Agenda - 10-14-1993 - D 1 - Laserfiche WebLink

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3.5.7 Blasting The sole purpose of blasting at rock quarry sites is to break the rock loose from its geologic formation and permit easy processing. Blasting is still the only means available to free the rock and is undertaken only when the need for material requires it. A common misconception is that blasting is an uncontrolled activity. Blasting is planned and engineered to produce the quantity, quality and size of product needed. Each blasting event is specifically designed on the type of material present in the blast area and the size of stone required. These requirements determine the number, depth,and spacing of the 5-1/2"bore holes to receive the explosive charge, the amount of charge to be placed in each bore hole and the sequence of the explosions. Sequence of explosions is the interval and order in which each bore hole will be detonated. All bore holes do not detonate at one time but detonate milliseconds apart. Sequencing is accomplished at the quarry by two mechanisms: time delayed blasting caps and computerized detonation system. The purpose of each is to produce a number of smail explosions rather than one large one, thereby reducing vibrations and hazards and intentionally creating blast waves which cancel each other out. To accomplish this, the engineered bore holes are filled with approximately 5 feet of high explosive material followed by a time delayed cap (normally 8 millisecond delay). On top of this cap is placed approximately 23 feet of insensitive explosive material consisting of fertilizer and fuel oil. This material is hauled to the site the morning of the blast in a specialized truck which keeps the fertilizer and oil separate until it enters the bore hole. A second time delayed blasting cap is placed on top of the fertilizer followed by another 5 feet of high explosive and capped off with 15 feet of aggregate and fine screening to form a plug. The purpose of the second time delayed blasting cap is "fail safe" to insure detonation. During detonation an electrical current is sent to the blasting cap which detonates the high explosive, thereby detonating the insensitive explosive. With 8 milliseconds,a second bore hole would detonate and so on until all bore holes are detonated. During normal operations of the quarry, blasting will occur approximately 1-1/2 times per week with a duration of approximately 1/4 second per blast. Prior to blasting events, a seismograph is placed in the quarry area at different locations for different shots to determine ground motion and air pressure pulse caused by the event. Although locations of these instruments vary, one is located on quarry property while others are located off site. The U.S. Department of the Interior, Bureau of Mines Reports of Investigations, RI 8506 3-27 „