Mining is one of Canada’s most important sectors and plays a vital part in the country’s economic prosperity. Canada is blessed with a wide variety of resources, and while the industry has had some serious ups and downs over the past five years, particularly on the exploration side, growth is expected once more.
"Canada has an extremely active mining industry, many, many deep mines which are located in different provinces but there are a number of major mining fields," says John Elliott, managing director, Alan Auld Company, which designs deep mine shafts. "The country has pretty much everything; gold, copper, diamonds. The shafts for these mines are typically between 500m and 2km deep. There are quite a few new projects ongoing – some of these are under construction, some are in planning. There is also a lot of ongoing upgrade and maintenance work."
Potash
Potash is mainly found in Saskatchewan; it was first discovered in the province during the early 1940s while drilling for oil. Around this area gold, copper and nickel can also be found. Whereas some of the base metals are over in the east of Canada and diamonds are in the far north. What is potash? The mineral’s name refers to several forms of potassium salt, the most important being potassium chloride or KCL. The mineral is found in thick massive seams and is pink and crystalline. It is found within evaporative sequences including thick beds of halite and other related minerals, such as carnallite.
"Potash is mined by continuous mining methods using cutting machines, which excavate long rooms in more than one pass," says Elliott. "Most mines are located around 1km or more beneath the ground. Once you get below this depth ground control begins to become a problem due to high ground loads and relatively weak strata."
In terms of the world potash production, Canada is a major player – nearly half of the world’s potash production comes from the country. "A load of it comes from Russia [34.7 per cent] and Belarus [7.9 per cent] as well. But if you go on Google look at ‘Russian sink hole’ you’ll see the Russians are a little less careful; they have a lot of problems with collapses. There isn’t this problem in Canada as it’s generally deeper. However, there are a few new mines in Russia and Belarus that are now of a similar depth," says Elliott.
Potash offers huge potential. BHP Billiton, the world’s biggest miner, has shown its ambitions in this area. In 2010 BHP Billiton offered PotashCorp of Saskatchewan (PCS) CAD 39bn (USD 29.5bn) takeover offer, signalling the seriousness of its ambition. While this attempt failed, it was a boost that the potash market needed. Since then, BHP has continued with its Jansen project in Western Canada, the cost of which has been put at CAD 14bn (USD 10.6bn) and which would be the world’s largest potash mine whenever it opens.
The existing potash mines in Saskatchewan are Vanscoy, Cory, Patience Lake, Alan, Colosay, Langian, Belle Plain, Esterhazy, and Rocanville. "There are currently some new mines going down," adds Elliot. "Picadilly is now ramping it up to full production. BHP is are building Jansen and we’re currently designing the shafts on that one. There’s a new ventilation shaft going down at Rocanville while K3 is a new mine that will replace K1 and K2. There’s a lot of activity."
When PCS purchased the Penobsquis mine, mill and port facility as part of a larger USD 112M transaction from Rio Algom, it was another example of potash making a comeback. The future of potash mining looked promising, but even more promising was what lay about 1km beyond the Penobsquis mine.
Picadilly
The Picadilly mine is a new mine in New Brunswick, situated adjacent to the old Penobsquis mine. The overall scope for Picadilly was to ramp up for production over three years to produce two million tonnes per annum with a mill plant and brine pipes. The project was an engineer, procure and construct project with 30 active subcontracts at any one time. Cementation Canada Inc was approached to carry out the design and construction of the shafts for the new Picadilly mine and engaged the Alan Auld Company as the shaft liner designer.
"PCS are the largest producer in Canada and own and operate the Penobsquis mine," says Elliot. "They currently are underway with a production expansion programme and are gearing up to produce a lot more potash, as are the other players. As part of this programme PCS planned to construct a new mine in Sussex, New Brunswick, which is next door to Penobosquis. The Picadilly mine be the first new potash mine in over 30 years. The mines are less than 1km together – when I first saw it I couldn’t understand why you’d want to sink the mines so close, it seemed bizarre to me."
Water is the nemesis for most potash mines. Flooding can eventually lose the mine after years of operation – as what happened to the abandoned PCS’s Cassidy Lake mine about 50km from Sussex.
"If you look at the geology around Picadilly, you’ll see it’s a salt dome, but it’s actually more of an anticline. It’s a salt dome that has caused the rocks above it to form an anticline. Penobsquis has a problem and the problem is water – they were shipping 120 water tanks a day of brine and putting it in the Bay Fundy so the mine is slowly drowning. They can keep it open but one day it will probably become too much, so the new mine is on the opposite side to the anticline and isolated from water problems associated with the Penobsquis mine." The project has two shafts – a production shaft and a service shaft – some 900m deep with a diameter of 5.5m. The shafts are concrete lined with a single station entry in each shaft rather than multiple access points.
The predominant geology is the Mabou mudstone – similar to Mercia mudstone – into siltstones lower down but with less water problems before entering the basal halite and anhydrite. "Mabou mudstone is good when it’s dry but not very nice when it’s wet," says Elliot. "Below 420m the geology changes and there are more siltstones, it’s very tight – there are still fractures but they are filled with jitson and you don’t get water problems. While we could have designed the lining for deeper than that, the deeper you go the more problematic it becomes and the more expensive it becomes."
The shaft lining was self-levelling 60Mpa concrete with particular attention paid to matcher joints between each 6m height pour. Heavy duty PVC grout and stainless steel grout injection points were used with the lower shaft lining through the halite and potash being carried on a pylon structure.
The salt creeps over time so historically a steel tower would be placed to provide support but created difficulties as the salt squeezed. Though polyurethane was originally to be used a liner for Picadilly, the risks involved with fire were believed too great and cellular concrete was adopted instead.
Analysis was undertaken on creep predictions as the mine is to have a 40-year life before the loads on the lining will begin to cause damage.
At station level the openings are larger hence the need for a pylon structure. The pylons sit on a horseshoe shaped mass concrete structure on a compressible fill. The ground is unlined in the anhydrite.
Sinking the Shafts
The shaft collar for the service shaft had a shallow overburden of 3m to 4m and was open cut with excavators while the production shaft had a deep overburden of 9m with artesian water and a secant pile support. There was pre-excavation grouting down to 40m depth. Once the collar was placed, the shaft was excavated by hand until there was sufficient depth to install a Galloway frame over the shaft. The Galloway installation is a secure structure equivalent to a five storey building. "It is clear of any columns so we can work freely and have easy access," says Alun Price Jones, technical director, Cementation Mining. "The Galloway installation is a busy place, there’s not a lot of elbow room. Once the Galloway installation was erected we picked it up with a crane and lowered it into the shaft."
Next, the construction of the headframes for the production shaft took place. The headframes at Picadilly went up quickly, over two and a half weeks. The headframes were constructed using slipform methods and are 100m high. Once in position, crews installed hoisting equipment to sink the shafts. The muck was brought to the headframe then into a chute and muckbin before removal.
The Galloway was supported on three ropes and act as guides for the muck bucket to prevent swinging. "You cannot lower an unguided conveyance down a deep shaft, it will just swing out of control and hit someone," says Price Jones. "So, the Galloway is suspended on ropes and you can run the bucket out at very high speeds, it goes where it’s meant to, and when it gets to the Galloway it goes through the bucket well."
The sinking hoist was a Hepburn 9ft (2.7m) diameter single line drum hoist able to support the self-weight of the rope and controlled from a hoistroom with cameras within the shaft and at stations. Each rope has a load cell and the proven system of bell signals used to communicate. The bottom deck of the Galloway was the concreting deck with the mucking deck below that.
The excavation cycle was generally as follows: The advance grouting was undertaken which could take up to two weeks; the bench was then drilled; the rock was blasted; the rock was mucked out and ground support placed; the bench was then cleaned and the benching cycle repeated on the other half of the bench; the curb ring (formwork) was lowered and poured with matcher joint grouting; and the mains were lowered, and contact grouting of the back wall undertaken every five sets.
Care was taken not to over pressurise the grout to avoid fracturing the ground. Grout hole deviation plans were produced.
The drill jumbo was stored in the headframe with a gantry for maintenance purposes. "Once you drill the bench, you blast, and the next stage is to muck that out," says Price Jones. "This style of shaft advance is called benching because we take half the shaft to begin with and then the other half later on. It has its advantages because if you’ve got very wet conditions you can let the water pummel into the deeper parts and when you need to clean up the bench ready to drill next time it’s easier to get below the loose material."
The vertical shaft mucker(VSM) was a long telescopic arm with a grab. Two could sometimes be used together but for Picadilly only one was required. With only one bucket well, the buckets would be switched with a filled bucket being transported to the surface, emptied then returned to the shaft bottom and replaced.
Price Jones says: "In this shaft, it’s only 5.5m diameter so one is all that is needed. We have only one bucket well so we send one empty bucket down and unhook it while the BSM fills it and the hook goes back to surface and brings down an empty bucket.
The ground support was initially short rock bolts and weld mesh. Sometimes in the Mabou mudstone the bolts would be ineffective if the rock was too soft so a permanent oversize formwork would be required with a concrete lining.
When the curb ring was set, accelerated concrete was used with the ring supported on Dywidag rods. When sinking through the salt layers a spray on material called Rock Web was used to prevent the salt hydrating and disintegrating. Full safety gear with face masks was required for application. At the oversize locations the shaft diameter became about a metre greater in diameter than the Galloway itself. A larger ring suspended off the Galloway was taken 100m through the halite and stabilised by rods. The halites were relatively stable to excavate through. As the proposed station was approached the benches were constructed by enlarging out.
A drawbridge from the bench was then needed from the Galloway to the shaft station. The pillars were eventually constructed at the entrance at the shaft station. The foam concrete liner could be placed top down as the self-weight of the liner would not support the weight of the lining itself. On the way back up the shaft, the formwork was placed to provide a sacrificial liner to provide a temporary lining before the permanent lining was in place.
Grouting behind the liner was then undertaken by casting stainless steel tubes in the concrete to deliver grout to the matcher joint and rock/concrete boundary. For finer fissures, microfine cement grouts would be used or chemical (urethane) grouts for short term water stoppage.
Finishing the job
"So we’ve done the shafts and part of our contract was to get this mining equipment in," says Price Jones. "There was a lot of engineered lifts using the hoists.
"The equipment had to be broken down into chunks and lowered down and assembled in workshops we had excavated underground ready to receive them."
The mining machines used were the Sandvik MF420 and the MB670 Borer Miners weighing over 200t. The M420 is currently the world’s heaviest and the most powerful, it can cut a mining width of 6.5m by 3.6m high.
The Sandvik MF420 Borer Miner is a variable height and width, boring-type machine that cuts and transports material to auxiliary haulage equipment in one continuous operation. The machine drives entries, excavates mine rooms and extracts pillars as rapidly as haulage equipment can remove material to the main passageway. Depending on the haulage system being used, the Sandvik MF420 can cut and convey approximately 15 metric tons of potash ore per minute.
The MB670 (four machines) are not quite as big but still a decent size. It has a ranging drum and it can cut a bigger hole than the M420.
"We had to do all the engineer lifts and we worked closely with the manufacture to get it down the shaft," says Price Jones.
The project boasts zero lost time for injuries. Price Jones explains that this was achieved by constant training of the staff, combined with regular audits and a positive safety culture.
"Shaft safety is a big deal to us. We had zero lost time on this project because of injuries, nobody failed to come to work because they were injured. In order to keep up this level of safety it involves constant attention."
The best shaft sinking advance was 3.98m per day with the bottom of the service shaft being reached by March of 2012 and a further year to fit out to a permanent state. The production shaft lagged the first by about three months and reached the bottom by July 2012. The fitout was completed by the end of the year, 2013.
The new mine is now complete and in production.
