The Ube-Isa Limestone Mine in Japan is surrounded by many residents, and blasting events causing airblast (infrasound and low-frequency sound; 1 to 80 Hz) have been problems to solve. Strategies to reduce the airblast should be studied for both influences on humans and impacts on homes in the area. The matters are dependent on not only the airblast (sound) level but also the frequency. For reduction of airblast level, we have focused our studies on blasting free face area and blasting direction. And for control of frequency, we have focused on time delay of shot by using non-electric detonator and electronic detonator. By applying the above strategies, blasts over 95 dB has decreased by half, and our studies indicate that a rattling of fixtures can be controlled by adjusting time delay shot.
Ube-Isa Limestone Mine lies in MINE city in Yamaguchi Prefecture, Japan. The annual limestone output was 8 million tons in 2008. The total production is 440 million tons (Japan's top) for 60 years since 1948. As the mine is surrounded by many residents, blasting events causing ground vibration and airblast (infrasound and low frequency sound) have been problems to solve. And recently, the impact of the airblasts have been getting larger because gradually our mining operation have been moving closer and closer to resident homes. Before this airblast study, ground vibration has been controlled by the explosive's volume and time delay shot. But an effective method of airblast control had not been set up. Strategies to reduce the airblast should be studied for both influences on residence such as stress, headache, ringing in ears and for fixtures in the home such as rattling of doors and windows. The above matters are dependent on not only the airblast (sound) level but also the frequency. Blasting free face area and blasting direction are studied for reduction of the airblast levels. Time delay of shot is studied for the frequency control by using non-electric detonators and electronic detonators. After our study, several definite results of the airblast reduction have been given as follows.
For reduction of noise, there are 3 measures from the following viewpoints,
source of sound
conveying process
receiving side.
However, measure-2) is not expected to be effective to reduce the noise because airblasts have long wavelengths that diffract even soundproofing walls. Measure-3) is also not realistic because of the number of factors that exist at the sound receiving side. For these reasons, measure-1) source of sound, which is the blasting itself, has been the focus strategy.
(1) Area Reduction of Blasting Free Face We tried to validate that the area reduction of blasting free face can lower the airblast level(dB). We compared airblast levels that occurred from 4 types of blasting as the ratio of free face area; standard:1,low bench:0.8, multiple row:0.6 and face covered:0.2 under the conditions of the same volume and powder factor by arranging them in parallel as shown in Fig.1.
