Engineering & Mining Journal

AUG 2017

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Page 38 of 99

BLASTING AUGUST 2017 ā€¢ E&MJ 37 along the rib and back of the round as a fi- nal row of holes. These holes are designed with a trim shot mechanism to smoothly contour and control the rib and back, re- duce blast damage and minimize the total amount of scaling after the round is fired. Relief Holes The relief holes used in the burn cut round are one of the most important design cri- teria for controlling and maximizing the face advance. Two main factors to consid- er with the relief hole is the diameter or equivalent diameter and the length of the relief hole. These both influence the stress relief of the cut and the distance from the relief hole(s) that the first cut hole will be placed. The second factor that will influ- ence the total pull is the length of the re- lief hole, and it has been found that the re- lief hole should optimally be drilled 0.5 ft to 1 ft beyond the length of the cut holes. In many cases, a mine does not have the drilling capabilities to drill larger diameter holes for the relief holes, yet would like to decrease the amount of loaded holes in the shot. To accomplish this, multiple holes are drilled close to- gether, giving the effect of a larger di- ameter borehole with a diameter called the equivalent diameter. To calculate the equivalent diameter one can use: Where: D H = Equivalent Diameter of Single Hole d H = Diameter of Drilled Holes N = Number of Empty Holes For three, 2-in. holes drilled in very close orientation to each other, one can then solve for the equivalent diameter as follows: Cut Holes The cut holes in the burn round are the first holes fired, breaking, and pushing out the material between them and the relief holes. The spacing between the relief holes and cut holes are critical. If improperly spaced, the round will freeze and there will be minimal fragmentation. The spacing can be improper in two ways. If the cut and relief holes are too close to each other, the material will move into the relief hole (while swelling from break- age) and recompact, the material will then not be properly ejected, causing the round to subsequently freeze. If the cut holes are too far from the relief hole, then the material between will be too large of a volume and fill the open space, also causing the face to freeze. In some pat- terns, numerous layers of cut holes are used with each layer having a larger bur- den from the previous cut (Figure 2). This allows for volume expansion of the mate- rial and will take place until the cut hole would have a similar or larger calculated burden than that of the production holes. In general, cut holes should have at least 50 milliseconds (ms) between them firing for the material to be broken and cleared. There are many dozens, if not hun- dreds of different orientations for the placement of relief and cut holes, how- ever, the distance from the first set of cut holes to the relief holes has been stan- dardized based on the geometric layout of the round. The calculation of the initial burden between the relief hole and the cut hole is shown in equation 2. Where: B 1 = First Cut Hole Burden (measured center to center) D H = Equivalent Diameter Drill Hole Subsequent layers of burn holes such as B2, B3 and B4 use equation 3. Where: B n = Subsequent cut layer hole burden (measured center to edge of the hole form the previous cut layer) W nā€”1 = The width of the previous cut layer. Note it is assumed for the design to work that the material from the previous cut layer to be blasted has been ejected from the cut before the next layer fires, thus developing progressively larger relief Diagrams 1-11 ā€” Burn sequence (red stars indicate detonation of borehole; gray area is an empty area available for relief).

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