Underground Perimeter Control Basics: Post-Blast Perimeter Control Methods

This post focuses on perimeter control methods that focus on firing the last row of holes last or not firing them at all. The methods include line drilling, smoothwall/trim blasting, and cushion blasting. The next post covers pre-blast perimeter control, such as presplitting.

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 reducing confinement, 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.

Line Drilling

Line Drilling

Line drilling is a technique where holes are drilled along a design perimeter so that blast damage is limited at the line of holes. The approach requires tightly spaced holes - on the order of 2 to 4 times the hole diameter. The holes are not charged with explosives. I have seen a lot of drillers use line drilling in soft rock to protect from overbreak, but usually the holes are not drilled close enough together, so the results are not consistent.

As noted above, the design recommendation is 2 to 4 times the diameter. A guide tube can be used if the drill holes need to touch. The downside is that this approach is high cost and time consuming. Plus drilling must be accurate. The benefits of this approach is that specialty explosives products are not needed and the walls will be smooth if successful.

Smoothwall/Trim Blasting

Smoothwall blasting is probably the most common underground perimeter control method for tunneling. Smoothwall blasting uses a combination of tigher-spaced and lighter-loaded perimeter holes. In this approach, the perimeter holes are shot last and they are used to trim off the worst of the overbroke rock around the main production or stoping holes.

Smoothwall Blasting

The design for smoothwalling can vary based on the hole diameter, the product you’ve selected, geology, and other factors. You can use many of the products discussed in previous posts, such as trim powder, detonating cord, and string loading.

I usually approach a smoothwall blast by designing to a specific powder factor. I try to match a certain mine or tunnel’s known production/stoping hole powder factor. For example, if the production holes have a burden and spacing of 0.8 m x 0.8 m, the round is 4.3 m long, and it is loaded with 8 kg of explosive, the powder factor (PF) is 2.9 kg per cubic meter. Assuming this PF is acceptable for the perimeter holes, the same value can be used for the perimeter design. Assuming a perimeter charge of 2 kg, the burden and spacing coverage needs to be reduced by 25% to maintain the same powder factor. The perimeter holes would have a burden and spacing of 0.4 m x 0.4 m to keep the PF constant.

Perimeter holes have greater relief, so the example above may provide good results with an increased burden and spacing of 0.6 m x 0.6 m. Or just the spacing can be increased to 0.6 m. It depends on the rock and the explosive product.

One important design aspect is that you want to make sure the production holes don’t overbreak into the perimeter and damage the smoothwall charges before they detonate. Additionally, drilling accuracy is important - the production drillholes need to maintain a consistent distance from the perimeter holes.

A final note on timing - shooting the perimeter holes on the same delay…or better yet, with electronic detonators with high accuracy, will yield the best smoothwall results.

Cushion Blasting

Cushion Blasting

Cushion blasting is similar to smoothwall blasting, except that an extra row of cushion holes are designed between the stoping/production holes and the perimeter holes.

The image shows the reduction in damage radius from the stoping holes (bottom row), the cushion, or buffer, row, and the smoothwall, or trim, row (top). The cushion row helps reduce perimeter damage.

The cushion row can be designed similar to the approach discussed in the smoothwall section, but the charge weight is larger than the smoothwall charge weight and/or the charges are not as decoupled as the smoothwall charges.

Others

There are other perimeter control applications that can be discussed, but are less in use. The first is fracture control technique (FCT). For FCT, the perimeter holes are mechanically notched on each side of each hole along the axis of the perimeter. The notch acts as a stress-focusing area to help drive a split along the perimeter. This option is really more of a presplit type approach. However, it is not in common use today.

Resueing is another approach that doesn’t really fit as a perimeter control method, but it could support perimeter control. To my knowledge, this is a Canadian method used with drives along veins. In this case, the vein is blasted and mucked out of a heading and then the remaining face in waste rock is drilled and blasted. This approach reduces dilution. But it also gives a good relief for the second wasterock blast. More relief means more opportunity to design a successful perimeter control blast.

Finally, air decking is another option. This will be discussed in a later post.

This article really just scratches the surface on underground smoothwalling and cushion blasting. 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.