| With the rapid development of underground engineering in China,with the rapid growth of the total number and total length of tunnels in major areas such as highway,railway,subway and water conservancy,the construction of tunnels in karst areas is facing great challenges.The biggest hazard of tunnel excavation in karst area is the destruction of tunnel and surrounding rock caused by blasting,which leads to cracks between adjacent karst caves and tunnels,reduces the bearing capacity of surrounding rock,destroys the original stress field,and causes serious deformation and destruction of tunnel.Blasting excavation is often used in tunnel excavation in mountainous areas.The disturbance of surrounding rock and the existence of karst caves will aggravate the phenomenon of water and mud inrush in tunnels.Potential safety hazards seriously threaten the lives of tunnel constructors in Karst areas.In order to ensure the safety and stability of surrounding rock of tunnel,it is necessary to study the stability of surrounding rock of blasting excavation of karst tunnel.According to the actual engineering geological conditions,construction methods and characteristics of Juneyuetan Tunnel of Yongji Expressway,this paper carries out the analysis of surrounding rock stability by different blasting excavation methods and different cave locations,and seeks for the nearest safe distance that secondary lining can follow up the face of the palm.Using Midas-GTS to establish two-dimensional and three-dimensional model analysis,the velocity,displacement and stress of 10 working conditions of karst cave located at upper,lower,left,right and lower right sides of tunnel and 50 m,40 m,30 m,20 m and 10 m distances from the face of secondary lining are discussed by two blasting excavation methods of full-section method and positive bench method.Analysis.Then the influence rule of cave location on surrounding rock and the nearest safe distance of secondary lining can follow up the face of the palm are obtained.The main research work is as follows:1)The mechanism of rock breaking by blasting,the classification,characteristics and attenuation law of stress wave are expounded,the parameters of tunnel blasting excavation are calculated,the safe vibration velocity of surrounding rock in karst area is analyzed,and the parameters of boundary conditions of blasting model are calculated.2)Based on the Yongji Expressway Juneyuetan Tunnel Project,using Midas-GTS software,a two-dimensional finite element analysis model is established to calculate the influence of different locations of karst cave on the stability of tunnel under blasting load and to consider the technological effects of full-face excavation and the construction of upper and lower steps.The calculated results of vibration velocity,displacement,principal tension stress and shear stress are analyzed and concluded.3)Combining with the example of Juneyuetan Tunnel Project,using Midas-GTS software,a three-dimensional finite element analysis model is established to explore the impact of blasting vibration on the secondary lining.The closest distance between the secondary lining and the palm surface is obtained through the analysis of vibration speed,arch waist displacement,vertical horizontal stress and shear stress. |
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