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40 E&MJ; • SEPTEMBER 2017 www.e-mj.com BLASTING The V-Cut is a method for blasting under- ground headings that has become increas- ingly popular, especially with underground aggregate producers due to its effective- ness in reducing the cost per ton of rock produced and reduction of fi nes. In gen- eral, a V-Cut will utilize fewer holes than a burn cut by removing the overloaded burn holes and replacing them with angled holes that break to the natural free face. These angled holes are typically designed to have an appropriate burden; however, do incur a slightly larger cost than normal production holes due to setup time and drill accuracy and reduced depth of pull. While the V-Cut has become popular with underground aggregate operations, it is also used in all other areas of underground mining and in some underground construc- tion where large opening is available for the angled drilling. For example, the V-Cut has been successfully employed and used in Carbonifera Criciuma coal mine, Santa Caterina State in Brazil. The V-Cut normally produces decent fragmentation, except in the middle of the drift where large boulders are generally found in between the fi rst V. (Techniques to reduce these boulders will be discussed later in this article.) The V-Cut also generally throws material further down the drift and leaves a more scattered muck- pile that can reduce bucket fi ll factors and increase muck-haul cycle times. General Design Critera The V-Cut design criteria most often refer- enced is to have the fi rst set of holes at a 29° angle from perpendicular to the face, which is a total angle between holes of 58°, as illustrated by Bullock in Figure 1. The depth of the cut then will either be based off the maximum drill steel length available at this angle or the room width and the set- up point of the drill. The rule of thumb is in general 45%-50% of the room width can be achieved in face advance (Langfors & Kihlstrom, 1967). In older times these holes would meet and when using dyna- mites sympathetic detonation often oc- curred. With modern explosives, instead of sympathetic detonation precompression or deadpressing will occur due to cap scatter, which reduces the energy of the explosive or causes it to not detonate. In order to eliminate this, the back of the boreholes can be spaced apart 2 feet, as shown in Figure 2; however, this will increase over- size in the middle of the face and addition- al special techniques should be employed to reduce oversize. Another option for de- terioration of explosive energy with closely spaced holes is to use deadpress-resistant explosives, such as an emulsion sensitized with high-strength microspheres. After the initial V is set, the remain- der of the holes will have an equal back burden with a decreasing angle until the production holes are parallel and the rib holes have a slight look out angle. This re- duction in angles from subsequent holes is a choice of the design engineer and can in- fl uence muckpile dimensions and face ad- vance, with a general decrease in 8° to 10° for each borehole. It also results in a pe- culiar hole pattern layout on the face. The back burden can be calculated using the following burden formula (Konya, 2015): Where: B = Burden (feet) SG e = Specifi c Gravity of Explosive SG r = Specifi c Gravity of Rock d e = Diameter of Explosive (in) The spacing between the layers of these holes should then be 1.1 to 1.2 times the burden at the back of the hole, with longer holes generally having slightly larger spac- ing. The timing between the layers/rows in the Vs should be at least 50 milliseconds (ms) or greater in the burden [distance from free surface to borehole, normally de- fi ned on an angle with the V cut] and the Vs can be shot instantaneously or with an 8 ms delay if vibration is a concern. Modern V-Cut Design By Anthony Konya, Dr. Calvin J. Konya and Dr. Paul Worsey Figure 1—V-Cut plan view (older design) (Bullock, 1982).