Research On The Effect Of Dynamic Load Of Underlying Metro Tunnel In Operating On Deep Foundation Pit

Research on the effect of dynamic load of train on the adjacent retaining structureof foundation pit gradually become one of the difficult and hot issues in the field ofgeotechnical engineering. Based on the foundation pit upon No.2subway, deformationof the existed metro tunnel during the excavation and the effect of train dynamic load onfoundation pit retaining structure were studied in this paper. FEA software MIDAS/GTSis used to establish a model to simulate the whole process of the deep excavation.Reasons of deformation of existed metro tunnel under the foundation pit and its controlmeasures was researched, and influence of adjacent foundation pit caused by train’stravelling load was studied within surface settlement, deformation and vibration ofretaining structure of foundation pit and stabilization of existed metro tunnel. The workand conclusions of this paper are as follows:(1) With the background the project of Nanjing Chengxi Road comprehensivereconstruction, monitoring program was designed and accomplished, and thedeformation laws of retaining structure of foundation pit was obtained by analyzing themonitoring dates. The study indicates that maximum deep horizontal displacement ofretaining pile was found in pile headpiece but horizontal displacement of pile bottom isvery small (almost zero); support axial force and pile headpiece displacement becomelarger along with the increase of the depth of excavation; support axial force and piledisplacement increase to maximum and keep steady roughly after the bottom offoundation pit completed.(2) The software MIDAS/GTS is used to establish a3D model to simulate thewhole process of the deep excavation. The calculated results are consistent with themonitoring data. The reason of tunnel’s deformation during the excavation andmeasures to control deformation of existed tunnel was studied. After the analysis of thecalculation results and the monitoring data, some conclusions about the deformation ofthe tunnels below influenced by the foundation pit excavation can be obtained. Forexample, the longitudinal heave curve of the tunnels is nearly a normal distributioncurve, the range of tunnel’s longitudinal ridges is2-3times the length of the excavationof foundation pit. All the research findings indicate that measures of excavation adoptedcan effectively control the lift deformation of tunnel.(3) Effects of the train moving loads on foundation pit retaining structure can notbe ignored. Analysis indicate that internal force of retaining structure was small when train was in motion, accordingly it can be ignored; vibration of retaining structurecaused by dynamic load of train in Z direction is obvious, but enclosure wall was safe.Dynamic load of train was adverse to normal use and durability of existed tunnel over along period of time, which was predicted through FEM calculation. Therefore, vibrationreducing measure, like fixing some damping fasteners on the rails, should be taken tolower the vibration of existed tunnel caused by dynamic load of train.