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24 www.coalage.com December 2018 blasting continued to use long rows with many holes yet narrow patterns (minimum number of rows) in order to maximize the throw. In order to determine optimal bench width, a site-specific study should be conduct- ed analyzing the cast percentage achiev- able versus the economic considerations of the overburden excavation and the coal production. At a certain point, small bench widths can provide problems op- erating equipment and slow down the process of coal removal. Bench Height-to-Pit Width Ratio The final bench height ratio that will be discussed in this article is the Bench Height to Pit Width, which will work in tandem with the Bench Height-to-Blast Width Ratio. In this situation, reductions in Pit Width (or excavation width) will be considered on the casting effectiveness. The pit width will strongly influence how far the material being cast will have to be thrown. The narrower the pit width, the higher the casting effectiveness. Figure 6 shows a site-specific graphic that was made to illustrate the differences in cast- ing effectiveness compared to pit width and the powder factor used on-site. It can be seen that with a bench height equal to the pit width, the casting is much better than when the bench height is half of the pit width. It is critical to understand that the blast width and pit width will, in gener- al, be the same. The blast width is typi- cally set based on the pit width, and the pit width is set based on the equipment used and the area needed for excavation of overburden and coal. When a mine is already in operation, this pit width should be fully analyzed and when pos- sible minimized in order to facilitate bet- ter casting and a more efficient opera- tion. When a new operation is specifying equipment, it is critical to understand the relationships between blast width, pit width, and bench height to ensure that the proper equipment for extraction of the coal is specified so the pit width is not too large and proper estimates can be made for cast percentages. To optimize the casting effectiveness based on these parameters and others, mines should have site-specific models developed to ensure they are minimizing their overall costs. These models should be developed using two factor studies to understand the full-scale interaction of variables. Timing The last design variable that will be ana- lyzed is the timing of the blast. In today's new electronic detonators, especially in large boreholes where the increased cost is minimal compared to total blast cost, give blasters and mines the ability to refine and tune in cast effectiveness, fragmentation, ground vibration, and numerous other blast outcomes. How- ever, many using electronic caps are not benefitting from the full effectiveness of the caps because site-specific studies have not been completed and true un- COMPANY PROFILE-PAID ADVERTISEMENT For over forty years, Polydeck Screen Corporation has been the leading provider of modular screening solutions for the aggregate, coal, and mining industries. Their extensive selection of screen media, frame systems, and accessories has helped companies save time, increase production output, extend screen service life and improve worker safety. Their screen media product lines include: Polydex ® – Modular polyurethane screen media in 1' x 1' and 1' x 2' sizes that feature the most extensive range of opening sizes, shapes and configurations available today. Since all polyurethane screen panels are made with a high quality injection molding process, surface features like dams and restricted flow bars are an integral part of the panel, not a glued or bolted on option. 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