Research On Deformation Monitoring And Mechanical Performance Of Retaining Structure For Deep Foundation Pit Of A Highway Tunnel In Shenyang

In order to ease the traffic pressure in the urban area of Shenyang and facilitate the public transportation,the North-South Expressway in Shenyang was constructed,southern using underground expressway tunnel.As Shenyang underground expressway tunnel was built under the existing highway,the restricted construction site and the complex surrounding environment brought some difficulties and risks to the excavation of deep foundation pit.In order to ensure the safety and stability of foundation pit,and ensure the project smoothly,need to monitoring the bored piles + steel support of foundation pit support system.In this paper,the 17.8m deep foundation pit project of Shenyang North-South Expressway Tunnel is taken as the research object.The sensor elements are embedded during the excavation of the foundation pit to collect the monitoring data of the horizontal displacement and the reinforcing steel bar's internal force of retaining piles,as well as the axial force of the steel supports,which were used to study the force and deformation of the retaining structure.Then the numerical simulation was carried out with the finite element software ABAQUS,and the two data were compared and analyzed,Wish the research result can provide references for the construction and monitor of similar foundation pit engineering.The main research results are as follows:(1)The maximum horizontal displacement(deformation)of the retaining wall is closely related to the excavation depth of the foundation pit,and increases nonlinearly with the excavation depth of the foundation pit.During excavation of the entire foundation pit,the shape of the pile body gradually evolved from the initial forward curve to the arcuate curve,and finally reaches the maximum of 14.30 mm at 2/3 of the total depth of the foundation pit excavation.(2)According to the pure bending component assumption theory,combining the internal force envelope diagram with the measured horizontal displacement discrete point of the pile body,it can be fitted to obtain the functional relationship between the pile horizontal deformation and depth.After the second derivation of the deformation curve of the pile body,the bending moment of the whole pile body can be deduced from the curvature at any point on the pile body.(3)The internal force variation of the reinforcement bar in the retaining pile is affected by the erection and disassembly of the steel supports.The internal force of the steel meter varies obviously from the end of each layer of soil excavation to the erection of the steel supports,which is the most adverse period.Throughout the construction monitoring process,the axial force of pile reinforcement is small,only 66.4% of the design value,indicating that the reinforcement design of retaining pile is safe and reliable,but conservative.(4)After completing the excavation of each layer of soil in the foundation pit,the steel supports shall be erected in time and a reasonable prestress shall be applied so as to improve the stress conditions of the retaining wall and prevent the instability of the foundation pit.The maximum monitored support axial force for this time is 1150 k N,accounting for 63.9% of the designed axial force,indicating that the design of the retaining system of the foundation pit is conservative and the design of the axial force of steel support can be optimized.(5)The results of ABAQUS numerical simulation and monitoring results are generally close in numerical value,and the trend of change is consistent.It shows that the model is reasonable and the parameters are chosen properly,which can reflect the stress and deformation of the retaining structure during excavation,can provide reference for similar engineering analysis.