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Impacts & Controls for Blast Noise , 2.3 cons' <br />Overpressure Effects and Equivalent Units <br />d8-L psi kPa <br />180 2.90000 20.0000 <br />170 0.91706 6.3246 <br />160 0.29000 2.0000 <br />150 0.09171 D.6325 <br />140 Q,02900 6.2000 <br />134 0.01453 0.1002 <br />120 0.00290 0,0200 <br />110 0.00092 0.0063 <br />1 pp 0.00029 0.0020 <br />g0 0.00009 0.0006 <br />60 0.00003 0.0002 <br />Effect <br />Structural Damage <br />Most Windows Break <br />Some Windows Break <br />osr>p. Maximum for Impulse Sound <br />Pressure Produced by 20 mph wintl <br />OSHA Maximum for 8 Hours <br />Figure 2.6--Overpressure Effects and Scale Comparisons <br />2.3-2 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 ]east 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 (RI 8485), airblast can <br />be used to estimate overpressure, based on charge weight and distance. <br />333 1.467 <br />Overpresswe (psi) = 52.7(D/W° )" Equation 3.2 <br />22 <br />