Study For Risk Identification And Control Of Building A Metro Station By Enlarging A Existing Large Diameter Shield Tunnel Using PBA Method

The Jiangtai Station of Beijing Metro Line14is constructed by a large diameter shield machine, then the tunnel is enlarged by using Pile-Beam-Arch method (PBA method) to form main structure of the station. The method can effectively solve the contradictory between the interval shield tunnel construction and metro station construction. This is a new method and there is not similar engineer to be referred, therefore, it is necessary to do further research on construction risk, risk control technology, transformation of structure stress, influence on environment in the construction process, and so on.(1) The enlarging construction schemes are compared and selected in the aspect of engineering economy and technology, and the numerical simulation method is used to select the optimal scheme. On the basis, the risk sources are identified and risk control technology measures are put forward in the process of enlarging large diameter shield to build metro station by PBA method.(2) Due to the main structural body of enlarging station locates in the shield construction disturbance region, in order to analyze reasonably construction mechanics in the enlarging process, the strata mechanical parameters are anti-analyzed and obtained according to the monitoring data of ground surface settlement caused by shield construction. The result shows the parameter accords the actual engineering and the conclusion has theoretical and practical significance for subsequent enlarging construction process.(3) The three-dimensional discontinuous contact model is used to precisely simulate the enlarging construction process and study the risk sources of the main structure of station——the force of the four kinds key joints including segment joints, connection between locks of Key segments, connection between longitudinal beams and segments, connection between primary linings or steel support and segments. The result shows, stratum load and structural force are transferred by connecting joints and the structure is almost steady.On the basis of ascertaining the force and deformation of structural key joints, the force transformation rules in the critical construction steps and the influence on the whole structure, the risks are controlled in the aspect of engineering technology and the technology measures ensure the station structure safety and stability in the enlarging construction process.(4) The influence of enlarging construction to the adjacent underground pipelines is studied in detail. The corresponding technology control measures are put forward. The representative pipelines above the station are selected, and based on the construction monitoring data, the numerical simulation method is used to analysis the settlement, lateral displacement, deformation and stress of pipelines, the influence on adjacent pipelines is detailed in the construction process of station. The monitoring data shows, the ground surface settlement above the pipelines is less than the control standard of absolute settlement change value and tilt value. The numerical simulation result shows, the stiffness of pipeline is large and the pipelines can resist the stratum deformation in the excavation process. The separation phenomenon between pipeline and soil appears. The differential settlement and joint angle of pipeline nodes are less than control standard, and calculation values of maximum tensile and compressive stress are also.(5) The rule of ground surface settlement by the enlarging construction is analyzed deeply, and risk countermeasures are put forward. In the premise of the monitoring data of shield construction process, the reasonable ground settlement control standard is put forward combining with the ground surface settlement control reference value of Beijing metro station and the statistic data of existing metro station constructed by PBA method statistics data in Beijing, and referencing the result of numerical simulation. In accordance with the three class control management methods, the ground surface settlement is controlled step by step. The result has been used to guide compilling the engineering construction scheme.