Laserfiche WebLink
Blasting Impacts and Rock Slope Stability Assessment for Cottonwood Quarry in Grand County,CO <br /> 4.3 Damage to Neighboring Structures <br /> The nearest residential structure, shown as GE-3 in the Figure 4.1, is located more than 3,800 <br /> feet from the boundaries of the proposed mining area where rock blasting operations might <br /> occur. At this distance, State of Colorado regulations stipulate that the minimum scaled distance <br /> (Ds) must be 55 or greater. With this limitation, the weight of any individually fired charge, <br /> delay-timed to fire with at least 8 milliseconds of separation, must not exceed 4,773.6 pounds. <br /> [(3,800/55) ]. Due to other normal blast design constraints, the size of charges will be much <br /> smaller than this limit. <br /> At quarries of this type,the diameter of blastholes will not exceed 6.75 inches and the maximum <br /> height of mined benches would not exceed 50 feet. For these maximum geometrical conditions, <br /> assuming the top 10 feet of blastholes are filled with crushed stone stemming, which is normal <br /> practice, the maximum charge height would be 40 feet. Assuming a very dense wet-hole slurry <br /> explosive is used, with a density of 1.25 g/cc, the weight of an individual charge would be <br /> around 775 pounds[6.75Z x 1.35 x 0.34 x 80]. <br /> Peak Ground Motion <br /> If the maximum charge-weight-per-delay is indeed limited to no more than 775 pounds, the <br /> intensity of vibration in the ground at the nearest residence would most likely be less than 0.092 <br /> in/s [240 (3,800/775 °.S )'1'6] Ground motion at this level will not damage any type of structure <br /> including surface buildings and in-ground water wells of any type. <br /> Air-overpressure <br /> Intensities of air-overpressure(blast noise)for the maximum 6.75-inch charges,stemmed with at <br /> least 10 feet of crushed stone stemming, should not exceed 122 dBL at distances of 3,800 feet <br /> and greater.This estimate is based on the authors' measurements made at similar operations.For <br /> practical purposes, levels of air-overpressure would likely be acceptable if the height of <br /> stemming is at least 15 charge diameters,e.g. if 6.75-inch holes are loaded with a bulk explosive <br /> at least 101.3 inches or 8A feet [15 x 6.75] of stemming should be used to provide adequate <br /> charge confinement. This stemming practice will also assure that excessive rock movement <br /> (flyrock) does not occur and the improved charge confinement will limit gas-pressure losses, <br /> which will certainly improve overall rock fragmentation. The onset of potential air-overpressure <br /> damage (broken or loosened glass window panes) does not occur until air-overpressure exceeds <br /> 145 dBL.In real pressure terms, expected air-overpressure level of 122 dBL generates a pressure <br /> of 0.0037 psi, which is more than 12 times lower than the 0.052-psi pressure at 145 dBL. If <br /> cautious blast design methods, like those used for these predictions are rigorously applied, it is <br /> extremely unlikely that any damage would result from blast-induced air-overpressure. <br /> REVEY Associates,Inc. Page 14 April 2004 <br />