PHOTO: Showing the smaller columnar structure of the Basalt at Smeeds Quarry.
Andrew Robertson explains the old blasting practice used in areas where there were no problems with vibration or noise in the past.
Coyote or tunnel blasting practice was a great way to get a great amount of rock broken for operations in remote areas.
The definition of Coyote blasting is from The Dictionary of Mining Mineral and Related Terms put out by the US Bureau of Mines in 1968:
“A term applied to the method of blasting in which large charges are fired in small adits or tunnels driven, at the level of the floor, in the face of a quarry or slope of an open pit mine.”
The first place I heard where this method was used was at Smeeds Quarry (Winstones’ Pukekawa Quarry today). This was carried out in the 1940s to 1960s. Using around seven tonnes of explosives, it was a spectacular way of getting up to 54,000 tonnes of rock in one hit.
The system has been used extensively overseas as well as also in Taranaki at the ill-fated Ballast plant in York Road (near Midhirst) for NZ Railways in the 1920s.
Preparation
First thing was to drill and shoot at floor level, a drift, drive or adit, of 1.5 metres by 1.5m into the face of rock for about 10 metres. At this point a “chamber” of around two metres by 2.5m is created, then a branch is driven parallel to the face, on either side of the chamber. At the end of these drifts a chamber is created of around the same proportions as the first chamber.
To drill and blast these tunnels requires a driller and mucker to basically lie on their back, or kneel while using a rock drill to drill a round of holes, fill them with explosives then blast the rock. The mucker then lays sets of tracks and sleepers of around a 40-45cm gauge track system, and then shovels the shot rock into wagons run in on the tracks, take them out when loaded, and dump the shootings. Then they do it all again as they only advance around 90cm each blasting round in the tunnel.
The next stage is to drive the central drive another 10m back, and repeat the process as described above. So now there are two ‘T’ shaped tunnels with chambers in the middle and the wings of each set.
In the case of Smeeds Quarry, I understand that the chambers were filled with explosives and the drifts and drives filled with shot rock as backfill to the single entry. As it was explained to me, the six lots of explosives were initiated by 10gm detonating cord started by one electric detonator. The other way would be using a cap and fuse.
While this method of blasting basically takes a piece out of the bottom of a 60m face, and relies on gravity to collapse the rock down to form the rill from which the crusher feed is taken, it would not suit large blocky rock structures, as the chance of hang ups and large rocks would be very high.
But, it does suit Smeeds Quarry as the columnar basalt has a column diameter of around 20cms to 30cms, and only gradually increases column diameter to around 75cms to 100cms over the height of 60m.
This means that when coyote blasting, the main charges are concentrated in the toe area and breaks the larger rocks down to a sub 40cm rock size, with very few larger than that.
So, this was a very effective way of getting a great deal of rock with very little cost in explosives. The points against were noise, vibration and a large one off cost for preparation and blasting of the rock. Also there is the requirement for a tunnel manager’s certificate for all of the tunneling, as well as having sections of prepared rail to transport the waste material out of the tunnels.
Today, we are a little more careful of noise, dust and vibration, and have found that down hole drilling and blasting are much more cost effective through lower noise levels and more even cost spread from regular smaller blasts.
The reasons Coyote blasting was so effective at Smeeds Quarry was its unique geology and the safety factor of clean and straight faces left after blasting. If the columns had been of a large diameter the Coyote system would have created a secondary breaking nightmare, and would not have been as cost effective as it was. The lower areas of the quarry have similar structure to the Giant’s Causeway in Northern Ireland. This makes for a strong rock structure that holds together well in the tunneling process and will give clean side break when blasted so is a relatively safe system in columnar basalt.
Block Caving
Coyote Blasting was very small beer when the big mining companies got hold of it and turned it into an amazing self-collapsing process called “Block Caving”.
This process as used at Magna Copper in the USA, causes a whole square kilometre collapsing on itself through driving cross drifts underneath, and close together, then drawing the collapsing material down through grizzlies to a series of draw points to be carted away to be processed.
The rock type causes the weight above to act as a primary crusher down to the draw points, so the material can be bulk carted by electric trains to the shaft lifts which carry a 50 ton payload wagon to the shaft head for further processing to extract the copper.
Relegated to history
Technologically, we are now way beyond that and other advances in delay blasting and drill hole accuracy have seen coyote blasting disappear as a means of getting a lot of rock relatively quickly.
Modern day blasting with electronic delay patterns is so much more efficient in both labour use and explosive quantities, plus the benefits of noise reduction and vibration control are several of the many reasons that coyote blasting is no longer used. The really great benefit is the safety factors of more stable faces and less exposure to danger for staff drilling the tunnels, and their exposure to dust and noise as well.
References
Dictionary of Mining Mineral and Related Terms (An SME Publication) by Maclean Hunter Publishing (USA). Journal of the Industrial Explosives Society – Japan, Paper by Kumao Hino “The Theory and Practice of Coyote Blasting” for the attached plans and profiles of some Coyote Blasting methods
