Study On Settlement Law Of Tunnel Pile Method In A Station Of Beijing Metro

PBA(Pile-Beam-Arch)method is one of the representative construction methods in urban metro rail transit.On the basis of inheriting the merits of shallow buried excavation method and cover excavation method,it has been widely used in subway construction with many advantages such as good stratum adaptability,construction flexibility,small settlement impact and no effect on ground traffic.PBA construction method inevitably affects the normal operation of underground pipelines,but the theoretical research of PBA method is not perfect,and the settlement and deformation mechanism in construction is not clear.Therefore,it is of great significance to study the settlement law and pipeline deformation of PBA method construction.Based on the Lagrangian finite difference principle and the background of PBA method construction in a station of Beijing Metro,this paper uses the finite element analysis software Midas/GTS to calculate the construction process,and establishes the construction model of the pile method.The surface settlement and pipeline deformation in each construction stage are analyzed.On this basis,the variation laws of stratum deformation,pipeline deformation and stratum plastic zone in each construction stage are discussed,and the measures of ground settlement control of PBA method construction are put forward,and following conclusions are drawn:(1)The maximum value of surface subsidence occurs at the left of the central line of the station during the subway construction.The left and right sides of the curve are nearly symmetrical,and the variation law and shape of the numerical simulation curve of surface subsidence are basically consistent with Peck's settlement theory.(2)The numerical simulation results show that the settlement during the construction stage of the guide tunnel is the largest,which is due to the cave effect caused by the construction,while the later settlement gradually decreases because the pile-column system forms a good force structure,which has a very significant effect on restraining the settlement of the soil,so it has little influence in the subsequent construction process.(3)In the process of station construction,the horizontal displacement of surface increases first and then decreases.The maximum horizontal displacement occurs in the construction stage of the mid-plate,which is located about 10-15 m around the midline of the station,reaching a maximum of 12.6 mm.The construction stage of the mid-plate has a great influence on the horizontal displacement.(4)Research on settlement of thermal trench shows that the maximum settlement of pipeline is in the excavation stage of guide tunnel and the maximum settlement is in the construction stage of mid-plate;the vertical displacement of pipeline rises after construction,which indicates that the soil unloading rebound effect is significant;compared with surface settlement,it is found that pipeline settlement has lagged.(5)The study of the plastic zone shows that the plastic zone in the excavation stage of the guide tunnel has a large distribution,and should be reinforced and supported in time.During the whole construction stage,the plastic zone is 16 m on both sides of central line of the station,and the advance grouting reinforcement should be carried out in time in the construction.