Sometimes blasting can be as simple as a yearly check up, but sometimes it can seem like open heart surgery. Our unwilling patient, the rock bench, might be riddled with jointing and cavities or have excessive toe left over from previous operations. As drillers and blasters we put on our PPE and use all the tools and techniques available, developed through rigorous trials (and sometimes errors).
So what really makes up the anatomy of a healthy blast? What are the key principles that you need to get right across most types of blasting applications?
The Head - Pit Planning
Properly planning out your benches can help avoid countless issues before they occur. It’s a bit like eating healthily and keeping active.
Establishing the right face orientation can help improve wall conditions, preventing slips and excess toe. Planners should assess the dip direction of the major joints and bedding planes and plan blast blocks so that these dip into the face rather than out.
Ensuring adequate subdrill on the bench above can help prevent poor fragmentation in caprock by pre-fracturing this top layer. The benefit is, however, dependent on digging to the correct floor elevation. Overdigging through this subdrill layer will leave you back with hard virgin rock in the stemming zone with no explosives to crack it.
The Chest - Energy Distribution
Energy distribution impacts heave and fragmentation right from the heart of the shot. It’s important to achieve even energy throughout the body that’s matched to the rock properties and the desired outcomes.
For face holes, this means positioning each hole to have optimal burden all the way up and down. It also means face profiling and adjusting loading to achieve adequate confinement for all explosives.
For body holes, this means drilling even patterns and checking toe-to-toe distances. Large gaps between toes should be addressed by moving or adding holes. For example, if the face holes are all angled but the body holes are vertical, it may be necessary to also drill the second row at a slight angle or to add a set of vertical toe charged holes to the front row.
The Arms - Drilling and Loading Accuracy
A perfect shot design is useless if it isn’t executed properly. It’s like trying to paint a masterpiece with a broken arm.
Drilling accuracy is the most important factor in setting up a blast for success. Measuring hole depths and backfilling / redrilling is critical for achieving good floors. Boretracking can help ensure holes haven’t wandered too close to the face or too close to each other. Artificial intelligence can even help now in automatically measuring collar position deviation.
Loading accuracy is the last line of defence against disaster. Overloading holes can result in inadequate confinement, producing excessive airblast and dangerous flyrock. For gassed emulsions, it’s especially important to be conscious of how much column rise you’re holes are experiencing to avoid understemming.
The Legs - Burden Relief
Finally, when timing a shot, it all comes down to burden relief. Run too fast and you’ll trip over; run too slow and something else might catch up.
Insufficient burden relief will cause the blast to choke up, leading to cratering and flyrock from the back rows as well as excessive vibration and wall damage. It will also produce less heave and slow down dig rates.
Too much burden relief can produce airblast and flyrock if the first row of holes has moved too far to provide enough confinement when the second row fires. In some situations it can also generate too much heave which can increase ore dilution.
Just like the human body, a blast only functions properly when every part of its anatomy is in good shape. It’s important to get all of these principles right to ensure a healthy blast.
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