Field Test of Long Footage Burn Cut Blasting Excavation and Control of Overcut and Undercut in Medium Section Tunnel

Due to the limitations in construction scope, blasting a medium-section tunnel is challenging as it often results in short circular footage and significant over and under excavation. To reduce costs and increase efficiency, it is essential to focus on long footage excavation and fine control of over and under excavation. In this study, a straight hole cutting blasting scheme was designed for a medium section tunnel project and 40 cycles of blasting excavation field tests were conducted. The results revealed that when designing the long footage blasting parameters for a medium section tunnel based on the blasting design manual, issues such as high block rate and uneven face frequently arise. However, by appropriately increasing the charge of the cut part(the proportion of charge of the cut part increased from 12.8% to 18.1% in our field test), better blasting effects were achieved. Additionally, by reducing the charge amount of peripheral holes and adjusting their distance from each other, smooth blasting effect was effectively ensured. During the field test, adjustments were made to the charge amount of peripheral holes based on preliminary design for blasting parameters. This resulted in good contour forming effects with a half-hole rate exceeding 90%. However, an average overcutting value of 18.6 cm was observed across all 40 excavation sections during the blasting cycle. The main cause for this overcutting was identified as platform irregularities along the contour line. To address this issue, it is necessary not only to reduce external drilling angles but also control platform width alongside reasonable parameter designs for surrounding holes. The straight hole cutting scheme proved compatible with three-arm rock drilling truck construction methods while enabling mechanized long-shot blasting excavations. Nevertheless, precise control over overcutting and undercutting remains challenging along with cost management during blast construction. It is necessary to optimize and improve the operation technology of drilling personnel and the management mode of site construction.