8.1.2: How do We Blast?

Very carefully! Actually, I am serious!!! There’s a difference between civil and military blasting. In the latter, their goal is usually to “blow things up.” In civil projects, such as mining, our goals are to use as little explosive as possible, as safely as possible, and to have no collateral damage, while achieving the design outcome for the blast. This is not easily accomplished and requires both good engineering and faithful implementation of the blast design by the drillers and blasters. The short answer to the question of “how do we blast” is as follows.

We create a cavity, typically a drilled hole, to accept an explosive. The diameter of the hole will depend on the amount of explosive energy that we want in the hole.
We will drill multiple holes, and the distances among the holes will be an engineered parameter.
We fill the cavity, partially, with a blasting agent, i.e., an explosive. The type and formulation of explosives is an engineering decision.
We place initiators, primers, and sometimes boosters in the hole with the blasting agent.
- The initiators trigger the process. They can be nonelectric or electric, and some are commonly known as blasting caps. These initiators may include a time delay, which allows us to design a specific sequence for the initiation of the blast.
- The primer is a small explosive charge that is set off by the initiator, and which has sufficient energy to set off the blasting agent. Explosives commonly used today are not cap-sensitive, meaning that the initiators do not have enough energy to set off the blasting agent. For this reason, we need to insert the initiator into the primer before dropping it into the hole.
- Boosters are used to ensure that the explosion propagates through the entire column of explosive. This is a concern in longer holes and holes of smaller diameter.
We connect the initiators in each hole together using nonelectric tube/cord or electric wires, and these terminate at the point where the blast will be fired, i.e., the blaster will “push the plunger” or “throw the switch.”

The placement of the holes and the timing sequence of when the holes are “fired” constitute the blasting pattern.

Collateral Damage

I said that one of the goals of blasting is to avoid collateral damage, which we do through proper design and execution of the blast. But, what do we mean by collateral damage? Here are the primary ones that we constantly have to assess.

Overbreak

This is when fractures from the blast propagate beyond the intended region. Imagine that you want to drill and blast a tunnel opening through a mountain, and you are designing for an opening that is 30’ wide and 15’ high. However, due to improper design or execution, cracks have propagated to 18’ high. Over time, it is likely that pieces of rock will begin to fall out of the tunnel roof, creating a safety hazard as well as delays in using the tunnel.

Flyrock

Flyrock is a large chunk of rock that is propelled well beyond the throw region for the blast. These chunks can weigh hundreds of pounds or even more than a ton, and travel distances of several hundred feet. Over the years, flyrock has caused numerous fatalities and millions of dollars of property and equipment damage. Proper design and execution are necessary to prevent flyrock.

Gas production

Gas production is an intended action during a blast, but the production of excess quantities of toxic gases, notably CO and NOx, is to be avoided. Improper on-site formulation of the blasting agent and problems with loading are often responsible for this problem. It should be noted that this is a hazard in both surface and underground mines.

Ground vibration

Like gas production, ground vibration is an intended consequence of blasting. However, excessive ground vibration can damage structures, and the level of vibration at the boundary of the mine’s property is regulated. Careful design of the blasting pattern, and especially the timing of the holes, is required to keep the ground vibration within prescribed limits.

Air blast

This occurs when excess energy from the blast creates a shock wave in the air. Certain weather conditions will cause the air blast to be bounced back to the surface but at some distance from the blast. There is little danger of personal or structural damage from an air blast, but it can precipitate a barrage of angry complaints from people who live in proximity to a surface mine. Nothing good ever comes from irritating the locals! Air blast can be reduced through proper design the blasting pattern and the choice of detonators used to link the holes together.

Misfires

These are not really “collateral damage” per se, but they are an unintended consequence. A misfire occurs when some of the blasting agent in a hole, or multiple holes, remain undetonated after the blast. This explosive could go off at a later time, such as during loading, and cause serious personal injury or death. Misfires can be avoided through careful attention to the execution, as well as the design, of the blast.

Good Engineering and Execution

Throughout this discussion, I have emphasized the proper design and execution of the blast. By execution, I mean the drilling and loading of the holes. Poor drilling or loading procedures will compromise the best design, and similarly, proper drilling and loading cannot compensate for a poorly engineered blast round.

It starts with good engineering, and specifically:

Design of the blasting round is concerned with:

the choice of the blasting agent;
the choice of detonators, primers, and boosters;
the use of free faces. A free face is an unconfined surface, through which the blasted material is free to move unimpeded;
the distance between holes, known as the spacing, and the distance between a hole the nearest free face, known as the burden;
the number of holes;
the diameter of the hole;
the depth or length of the hole;
the use of decking, when needed, and stemming. Decking is an inert material used at certain locations within the column of explosive when it would be undesirable to have explosive at that part of the drill hole. Stemming is an inert material placed in the top or front of the hole. It is necessary to help provide proper confinement for the blast, ensure safety, and reduce airblast;
the timing of when each hole is detonated;
the overall positioning or arrangement of the holes, including the use of unloaded holes;

and ends with good execution by the drillers and the blasters. Specifically for the driller:

accurate positioning of the collar, i.e., drilling the hole exactly at the specified location;
control the drilling angle, i.e., drilling the hole at exactly the specified angle;
control the depth or length of the hole;
accurate logging of the holes during drilling (applicable primarily to surface mining applications);

specifically for the blaster:

correct mixing of the blasting agent at the hole (if batch loaded);
proper loading procedures if there is water in the hole;
correct loading rate;
correct placement of decking, boosters, and primers;
proper placement of stemming and stemming to the design depth.