Blast Design and Physical Layout of Explosives
Rock Blasting is usually achieved by the drilling of holes into the rock. These holes are spaced to achieve the appropriate rock fragmentation, shearing, and heave needed for the project. Several common terms are used to refer to the blast design and physical layout of the explosives and holes. Rock blasting is the science and art of the use of controlled explosive energy to fragment, displace, and shear — thus facilitating the removal of rock. It can be used both for surface and subsurface rock excavation and for rock removal underwater. When this explosive energy is released inside rock, it produces both fragmentation of the rock and heave (displacing the rock from its in-situ condition). Blasts can be designed to fragment rock only for ease of removal, but can also be designed to fragment rock into smaller sizes useful for the production of rock products such as rip-rap.
Backbreak and Overbreak are fairly self-explanatory terms that denote rock breakage beyond the intended limits of excavation. In some usage, these terms are distinguished in that “backbreak” refers to fracturing beyond the limits of excavation, “endbreak” refers to fracturing beyond the edge or side limits of the blasting pattern, and “overbreak” refers to the actual removal of rock beyond the intended limits of the excavation.
Boreholes are holes drilled in rock into which explosives are placed (Figure 1-2). These are generally drilled using “destructive” drilling techniques that do not leave a rock sample such as core behind.
Burden is the volume of rock to be fragmented and displaced by blasting. There are two kinds, the drilled burden and the shot burden. (Figure 1-3) Illustrates this and next four terms.)
Drilled Burden is defined as the distance between a row of boreholes and the nearest free face. It is always measured perpendicular to a row. It is also the distance between any two rows of boreholes. When laying out a blasting pattern for a shot, this is the term usually meant when using the word “burden.”
True Burden is defined as the perpendicular distance between a single borehole containing explosives and the nearest free face.
Spacing is defined as the distance between holes that are located in a row. Drilling patterns are always defined as this spacing and the burden and spacing (e.g., for a 5 x 6 pattern, the blast design has a burden of 5 ft and a spacing between boreholes of 6 ft).
Relief is the presence of a free face in the rock mass such that the blasted rock can displace into that space as it heaves and expands due to the detonation. It can be a ledge or bench face or an internal face created by previous holes firing.
Free Face is defined as the nearest open face or relief. In rock blasting, this is at the edge of the rock face or relief created by previously fired blastholes. It is also the top surface of the rock that will be blasted. Features such as joints, faults, bedding planes, voids, and other discontinuities are not considered free faces because they do not allow for relief.
Decking is a method to create unloaded zones in an explosive column in a blasthole. “Decks” are often created by using stemming to separate several layers of explosives in a loaded hole. Decks may be used to increase the efficiency of the blast, to limit the amount of explosives at any given delay, or to accommodate a weak layer or void that has been encountered in the rock. Air decks are unloaded portions of the explosive column that contain no explosive or stemming materials.
Depth of Advance (underground blasting) is the total length of the borehole that broke from the formation or the distance a blaster wants to break down to the intended grade of the blast.
Stemming is the inert material put in a borehole to provide confinement along the axis of the borehole. Material used for stemming is commonly small sized crushed aggregate (Figure 1-4). Note that Corps’ practice generally forbids the use of drill cuttings as stemming.
Subdrill is the length of borehole drilled below finished grade or the bottom grade of the intended blast.
Swell is the term used to account for the increase in volume of rock that has been blasted or otherwise excavated. The volume increases from the in-situ or in-bank condition because the piled rock fragments take up more space after the blast because when there is considerably more void space between the rock boulders and fragments than in the intact (pre-blast) condition.
Swell Factor is the percentage of increase in volume expected due to blasting or excavation.