Safety Evaluation And Support Optimization Of Deep Tunnel Under Impact Load

The frequent occurrence of earthquakes and the rapid development of high-tech drilling weapons threaten the safety and stability of underground protection projects.How to reduce the harm of impact load on underground engineering,especially deep underground engineering,has become a hot topic for researchers.In this paper,the impact isolation performance of traditional support(such as bolt,steel frame and their combination)and novel metamaterial support are studied under the consideration of the high in-situ stress generated by surrounding rock of deep tunnel.The main research contents and results includes:(1)Taking the deep tunnel without support as an example,the modeling method combining finite element and infinite element is introduced,and the impact response of tunnel under high in-situ stress is calculated.Meanwhile,the band structure and steady as well as transient response of metamaterial are calculated.The results indicate the possibility of metamaterial to act as impact support of tunnels.(2)The impact performance of bolt,steel frame and combination of bolt and steel frame under high in-situ stress in deep tunnel is studied.The results show that due to high in-situ stress,increasing the length of bolt and decreasing the distance of bolt can reduce the displacement in a certain range,and improve safety of the tunnel.If the bolt length is fixed,the reasonable bolt spacing arrangement can also reduce the displacement and plastic strain of the tunnel.Increasing the number of long bolts at the arch foot can reduce the overall displacement of the tunnel and improve the safety of the tunnel.The steel frame support can reduce the displacement of the tunnel without increasing the peak stress of the tunnel and has good impact isolation performance.The impact isolation performance of combined support is better than that of single support.(3)The impact isolation mechanism of metamaterials is analyzed,and the influence of metamaterial layer number,gradient distribution,structure form and buried depth on the safety of tunnels is studied.The results show that compared with uniform rock or phononic crystal,metamaterial support can significantly reduce the peak stress around the tunnel and has better impact isolation performance.The mechanism of impact isolation is a combination of wave slowing and local resonance.Increasing the number of metamaterial layers,decreasing the center frequency of bandgap,and widening the band gap can reduce the peak displacement and stress of the tunnel,and improve the safety of the tunnel.For a group of metamaterials with the same band gap,the impact isolation performance is independent of the gradient arrangement.The closed structure is better than that of open structure in the reduction of peak stress.The effect of arc structure on reducing peak displacement and stress is better than that of linear structure.For deep tunnel,the effect of stress reduction of closed straight line structure is still higher than that of open straight line,which is consistent with that of shallow tunnel.The increase of layer number and gradient arrangement(frequency range expansion)can still reduce the peak stress,but the tunnel displacement is not reduced obviously.It may be affected by the high in-situ stress of the deep tunnel.