Rock Products is the aggregates industry's leading source for market analysis and technology solutions, delivering critical content focusing on aggregates-processing equipment; operational efficiencies; management best practices; comprehensive market
Issue link: https://rock.epubxp.com/i/673147
24 • ROCK products • May 2016 www.rockproducts.com D riving through the mountains you may look up and see a wall nearly falling over with hanging boulders and loose rock. As you get closer, you may see small portions of half casts scattered along the wall. We all think the same thing: "Someone must have underbid the presplitting on this project." However, is this really the result of a bad job of drilling and blasting or is there a problem with the current methods of explosive presplitting used in today's blasting industry? The normal presplitting used in quarries and construction projects involves loading a drillhole with a presplit powder, small-cartridge emulsion or dynamite. This is done with little consideration to the rock types and the geologic dep- ositional environment with typical presplit powder loads able to break the strongest of granites and limestones. Generally, when one chooses to presplit the weak or heav- ily jointed with typical presplit blasting, the blasts are overloaded and the results can be poor. It makes no sense then to use these high powder loads, which leave the wall in bad condition with scattered half casts, bad backbreak and hanging boulders. This becomes a safety issue when loading and guarding must be put up to keep employees away from the crumbling face. For example, a highly jointed limestone quarry with 35-ft. benches needs to be presplit to provide a smooth final wall near a structure such as a crusher. The drill rig is capable of drilling a 3-in. hole. The question then becomes, what blasthole spacing should be used and what would be the powder load? A rule of thumb for normal presplitting would be to use a spacing that is 12 times the drillhole diameter, which in this case is 36 in. (3 ft.). Next the powder load needs to be determined, which unfortunately is normally the presplit powder that is available from the manufacturer. Figure 1 - Normal Presplit In this example, the available powder is 0.32 lb. per ft. The stemming for a normal presplit is estimated to be 12 to 14 times the drillhole diameter. Using a 3-in. hole, this is a stemming of 36 in. (3 ft.). The main concern with this is that while a large amount of stemming may reduce blowout, a smooth crack does not form to the top of the presplit, caus- ing overbreak at the top of the blast. Little attention is paid to timing and generally the presplit is fired instantaneously. Table 1 - Normal Presplit Design Bench Height 35 ft. Drillhole Diameter 3 in. Spacing 36 in. (3 ft.) Powder Load 0.32 lb./ft. Stemming 36 in. (3 ft.) Timing Instantaneous This presplit is then drilled out and loaded and the blaster hooks up the initiators. The blaster is waiting in eager antic- ipation that a smooth wall will result and that will eliminate all the safety concerns with the highwall. Precision Presplitting: The Basics Precision Presplitting Is Being Used In The Blasting Industry To Increase Safety And Prevent Overbreak In Weaker Rocks. By Anthony J. Konya and Calvin J. Konya This is the first in a two-part article about Precision Presplitting. –Ed. PRESPLITTING