The Research Of Center Pillar Type Of Twin-Bored Tunnel

Highway twin-bored tunnel is a kind of special structure of highway tunnel in express highway construction, especially in mountain area express highway construction. Through the survey of tunnels which have already been finished and which have still under construction, it shows that center pillar crack and leakage of twin-bored tunnel are outstanding problems in design, construction and service of twin-bored tunnel. According to the survey, study and design summary of twin-bored tunnel, this paper focuses on center pillar design style, center pillar construction method, center pillar water-resistence and abstraction of water, connection technology and comprehensive application of twin-bored tunnel. Finally, design and construction technology of twin-bored tunnel is studied and generalized to solve the outstanding problems, such as center pillar crack and leakage, in recent construction of twin-bored tunnel.The survey of design and construction of twin-bored tunnel shows that center pillar style of twin-bored tunnel is straight center pillar type in our country in early times; straight center pillar type and flexural center pillar type coexist in recent period; flexural layer type center pillar will be mainly adopted in the future. In order to solve leakage of center pillar, center pillar constitution is developed from integral type to layer type, especially to flexural layer type center pillar. In response to the different grades of wall rock, the construction method for twin-bored tunnel is changed from emphasis on three pilot holes method to emphasis on three pilot holes method, central pilot hole method and null pilot hole method.The paper studies and generalizes design and construction method for twin-bored tunnels with different center pillar types, such as straight integral type center pillar, flexural integral type center pillar, flexural layer type center pillar and hollow center pillar etc. Center pillar constitution, condition of compatibility and inside-outside connection method for twin-bored tunnel with different center pillar type are proposed. When thetwin-bored tunnel is on a curve, tunnel structure with flexural layer type center pillar and different inside-outside radius of curve, separated type subgrade (double-line) location method should be adopted to design the inside-outside transition style of the tunnel; when the twin-bored tunnel is on a straight line, tunnel structure with straight integral type center pillar , integral type subgrade (single-line) location method and rectigradation transition method should be adopted. The thickness of the sandwich type wall of flexural layer type center pillar tunnel should be greater than 1 meter, otherwise, twin-bored tunnel with flexural (or straight) integral type should be adopted.The paper preliminarily studies the boundary cross-section design method and proper sandwich type wall's thickness which is related with condition of wall rock and construction technology, hereby boundary cross-section of the tunnel which is gradually changed from separated double-tunnel type to small clear distance type then to twin-bored type is proposed. The paper also discusses the possibility that twin-bored tunnel can be not only applied in medium short tunnels but also in long tunnels, which promotes the application length of the twin-bored tunnel.