Overcoming Oversize: Caprock in blasting (Part 1)

Pit & Quarry’s “Overcoming Oversize” series will discuss the generation of oversize in the blasting process and techniques to reduce or eliminate oversize. Next month’s article will discuss oversize as a result of insufficient bench dimensions and how to optimize stiffness ratio for a reduction in oversize. Here’s part one of the four-part series.

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I had just arrived to a new mine. This one was having problems controlling the oversize produced from the blast.

Videos I viewed of a blast at the site showed extreme violence, with material ejecting over two times the bench height into the air. Photos of the muckpile, however, seemed to show appropriate fragmentation. From a quick observation, no oversize existed from the shot.

After a blast, the mine foreman and I sat in the pit all day watching the excavation progress. As soon as the loader began digging into the pile, the problem was apparent: Between holes, boulders existed that led to very slow dig times and a tremendous amount of secondary breakage. It was evident that this was the result of cratering (oversize at grade between holes along with vertical uplift is often a telltale sign).

That night, I began redesigning the pattern, including the rework of numerous design ratios for that bench. The next day, the mine foreman reviewed the plan and was skeptical. The stemming length was increasing, and the powder factor was dropping. He thought surely this was a sign that more oversize would be generated. Still, we went ahead with the redesign and began drilling the new pattern.

After the blast, the stemming zone seemed to have coarser material along with a few boulders. The mine manager was sure the entire shot would contain boulders. We sat there watching all day, counting the number of boulders. After some work by the loader on the top of the muckpile, the bottom contained no oversize and 95 percent of the material would pass 12 in.

The foreman was stunned. In a single blast, the oversize was reduced to less than 30 percent from the previous shot – but it shifted its placement. Previously, the oversize was at grade. Now, the oversize was in the stemming zone. The next question was could we reduce it further?

Historical dilemmas

The problem of oversize material has plagued blasting for centuries. In fact, it has been a problem since blasting began in 1627.

Oversize is expensive. It increases costs by requiring secondary breakage. It increases the cycle time, as loading takes significantly longer. It adds unnecessary stress on equipment and, in certain operations like run-of-mine leaching, it creates a waste product from good ore.

Oversize is also subjective. A small aggregate operation that’s using a small front-end loader and an impact crusher may consider material oversize when it is larger than 24 in. A large mining operation, on the other hand, may not consider material to be oversize until it exceeds 48 in.

The exact dimension, at least for now, is not the major concern of this article. Instead, we will focus on the location and treatment of oversize in the stemming zone.

Location

The biggest challenge when facing oversize is how it can be reduced during normal drilling and blasting without significantly increasing costs. The first step to being able to treat oversize is understanding where it comes from. This is typically lacking from any oversize monitoring program.

Oversize from the stemming zone must be handled differently than oversize at the grade or oversize between boreholes. A thorough monitoring program should include the documentation of the location of the oversize. This doesn’t have to be completed with GPS monitoring, as even crude sketches may solve the problem.

Without monitoring the location of oversize, two common but often inappropriate solutions are always chosen by blasters to deal with oversize. The first is to reduce the stemming. When a blaster is told the shot had oversize, it is almost second nature to assume it came from the stemming zone. This is because in a well-designed blast oversize is almost all entirely from the stemming zone.

Still, this is the case with a well-designed blast. A poorly designed blast can have oversize occur from almost anywhere in the shot – and the location will be critical in fixing problems.

The issue with reducing the stemming is that in many situations the oversize is not from the stemming zone. Reducing the stemming leads to blowout of boreholes. This blowout reduces the work energy of the explosive by up to 40 percent. This then generates more oversize in the interior of the shot, making the problem worse.

The stemming is then again reduced, again worsening the problem. Eventually, the blaster says it’s the geology causing the problem and that oversize is a natural occurrence that cannot be corrected.

The second normal reaction is to increase the powder factor of the bench. This causes a number of unwanted problems, along with increased blasting costs. Most of the time, increasing the bench’s powder factor does not treat oversize.

Additionally, without analyzing the location of oversize, the problem becomes hidden. Violent blasts that lift the entire top of the shot and throw it hundreds of feet in the air may seem to have little to no oversize, but this is because it is now hidden at the grade.

As the mine foreman mentioned earlier found out, hiding the oversize often leads to a much larger percentage. So if we are going to have a problem, it is better to make it visible and apparent.

Treating caprock

The transition of oversize from within the muckpile, which is a sign of poor blast design, to the top of the muckpile, which is often a result of the stemming, typically leads to a reduction in oversize. But it also provides a huge added benefit, as we now have tools that can be used to reduce and further eliminate oversize.

This is, again, where tried-and-true methods can often fail. The most common technique that has been used in the past is stab holes, or short-length holes that are placed in between holes that go through the stemming zone. These holes are extremely expensive, as they contain little explosive but take a long time to drill – and each must have an initiator.

Stab holes can be a great tool in some situations – if, for example, we have a presplit drilled on an angle and we have a large amount of rock between the crest of the presplit and the buffer holes. In this case, the stab hole functions as a miniature hole that is specially designed to still break under the borehole effect and move rock away from the presplit (or buffer) row.

Stab holes, however, can also wind up increasing blasting costs much more than the original cost of secondary breakage – especially when you consider not only the production cost but the schedule impacts, which tend to result in a significant reduction to the amount of shot material per week. This, obviously, depends on your drill utilization, along with your current setup. But these are reasons mines normally do not use stab holes to a significant extent.

Three other techniques exist that can be effectively applied to reduce oversize from the stemming zone:

1. Pretreatment of caprock
2. Stem charges
3. Reduction in borehole pressure

The first situation is a technique that’s been used for nearly 100 years. It is selective drilling on benches, having the subdrill of one bench break the stemming zone of the bench below it. This technique works by increasing the subdrill of the top bench to around 75 percent of the stemming zone for the next bench. This fires with the bench above, typically using offset holes so the bench below does not have holes in the same position as the above bench.

The technique can be excellent and results in a minimal cost increase, as the increase in subdrill typically leads to about 10 percent more drilling and explosives – with no change to the number of initiators.

There is one word of caution, though: This can result in stemming ejection on the next bench and, in some cases, may require additional stemming to be held.

The second technique of stem charges has been underutilized of late, specifically due to poorly utilized design techniques.

In the application of stem charges, the goal is not to have a depth of burial to have a crater form, but to use the borehole effect to break off the side of the charge. The problem with trying to crater is that if the depth of burial is too small, the stemming will eject and the entire hole will blow out, creating more oversize.

If the depth of burial is too deep, the crater closes its angle and will only break 1 to 2 ft. around the top of the borehole. Instead, the stem charge should be properly designed and positioned to break outward – perpendicular to the charge. This stops it from blowing out the stemming and causes the force of the explosive to act in the burden.

An extra tip

The best way to time a stem charge is with detonating cord and dynamite so it goes off at the same time as the main charge. Even one millisecond of cap scatter can be too much to effectively apply stem charges.

The final method of treating oversize in the stemming zone is to reduce the borehole pressure at the top of the borehole. While simple rules of thumb are available for designing stemming, true stemming design is based on the internal borehole pressure. Therefore, if the borehole pressure can be reduced below the stemming zone, then less stemming can be used.

The major problem is how to practically achieve this. What has been commonly used is air decks at the top of the borehole. An air deck is simply leaving part of the borehole empty with no explosive or stemming. This allows the gasses to travel into this region but will reduce the overall pressure in the region. Air decks can work, but the design has to be nearly perfect.

Too long of an air deck and you risk reducing the pressure through the entire borehole. Too short of an air deck and no pressure reduction is obtained.

A concept that can be even better is to use a smaller-diameter charge at the top of the column. With the proper design, the stemming can be reduced while keeping gas pressure in the top of the borehole to cause rock to break.

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Anthony Konya is vice president at Precision Blasting Services, consulting around the world in rock blasting and vibration from blasting. He is also the founder and CEO of Academy Blasting, an explosive engineering education company, and the host of AcademyBlasting.TV podcast.