New methodology to optimize quarry blasts

By |  January 13, 2021
A look at the fragmentation created through the blasts at the test site. The fragmentation pictured at left occurred before Multivariate Blast Design (MVBD), while the fragmentation at right resulted from use of MVBD. Photo: Anthony Konya

The fragmentation pictured above occurred before Multivariate Blast Design. Photo: Anthony Konya

The mine manager approached, bearing the news we had all been waiting for over the last few days: What was the oversize percentage from the previous blast?

Our team left nearly two weeks earlier, and after nearly 30 hours of travel we arrived at the site. We spent the next two weeks in a camp in the deserts of the Middle East.

We were called in for what promised to be a great challenge. The blasting on-site was producing more than 10 percent oversize, which was then being broken by a hydraulic pick before going to primary crushing – but this was if the oversize was small enough to be easily broken. Otherwise, the ore was being stacked across dozens of acres in fields as waste material.

A look at the fragmentation created through the blasts at the test site. The fragmentation pictured at left occurred before Multivariate Blast Design (MVBD), while the fragmentation at right resulted from use of MVBD. Photo: Anthony Konya

The fragmentation pictured here resulted from the use of Multivariate Blast Design. Photo: Anthony Konya

Not only was this a significant cost for the mine, but it also caused slow production.

This was also a challenge because the mine was heavily regulated, as it was owned by the government with numerous arbitrary limitations put in place. This included a maximum regulated powder factor – which the mine was already operating at – meaning no change to drill size or explosive diameter.

There was only one explosive supplier, so no changes could be made to the initiators or explosive type. The mine was also blasting at a stiffness ratio near 1.0 and could not change the bench height to improve performance.

In all aspects, this mine looked doomed to never achieve real success in the drilling and blasting program. This wasn’t for a lack of effort, as the mine had brought in consultants from around the world – all without any meaningful change to the blast performance.

Now, our team ventured from America with the hope of improving the results. We had a new tool to perform the work, though, in Multivariate Blast Design.

Multivariate Blast Design

This new tool, Multivariate Blast Design, was a technique we had spent the last few years perfecting.

It was a radical change to the blast design process, which traditionally had relied on basic rules of thumb or arbitrary variables such as powder factor. Blasters using these methods knew they did not work in a large enough percentage of cases and that they failed to be meaningful design techniques.

The problem was that blasters had no better tools, until the development of Independent Variable Blast Design (IVBD). This approach allowed blasters to understand basic relationships and apply a design philosophy to the bench, ensuring each new blast produced good results and could be further optimized. This system worked great and was widely adopted in the industry, and it is being applied to different operations worldwide.

Still, two questions remained with this incredible system: how can a blaster optimize the blast to improve from good results to the best site-specific results? And what if one of the blast design variables is forced to a certain standard that deviates from the recommendations of IVBD?

These are the solutions that Multivariate Blast Design intends to solve. This is why we had spent 30 hours on planes and driven for hours through the deserts, all to bring this new tool to one of the most difficult blasting situations in the world – a situation in which all other methods of blast design had failed.

Optimizing blasts

Previous regimes of blast design have worked to give good blast performance in all manners of sites, when they can be followed entirely.

The development of site-specific optimization was then a process that was made over a long period by slowly adjusting variables in the blasting process. However, most sites did not complete this part of the process, as the results were good enough to continue forward and, in many situations, the expertise did not exist on-site or within a company to perform meaningful optimization.

Some sites would bring in blast consultants to help with varying degrees of success. This often correlated with the consultant’s education and experience in blast engineering, and this was typically only completed when the blasting proved to be a problem and was directly costing large amounts of productivity or money.

The majority of sites continued with “good enough” blasting. But today, good enough is not good. New methods must be employed to keep sites cost competitive and productive. This is especially true in locations with high labor costs and expensive capital expenditures.

It is no secret that most sites know they can get better; they just don’t know how. That’s where the tool of Multivariate Blast Design can be applied.

More on the design

Multivariate Blast Design breaks blasting into three distinct principles which govern a blast: breakage mechanisms, confinement and performance.

These are all interrelated and should be optimized in the order that they appear. For example, it is well documented that cratering (vertical uplift of a blast) in almost all situations has extremely negative impacts on the site, including significant overbreak, poor fragmentation with a mix of fines and boulders, toe left at grade, and large secondary costs.

If we optimize the performance of the cratering, we will never achieve the results that could be attained with proper bench breakage under the borehole effect. For this reason, we need to first change the blast’s breakage mechanics from cratering to the borehole effect, then optimize the performance.

Multivariate Blast Design then utilized the interdependence of variables in a blast design to achieve the proper results. For example, we clearly understand that the burden, bench height and delay timing between holes affects the spacing of the boreholes. But how do changes outside the parameters that are set up for an IVBD approach affect this?

Another simple relationship to consider if we change the stemming length or material: How does the burden change to ensure we still get appropriate breakage?

In blasting, we cannot use approaches such as multivariate analysis of variance, a common statistical methodology for dealing with these items, because we cannot get a large enough sample size with appropriate ranges at any single site. The first step of Multivariate Blast Design was to then overcome this and develop new approaches for determining site-specific multivariable optimization.

This process of optimization goes through the stages until reaching the performance phase, which ranks the desired outcomes of a blast. For example, one site where this technique was applied was a large coal mine that used about 1 million pounds of emulsion per blast. The site typically had a radius of flyrock of about 2,000 ft. in distance.

In order to continue, the operation had to blast within 60 ft. of an environmentally protected waterway. If a single piece of flyrock went into the river, the site would be shut down and likely stop mining that area.

So, the technique was applied, with the primary goal of eliminating flyrock. It worked perfectly, allowing the mine to blast adjacent to the riverway. Prior to this, we had worked with the site to develop appropriate blasting for fragmentation and protection of the coal using IVBD.

Deviation of design

One of the major problems with the IVBD that often left blasters and blast engineers with headaches was when a certain variable in the design was fixed and could not be altered.

The famous example of this is always when a site has a large-diameter drill and short benches, resulting in a low stiffness ratio. It is well understood that a low stiffness ratio leads to poor blast results, so how was a blaster supposed to overcome this dilemma?

Often, the tools did not exist to help, so secondary measures were taken to mitigate problems such as secondary breakage of oversize, increase in the size of site muck-haul equipment, the use of blasting mats to mitigate flyrock, or the use of a significant number of decks to reduce ground vibration and air overpressure. The blaster had to treat the problems and not the root cause, which was ineffective blasting.

This is really were the Multivariate Blast Design technique is appropriate and allows for the blaster to solve the problem. When a blast is outside the set boundaries that are defined for it in the IVBD, the IVBD cannot account for this because it relies on the assumption that all variables are properly designed in a sequential order. Multivariate Blast Design, however, utilizes the interdependency of the variables to optimize with the given results, following the same sequence as above.

Back to the Middle East

As we awaited the mine manager’s report on oversize, we went back through the data.

We had spent two weeks in the camp and successfully fired eight test blasts in two different rock types (ore and waste), which, for this site, totaled more than 1 million tons of material blasted.

When we arrived, the blasting was violently cratering, with a violence factor of 4. The material was generating 10 percent oversize, the blasts’ stiffness ratio was 1.0, and the explosive type, initiators and drill diameter could not be changed. The powder factor was set and could not be changed, as well.

Over these test blasts, we determined the interdependency of the blast design variables using the N-Factor blast design technique – a Multivariate Blast Design tool. We then optimized the performance to stop the cratering and ensure that the borehole effect was utilized.

We had redefined the site’s blasthole confinement, altering key variables to achieve appropriate confinement on each borehole. The last test blast in each ore type was going to be the final proof with all changes. Could we achieve the desired performance with all of these site limitations?

The mine manger approached us and handed us the report. I flipped directly to the section on oversize – the final blast had less than 1 percent of total oversize. Without changing the stiffness ratio, drill diameter, explosive type or powder factor, we successfully cut the oversize from more than 10 percent to less than 1 percent. We cut the back break from up to 10 meters to less than 1 meter.

It was now time to punch our plane tickets home, following another success of the Multivariate Blast Design.

Anthony J. Konya is the 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.

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