INTRA-BOREHOLE SLOWDOWN (EN: interhole slowing-down, interhole retardation; DE: Intervallzundung im Bohrloch, Explosionsverzogerung im Bohrloch; FR: tir à cour retard des parties d'une charge fractionnee dans le trou; ES: retardo en la explosion de los barrenos; RU: внутрискважное замедление) is the method for the blasting of the charge of the explosive substances, which is distributed within the borehole, with the different times (from the thousandths parts to several milliseconds) for the initiating of the individual parts of this charge.
This method has been developed within the USSR during the 1965-67. This method is most effective in case of the multiple-row blasting at the open-pit mines, because this method permits us to enhance the uniformity of the crushing within all the zones of the massif of the rocks, which is being broken. The major parameters for the intra-borehole slowdown (the structure of the charge, the duration of the interval of the slowdown, and the general scheme for the placement of the retarders for the charges, which are being exploded) depend mainly on the height of the bench, on the physical-mechanical properties of the massif, which is being blasted, on the specific energy of the explosive substance, which is being used. In case of the blasting with the intra-borehole slowdown, the charge of the explosive substances is distributed within the borehole (using the tamping or air gap) into the two or three charges (in case of the high benches). The major charge is the lower charge with the mass (in case of the division into 2 parts) of 70-80% of the total mass of the explosive substances within the borehole. The decreased upper charge permits us to diminish the height of the tamping, and to enhance the effectiveness of the crushing for the upper portion of the bench. The length of the tamping between the charges (usually 15-20% of the length of the borehole) depends on the interval of the slowdown. For example, for the rocky rocks, this value amounts to 10-25 milliseconds. The most simple and reliable metho d for the performance of the slowdowns within the borehole is the usage of the different-speed detonating cords, electric detonators, and pyrotechnic retarders.
There is optimal for the intense crushing of the rocks the scheme for the placement of the slowdowns, in case of which the upper and lower charges are exploded simultaneously within the adjacent boreholes within the row and between the rows; the improved working of the sole of the bench is achieved during the simultaneous exploding of all the lower charges, and later of all the upper charges within the row. The effectiveness of the intra-borehole slowdown is enhanced in case of the cushioned blasting.
They use successfully the method for the intra-borehole slowdown at the iron ore and limestone open-pit mines, which permits them to enhance the intensity of the crushing by 20% on average, to decrease the specific consumption of the explosive substances, to improve the shape of the ejection (the compactness) of the mining mass, which has been blasted. Abroad, they also use the blasting with the intra-borehole slowdown for the underground breaking of the ores using the deep boreholes with the usage of the multi-channel detonating machines; this method permits them to decrease the seismic effect, and to increase the intensity of the crushing.
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