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Blasting Impacts Assessment for the Proposed GCC Rio Grande, Inc. Quarry in Pueblo County, Colorado <br />4.2 Stability of the DRG -RR Bridge and Embankments <br />For the blasting at the Red Rock site, where GCC has used a charge size to 360 pounds as an <br />example in the mining permit application, vibration intensity at the nearest part of the DRG -RR <br />line, located 2,300 feet from the closest blast area will not exceed 0.11 in/s [240(2,300/(360) 0.5)- <br />1.6] Vibration at the heavily constructed concrete bridge over the St. Charles River, located over <br />7,800 feet from the blast areas will not exceed 0.015 in/s. Vibration at these levels is below <br />threshold levels of damage for rock -fill embankments (4.0 in/s) and heavy concrete structures (20 <br />in/s). Hence, it is extremely unlikely that the proposed blasting at the Red Rock Project could <br />harm these structures. <br />4.3 Damage to Neighboring Structures <br />Investigations of the site have revealed that several abandoned ranch structures exist about 1,500 <br />feet northwest of the mining area beyond the DRG -RR tracks. Two stone kiln structures are <br />located against a rock bluff about 2,600 feet west of the site and the Blake Ranch home and other <br />associated buildings are located about 4,000 feet east of the mining zone. Ground motions caused <br />by blasting with charges less than 360 pounds will not even approach the lowest published <br />cosmetic damage threshold level (0.5 in/s) at these structures. At the closest structure (kilns), the <br />intensity of ground motion will not exceed 0.09 in/s [240(2,600/(360) Vibration in ground <br />near the Blake Ranch structures should not exceed 0.05 in/s [240(4,000/(360) 0.5)4 <br />Intensities of air - overpressure (blast noise) for the calculated 6.75 -inch charges, stemmed with at <br />least 6 feet of crushed stone stemming, should not exceed 115 dBL at distance greater than 4,000 <br />feet. Note, that the author concurs with the CO- DNR -DMG comment that GCC should use more <br />than 2 feet of stemming to confine blast charges. At other quarry operations using blastholes in the <br />range of 6.75 inches, adequate charge confinement is achieved when at least 6 feet of clean <br />angular- crushed -stone stemming is used. For practical purposes where smaller holes might be <br />used, the stemming should be at least 10 charge diameters, e.g. if a 3 -inch holes are loaded with <br />ANFO, at least 30 inches of [10 x 3] stemmed should be used. This stemming practice will also <br />assure that excessive rock movement (flyrock) does not occur and the improved charge <br />confinement will limit gas - pressure losses, which will certainly improve overall rock <br />fragmentation. The onset of potential air - overpressure damage (broken or loosened glass window <br />panes) does not occur until air - overpressure exceeds 145 dBL. In real pressure terms, expected air - <br />overpressure level of 115 dBL generates a pressure of 0.016 psi, which is 30 times lower than the <br />0.052 -psi pressure at 145 dBL. Since levels of air - overpressure will be very low at points of <br />concern around the Red Rock site, the author is quite sure that it will not harm neighboring <br />structures or cause undue annoyance to neighbors. <br />4.4 Human and Animal Impacts <br />As established in the prior section, vibration and air - overpressure intensities at the nearest <br />dwelling (Blake Ranch — 4,000 ft) are not expected to exceed 0.05 in/s and 115 dBL. <br />REVEY Associates, Inc. Page 14 July 2002 <br />