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5.0 Environmental Analysis 5.6 Noise <br />Avion Project SEIR <br />Page 5.6-7 <br />Blasting <br />Noise <br />Blasting involves drilling boreholes and placing small amounts of explosives in each hole. By limiting <br />the amount of explosives in each hole, the blasting contractor can limit the fraction of the total <br />energy released at any single time, which can limit noise and vibration levels. When explosive <br />charges detonate in rock, almost all of the available energy from the explosion is used in breaking <br />and displacing the rock mass. However, some blast energy escapes into the atmosphere as a <br />sequence of airborne sound waves, a phenomenon known as “air blast over-pressure.” These sound <br />waves are very low frequency, below the audible range. Very high blast over-pressure levels can <br />rattle or in some cases break windows. However, air-blast over pressure rarely reaches levels that <br />could cause building damage with modern blasting practices. <br />According to the FHWA, within the audible frequency range, a blast generates maximum noise levels <br />on the order of 101 dB(A) Lmax. However, the total time for a blast would be seconds and only one <br />blasting event would occur in a given hour. Consequently, hourly noise levels from blasting are <br />calculated at 74 dB(A) Leq at 50 feet, and would not exceed 75 dB(A) Leq at the nearest residence <br />175 feet to the northeast. <br />Vibration <br />Vibration levels associated with blasting are site-specific and are dependent on the amount of <br />explosives used, soil conditions between the blast site and the receptor, and the elevation where <br />blasting would take place (specifically, how far below surface elevation where bedrock would be <br />encountered). At the current stage of project design, a blasting and monitoring plan has not been <br />completed. Therefore, specifics, such as the explosive, blasting quantities, and exact locations, have <br />not been identified. However, it can be assumed all blasting locations would be within the <br />boundaries of the non-rippable rock, and to be conservative, all the non-rippable rock is considered <br />a blasting location. Consequently, noise and vibration impacts from blasting are calculated from the <br />nearest location of the non-rippable rock to the nearest receiver, which is approximately 175 feet to <br />the northeast (see Figure 5.6-1). <br />Ranges of vibration levels have been predicted at various distances from potential blasting sites for <br />quantities of explosives ranging from 0.25 pound to 12 pounds per charge weight. The range of <br />vibration levels in this analysis is due primarily to the quantity of explosive, as all other parameters <br />were held constant. As shown in Table 5.6-2, blasting is predicted to generate vibration levels <br />ranging from 0.06 in/sec PPV (from a 0.25-pound charge) to 1.34 in/sec PPV (from a 12-pound <br />charge) at the nearest residence. Calculations are based on a receiver distance of 175 feet, which is <br />the approximate distance to the nearest receiver from a potential blasting location. Calculation <br />details are provided in the Noise Analysis (see Appendix E) <br />The resulting PPV from blasting can be decreased through best engineering practices used by <br />professional, licensed, blasters, including, but not limited to, orienting the progressions of the charges <br />away from receivers, decreasing confinement of the explosive energy, increasing spatial distribution of <br />the charges, and increasing time of energy release or detonation. The County of San Diego Fire Code <br />includes a minimum energy release time for individual charges of 8 milliseconds to limit vibrations.