14 MAY 2023 WorldWide Drilling Resource® A Good Blast Starts with Proper Planning Adapted from Information by The World of Explosives The blast design process begins with a good understanding of the existing site conditions. Blasting professionals determine how explosives will be applied to the rock set to be blasted. The goal is to distribute the explosives throughout the rock mass so the rock breakage achieves the desired result, is easily movable by the excavation equipment, and generates minimal adverse environmental effects. Blastholes - The blasthole diameter is often selected based on the depth of the excavation. Shallow excavations generally use smaller diameter holes than deeper excavations. Selecting an Explosive - There are many types of explosives and each are designed to address specific site conditions. Selection of explosives is typically based on: ( Diameter of the blasthole ( Hardness of the rock ( Depth of rock ( Presence of water ( Type of excavation Determining the Blasthole Pattern - Once the diameter of the hole and the explosives type have been selected, the next step is to determine the layout of the holes - the blasthole pattern. With the proper layout, the energy from the explosives can be optimized to break the rock. To distribute the explosives throughout the rock mass, holes are drilled in the rock in a grid-like pattern. The distance between the holes is critical. Each blasthole is intended to break the rock beside the hole, towards the open excavation. Regardless of the type of explosive used, there is only so much energy available to break the rock. If the blastholes are too far apart, the energy will not be enough to create the desired breakage. On the other hand, if the blastholes are too close together, energy will be wasted. Another important part of the blasthole pattern is the number of holes which is largely dependent upon how much material needs to be removed by each blast. Detonation Sequence of the Blastholes - While a blast may feel and sound like all the blastholes explode at once, in reality, each hole detonates separately, just a few milliseconds apart (1 millisecond equals 1/1000th of a second). This allows the blastholes closest to the open excavation to detonate and move rock into the open space first. Then the blastholes behind the first holes can move rock horizontally into the new open space. Blasting professionals accomplish this timing between holes using detonators or blasting caps. Each blasting cap contains a timing element which controls the firing time. The timing sequence in the blast is created by using specialized detonators which either have specific timing built into them, or are programmed to detonate at the intended time. There are two main reasons for setting off the blastholes in a planned sequence: Improved Rock Breakage - The timing sequence can be designed to control how the rock moves and the size of the broken rock. Controlling Off-site Effects - Separating the detonations also helps minimize ground vibration and air overpressure. These are some of the basic design elements from the planning phase of a blast. With a properly calculated and welldesigned plan, the blasting professional is ready to execute a productive blast. EXB
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