Study On Unloading Mechanism Of Fractured Rock Mass And The Amendment Of Rock Classification In Strong Seismic Area

With China's"Western Development"strategy and the steadily advancing"The Belt and Road"and the development of underground space,tunnels built in our country increases year by year,and more and more tunnels will be built in the west.In this paper,unloading mechanism of the fractured rock and the stability of the tunnel in the fractured rock has been studied,and discussed the surrounding rock classification considering the seismic effect.The results can provide certain theoretical and engineering practical value for improving the safety and stability of tunnels be built in the fractured rock in West.First of all,this paper summarizes the current status of the researches on unloading mechanism of fractured rock mass at home and abroad,the stability study of tunnel surrounding rock,and the engineering rock mass classification.Through the analysis of the variation characteristics of the stress field around the crack during the excavation and unloading process,the paper obtains the stress state around the crack during the unloading process from the initial compression shear state to the later tensile shear state;the BBM model in the discrete element method is used to simulate the crack expansion mechanism and fail mechanism of the fractured rock during excavation,loading and unloading processes.It is recognized that the unloading process of the cracked rock mass has a greater impact on the stability of the tunnel;It is also necessary to support or advance support the tunnel to ensure the stability of the over-broken rock mass.Secondly,based on the 12#construction branch of the diversion tunnel of the Yellow River Diversion Project in central Shanxi Province,tunnels in the III,IV,and V surrounding rock sections are selected for discrete element modeling and analysis,the stress distribution under different surrounding rock classifications are discussed.The displacement change of characteristic points and the range of plastic zone indicate that the difference of rock classification has a great influence on the deformation of unloaded surrounding rock,so that different excavation methods should be considered for different surrounding rocks.Finally,in the construction of tunnels and other underground projects in the strong earthquake regions in western China,if the damage of the earthquake to the rock mass can be reflected in the rock mass classification in advance,then the tunnel project is supported based on the results of the rock mass classification considering the potential seismic damage.The support design may become a new idea to ensure the stable operation of tunnel engineering in strong earthquake zone and the safety of the whole life cycle.Therefore,based on the National Standard Rock Mass Classification Method,this paper carried out a quantitative reduction study of the rock mass integrity factor K_vconsidering the potential seismic damage results,and proposed the modified BQ formula of the rock mass classification considering the fortification intensity,and the BQ correction formula was verified with the site rock data of Yangjiashan Tunnel and Yingwen Tunnel.Through the revised formula,the modified surrounding rock classification of the tunnel after considering the potential seismic damage can be obtained,which can provide some guidance for the seismic design.This research work has certain reference significance for the safety design and construction of underground engineering in strong earthquake areas.