Study On Reasonable Support Types For Tunnels In Soft Surrounding Rock

Combined with the Baojiashan extra-long highway tunnel, acoording to theoretical analysis and construction situations on the spot,giving a viewpoint that it is feasible to take steel arch centering and shotcreting support as primary lining when complex lining used in soft surrounding rock.In steel arch centering and shotcreting support pattern ,the systematic bolts were canceled.the whole supporting structure was consist of steel arch(steel grid),shotcrete, reinforcing mesh,locking stringer bolt (anchor pipe) and vertial connecting steel bar. The effect of locking stringer bolts (anchor pipes) in structure which were installed and welded in connectiong of steel arch was to fix steel arch and keep the stability of supporting structure.To study the feasibility of taking steel arch centering and shotcreting support pattern in soft surrounding rock, taking two methods, which were field test and finite-element simulation, to make comprehensive evaluation of tunnel structure on its safety, stability and economy.The content of monitoring measurement included surrounding rock pressure, convergence measurement, steel arch stress, shotcrete stress, locking stringer bolt stress, vertial connecting steel bar stress, pressure between primary lining and secondary lining, concrete stress of secondary lining and convergence measurement of secondary lining. Using finite elements analysis soft (MIDAS/GTS) to simulate the construction process of Baojiashan tunnel and analysis the stress and deformation of supporting structure.The results of monitoring measurement and simulation calculation show that the surrounding rock was stable; the tunnel lining was safe and had enough safety margins. If the system rockbolts are abandoned, on based of making sure the stability of supporting structure and construction safety, construction processes can be reduced, processes cycle time can be shortened and engineering cost can be reduced in some extent. The above analysis show that it is feasible to take steel arch centering and shotcreting support in soft rock, the support pattern which is consist of steel arch centering and shotcreting support and secondary lining is the reasonable support pattern in tunnel in soft surrounding rock(when taking complex lining).