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www.rockproducts.com ROCK products • May 2016 • 25 The blast goes off and after the dust settles everyone watch- ing is upset that the wall is still badly broken and the high- wall still poses a risk to employees. In many cases the blaster would just give up believing that bad geology is to blame and that this type of rock cannot be presplit. However, it is not the geology but the problems with our typical presplit design that is to blame. About 30 years ago we were facing these same problems when trying to presplit highly weathered and very soft rocks. Needing to create a design that would work in all situations, we tried what is now called "precision presplitting." This new form of presplitting would use the rock characteristics to help determine the spacing and explosive load. Figure 2 shows the original test of precision presplitting in a sand- stone quarry. Figure 2 - Original Presplit Test While testing this new method, it was realized that the con- tinuous powder loads used in traditional presplitting was too powerful and this is what was causing the large destruc- tion (overbreak) of the back walls. By switching the explo- sive from continuous lengths of presplit powders to deto- nating cord with small dynamite, charges spaced out on the detonating cord. Later it was found that multiple strands of detonating cord were easier and faster to load and the exact explosive requirements can be set and accurately loaded. Using this method, even weak siltstones can be presplit to form smooth, solid walls. In the previous example the blaster decided to try this new technique of precision presplitting. He again had a 35-ft. highwall and a drill rig that could drill a 3-in. hole. With pre- cision presplitting that typical spacing used was 24 in., and this proved to work in almost all situations. The explosive load for this blast was 550 grains of detonating cord per ft. (0.08 lb. per ft.) This time a stemming of only 10 times the drillhole was used; 30 in. and the entire presplit will be fired instantaneously. Table 2 - Precision Presplit Table Bench Height 35 ft. Drillhole Diameter 3 in. Spacing 24 in. (2 ft.) Powder Load 0.08 lb./ft. Stemming 30 in. Timing Instantaneous Figure 3 - Precision Presplit The blast was tied in and set off, and after the dust settled a smooth wall was shining in the distance. They have done it and the blaster cannot be happier! In this example, the presplit was in homogenous rock, but in many cases the presplit has to run through multiple rock lay- ers: siltstone, shale, limestones and sandstones. In this situ- ation one can vary the explosive load at the proper intervals to account for all the different rock types. This can be done by taping different lengths of detonating cord to increase the explosive load where needed and having the main strand of detonating cord be for the weakest zone. For example: Table 3 - Varying Explosive Loads Rock Type Elevation Explosive Load Limestone 0-20 550 grains per ft. Shale 20-25 300 grains per ft. Limestone 25-35 550 grains per ft.