| During the construction of tunnels in Karst areas,the hidden danger of karst caves has seriously threatened the safety of tunnel workers.Therefore,the study of the influence of tunnel blasting excavation on surrounding rock in karst area is an important issue that must be faced in the construction of tunnel underground engineering.Taking Bantao tunnel as engineering background in this paper,corresponding finite element analysis model was established.Vibration velocity of each measuring point on surrounding rock of tunnel side wall was obtained by blasting monitoring,and was compared with the results of finite element method.The variation of stress and vibration velocity was discussed in three condition,that is when ellipsoid caves above the tunnel,over the side of the tunnel and belove the tunnel.The influence of excavation feed,blasting source distance and surrounding rock grade made on the stability of the wall of karst cave was studied.According to the analysis results,the safe distance between blasting source and karst cave was determined,and some treatment measures were put forward.The main contents and achievements of this paper are as follows:(1)The field monitoring data of vibration velocities was compared with the finite analysis results,it can be concluded that the peak errors of the surrounding rock of the tunnel were within ±1cm/s in horizontal radial,horizontal tangential and vertical directions.The results show that the numerical analysis is reliable.(2)The dynamic response of surrounding rocks at different locations of karst caves was analyzed,it can be concluded that variation of vibration velocity and stress of surrounding are basically the same in three direction.And the maximum combined vibration velocity is much higher than the control standard value.At the same time,when the explosion source is near to the karst cave,the surrounding rock on the wall of the karst cave may be destabilized.It is suggested that grouting and reinforcement should be taken in advance.(3)The influence of excavation feed,blasting source distance and surrounding rock grade made on the stability of surrounding rock on above karst cave wall was studied.The maximum values of velocity,principal tensile stress and displacement of karst cave are all increase gradually from below to above.(4)The safe distance between explosion source and cave under each excavation feed was calculated by numerical simulation.When the surrounding rock is grade IV,the safe distance R is 9.48 m when the excavation feed is 2.4m,is 6.22 m when the excavation feed is 1.6m.When the surrounding rock is grade III,the safe distance R is 7.19 m when the excavation feed is 2.4m. |
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