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60 • ROCK products • March 2018 www.rockproducts.com Modern Design in Blasting W hen starting a new project from a flat surface or moving down to another bench, a sinking cut or drop cut (words used interchangeably) is used to open the new area up for production blasting. This is very different from a pro- duction blast as only one free face exists for the material to initially move towards. Therefore, the initial holes only have one method of relief and must not only heavily fragment the rock but also eject the material to allow subsequent holes to fire under more optimal conditions. To understand how a sinking cut is different from a typical bench blast, one must first understand how bench blasting works and a term called the borehole effect. To understand this and clear up and confusion that may purvey a review back to the early 1950s is in order. In the beginning of the 1960s, high-speed photography appeared on the scene and as a new scientific tool that could study fast events it was quickly applied to explosives and blasting. One of the first to do this was Dr. Hino who published information unconfined charges breaking concrete beams through shock breakage, the only issue was he never was a field blaster and used over eight times the powder load of rock blasting and never confined his charges as is done in rock blasting. However, his work was then taken and expanded by Duvall and Atchinson into a shock breakage theory for rock blast- ing. While this propagated through the 1970s and 1980s as a method to sell more expensive explosives and allow for researchers to get large federally funded grants; it had no practical application. Today the industry understands that shock energy is not what breaks rock in a typical bench blast, but that the gas pressure breaks and moves the rock from a bench blast or situation where a charge is confined. This leads to the phenomena of the borehole effect, which states that explosives in a borehole will break along the side of the borehole, as long as the borehole is long enough that the charge will not crater. This breaks under the gas pressure that is generated by the explosive. Why is this important? Well a sinking cut does not contain a free face that is parallel to the borehole, at least initially. It is then important that a sinking cut quickly create a free face in order to be efficient and cost effective. Old Methods for Sinking Cuts In the past, sinking cuts used to be shaped like a large rect- angle and be fired in full rows. Two rows of holes would be placed close together (one burden distance apart) with the thought that this will double the powder factor in this region and cause a bit of movement of the material. While it is true that this does increase the powder factor in this region, it does not necessarily force the material to eject from the center area. If this center material does not eject then the subsequent rows firing will either shoot and have poor fragmentation and movement, or they will crater caus- ing flyrock and a violent blast with poor fragmentation. This can also lead to problems with toe being left after the shot. In addition, this method of firing a sinking cut is not very eco- nomic. Two entire rows of holes are firing at half the burden, leading to large cost increases. With the cost and unreliable nature of this type of sinking cut a new, modern type of sink- ing cut needed to be developed. Modern Design of Sinking Cuts The modern design of a sinking cut requires understanding of the borehole effect and how blasting actually works. Armed with this knowledge one can begin to change the blast pat- tern to ensure optimum breakage, blasting to proper depth, and efficient economically. The sinking cut will now be a square drilled pattern instead of a rectangular pattern and instead of shooting rows squares or spirals will be shot. The pattern will now have two types of holes, opening holes and production holes with all holes being drilled exactly one burden distance apart. While the production holes will have normal production hole burden they will have slightly less than normal spacing, creating a slightly worse economic condition than a typical production blast. Modern Design of Sinking Cuts The Modern Design of a Sinking Cut Requires Understanding of the Borehole Effect and How Blasting Actually Works. By: Anthony Konya and Dr. Calvin J. Konya