Hard rock tunnel blasting becomes more efficient with the state-of-the-art technology advancements of the computeraided drilling machine which minimises the drilling error. However, overbreak is still considered as the primary challenge of any tunnelling project using drilling and blasting methods. This study evaluates a standard tunnel blasting design for 1 m advance used in a tunnel construction site in Japan. Blast damage zone (BDZ) of a single buffer hole blast design of the tunnel is estimated using Ash's modified energy and pressure-based approaches which calculated as 0.81, and 0.95 m respectively. In succession, five numerical models with different tunnel perimeter spacing-to-burden (S/B) ratio were simulated using ANSYS AUTODYN to evaluate S/B ratio effects to damages beyond the tunnel contour. The average practical BDZ of the AUTODYN analyses was estimated at 0.83 m which is reasonably corresponding with the Ash's BDZ approaches. The results show that more damages caused when S/B ratio increases. When the perimeter space is fixed to 0.83 m, the most appropriate S/B ratio is 1.0 for the tunnel construction site. The presented process of the BDZ evaluation can be applied in tunnel blasting designs in practice to minimise possible overbreak.
blast damage zone, buffer concept, tunnel, overbreak, AUTODYN