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Blasting Impacts and Rock Slope Stability Amt for Cottonwood Quarry in Grand County, CO 4.3 Damage to Neighboring Structuress The nearest residential structure, shown as GE -3 in the Figure 4.1, is located more than 3,800 feet from the boundaries of the proposed mining area where rock blasting operations might occur. At this distance, State of Colorado regulations stipulate that the minimum scaled distance (DS) must be 55 or greater. With this limitation, the weight of any individually fired charge, delay -timed to fire with at least 8 milliseconds of separation, must not exceed 4,773.6 pounds. [(3,800/55)2 1. Due to other normal blast design constraints, the size of charges will be much smaller dm this limit'. At quarries of this type, the diameter of blastholes will not exceed 6.75 inches and the maximum height of mined benches would not exceed 50 feet. For these maximum geometrical conditions, assuming the top 10 feet of blastholes are filled with crushed stone stemming, which is normal practice, the maximum charge height would be 40 feet. Assuming a very dense wet -bole slurry explosive is used, with a density of 1.25 g/cc, the weight of an individual charge would be around 775 pounds [6.752 x 1.35 x 0.34 x 80]. Pepk growldM ti n if the maximum charge -weight -per -delay is indeed limited to no more than 775 pounds, the intensity of vibration in the ground at the nearest residence would most likely be less than 0.092 ids [240 (3,8001775 °S ) 't'61. Ground motion at this level will not damage any type of structure including surface buildings and in -ground water wells of any type. Air-ov_emresstsr_cc Intensities of air -overpressure (blast noise) for the maximum 6.75 -inch charges, stemmed with at least 10 feet of crushed stone stemming, should not exceed 122 dBL at distances of 3,800 feet and greater. This estimate is based on the authors' measutrements made at similar operations. For practical purposes, levels of air -overpressure would likely be acceptable if the height of stemming is at least 15 charge diameters, e.g. if 6.75 -inch holes are loaded with a bulk explosive at least 101.3 inches or 8.4 feet [15 x 6.75] of stemming should be used to provide adequate charge confinement. This stemming practice will also assure that excessive rock movement (flyrock) does not occur and the improved charge confinement will limit gas -pressure losses, which will certainly improve overall rock fragmentation. The onset of potential air -overpressure damage (broken or loosened glass window panes) does not occur until air -overpressure exceeds 145 dBL. In real pressure terms, expected air -overpressure level of 122 dBL generates a pressure of 0.0037 psi, which is more than 12 times lower than the 0.052 -psi pressure at 145 dBL. If cautious blast design methods, like those used for these predictions are rigorously applied, it is extremely unlikely that any damage would result from blast -induced air -overpressure. REVEY Associates, Inc. Page 14 April 2004