Study On Blasting Construction Method Of Large Span Tunnel With Small Spacing And Vibration Characteristics Of Intermediate Rock Wall

With the rapid development of transportation in China,large-span and small-spacing tunnels have become an important part of highway tunnels.The stability of the intermediate rock in large-span and small-spacing tunnels determines the safety of the whole tunnel.However,the thickness of the intermediate rock wall of such tunnels is much smaller than that of general tunnels,and its stability is highly susceptible to blasting.Therefore,research on the blasting construction method and the blasting vibration response characteristics of the intermediate rock wall is extremely important for the large-span and small-span tunnels.Based on the No.1 long-span and small-spacing highway tunnel project at Qichongcun in Guiyang,the blasting construction method and the response characteristics of intermediate rock wall are studied by field test,theoretical analysis and numerical simulations.The main research contents and results are as follows:(1)Vibration safety standards are determined by on-site monitoring.The monitoring results show that the main frequency of the lining blasting vibration of the former tunnel is greater than 10 Hz.According to the monitoring results and the Blasting Safety Regulations,the safety standard of blasting vibration is determined to be 12 cm/s.(2)The influence of free surface on the blasting stress wave propagation and blasting effect are studied through theoretical analysis.Numerical simulations are used to further study the influence of free surface on blasting vibration velocity and vibration time.Furthermore,the vibration damping principle of free surface is explored,and the necessary conditions for the vibration reduction effect of the free surface are obtained.The damping effects of the free surface with different shapes are also discussed.(3)Based on the No.1 tunnel project of Qichongcun,the factors affecting blasting vibration are analyzed by numerical simulation.The vibration velocities of the intermediate rock and former tunnel are compared considering different blasting construction methods,and the order of the blasting construction methods by vibration intensity is obtained.At last,the best blasting construction method for large-span and small-spacing tunnels is determined according to the safety standard of blasting vibration.(4)To expedite the construction progress and reduce blasting vibration,according to the principle of vibration reduction by free surface and the method of synchronous excavation by parts,synchronized blasting construction of three parts which locate separately in tunnel axial direction at upper step of CD method is put forward.The effect of vibration reduction and construction progress are analyzed by numerical simulation and scheme comparison.The results show that,under the same excavation footage,the blasting vibration produced by the synchronized blasting construction of three parts which locate separately in tunnel axial direction at upper step is reduced by more than 62%compared with the double-sided Guide-Pit method,belonging to micro-vibration blasting,and the construction is faster than CD method.(5)The vibration velocity of the surrounding rock of the intermediate wall under the synchronous blasting construction of step "front and back staggered three parts" in CD method is monitored.The results of field monitoring are basically consistent with those of numerical simulations.According to the monitoring results,the influence of different blasting holes on the intermediate rock wall behind and in front of the palm is studied,and the blasting vibrations of the intermediate rock wall in different zones are obtained.The law of velocity propagation and the variation of K and α under the influence of free surface and geological factors.