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Janice Lynn Bennett - 81422.PDF Page 19 <br />Blasting Impacts Assessment for the Proposed GCC Rio Grande, Inc. Quarry in Pueblo County, Colorado <br />3.3 Stability of the DRG -RR Bridge and Embankments <br />For the bla at the Red Rock site, where GCC has agreed to limit maximumused a <br />charge size to 360 pounds as an example in the mining permit application, vibration <br />intensity at the nearest part of the DRG -RR line, located 2,300 feet from the closest blast <br />area will not exceed 0.11 in /s [240(2,300/(360) °• Vibration at the heavily con <br />concrete bridge over the St. Charles River, located over 7,800 feet from the blast areas will <br />not exceed 0.015 in /s. Vibration at these levels is below threshold levels of damage for rock - <br />fill embankments (4.0 in /s) and heavy concrete structures (20 in /s). Hence, it is extremely <br />unlikely that the proposed blasting at the Red Rock Project could harm these structures. <br />3.4 Damage to Neighboring Structures <br />I nvestigations of the site have revealed that several abandoned ranch structures exist about <br />1,500 feet northwest of the mining area beyond the DRG -RR tracks. Two stone kiln <br />structures are located against a rock bluff about 2,600 feet west of the site and the Blake <br />Ranch home and other associated buildings are located about 4,000 feet east of the mining <br />zone. Ground motions caused by blasting with charges less than 360 pounds will not even <br />approach the lowest published cosmetic damage threshold level (0.5 in /s) at these structures <br />At the closest structure (kilns), the intensity of ground motion will not exceed 0.09 in /s <br />[240(2,600/(360) O. Vibration in ground near the Blake Ranch structures should not <br />exceed 0.05 i n/s [240(4,000/(360) 0 5) -,.5] <br />Intensities of air - overpressure (blast noise) for the calculated 6.75 -inch charges, stemmed <br />with at least 6 feet of crushed stone stemming, should not exceed 115 dBL at di greater <br />than 4,000 feet. Note, that the author concurs with the CO- DNR -DM G comment that GCC <br />should use more than 2 feet of stemming to confine blast charges. At other quarry operations <br />using blastholes in the range of 6.75 inches, adequate charge confinement is achieved when <br />at least 6 feet of clean angular - crushed -stone stemming is used. For practical purposes where <br />smaller holes might be used, the stemming should be at least 10 charge diameters, e.g. if a 3- <br />inch holes are loaded with ANFO, at least 30 inches of [10 x 3] stemmed should be used. This <br />stemming practice will also assure that excessive rock movement (flyrock) does not occur <br />and the improved charge confinement will limit gas - pressure losses, which will certainly <br />improve overall rock fragmentation. The onset of potential air - overpressure damage <br />(broken or loosened glass window panes) does not occur until air - overpressure exceeds 145 <br />dBL. In real pressure terms, expected air - overpressure level of 115 dBL generates a <br />pressure of 0.016 psi, which is 30 times lower than the 0.052 -psi pressure at 145 dBL. Since <br />levels of air - overpressure will be very low at points of concern around the Red Rock site the <br />author is quite sure that it will not harm neighboring structures or cause undue annoyance <br />to neighbors. <br />3.5 Human and Animal Impacts <br />1 REVEY Associates, Inc. <br />Page 16 July2002 <br />