Blasting precautions you must take

Blasting is essential to the mining industry.

Explosives provide the energy to break and loosen the rock necessary for processing. To effectively blast rock, a tremendous amount of energy must be released in a very short period of time. If not done correctly, this energy release can cause damage, injury or death.

The challenge in blasting is to control the energy to break and move the rock without causing flyrock, high vibration or airblast.

Proper planning

When blasting, there are at least two major safety concerns. One is premature detonation. The other is flyrock.

Premature detonation involves an unplanned detonation during the loading or tying-in procedures, or while awaiting scheduled firing. Explosives require heat or shock energy to begin the detonation process. Therefore, preventing contacts with these sources of energy is of utmost importance.

Examples of unwanted contact could include explosives being run over by equipment, being exposed to a fire on the blast pattern, being exposed to lightning strikes or having a drill intersect a loaded hole. To prevent such hazards, consider the following:

• Using cones or berms, demarcate the blast site (the area where holes are loaded or in the process of being loaded).
• Within the blast site, have only the personnel and equipment necessary to load the blast.
• Complete drilling prior to commencement of loading, or have the drill at a distance where it cannot possibly intersect a loaded hole. Also, consider drills have been prone to fires when hydraulic lines burst onto a hot engine or compressor.

Regardless of the initiation system being used, always evacuate the area at the approach of an electrical storm. Have an evacuation plan in place, and make sure everyone is familiar with it.

Avoiding flyrock

A second blasting safety concern is flyrock. Flyrock is more common than premature detonation and poses a threat not only to those on the quarry property, but also to those in surrounding communities. This is why tremendous emphasis is placed on preventing flyrock by training and technological advancements.

Rock is fragmented and moved by applying explosives energy to the rock. The relationship between the energy and the volume or weight of rock is called powder factor. It is stated in either tons of rock per pound of explosives or pounds of explosives per cu. yd. of rock.

Industry standards define the range of powder factor based on geology and/or the type of blasting being performed. The powder factor of the overall blast is important, but the powder factor of every hole segment drives the possibility of flyrock.

Most flyrock incidents are a result of a high powder factor in a localized area of one or several holes. This may cause rock to be thrown more than 2,000 ft.

The cause of the high-localized powder factor usually occurs as a result of light burdens on the front row of holes. Burden is the measurement from the hole to the nearest free face.

In most cases, due to either geology or backbreak from a previous blast, burdens are variable along the length of the explosives charge. Adjusting the explosives charge to match the amount of rock (burden) is key to maintain a proper localized powder factor and prevent flyrock.

Over the past couple of decades, technology was developed to aid in measuring this variable burden. Two-dimensional and three-dimensional laser profilers, photogrammetry and now drones can accurately measure burden and display it to the blaster in graphic and table form.

To use these technologies, the open face must be completely clear and visible prior to loading holes. In fact, for best results, the face should be mucked out prior to layout so holes can be placed in the proper position and require less loading adjustments.

Another reason for a high-localized powder factor may be that the amount of rock present may be proper, but too much explosives charge is present in an area due to cracks, seams or voids.

The proper action to prevent this is to maintain an accurate drill log. No high technology here – just accurate note-taking of these conditions by the driller. The drill log should also provide information on the amount of loose or soft material in the collar zone so the blaster can make loading adjustments and prevent vertical flyrock.

Other details to consider

Improper blast area security during a blast event has been responsible for a number of injuries and fatalities over the years. The blaster and quarry management should develop a plan ensuring that, prior to the blast, an inspection is made to determine that the area is evacuated, that guards are posted at all entrances to the pit, and that the guards are in communication with the pit foreman and blaster.

Recently, it seems more operations rely on cell phones for communication during a blast event. Personally, I believe that two-way radios are a better choice for communication with the guards, pit foreman and blaster. This allows for instant access to the blaster should a breach in security be detected just prior to detonation.

As in any venture, the quality control and attention to detail put into a task will directly affect the results. Blasting safety is no different. Properly trained blasters, accurate measurements and proper attention to every safety regulation will ensure consistent, safe blasts.

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Joe Nawrocki is senior technical manager at Dyno Nobel.