Load-bearing Mechanism And Design Method Of The Pipe Shed Under The Tunnel Passing Through The Existing Station Under The Action Of Train Dynamic Loa

In recent years,the construction of subway has been developing rapidly,and the expansion projects of subway transfer stations have been increasing gradually.When the new station is going through the existing station,the settlement of the existing station will be too large and the normal operation of the station will be affected by not taking reinforcement measures.In the construction process,the existing station is in operation,and the vibration load of the train at the existing station will be transferred to the new tunnel,which will have an impact on the construction process.Pipe shed method has the advantages of high rigidity and high safety,and has been widely used in tunnel engineering.The load transferred to the pipe shed should be considered in the design of the pipe shed support scheme,but there are few analyses of load transfer in the soil layer at present.In this regard,relying on the existing station project of Guangqumen Wai Station of Beijing Metro Line 17 under Line 7,this study adopted numerical simulation,field measurement and theoretical analysis to analyze the stress change of pipe shed in the construction process,the transfer mechanism of dynamic load,the calculation method of soil transfer load,the stress model of advanced pipe shed and the construction design method,and obtained the following results:(1)The advanced pipe shed itself can bear the load,and the miniature soil arch can be formed between the pipes to support the existing structure,which can effectively inhibit the settlement of the existing station during the excavation of the lower tunnel.The settlement of the ground surface and the existing station is reduced by 22.6% and 59.1%,respectively,compared with that without the pipe shed.The vibration velocity and vibration acceleration at the arch of the new tunnel are 49.3%,49.1% and 51.6% lower than those without the advanced pipe shed support,respectively.(2)In the construction process,the deflection deformation curve of pipe shed presents three forms of "V","W" and "S" under the influence of excavation footage and existing stations.The stress model of pipe shed can be divided into two modes: the pipe shed can be regarded as simply supported beam when the distance between the palm face and the end of pipe shed is more than 1.5 times the height of the tunnel,and the pipe shed can be cantilevered beam when the distance between the palm face and the end of pipe shed is less than 1.5 times the height of the tunnel excavation.(3)Considering the train vibration load and based on the elastic foundation beam theory,the mechanical transfer model of the existing station-soil sandwiched-new tunnel is established.Then,through the function conservation relationship,the corresponding relationship between the settlement of the existing station and the support load of the new tunnel is defined,and the calculation method of the support load based on the settlement deformation of the existing station is established.The calculation structure shows that the smaller the settlement of the existing station,The greater the required support force,the greater the thickness of the sandwiched soil,and the smaller the static load transfer.(4)Based on the deformation and stress modes of pipe shed under different excavation penetration,two kinds of mechanical models of pipe shed deformation and stress elastic foundation beam are established.Based on this,the calculation method of pipe shed deformation considering the initial support and excavation penetration is derived.Combined with the deformation coordination theory of the soil in the middle,a pipe shed design method based on the existing station control is formed.It is concluded that the diameter of pipe shed steel pipe should be controlled at 108-200 mm,the spacing should be controlled at 250-400 mm,and the single excavation footage should be controlled at 0.5-1.5m.