Downstream Impacts of Drilling and Blasting
Impacts of Drilling and Blasting
Drilling and Blasting is a complex process, that in many situations is not given the full emphasis it deserves in process development and controlling the costs throughout the mine. For example, the blasting process dictates muckpile configuration which effects shovel and loader fill factors. The fill factor of the shovel effects how many trucks are required on-site, how fast those trucks can be loaded, and the truck fill factor. The fragmentation of the blast effects the crushing costs and leaching efficiency. The overbreak of the blast effects slope stability, slope steepness, pit safety, and scaling costs. The blasting process effects every aspect of the operation and poor blasting can lead to millions of dollars in direct costs and tens of millions in indirect costs at larger operations, such as copper and gold mines. In many cases, the drill and blast team may not need to understanding the haulage, as the haulage does not have an impact on blasting (in a direct cost sense). This is why every person responsible for any operation at a mine must at least understanding drilling and blasting, enough to explain what they require of the drill and blast team.
To begin one variable, stemming, will be considered to see how this can affect a mine. Stemming, while important, is not nearly as important as other variables such as burden and stiffness ratio. However, one can appreciate how just stemming can dramatically change a blasts function. To begin, stemming can reduce the air overpressure from a blast by over 98% which dramatically reduces noise. However, proper stemming can also decrease the P80 of a blast by over 10% and reduce mucking cycle times by over 18%. Yes, a few feet of gravel can really change the blast this much. Poor Stemming (as shown in the picture) will blow out causing poor results for fragmentation, muckpile control, muck-cycle times, and environmental factors.
What are the downstream impacts of drilling and blasting?
The drilling and blasting at a mine influences nearly every aspect and is the starting point for other processes. Poor drill and blast results lead to poor mine results and very expensive operating conditions. While many mines try to reduce these costs on there balance sheets, the ramifications for this can be an increase of expenses in all other departments.
Let’s look at the basic ways a mine will begin to reduce there drill and blast costs, we will go over each of these points next:
- Larger Boreholes
- Larger Burdens
- Larger Spacings
- Reduce of elimination Subdrill
Increasing Drill Diameter
Lets first take a look at how increasing the drill diameter reduces blast costs. With an increase in drill diameter (assuming bulk loading) there is a equal proportion increase in the blast burdens. For example, using a 10″ (250 mm) drill loaded with bulk emulsion explosives a burden would be calculated of about 24 feet (using Konya burden equation). An increase to a 12″ (300mm) drill would be a 20% increase in the drill diameter. This leads to about a 20% increase in the burden of the blast, to around 29′ (after rounding). Let’s assume an equilateral triangle pattern is used as the bench is around 50′ (15 meter) and the powder factor remains similar and typically the mine will not change the timing of the blast either [this is an old school design approach that is not correct, but is often done].
What is the problem? Well first the stiffness ratio of the blast has been decreased reducing the explosive efficiency and resulting in cratering or uplift movement of the shot instead of proper flexural failure. This causes:
- Increases in boulders and fines
- Decreased movement of the muckpile
- Very hard toes
- Severe Backbreak
- Increased Ground Vibration and Air Overpressure
Why will the drilling and blasting team be praised for this then when all other downstream processes (muck-haul, crushing, processing, slopes, etc.) will greatly suffer. Well the bottom line is that the costs for blasting were decreased on the balance sheet and we have very poor methods at most operations to monitor any of these outcomes (besides the environmental aspects).
Increasing Blast Burden
Lets now look at the case where a mine wants to decrease their drill and blast costs by using the same diameter drill. In this case we will assume a 10″ (250mm) drill bit is again being used with a 24 foot burden. Now suppose the mine switches to a 26 foot burden again spreading out the pattern but now with the same amount of explosives. Here the powder factor is reduced (Note: we do not use powder factor to design with modern techniques, powder factor is mostly an economic tool!) and the ramifications are terrible. The outcomes of this include:
- Large increase in boulders
- Minimal movement of muckpile, most likely finding part of muckpile on the next bench
- Violent shots blowing out on top of the bench
- Solid toes and inconsistent floors
- Severe backbreak
- up to 5x increase in ground vibration and 5dB increase in air overpressure
Unfortunately for the other downstream processes, it is relatively easy to put an explosive into the ground and break rock. What is not easy is using that explosive as an efficient and economic tool. Again the drill and blast budget has decreased and the remainder of the mines costs have increased.
Increasing Blasthole Spacing
Maybe the mine recognizes that increasing the burden would have dramatic results, but surely increasing the spacing (above normal acceptable design levels) would not have that large of an impact but would again reduce the powder factor of the blast. What actually happens is the borehole interactions stops and the areas between the blast holes are not broken properly. In addition to this the interaction of radial cracks is minimal, leading to larger average size of broken rock overall. This leads to:
- Larger average size of material
- Large boulders throughout shot
- Toes left between blast holes
This then leads to increased muck-haul times, crushing costs, and secondary blasting processes; again increasing the overall operating cost of the mine.
Reduction or Elimination of Subdrill
Finally the big trend today is to eliminate the subdrill or dramatically reduce it. To understand why the subdrill of a blast is important, one needs to understand what it does. The subdrill is normally drilling and loading of explosives past the floor of the shot in order to break the shot. What the subdrill actually does is increase the maximum tension zone of the blast and bring this to the proper grade, effectively shearing the floor and leaving no toes or humps. In cases where a bedding plane or soft seams is at the intended floor height, a site may not need a subdrill. Otherwise, the drilling of the subdrill is required, sometimes the loading of the subdrill is not. This can reduce the total powder bill at some operations without effecting production; other operations, especially those with already large patterns, will need this subdrill to be loaded. From the authors experience, unless a geologic feature is already present that eliminates the need for a subdrill, the subdrill has to be there and no device can be put in the bottom of the hole to stop this requirement. Improper elimination of the subdrill will lead to:
- Poor floors with toes
This will lead to possible secondary blasting to treat floor conditions, reduced haulage times as trucks need to go slower over these bumps, and increased maintenance on equipment driving over these areas.
Drilling and Blasting is an extremely complicated process that relies heavily on proper understanding of design, loading procedures, geology, and safety. While many drill and blast teams seek to optimize there results, they can often be pushed to reduce drill and blast costs at a mine. The normal operating procedures that are seen all over the world have been mentioned above, but have severe ramifications to all other downstream processes. If you do not work in drill and blast but are involved in any other operation at a mine, basic understanding of drilling and blasting is critical to ensure that your process is not negatively impacted and your performance is diminished. If you are on the drill and blast team, you must either convince your management that a mine-to-mill optimization approach is best or look for cost reduction opportunities that do not impact the other mine processes.
If you need help with any of these, please feel free to contact us and ask about our training programs, consulting packages, or our advanced blast timer and six sigma drill and blast programs!