Blasting is the breaking of rock masses using explosives to obtain the fragments and separate rock materials for further processing. There is always a need to have effective free face for blasting. In essence blasting is done to blast overburden, ore deposit and coal seams so that the power requirement for excavators to extract the materials becomes less. It also reduces clearing time of materials and decreases operation costs. Blasting operations involve (1) setting up a drill pattern (2) drilling blast holes (3) placing explosives in the blast holes (4) detonating the explosives (5) getting rid of the fragmented material (6) secondary blasting if there is overburden. Different blasting methods in mining are used to fragment bulk rock masses into loose forms, primarily using explosives. The explosives are used to produce a volume of rapidly expanding gas that exerts sudden pressure on its surroundings and break the mass into pieces. Selection of explosives factors in critical diameter, hydrostatic pressure, temperature and shelf life amongst many others. An optimum blast is associated with the most efficient use of blasting energy in the rock-breaking process. Regardless of the blasting method chosen, a good blast design and proper execution are essential for a successful mining operation. Key factors affecting blast design are:
The article will cover types of blasting methods and patterns including blasting techniques in opencast mining.
What are the types of blasting methods?
The major method adopted in mining is controlled blasting. It is a method of blasting for the purpose of reducing the amount of over break and to control the ground vibrations. There are different types of controlled blasting methods. Pre-Splitting is an old but highly recognised method with is done to create a fracture plane beyond which the radial cracks from blasting cannot travel. It involves loading a single row of holes that have been drilled along a desired highwall crest or excavation line. Pre-splitting reduces the crushing effect around the borehole and are shot before the main production shot. The idea is to minimize or eliminate overbreak from the primary blast and to produce a smooth rock wall. Other methods include smooth blasting also known as contour or perimeter blasting. It is used for underground mines and muffle blasting as a solution to prevent fly-rock from damaging human habitants and structures. The term smooth blasting refers to lightly loaded holes that have been drilled along excavation limits and are shot after the main excavation is removed. Such holes are shot instantly or with little delay. They leave a smooth wall with minimum overbreak. Cushion blasting method or trim blasting method is similar to smooth blasting. In this method, the holes are shot after the main production shot. Cushion blasting involves backfilling the entire borehole with crushed stone to cushion the shock from the finished wall. Next, we discuss blasting patterns in depth.
What are the types of blasting patterns?
There are three major types of standard blast patterns. These are square grid, rectangular grid and staggered patterns. In staggered pattern the alignments are diagonal. First row blast holes and third row blast holes are in alignment. The drill holes may be vertical, inclined or horizontal. In opencast mines, both vertical and inclined holes are parallel to the bench face. For staggered patterns, the row of the holes may be a single row blasting pattern or a multiple-row blasting pattern. In single row blasting, the fragmentation is low and the explosive consumption may be more than multi-row blasting. Preference is usually for multi-row blasting pattern. The blast timing and triggering sequence plays a dominant role. It is called a delay pattern or firing sequence. The delay patterns, and variation of the hole array are set to fit natural excavation requirements. This allows for an efficient use of the explosive energy in the blast. The types of blast patterns are for different blast designs which depend on the kind of the rock materials. The alternate delay pattern is used for softer rocks. The consecutive short delay pattern is used for rocks with medium hardness and the short delay firing with a cut is used for hard rocks.
What blasting techniques are used in opencut mines?
Bench blasting is a common technique used in open-pit mines. It is blasting in a vertical or sub-vertical hole or a row of holes towards a free vertical surface. More than one row of holes can be blasted in the same round. A time delay in the detonation between the rows creates new free surfaces for each row. There are three methods in the process. These are short-hole blasting, long-hole bench blasting and ring drilling and blasting methods. Short-hole blasting is usually limited to drilling rounds of 1.2 m to 5.0 m length and hole diameters of up to 43 mm. Cut and fill and room and pillar underground mining methods commonly utilize short-hole blasting. The long-hole bench blasting is similar to bench blasting in open pits, using long holes drilled downward either parallel to each other or in slight rings to cover the stope area. Initiation of the blast is with a booster down the hole. Ring drilling and blasting is done from a series of sub level drifts developed in the ore body. The drill pattern is designed to cover off the extent of the ore in the stope. This type of blasting will cause the ore to swell by 30% and this must be allowed for when blasting otherwise the blast may freeze.
Bringing everything together – before it blows apart!
Integration of all factors is key to achieving good fragmentation. It considers the internal environment, external environment, drill pattern, loading and haulage of the fragmented ore materials. The internal environment is set to achieve minimum wall damage. Externally you want to obtain minimum fly rock, noise, air blast and ground vibration. Drilling should be done in a specified drill pattern. Finally, loading and haulage is much easier if good fragmentation occurred. This makes pile shape much accessible and “diggability” much more effective. Planning is a must to achieve success in the implementation of any blasting method in mining. A good blast design will produce good result as is evident in the release of broken rock masses. In summing up we cement the discussion with mention of why blasting is performed. The main technical objective of blasting are to excavate a planned volume of rock within defined boundaries, to achieve acceptable fragmentation and to avoid scattering broken rock. Finally, it is critical to avoid overbreak in order to develop neat and stable excavations.
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