Underground Perimeter Control Basics: Pre-Blast Perimeter Control Methods

This post focuses on perimeter control methods that require the perimeter row of holes to detonate prior to the main blast or as a presplit. The methods include presplitting and fracture control techniques, both of which are relatively uncommon underground. The next post covers budget perimeter control methods, such as air decking and presplitting with nonelectric detonators.

As discussed in a previous post, perimeter control blasting methods use a combination of design tools. The designs discussed in this article include concepts such as decoupling, reducing charge weight, and timing. This post focuses more on development and burn cut blasting perimeter control; however, nearly everything in the article applies to underground longhole blasting and surface blasting.

Presplitting

Presplitting trial results in very soft rock that normally produces high overbreak.

Presplitting is a technique where holes are drilled along a design perimeter, similar to a smoothwalling pattern, and blasted on the same delay prior to the production holes so that blast damage is limited at a fracture plane created along the line of holes. The approach requires tightly spaced holes with decoupled charges. The following design guidelines are good rules of thumb to use if you’re designing a presplit the first time.

• Decoupled charges

  • The rule of thumb is that the explosive diameter should be around 25% of the hole diameter. This reduces the fracture distance around the hole to protect the perimeter.

  • Typically, packaged explosives are not manufactured to such a small diameter for underground development, so a higher percentage (less decoupling) diameter explosive is typically used

  • If using detonating cord, a high strength single strand detonating cord may be able to fit the 25% decoupling ratio (or two high strength cords)

Close-in view of presplit pattern.

• Hole Spacing

  • The rule of thumb is 10 times the hole diameter. This is a guideline and the best spacing may be more or less, especially depending on drill accuracy and geology.

  • If a larger-than-desired explosives charge is used, then the spacing may need to be increased or decreased, depending on geology, to account for the heavier charge weight.

• Hole Diameter

  • Underground hole diameters are typically consistent throughout the blast.

  • In some cases, such as when only larger diameter packaged explosives can be used in the perimeter, it may be worth increasing the perimeter hole diameters to improve the decoupling ratio.

Presplit timing example.

• Timing

  • The presplit holes must be fired on the same delay with detonators that do not have cap scatter. Even a few milliseconds of scatter will impact the efficacy of the presplist design.

  • Typically, electronic delays must be used (sometimes nonelectrics can work if the right product is selected - see next post).

  • The perimeter must be timed on the first delay or an early delay. Typically, all holes should be timed at 0 ms. Or the walls and roof can be timed separately to reduce the charge weight per delay if they are still all fired first.

• Other notes

  • Presplitting reduces the chances of charges rifling from holes compared to smoothwalling. It is still important to place a birdie or other device about 1 ft (0.3) m inside the collar of the hole to retain the charge.

  • Detonating cord can be traced in the hole with packaged explosive to ensure it detonates along the entire charge column.

  • Detonators should be placed at the toe of the hole and be pointed toward the collar of the hole.

The downside of presplitting is that this approach is potentially more costly than standard smoothwalling, but the loading time should be similar to standard smoothwalling. Typically, the pattern designs and explosives/charging do not vary much from standard smoothwalling.

Another concern for presplitting is the potential for the higher-confined charges to produce more vibrations and airblast than a typical development blast. Therefore, utilities, such a ventilation ducting, may need to be pulled further back from the face to protect them.

Fracture Control Technique

Mechanical notching tool for fracture control technique.

Fracture control technique was developed from around 1975 at the University of Maryland under Dr. Fourney. The concept was suggested as early as 1965 by Langefors and Kihlstrom and in 1905 by Clement Foster. In this method, perimeter holes are mechanically notched (waterjet has also been used) in the direction of preference for the perimeter fracture line. A small charge is also incorporated, similar to a presplit, to extend fractures between the notches along the perimeter. The team that developed fracture control found that a charge of around 1/40th of that required for smoothwall blasting could be used. Details on the research can be read about in the 1984 paper by Fourney and his team (accessible on OneMine.org).

Waterjet notches for fracture control technique.

One design example from the Maryland team used 1.75 inch (44 mm) diameter holes. The holes were spaced 2 ft (0.6 m) apart and drilled 5 ft (1.5 m) deep. Notches were 0.25 inch (6 mm) and cut 30 inches (0.76 m) deep. Holes were loaded with 4 ft (1.2 m) of 400 gr/ft (85 g/m) detonating cord. Stemming was 1 ft (0.3 m) of fast setting grout. One should also note that the notches were oriented parallel to the dominant joint direction. This design was also shot with the same parameters except the holes were not notched. The notching showed fracturing between holes without fracturing in other directions, while the control (non-notched) test showed fracturing extending only a few hole diameters with some fracturing in the non-desired direction.

Reportedly, compared to smoothwalling or post-splitting, fracture control can allow an increase in perimeter hole spacing by 39 to 83 percent and allow a reduction of explosive weight by 43 to 69 percent, likely depending on the geology. In addition, stemming would impact gas retainment and allow for a greater increase in spacing. The half-cast factor (average length of half barrels remaining divided by the round length) was also reportedly 15 to 270 percent better for fracture control technique.

Results from fracture control technique.

The side-by-side test and improvements summarized in the above two paragraphs show that fracture control technique works. However, in my experience, this method is uncommon, if used at all in the industry. The team at Maryland report that drilling time increased and their notching tool had a tendency of jamming in the hole, obviously something that would prevent drillers from using the tool.

Summary

This article really just scratches the surface on underground presplitting. If you’re having issues at your operation with your perimeter control, feel free to reach out to your friendly Thoroughbred Drill and Blast Consultants! You can reach me at nathan@thoroughbreddrillandblast.com. I would be happy to support your team.