Squeezing talc zone forces review of haulage options
Goldcorp’s six-kilometre (20,000-foot) Cochenour-Red Lake Haulage Drift broke through to the Cochenour mine workings late last year, but the four-year underground construction project may not provide the gold miner with the highspeed rail line that it hoped for.
A 2,300-foot talc zone encountered 13,000 feet in from the Campbell shaft has prevented the laying of track from that point on and presented the company with a major ground control headache.
“It’s a very soft rock – like toothpaste and it’s moving all the time,” said Paul Healey, vice-president of Canadian operations with Redpath, the contractor hired to execute the project. “It’s constantly trying to come in on you at a rate of inches per week.
“We’ve had to go back in to recondition and resupport the area several times,” added Healey. “Goldcorp has all their geotechnical resources and some outside consultants trying to determine the best approach. They’re looking at all different kinds of ground support and have done several tests to determine what will stop it so they don’t have to go in and constantly rework the area.”
“It was a shock when we hit it,” said Rolf Arnold, a Redpath area manager who was heading up the project when the talc zone was first encountered. “None of us had worked in talc before. Most people I’ve talked to worked in small amounts of talc, but not in anything like that. “Everybody looked at it and had different ideas, but no one had a solution.
We put seismometers in 45 feet into the wall and we put some long support in at 25 feet in to see if we could contain it, but it was still moving. Every three or four months they have to go in and scrape the walls with a scooptram.
In one area of very soft serpentine talc, the floor and the ceiling were coming together, said Arnold. “We had to strip away all the services – the metal ducting, air, water, electrical cables – scrape it and put them all back. We never laid any track in that area because it was moving all the time.”
According to Elizabeth Howell, Goldcorp’s project manager for the haulage drift and the Cochenour shaft redevelopment, a final decision on whether it’s feasible to extend the track the remaining 7,000 feet is in the works.
“We’re still doing some testing on the floor. When we finish the study, we’ll make a determination. The intention is to put track all the way through, but we have options if the squeezing can’t be resolved. We could put in a conveyor or go with a civil approach.”
As a last resort, it might be necessary to haul the ore by truck for the first 7,000 feet and construct an ore dumping system at that point to transfer the material to the rail system.
The haulage drift was given the goahead in 2009 following Goldcorp’s $1.5 billion purchase of Gold Eagle Mines’ high-grade Bruce Channel deposit, which sits off to the side of Goldcorp’s former producing Cochenour mine.
The idea was to extend and refurbish the Cochenour shaft to access the Bruce Channel deposit and haul the ore by rail through the haulage drift for processing at its Campbell Mill.
The 14-foot wide by 16-foot high drift connects Goldcorp’s Campbell Complex with the Cochenour mine workings at a depth of 5,400 feet, and provided the company with drilling platforms to test for mineralization between Campbell and Cochenour. Aside from multiple drill bays, Redpath crews also excavated shops and refuge stations off the drift.
Redpath had 80 people working on the project beginning in 2010 – two crews totalling 55 people on site at any one time. Working 24/7, “we were typically cycling 10 to 12-foot rounds per shift, so somewhere between 20 to 24 feet per day,” said Healey.
Shotcrete
The drift was shotcreted 100 per cent as part of the primary mining cycle.
“Crews would muck out the drift with a scooptram, immediately spray shotcrete, bolt through it, then go in and drill and blast the next face,” said Healey. Screen was installed on the back for protection during bolting. Aside from the talc, Redpath also had to contend with a shortage of ventilation.
“We only had 43,000 cfm, so we had to stagger everything,” said Arnold. “We couldn’t run everything at once, so if we had a truck and a scooptram, you had to park the truck, turn it off, load the truck with the scooptram, turn it off, then jump in the truck and drive away. Once we got in to 10,000 feet, they added 20,000 cfm for us. That allowed us to run more equipment.”
A six-yard loader brought the muck from the face to a remuck, at which point a four-yard loader was used to load the trucks – once again to get around for the limited ventilation and reduce the heat. A new remuck was cut every 400 feet.
A push-pull ventilation system with metal ducting and 125 hp fans was used to accelerate the clearing of blast fumes.
“If we had to wait for the smoke to clear, we’d be waiting an hour,” said Healy. “This way, we only had to wait half that time to get back to the face.” The air was exhausted out an exhaust
raise at the beginning of the level. As the drift was completed, ventilation was able to come from the Cochenour side, ending the requirement for the pushpull system.
Loading systems
A third of the way through the drift, Redpath constructed a loading system and began laying 100-pound rail on concrete ties. That allowed haul trucks to dump waste into the rail cars for the remaining 6,000 or 7,000-foot distance to the Campbell Complex for skipping to surface. Another so-called transition area was constructed two-thirds of the way through the drift close to the beginning of the talc zone.
The rail system, running battery powered locomotives and pulling twelve 18-tonne cars, was a more efficient way to haul the waste and helped to get around the limited ventilation.
Goldcorp has two locomotives on site – a 30-tonne unit for hauling waste, or ore, and a smaller eight- tonne unit for transporting men and material. To refurbish the Cochenour shaft, Redpath had to dewater it, remove the timbers and slash it out to a more conventional circular shape. The headframe was removed and replaced, a new hoist was installed and the shaft deepened from 2,600 to 3,400 feet.
An alimack raise, completed in December, joins the haulage drift with the Cochenour mine workings, and will serve as a both a means of egress as well as for ventilation.
The Bruce Channel deposit is forecast to produce up to 250,000 ounces of gold per year, requiring the transport of 1,500 tonnes of ore to the Campbell mill every day.
Goldcorp expects to begin producing Bruce Channel ore in the third quarter of this year with commercial production expected by the second half of 2016.
