Initial Supporting Mechanism And Deformation Controlling Technology Of Loess Tunnel With Large Section

With the rapid growth of high speed railway construction in China, the corresponding basic construction experiences and theory are also developing greatly. Along the Zheng-Xi High Speed Railway,38tunnels with total77km length has been constructed. The tunnel sections have significant large excavation section as170m2. Although China has many construction experiences in loess area, but no tunnel section as Zheng Xi's has reached such big. So, if continuely follow the old design method, such as empirical design, it's hard to choose the suitable method for construction, advanced and primary supportting, and controlling the ground settlement and deformation. Undoubtedly, the existing design theory in loess area will restrict seriously the scientificity and standardlization of loess tunnel construction, and further have direct influence on the tunnel and the ground structures'security and economical efficiency. Based on the loess tunnel construction in Zheng-Xi High Speed Railway, systematic deep research on the deformation features of tunnel and tunnel supporting mechanism, has been done; and the most important conclusions are summarized as follow:(1) The deformation feature research on deep buried and shallow large section loess tunnel:According to field monitoring data, it is found that the ground settlement caused by the construction exceeds the allowable design value a lot; the settlement is hard to control but will easy cause ground crack; the support of tunnel has integral sinking both in deep buried and shallow tunnel, and also the arch springing isn't steady, etc.(2) The system bolts mechanism research deep buried and shallow large section loess tunnel:According to field monitoring data, it is found that cancelling the system bolts is more propitious than with system bolts to control the deformation of soil because of curtailing the construction time of different process in shallow large section loess tunnel; But for deep buried large section loess tunnel, with or without system bolts doesn't has influence on section deformation. Based on the theory of soil deformation and neutral point theory of bolt, the study indicates that the different distribution model of displacement lead to bolts play different role in different position of tunnel. Based on soil mechanism, bolt mechanism and the method of centrifugal loading finite element, designing parameter for system bolt in large section loess tunnel has been presented in dissertation. (3) The comparison of grid steel frame and section steel frame in shallow large section loess tunnel:The result indicates that the deformation of the two supports are almost equivalent; both the stresses of the two supporting forms are in the range of allowable value, but the stresses of grid steel frame is lesser than the section steel frame's; the earth pressure distribution in grid steel frame is better than the section steel frame's; there isn't any obvious differences between the two supports to control the vault crown settlement of tunnel, but the section steel frame has some advantages to control the horizontal convergence of tunnel. Based on the early strength of concrete, the support characteristic curves combined both supporting forms and ground response curve have been developed to study the mechanism between soil and support, and finally the applicable condition and design parameters for the different steel frame have been developed.(4) Ground surface settlement controlling technology of shallow large section loess: According to the field monitoring data, the law of soil deformation above the shallow large section loess tunnel and the influence factors to the deformation have been presented; also the control criterion for ground surface settlement of shallow Large section loess Tunnel is set up by research; Aseries of key technologies to control the settlement, such as reducing excavation area, increasing excavation steps of pilot tunnel, reserving core soil, quick closure, reinforcing primary support, big arch springing, etc, has been proposed and used successfully in the LianHuo Highway underpass Project.