Study On Safe Vibration Velocity Of Blasting Construction In Shaft Crossing Section Of Extra-long Tunnel

With the improvement of national economy,the number of tunnels is increasing,and the number of extra-long tunnels is also increasing gradually.In the construction of extra-long tunnels,it is necessary to increase shafts and connecting passages to bring ventilation to tunnels and increase working planes.When blasting excavation is carried out on the main tunnel,the crossing section between the main tunnel and the shaft is a part of sudden change of stress.Therefore,it is of great theoretical and practical significance to study the safety of surrounding rock and initial supporting structure of the crossing section between the tunnel and the shaft under blasting load.In this paper,the section near No.1 shaft of an extra-long highway tunnel in mountainous area is taken as the supporting project,and the maximum vibration velocity and stress of surrounding rock and primary support of the main tunnel and shaft crossing section or connecting passage crossing section are analyzed by MIDAS/GTS NX.The main research contents and achievements are as follows:(1)according to the field monitoring data,the vibration velocity formula of particle blasting is fitted,and the vibration attenuation coefficient α and site coefficient k are obtained by solving,which provides the basis for the subsequent blasting simulation;(2)Four models with excavation footage of 0.5m,1m,1.5m and 2m are established by finite element software,and the distribution laws of vibration velocity and stress at the interface between main tunnel and shaft or the interface between connecting channel and shaft are analyzed.The results show that with the increase of excavation footage,the vibration velocity and stress of tunnel surrounding rock gradually increase,and the reasonable safe excavation footage should be 1 m;(3)The model of primary support structure with spray layer thickness of 60 mm,90mm,120 mm,150mm and 180 mm is established,and the maximum vibration velocity and stress distribution law of primary support structure at the intersection of main tunnel and shaft are analyzed.The results show that with the increase of sprayed layer thickness,the maximum vibration velocity and stress of the initial supporting structure of the crossing section gradually decrease,and it is suggested that the reasonable sprayed layer thickness should be 150 mm;(4)The primary supporting structure models with concrete ages of 1d,3d,5d,7d,14 d and 28 d are established,and the maximum vibration velocity and stress distribution law of the primary supporting structure at the intersection of the main tunnel and the shaft are analyzed.The results show that when the age is less than 14 days,the initial supporting structure of the crossing section will be damaged in different degrees.It is suggested that the initial supporting concrete should be added with early setting agent to shorten the cyclic blasting time.