Watertight is the word as crews work to restore salt mine’s old shaft liners
Cementation Canada Ltd. of North Bay, Ont. bills itself as “one of the premier shaft sinking companies in the world,” and it has the track record to back up that claim.
With some 20 projects on the go in Canada, the U.S. and elsewhere around the world, Cementation is also on record for having sunk the deepest shaft in Canada at the Kidd Creek Mine in Timmins, the deepest single lift shaft in the U.S. at the Resolution Copper Project in Superior, Arizona, and the deepest single lift shaft in the world at the South Deep Gold Mine in South Africa.
But by the end of March, Cementation crews will start a completely different sort of project at the Sifto Canada salt mine in Goderich, Ont., on the shore of Lake Huron. They will begin refurbishing the liners inside two of the mine’s three shafts, which will take almost four years, and rank among the most challenging work the company has taken on in recent years.
“Technically, this is a very different project,” says President and Chief Executive Officer Roy Slack. “It’s not like designing a shaft or shaft liner from scratch. We have to adapt to what’s there.”
In both cases, what’s there is a concrete liner that has deteriorated and sprung leaks. In some places water is seeping in. Elsewhere, it is surging through the concrete as though driven from a garden hose.
The problem is partly a function of age. Shaft one, which is now out of commission, was sunk in the late 1950s, while shaft two, currently used to move miners and materials, was built in the early 1960s.
The construction standards of the day have also played a role, says Mike Marksberry, Director of Mine Engineering with Compass Minerals Inc., the Kansas-based company which owns Sifto Canada.
“They used thicker concrete where water was an issue and thinner concrete where it wasn’t,” says Marksberry. “But over the years with ground movement from mining and everything, water has migrated everywhere.”
A third shaft, sunk in the late 1970s, remains in good shape and is not in need of repairs. It is equipped with a cage to transport miners and materials as well as skips to haul salt to the surface and will remain in operation while the other two are refurbished.
All three shafts were sunk to a depth of 543m to tap a vast, nearly pure salt deposit which is some 25m thick and extends for hundreds of square miles beneath Lake Huron.
Miners currently have to travel some eight km from the bottom of the shafts to reach the mine face. In the future, they’ll be travelling even farther.
“The reserves under Lake Huron are almost endless,” says Marksberry. “We could literally mine all the way to Michigan if we kept going. One of the things driving this project is that we intend to operate this mine for 50 plus years.”
The headframes and other surface infrastructure are located on a narrow, L-shaped spit of land with the mouth of the Maitland River on the inside of the spit and Lake Huron on the outside and that adds another layer of complexity to the shaft rehabilitation projects.
Space is limited and Cementation must remove the headframes for shafts one and two, as well as several other aging structures, before the work below surface begins.
The mine will continue to operate throughout the project and salt destined for markets will be loaded onto ships and rail cars.
Compass initially intended to remove the old concrete liners in the two shafts and replace them with fresh concrete, but it would have had to stop the inflow of water until new hydrostatic liners were installed.
The company looked at two options for doing that and found problems with both.
The first was freezing in which a series of wells was drilled around the shaft and a refrigerant injected, a proven technology, but one that occasionally fails. Compass also considered using pumps at the surface to draw off enough water to prevent leaks, but concluded that that approach would not work.
Cementation came up with a solution that, while not unique, is also not very common.
Norm Rochon, who will supervise and manage the work, says the existing concrete will be left in place to a depth of 296m, which is the bottom of the water-bearing strata, and cylindrical steel liners will be installed to make the shafts impermeable.
First, though, Cementation must construct a concrete foundation ring to support the steel liners. It will be built into the native rock behind the existing concrete liner and will extend 230mm into the shaft, meaning that the widths of shafts one and two will be reduced to 4.5m from 5m when the project is complete.
Once the foundation ring is finished, Cementation can begin installing the new liner in sections, each of which will resemble an enormous steel ‘can.’
The steel will be 38mm thick and each ‘can’ will be 3.1m in height. Two sections (weighing a total of 42 tons) will be welded together at surface and lowered into place using giant cranes.
Cementation crews will be working in the shafts throughout the rehabilitation project and they will be lowered into place on a platform called a galloway. They will construct the foundation ring and will inject a thin layer of grout comprising cement, water and small amounts of sand-like aggregate between the exterior of the steel ‘cans’ and the existing concrete liner. As well, each 6.2m-high ‘can’ must be welded to the one below it and Cementation workers will perform that task as well.
“This project is anything but typical,” says Rochon. “We’re not typically working in an existing shaft with existing infrastructure, with production ongoing and with water flowing into the shafts. This is a salt mine. It really has to be watertight.”
From the foundation rings to the bottom of each shaft–a distance of 248m–Cementation will chip out the old concrete and replace it with a new liner that is 457mm thick.
The mine will continue operating throughout the rehabilitation project. Shaft three will continue to be the main production shaft and will serve that purpose even after the work is complete.
Cementation will be working on shaft one first and it will be put back into operation as a production shaft afterward, which means a new headframe will have to be constructed along with all the associated infrastructure.
Shaft two will be reserved for ventilation.
“We can’t have any conveyance equipment in there taking up space,” says Marksberry. “It has to be stripped of everything to ensure the least possible resistance and to meet the safety regulations, which have become more stringent over time.”