The drilling pattern (drilling geometry) depends on certain parameters of which the blast-hole diameter is one of the most important. The following is a simple guide for estimating a suitable drilling pattern for bench blasting. It includes the different variables that affect the blasting result.
d, blast hole diameter, mm
The selection of a blast hole diameter depends on the type of work and the desired fragmentation of the rock. Large diameter holes give a better drilling economy but may at a later stage of the preparation of the rock lead to added costs. In large-scale mining operations, it is possible to use blast hole diameters of up to 400 mm while in a small-scale construction operation a blast hole diameter of 30 mm may be the best choice. Normally the rock calls for 0.4 kg of explosive per cubic meter to be well fragmented. With smaller blast holes the explosive is distributed more uniformly in the rock mass which normally gives better fragmentation.
|B, burden, m The distance between the blast hole and the free surface is called “burden”. The amount of explosive that a blast hole can take depends on the blast hole diameter and the density of the explosive. What the hole takes per linear meter is called charge concentration and is expressed in kg/meter There is a relation between the charge concentration of a defined explosive and the distance to the free surface. Different explosives give different burden|
Good blast design and execution are essential to successful mining operations. Improper or poor practices in blasting can have a severely negative impact on the economics of a mine. The use of excessive explosives at a mine site can result in damages to the rock structures and cause unwanted caving and large increases in support costs.
Blasting is used in both open pit and underground mining operations. While traditional blasting utilized black powder and dynamite, there are many different types of explosives used today. Common explosives used in industry now are ANFO (ammonium nitrate/fuel oil), slurries, and emulsions. Many factors are taken into account when determining what type of blast design or explosive will be used. Rock type, density, and strength are all important factors, as well as fracture condition of the rock, and water conditions.