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Impacts & Controls for Blast Noise , 2.3 cont' <br />Overpressure Effects and Equivalent Units <br />psi kPa Effect <br />2.90000 20.0000 Structural Damage <br />0.91706 6.3246 Most Windows Break <br />0.29000 2.0000 <br />0.09171 0.6325 Some Windows Break <br />o,oz9oo 6.2000 ost-u Maximum for Impulse Sound <br />0.01453 0.1002 Pressure Produced by 20 mph wind <br />0.00290 0.0200 <br />0.00092 0.0063 <br />0.00029 0.0020 <br />0.00009 0.0006 OSHA Maximum for 8 Hours <br />0.00003 0.0002 <br />Figure 2.6--Overpressure Effects and Scale Comparisons <br />Controlling Air Overpressure: <br />It is difficult to accurately predict airblast with distance. Unlike vibration predictions <br />where seismic energy is generally proportional to the weight of a confined charge, airblast <br />is caused by venting gases, moving rock and the vertical component of vibrating ground, <br />which are hard to predict. It is well known that airblast intensity is reduced when blast <br />charges are well confined. Conversely, when charges are poorly confined due to <br />inadequate stemming or by prematurely breaching through open cracks, the level of <br />airblast is increased. As previously recommended in Section 2.1.4, AG has recommended <br />that at least two feet of dirt be placed over all blastholes before shooting. The confinement <br />provided by this dirt will reduce flyrock and it will also buffer the blast noise. <br />The following equation developed iii a US Bureau of Mines Study (12I 8485), airblast can <br />be used to estimate overpressure, based on charge weight and distance. <br />333 -1.467 <br />Overpressure (psi) = 52.7(D/W°' ) Equation 3.2 <br />22 <br />