subway tunnel will appear in the city.At present,the number of cases where foundation pits share underground diaphragms with subways is gradually increasing.Excavation of the foundation pit will cause uneven settlement or uplift deformation of the existing subway,which affects the safety of the subway.When the supporting structure of the foundation pit directly acts on the share structure of diaphragm walls,the deformation mechanism of the subway station is more complicated.At present stage,the researcher mainly focus on the study of the deformation of the adjacent shield subway tunnel due to the excavation of the foundation pit,and there are few studies on the deformation characteristics of the diaphragm walls shared by the foundation pit and the subway station.Therefore,this article mainly summarizes the relevant theories about the surrounding environment deformation caused by deep foundation pit excavation.And the theories mainly include the displacement of the retaining structure of the foundation pit,The uplift of the soil at the bottom of the foundation pit and the settlement of the ground.Then,we summarizes the deformation mechanism of subway station which is commonly shared diaphragm wall with the foundation pit caused by the excavation of the deep foundation pit.Based on the above theoretical research,taking the foundation pit project of the Qianhai early start area project in Nanshan District,Shenzhen as an example,this article analyzes the deformation characteristics of common diaphragm wall subway station caused by deep foundation pit excavation through actual monitoring data analysis and three-dimensional numerical simulation.The following conclusions are obtained:(1)The excavation of the foundation pit causes the retaining structure to incline the inside of the foundation pit,and the ground outside the pit to settle.The horizontal displacement of the shared diaphragm wall is much smaller than the deformation of the surrounding structures on the other sides of the foundation pit.The upper part has a small deformation and the deformation below the excavation surface of the foundation pit is large.Excavation of a deep foundation pit will cause ground settlement on both sides of the station,and uplift on t he side of the station.The maximum ground subsidence is about 8.41 mm,and the maximum ground uplift is about 3.2mm.(2)The excavation of the deep foundation pit causes a vertical uplift deformation of the subway station with a shared diaphragm.With the excavation of the foundation pit,the uplift gradually increases,but the uplift growth rate first increases and then decreases.Away from the direction of the foundation pit,the uplift deformation of the station gradually decreases.The maximum uplift of the right line of the station is about35% of the maximum uplift of the left line.In the horizontal direction,the station displaces to the side of the foundation pit.The horizontal displacement of the station floor is the largest,and the top is level.The displacement is the smallest,accounting for about 20.99% of the bottom plate deformation.(3)Factors such as the distance between the foundation pit and the side subway station,the excavation depth of the foundation pit,and the thickness of the shared diaphragm wall have significant effects on the deformation of the station.When the station and the foundation pit share the diaphragm wall,the deformation of the subway station is the largest.The distance between the foundation pit and the side subway station affects the station mainly within the range of 15 m from the foundation pit.In this range,the horizontal displacement of the station decreases by 47.78%,and the vertical displacement decreases exponentially by 99.82%.For the same foundation pit project,the greater the excavation depth of the foundation pit,the greater the impact on the deformation of the common diaphragm subway station.When the final excavation depth of the foundation pit changes from 12.5m to 20 m,the maximum horizontal displacement of the common diaphragm subway station increased by 58.37%.The station uplift first increased and then decreased with the increase of the maximum excavation depth of the foundation pit.When the floor of the station is higher than the depth of the foundation pit,the uplift of the station floor begins to decrease.The thickness of the common diaphragm wall of the foundation pit and the subway station has a significant impact on the deformation of the station.When the thickness of the diaphragm increases from 0.6m to 1.2m,the maximum horizontal displacement of the station decreases by 22.64%,and the maximum vertical displacement of the station decreases by 16.94%.