A firing pattern in blasting operations is the carefully designed sequence and timing of explosive detonations in boreholes to control the movement and fragmentation of the rock or material being blasted. There are mainly five types of drilling pattern: “V” Pattern, Echelon delay pattern, flat face pattern, channel pattern and sinking hole pattern [1]. Let’s explore each of these patterns.
“V” Pattern [1]
This type of firing arrangement is made on either a square or rectangular drilling pattern. The blastholes on two rows have the same delay timings. These rows make an angle of 45° with the bench face and 90° with each other. They are in V shape as can be seen from the figure and hence the name. This firing pattern is also called a chevron pattern. V pattern is the most commonly used firing pattern in large surface mines because it has many advantages as listed below.
- The fragments of rock are in the form of a tall heap. Material in such a heap can be easily lifted by shovels and loaded into the dumpers.
- Since rock fragments are thrown towards each other in a 90° angle they collide with each other and further reduce in size.
- The collision of fragments reduces the hazard of flyrocks. Sometimes, when the number of rows fired in the same blast is more than five, two centrally located blastholes in the first row are given the same delay time. This firing pattern increases the probability of collision to give even better fragmentation and throws the muck pile a little farther.
Echelon delay pattern
Echelon Delay Pattern is shown in Figure 25.25. It is sometimes referred to as half echelon pattern. It is particularly suitable for mine benches which have two free faces. This firing pattern gives a muck pile that is not flat but well spread on the bench floor. Such muck piles are suitable for loading by wheel loaders [1].
Flat face pattern
The flat face firing pattern is adopted when the number of rows in the blast is five or less. The drilling pattern of the blastholes is usually staggered or triangular because with such a drilling pattern the fragmentation is within acceptable limits. When the blast is meant for getting large stone pieces to be used in a dam or breakwater, a square or rectangular drilling pattern is better suited [1].
Channel delay pattern [1]
Sometimes mining activities have to start from the side of a hill. In such case there is no bench and so no bench top or bench floor. A classic excavation of this type of operation is a channel cut for some distance, made before the entry into a large tunnel. The blasted material must be made to accumulate in the blasted area where shovels can approach from one side and the dumpers also approach from the same side. For the purpose of loading muck into the dumper the shovel must slew through a 180° angle rather than the usual 100° to 120°.
Sinking hole pattern [1]
Many mines, particularly coal mines, are located on horizontal planes. To start a mine in such terrain, initially a huge excavation, much like the excavation for a foundation of a skyscraper, is started by blasting vertical holes. Blast starts with detonation of the explosives in centrally located blastholes.
Later blastholes surrounding these holes, but which lie on the periphery of the diamond shaped area, are detonated with the same delay. In this manner the blast spreads outward. In such a blast the fragments do not move in a horizontal direction because there is no space for such movement.
Naturally, movement of rock fragments in the upward direction is on a much larger scale. Ground vibrations and flyrock hazard of this type of blast is much higher than in other types of blasts with an equal quantity of explosive. To reduce both these hazards the delay intervals are kept large.
Reference
[1] B. V. Gokhale, “Tests on explosives,” in Rotary Drilling and Blasting in Large Surface Mines, CRC Press, 2010.
