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• -f <br />Evaluation of Proposed Blasting at [hc ACA Riprap Quarry near Buena Vista, CO ____•_ _. , <br />When historical or test blast data is not available, blasting engineers can use estimated values of <br />K, based on relative blast confinement and elastic rack modulus data, to predict intensity of <br />blast-induced mound motion. .E'or blasts occurring in cwnpetent and hard (20,000 psi) <br />grartodiorite, the following equation using a K-factor of 240, cart be used to estimate maximum <br />vibration intensity (PPV). The estimated 24(1-K-Constant assumes that blasts will be designed <br />with adequate burden relief for all charges. <br />IG <br />•_ D <br />PPY - 2401 / ~~ 1 Cquation 2.2 <br />Using this relstionsltip, if the maximum charge-per-delay (~V) is 50 pounds, the intensity of <br />gr•ottnd motion itt rock at the closest foundation location of the date, ] 50 feet away (D = 150 ft), <br />would likely not exceed ].8 in/s [240 x (150!50"')"' `]. <br />Itt prior work at close-in blasting projects, experienced blasters and engineers (Kilter, 1998; <br />Oriatd and Sheeran, 1998; Rcvcy atld Scott 1998, Rcvc}• anti liartison, 1985; Oriard, 1951; Tart <br />et al, 1980; and Matehesvs, 19781 have learned that heavy steel and concrete structures can <br />ss~itlrstand vibration at intensities that would seemingly destroy theta. Unfortunately, damage <br />criteria developed rn protect w=ood-frame buildings front cc,sntetic hairline crackint; are often <br />ittappropriately applied to protect massive civ!! stuctures. Criteria, with peak-particle-velocity <br />limits ranging froth 12.E to 51 marls (0.5 to Z.U tars) are taken from United Stales 13tus:au of <br />Mines (iJSBiVI) reports and ate written in into blasting specifications where the intent is to <br />protect heav7~ steel and concrete structures or buried pipes. However, the prior case histories <br />have proven thut ]ranvy concrete sad steel structttres like the Trout Cmelc RCC Dam ran easily <br />survive vibrations with intensities exceeding 5 in/s, especially when the frequency of ntotiort is <br />relatively high (> 30 Hz). Unlike earthquakes where the frequency of motion is azound 1 1•lz, the <br />maximum ground displacement created b high-frequency blast motion is very low. Pot instance, <br />for a PPV of 4 in's. at a frequency of 56 Hz, maximum displacement would be around 0.01? <br />inches. Whereas earthquake oration of tLc same intensity at 1 Ilz yields a maximum <br />displacement of 0.63 inches, which is ~3 times higher than that of the ~0-Hz blast motion. 'I'bis <br />lots=fr>rquency charaeterisric of earthquakes explains why they do indeed damage to strucrures. <br />REVF.Y Associates, lac. Page 6 July ?001 <br />