Explosive cast blasting has seen widespread use in recent years. It is as much a method of pit design and equipment utilization as it is a method of blasting. Cast blasting is a type of blast design which utilizes the surplus explosive energy to move overburden material across the pit. It is most frequently employed in coal mines to remove overburden from above the coal seam. This is achieved by arranging pits in long and narrow configurations that facilitate the movement of the blasted rock into the previously mined pit. This technique allows approximately 25% to 50% of the rock to be moved without the use of mine equipment. Cast blasting can be employed with a a dozer push operation that follows the casting operation. The combination of these two techniques can typically remove 60% to 80% of the overburden that originally existed over the coal seam.

 

Although cast blasting requires more kilograms of explosive for each cubic metre of rock to be moved, it does not necessarily mean that ground vibration intensity will also increase. More of the explosive energy is used in moving the rock across the pit. Blast design is very important to the performance of the cast blast. A properly designed cast blast often generates less vibration than a conventional blast design. This is because the explosive energy is moving the rock across the pit and less energy is available to be transmitted to the surrounding environment. A poor cast blast design can lead to the rock not moving across the pit as it should and, consequently, higher ground vibrations.

 

Cast blasting usually employs pre-splitting in order to develop a vertical face. A vertical face reduces the toe burden, thereby increasing the efficiency of the cast blast. The efficiency or "percentage blast-over" of a cast blast is a measure of the volume of overburden thrown by the blast over the coal or mineral seam, as a percentage of the total volume of the blast. (Atkinson, 2000). Based on measurements at a number of cast blasting operations, the cast blast efficiency can be estimated using the following relationship:

Cast Blast Efficiency = 57.5(d/w) + 18
Where, d = depth of pit to the base of the mineral/coal seam w = width of exposed strip