Blast performance based on violence

The mining and quarrying industries have often faced major problems in having a basic mechanism to assess the performance of a blast.

Since the late 1800s, those astute in blasting have known that if a blast has vertical displacement – or blowout from the top of the bench – the blast is considered to have performed poorly. Many blame this phenomenon solely on the stemming, and any vertical uplift is deemed to have occurred from improper stemming.

This leads to the inevitable increase in stemming until the rock fragmentation is extremely coarse and boulders are numerous.

Management is frustrated as drill and blast costs increase due to attempted improvements through more expensive blast components, such as electronic detonators. The blaster or blasting team is frustrated because nothing seems to work, and at the end they blame a unique site geology.

The problem is not the site’s geology, but rather it is a misunderstanding about the cause of vertical blowout from the bench.

The first mechanism

Two mechanisms exist for vertical motion from a blast. The first is stemming blowout, which can be identified as stemming vertically moving out of the borehole. Stemming blowout is not solely a function of the stemming material and stemming length. Stemming blowout is a complex problem that involves the stemming functions, as well as blasthole timing – not just sequencing, burden and numerous other minute factors.

The photo on page XX illustrates that the holes for the entire blast are blowing out before face movement – yet this is not because of an improper burden. The cause of the stemming blowout in this photo is not poor stemming or over-confinement from burden. The cause of blowout is the direct cause of timing that is too fast.

The blowout of stemming can be looked at not as rock and dust in the air, but as dollar bills burning above the pattern, as this can reduce explosive efficiency by more than 40 percent.

The second mechanism

The second mechanism of vertical motion is that of vertical displacement of rock. This is due to poor blast design on a number of levels, with many complex relationships causing this to occur.

This can be seen in the background of the photo on page XX, where actual pieces of rock can be observed to be vertically rising from the blast. This causes many problems in the blast results, including large boulders intermixed with fines, ground vibration up to five times a proper blast, air overpressure increases of up to 6 decibels, severe flyrock that puts workers and neighbors at risk, backbreak that causing destabilized faces, overbreak that causes floors to change elevation through the pit, and increased rockfalls for subsequent benches that lead to dangerous working conditions for workers.

Cratering is known to be the worst breakage mechanism and optimized blasts do not have any crater component or vertical blowout.

Violence factor

A number of articles on the science and nature of stemming blowout and optimization of these principles have been published over the years. These are considered efficiency problems, as poor stemming will reduce the total borehole pressure and can lead to states of over-confinement.

In this article, the interest lies in determining a key performance indicator (KPI) to assess the second mechanism of vertical motion: the vertical displacement of rock through cratering.

This cannot be accomplished through trivial methods such as powder factor, as the exact mechanisms that lead to cratering are numerous and complex. Generally, benches that are shorter (when compared to the burden) lead to cratering. Still, it cannot be stated that short benches will always crater, and high benches will never crater.

Instead research shows that short benches are more than three times more likely to crater, and the odds that they will produce good results are small. Yet, they can be made to work in the absence of changing certain parameters through methods like n-factor design.

The goal is to develop a system that managers can easily apply to a site’s drilling and blasting program to quickly assess the general performance of the blast and understand how this is impacting the mine’s costs, production and safety.

We recently developed the term “violence factor,” which provides exactly this information. More than 20,000 boreholes were studied from full scale bench blasting in actual mining and construction projects to determine this violence factor relationship. The violence factor can be used as a simple KPI to determine the blast function solely based off a video of a blast.

When determining the violence factor, only the vertical displacement from breakage of rock will be analyzed. Stemming blowout is not considered part of the violence factor. The violence factor variable is then an integer with values between 0 and 4. This scales based on the bench height and the maximum vertical height of rock.

Table 1 shows the values of violence factor and the method to define what is violence factor for the blast. The entire blast will typically be assigned a single violence factor based on the maximum vertical displacement. Very long blasts, or blasts with multiple parts, can be assigned multiple violence factors.

Violence factor performance

The importance of this for a manager would be in determining how the blast performs based on the violence factor.

High violence factors indicate that cratering occurred, resulting in terrible fragmentation and a mixture of fines and boulders along with flyrock, high ground vibration and high air overpressure. Cratering is a poor-performing and unsafe blasting method.

A low violence factor indicates that flexural failure occured with axisymmetric bending. This indicates that the blast has good performance with a generally uniform fragmentation size, and it does not contribute to additional flyrock, air overpressure or ground vibration for the blast design.

Solely having a low violence factor does not, however, necessarily mean a blast is well designed.

One point to consider is the difference in the variables defined by the violence factor from the blast. When considering changes to fragmentation, it is important to understand that the typical variables, such as a P50, are not the only consideration for fragmentation. The actual size distribution is important, as well.

The same P50 can be generated if you combined basketballs and golf balls to that of baseballs. This is like a combination of boulders and dust having the same P50 of 6 in. in the same way that a mixture of material that is 4 in. to 8 in. in diameter has a P50 of 6 in.

The goal is not just achieving the proper P50, P80 or other sizing metric, but in the actual distribution. Boulders and fines are difficult to handle and crush, and they significantly increase costs compared to a well-distributed product. Cratering will not only give a larger average size but can increase oversize by 20 percent and increase fines by 5 to 10 percent. The result is a product with a mix of boulders and fines, which leads to additional costs than just a basic handling and crushing economic model would indicate from the larger-sized product.

The violence factor also provides a critical indication of the safety of the blast. A blast is one of the few processes on a mine that can cause damage and injuries off of mine property through uncontrolled flyrock.

Flyrock is the top cause of death on mines from drilling and blasting over the last decade. Flyrock can occur for many reasons, including improper loading, poor design modifications to geologic conditions, drill deviations and too small of a burden. These are typically seen as flyrock projected from the face of a blast, which are more predictable and can be determined through other means.

Flyrock can also occur to the sides of the blast and behind the blast. This is typically the result of cratering. Furthermore, cratering will lead to a larger number of flyrock, whereas with flexural failure the actual number of rocks being projected from the site is much fewer. The mechanism of cratering is more violent and poses a greater risk of flyrock.

Final thoughts

Cratering will also cause significant increases to environmental factors such as backbreak, ground vibration and air overpressure. These have unique impacts on the public and can impact a mine’s social license to operate, as well as make conditions for workers more dangerous.

The mechanism of cratering also produces inconsistent results. Blasts that function under cratering cannot be optimized for any of these situations.

For example, some mines spend tens of thousands of dollars on signature hole studies and increase detonator costs by 500 percent to use signature hole studies to develop better timing practices. If the blast is cratering, these efforts are in vain and produce no results because the vibration is inconsistent for violent blasts.

These methods can only be completed with blasts that are of a low violence factor. The mechanism of flexural failure produces better results than cratering. A blast that is not violent, or one that has no vertical material displacement, performs better on all performance criteria compared to a violent blast.

This is not saying that a non-violent blast cannot get better; it is simply saying that it can get much worse.

This also is saying that if a blast has a high violence factor it should not have fancy strategies employed to fix performance. Instead, pick the low-hanging fruit and reduce the violence first.

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Anthony Konya is vice president and Calvin Konya is president of Precision Blasting Services, a drill and blast consulting firm operating worldwide. They can be reached at anthony@idc-pbs.com.