Study On The Influence Of Steel Casing Bored Pile Construction On Adjacent Existing Subway Tunnels

Subway tunnel is a very important infrastructure in urban transportation system,which has high requirements for the stability of the surrounding environment.However,with the rapid development of urban construction,large-diameter bridge pile foundation construction will inevitably occur in the neighborhood of subway,which will affect the safety and service life of subway tunnel.Steel casing bored pile is a common technology of pile foundation construction around subway tunnel.It is important to master the displacement of soil around the pile and the deformation law of subway tunnel structure during pile foundation construction,which is of great significance to ensure the safety of subway tunnel and save construction cost.In this paper,based on the Yindeng Overpass Project in Nanning,Guangxi,the influence of pile foundation construction on the horizontal displacement of soil around the pile and the subway tunnel displacement is studied by combining the construction measurement analysis and numerical simulation analysis:(1)A large number of pile foundation construction cases of adjacent metro tunnels are collected,and the basic research methods and main influencing factors of pile foundation construction on adjacent existing metro tunnels are summarized.Based on the interaction mechanism of pile soil tunnel and the deformation theory of metro tunnel,the essence of the influence of pile foundation construction on metro tunnel is elaborated,and the forms and internal causes of metro tunnel deformation are analyzed.(2)The construction monitoring of deep horizontal displacement of soil around pile and subway deformation is carried out.The monitoring data of soil displacement and subway deformation are obtained under different pile tunnel distance and excavation depth.The results show that the greater the distance between the subway tunnel and the pile center,the smaller the horizontal displacement of soil around the pile.The greater the depth from the surface,the smaller the horizontal displacement of soil around the pile.The rotary excavation construction at the interface between soil layer and rock in karst area is an important factor that causes the rapid increase of horizontal displacement of soil around the pile.Attention should be paid to the construction of similar strata.The displacement of subway tunnel increases with the increase of the digging depth of pile hole,and a little rebound after the pouring of pile body.The greater the distance between subway tunnel and pile center,the smaller the displacement is.(3)The three-dimensional finite element model of pile-soil-tunnel is established.Under the influence of two main factors such as the clear horizontal distance between pile and tunnel and the follow-up depth of steel casing,the displacement variation law of soil around the pile and subway tunnel is obtained.The reliability of the numerical simulation results is verified by comparing with the measured data of construction.In order to ensure the safety of subway tunnel,when the horizontal clear distance between pile and tunnel is less than 3 times of pile diameter,it is recommended to adopt the construction technology of full casing wall protection;When the horizontal clear distance between pile and tunnel is 3 to5 times the pile diameter,the follow-up depth of steel casing should exceed0.5 times the diameter of the tunnel at the bottom of the subway tunnel;When the horizontal clear distance between pile and tunnel is 5 to 10 times of pile diameter,the following depth of steel casing should be reasonably determined according to the stratum condition and the relative distance between pile and tunnel.When the horizontal clear distance between pile and tunnel is larger,or the steel casing follow-up depth is larger,the displacement of soil around pile and subway tunnel is smaller,and vice versa.This shows that the steel casing wall protection construction can effectively reduce the impact on the adjacent subway tunnel,but when the steel casing follow-up depth exceeds the bottom of the subway tunnel too much,the effect will be weakened,and the construction cost will be wasted.