2008-04-15_REVISION - M1980244 (198)

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• The combined Globe Hill and Ironclad data sets provide the most conservative scaled distance relationships. This combined data set } comprises all of the production blast data acquired at the mine site. Vibra-Tech bases all recommendations on the Ironclad and Globe Hill combined data set because of its conservative nature. The attenuation rates are similar in the combined and Cresson data sets and suggests similar ground motion attenuation characteristics. CONCLUSIONS Vibra-Tech recommends a scaled distance of 23.15 in order to not exceed 1.0 inch per second. A scaled distance of 34.2 is required to not exceed 0.50 inches per second. These are +95% confidence intervals, the U.S.B.M. suggested method of scaled distance determination. These scaled distance recommendations are site specific and are based on the conservative production blasting data set. Use of a scaled distance of 34.2 for all blasting in the proposed Cresson mine area will prevent any possibility of exceeding a 0.50 inch per second peak vibration limit. This equates to a maximum charge weight per delay period of 3420 pounds of explosive at a • 2000 foot distance from any point of concern. Lower charge weights per delay period will result in lower ground motions. The data collected in the Cresson area suggests that the use of a scaled distance of 34.2 will result in a maximum particle velocity of 0.32 inches per second. The use of a scaled distance of 34.2 does not impose a burden on typical production blasting and will assure regulatory compliance with a large margin of safety. Keeping vibrations to a maximum of 0.50 inches per second will minimize complaints and assure compliance with the Variable Particle Velocity versus Frequency Limits recommended in U. S. B. M. Report RI-8507 (Nov. 1980). The peak frequency in virtually all of the records collected is 10 Hertz. The use of 100 ms delays is the cause of the 10 Hertz dominant frequency. Records taken at all distances greater than 683 feet or a scaled distance of 19.3 satisfied all Variable Particle Velocity versus Frequency criteria. The highest recorded particle velocity at distances beyond 2000 feet was 0.02 inches per second. This is a vibration level that is beneath the level of human perception. This study is statistically accurate and was conducted with modern i equipment and in a manner that is accepted as "state of the art". It~is not a substitute for direct vibration monitoring of all production blasting. The best method of limiting liability and reducing complaints is to monitor all blasting at the nearest I • points of concern.