Research On Key Technology Of New Support Structure In Long-span Mined Tunnel And Its Engineering Application

Generally the form of single tunnel with double routes or double-arch tunnel with double routes will be used in inbound-outbound siding of urban rail transit.However,there are some problems like cockamamie construction technology,bother caused by tunnel form transformation from the normal route tunnel to the inbound-outbound siding and so on,which present in whether long-span tunnel or double-arch tunnel.Based on investigating the structural form and construction process of inbound-outbound route of metro transit at home and abroad,the tunnel of siamesed primary support with separate secondary lining and its construction technology as an innovative idea has been proposed.Through laboratory test of key junction,experiment section engineering of inbound-outbound siding of Beijing Metro Line No.16 and numerical simulation analysis by FLAC3 D,the feasibility of siamesed primary support with separate secondary lining tunnel has been verified,the structural section and the construction scheme has been optimized,and finally the applicability of this structural form used in multi-arch tunnel has been analyzed and summarized.The main research contents and relevant conclusions are as followed:1.It's the first time that the siamesed primary support with separate secondary lining tunnel has been applied in underground section of urban rail transit and the primary support uses supporting form of I-beam with shotcrete,the force condition of “Y” junction in mid-partition is complex,the phenomenon of moment concentration in arch foot of primary support.The research shows that under vertical load the “Y” junction structure in mid-partition of primary support is in elastic loading range;the stress states of primary support has been significantly improved after using quasi-circular cross-section instead of quasi-horseshoe cross-section in siamesed primary support with separate secondary lining tunnel.2.The parameters of core length(CL),distance between top heading and bench(BD)and distance between right and left excavation stages(TD)are determined by past design experience in experiment section engineering,but the usability of those parameters used in siamesed primary support with separate secondary lining tunnel is uncertain.And the secondary lining will be concreted after the two caverns have been transfixed,but it is not certain that the surface subsidence can be controlled within code requirements by using this construction procedure.Therefore,those excavating parameters of siamesed primary support with separate secondary lining tunnel will be analyzed and optimized through numerical model of formation inversion.The results show that the surface subsidence will dramatically reduce by choosing core length(CL)0.3D,distance between top heading and bench(BD)2.5D and distance between right and left excavation stages(TD)5.5D,in which the parameter “D” represent the diameter of single cave in siamesed primary support with separate secondary lining tunnel.In addition,the construction opportunity of secondary lining has been summarized in different formation conditions like hard soil,medium hard soil,medium soft soil and soft soil.3.The structural form of siamesed primary support with separate secondary lining can be applied in multi-arch tunnel such as three-arched or four-arched tunnel.But the problem of structural safety influenced by excavation sequence of multi-arch tunnel is unclear.After simulating the construction process of multi-arch tunnel with siamesed primary support and separate secondary lining cross-section,the siamesed primary support three-arch tunnel should use the construction procedure of side caverns excavating first followed by middle cavern prior,the siamesed primary support four-arch tunnel should use the construction procedure of side caverns excavating first followed by middle caverns prior in the condition of poor geology with bounteous construction period while use the construction procedure of interval caverns excavating synchronously prior in the condition of good geology with compact construction period.