| With devolopment of the high-grade highway in our country,it is inevitable that high tunnels will be constructed in karst area. Damage in surrounding rock of a tunnel and a karst cave caused by tunnel blasting is the maximal harm in the process of tunnel excavation in karst area,so as to make cranny peneqtrated between the sunnel and karst cave,reduce the bearing capacity of surrounding rock,destroy the original stress field,and cause local stress concentration in wall rock.All of that have negative effect on the stability of wall rocks and tunnel construction.Based on the process of Baixugong tunnel excavation blasting in the B4 bid section of Qing-Lian expressway in which a especially big limestone cave was found,main research contents include following several aspects:(1)stress redistribution of the surrounding rock and the reacting wall when the tunnel located at the karst cave was excavating.(2)Concrete variation of displacement of the surrounding rock of karst cave and the reacting wall in the process of the excavation of the tunnel with a small space to an existing karst cave.(3)The propagation law of blasting seismic wave in the surrounding rock under the blasting load was analyed.Based on field test data of the peak partical vibration velocity on the wall rock,preliminarily discussed investigate the safe peak partical velocity on wall rock of karst cave when the tunnel was excavating with a small space to it.By means of the numerical simulation calculation on the tunnel excavation blasting and compared with test data, the main achievements attained were as follows:(1)The horizontal displacement of the karst cave surrounding rock and reacting wall closed to the main tunnel was a little larger than the vertical displacement of it.The maximal horizontal displacement was present to the left lateral wall of the karst cave.But also the vertical displacement increased gradually during the excavation of the tunnel upper bench and the excavation of the lower bench had little influence on the settlement of surrounding rock.The settlement of suppoting wall was about 56.3 percent of that of that adjacent karst cave roof.The top of supporting wall was bearing big rock pressure.It was showed that the wall top was the stress concentration region.(2)The stress and displacement of anterior segment of the tunnel adjacent the karst cave were complicated change and larger comparatively obviously than that of"the mouth of tiger".Before the tunnel was excavated to appointed section,the displacement of vault and bottom surface was almost unchanged,but increased evidently after tunnel excavation.And also the displacement increased faster during the excavation of the upper bench compared with the excavation of the lower bench,then it tended to be stable until the excavation section was 20 meter far away from the measured section.(3)When the escape trunk was blasting,the particle vibration velocity on the rock face of the karst cave closed to it exceeded greatly the standard of the《Blasting Safety Regulations》in which the particle vibration velocity must be less than 10-20cm /s in traffic tunnels.The vibration velocity was controlled less than 10cm/s affer optimization of blasting parameters and thus to decreased disturbance of surrounding rock.(4)Peak particle velocity induced by blasting will be increase with the maximum charge quantity per blasting serial increasing or with distance decreasing from measuring points to explosion source.So the permitted maximum charge per blasting serial could be determined by safety vibration velocity and must be controled in the safety range,it was that the partice vibration velocity on karst cave wall rock would be less than 8cm/s during excavation of the excape trunk and less than 6cm/s during main tunnel excavation.
|
PDF Full Text Request