1989-03-17_REPORT - M1977378 - Laserfiche WebLink

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Mr. Larry Petrino August 15, 1988 Page Two • Gravity loaded with ANFO column 8 ft high. • Top 3 ft, no explosive and no stemming. • Detonation by NONEL with 0 to 15 ms delays. • Bottom primed. • Area blasted in single shot up to 15 ft wide by 200 ft long. Based upon the blasting specifications, we calculate a current powder factor of 2.7 lbs ANFO per cubic yard or 1.31 lbs ANFO per short ton. Powder factor varies depending upon the rock type, rock structure, and hole pattern, however, it appears to be high based upon open pit practice. An empirical relationship between powder factor, fracture frequency, and joint shear strength is attached as Figure 1.0, and may provide some general guidance. Your application is probably in the 0 to 2 fractures/foot range, with friction angles of 35 to 40 degrees. Note that the curves are very steep in this range; however, discussions with some of our other clients suggest that your powder factor should be in the range of 0.75 to 1.0 lbs per ton. The burden and spacing of the holes is small for open pit blasting, however, it is necessary to prevent cratering of the shallow holes. You may be able to reduce the hole charge, and thereby, reduce the blast damage sustained at the final wall. Changing this parameter will require some experimentation by your site personnel who have experience. Delays Between Holes and Rows Recent work has shown that increasing the delays between holes and rows will reduce the damage induced on the final wall. Blast damage is strongly correlated with the peak particle velocity generated by the blast. The initial pressure wave associated with detonation of the primer has the greatest impact on peak particle velocity. This effect is illustrated by Figure 2.0, which shows particle velocity as a function of distance from the blast and quantity of explosive detonated per delay. Based upon my conversation with Mike Fouts, your largest-scale blast (15 ft by 200 ft) could involve a last row of 33 holes with roughly 900 lbs of ANFO detonated by the last delay. Using Figure 2.0, this would be sufficient to cause the breakage of rock (particle velocities of 100 in./sec) a distance of 43 ft from the last row of holes. The size of the charge per delay can be reduced by using delays between holes on each row and delays between rows. I've attached a copy of USBM RI 9008, which describes the results of recent work to improve highwall stability in open cut coal mining. This work used 17 ms delays between holes and 100 to 150 ms delays between rows. The net effect is to reduce the instantaneous charge per delay to a single hole and to assure that the burden of the previous row is removed from the current holes. Stability is also enhanced when the direction of burden movement is perpendicular to the final pit wall.