Because blasting is a quite efficient and economical excavation method, it has been widely applied to tunnel excavation especially in hard rock. However, this method is rarely used to the tunnel site close to residential houses due to its environmental impact such as vibration and noise. Nowadays, the advanced electronic detonator, which was widely used in overseas mine site, was introduced to the tunnel site in Japan. This electronic detonator with accurate delay time (=ignition time difference between consecutive holes) has a potential to mitigate blasting vibration. In the past, many studies to lower the blasting vibration had been conducted and in some of them, several ways of simulation were implemented and the relations between vibration mitigation and arrangement of delay time were proposed. In this study, based on the past achievement and blasting test the authors had carried out, the appropriate method to determine the optimum delay time was proposed. In the case of electronic detonator in which each hole has different ignition time, the methods using autocorrelation and frequency analysis were found to be good expectation of delay time to lower the vibration. Also, in the case of conventional electric detonator in which ignition time has dispersion, the relation between vibration and ignition time dispersion was calculated, and the proper way to lower the vibration was proposed. This study gives practical method to set appropriate ignition time to lower blasting vibration.
