1999-01-19_REVISION - M1977326 (5)

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Review of Blasting Operations at Robinson Brick Company's Siloam Clay Mine Both governmental agencies and private sector investigators have conducted considerable research to determine how ground motion might influence the stability of rock and soil structures.. or cause damage to typical structures. For instance, from U.S. Bureau of Mines studies (Bulletin 656, 1979 and RI 8507, 1980) researchers have recommended conservative criterion that most government agencies, including Colorado, have adopted in commercial blasting regulations. To prevent blast-induced ground motion from even approaching damage thresholds, these regulations establish extremely cautious limits on peak particle velocity (ppv). Some damage prevention regulations also consider the frequency of ground motion. Other researchers have evaluated the effects of ground motion on people, buried pipelines, water wells, and many other structures. In. this investigation, GEOTEK has analyzed the potential risk of blast-induced vibration damage or' disturbances to structures and persons located near the Siloam Clay Mine. 2.1.2 Potential for Damage to Structures from Blast-induced Ground Motion: The United States Bureau of Mines, in RI 8507 (1980) made the following conclusions regarding the potential for damage caused by blast-induced ground motion. "Practical safe criteria for blasts that generate lour frequency ground vibrations are 19 mm/s (0.75 in's)for modern gypsum board houses curd 12.7 mm's (0.50 in.$) for plaster on lath interiors. For frequencies above 40 Hz. a safe particle velociry niasimum of 51 mm/s (2.0 in/s) is recommended for all houses. " It should be noted that the USBM RI 8507 recommendations are designed to prevent even threshold damage in residences of lower-quality construction. Threshold damage was defined as: "Threshold damage was defined as the occurrence of cosmetic damage; that is, the most superficial interior cracking of the type that develops in all homes independent of blasting. " In an earlier USBM study (Bulletin 656, Nichols et al., 1971), it was concluded that if at least 8 milliseconds of delay time separates charges, their effect on ground motion is not cumulative. The "8-millisecond rule" remains an industry and regulatory standard for defining "separate" delays. GEOTEK&Associates, Inc. Page 6 January 99