GENERAL35603 - Laserfiche WebLink

Home Browse Search

~~ ,~ • -2- L EXISTING AND PROPOSED BLASTING: The only change proposed from existing blasting is to occasionally use a maximum charge weight of 50 pounds, [he usual range bei~rg 15 [0 30 pounds. POSSIBLE AND PROBABLE VIBRATION EFFECTS: When any blasting operations are carried out, they effects of vibrations, both ground and air blast ~"air overpressure" not necessarily audible) will be greater as greater amounts of explosive:; are detonated, and will be greater as distances be- tween the blast and the observer decrease. A convenient way to combine these two factors into a single non-dimensional number is to introduce the single blast effect indicator known as the "Scaled Distance". In terms of ground vibration, [tie scaled distance is [he distance in feet divided by the square root of [he charge per delay in pounds. As the scaled distance increases numerically, the blast vibration intensity decreases. For many locations along the front range of the Rocky Mountains in Colorado, a scaled distance as low as 15 or ZO might indicate that vibra- tions were of a very high intensity, certainly high enough to cause alarm, discomfort and possibly threshold cosmetic structural damage such as plaster cracking. Scaled distances of 30 to 40 or so would probably cause vibra- tions that could be distinctly perceived, but would normally present no risk of damage. Cornplaints might occur dependent to great extent on human subjectivity considerations. Human beings can sometimes perceive very low levels of ground motion, dependent on factors such as state of health, state of mind, position and attitude of [he subject, and state of activity. Some persons are able to perceive vibrations as low as blasting seismographs can measure: in these cases, however, the vibration levels are lower than would be caused by normal day-to-day activity in a structure: walking, climbing or descending stairs, or openning and closing doors. Scaled distances of 60 or 70 are completely safe from a structural damage stand point, normally do not cause complaints, and few persons notice the vibrations. The Office of Surface Mining, Department of the Intecior has stringent and well-tried regulations covering the use of explosives on surface coal mining operations. Based on data from all over the United States, which includes geological areas more susceptible to the propagation of vibrations than the front range of the Rockies in Colorado, the OSM regulations allow blasting without seismographs at distances from 0 to 300 Eeet using a scaled distance of 50 as the sole vibration control. At distances from 30L to 5000 feet, the significant conditions which would apply at Arkins Park Stone, OSM allows a scaled distance of no less than 55. The closes[ structure to any of the Arkins Park blasting would be approximately 600 feet away. If the heavies[ weight of SO pounds were detonated at this distance, the scaled distance would be 84.9. This is the lowest scaled distance indicated apt any of the quarry areas, AC a greater distance, say 800 feet, when 30 pounds of explosive - the usual loading - was being detonated, a scaled distance of 146.1 would apply. It is most unlikely that any normal person would be able to feel ground motion at such a high scaled distance.