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Author : David Strydom

02 April 2015

Storing archives and other documents in a working salt mine has definite health and safety challenges. David Strydom went to Winsford Rock Salt Mine in Chesire to find out what they are. 

Touring the underground Winsford Rock Salt Mine, Britain's oldest working mine which lies almost 200m under the Cheshire countryside, is like taking a moonwalk. It’s surreal and undeniably eerie but so fascinating you’ll feel as though you’re exploring a different planet.

But the mine isn't just a geological curiosity – it’s an important storage area which is used to store documentation such as evidential material for police authorities and patient records for local hospitals. The mine’s biggest customer is the National Archives.

Compass Minerals, which operates the mine, is part of the Compass Group; DeepStore is the document storage part of the business. “We store the country’s treasures underground,” says James Noblett, environment health and safety manager for Compass Minerals UK. “We store for several architects, not just architectural paperwork but models too. We’ll store anything for any customer because we have the ability and the space.”

DeepStore, James explains, revolves around carefully handling boxes and being custodians of customer archives ; the salt business, on the other hand, is about using large pieces of kit to safely mine 1m tonnes of salt every year.  What this means is Compass Minerals is a ‘two part’ business in the sense it makes money from the salt but also from the underground storage area created by moving the salt.

“The good thing about the business is that although it’s dependent on the weather, if we have a really good winter – meaning it’s cold – we do more business because we’ll sell plenty of salt for de-icing. If it’s a warm winter we don’t sell as much but can rely on the DeepStore business to maintain our profitability.”

Specially designed machines cut the salt into large pieces. It’s crushed, anti-caking agent is added, and it’s then ready to go to customers. The small pieces of salt are then sold mainly to local authorities. “We’ll also sell to local gritting companies and small companies that want to bag it. But our main customers are the highway agencies.”

The salt is extracted in such a way that pillars are left behind – each one 20m2 – to hold the roof up. That means the mine is naturally supported; there’s no need for additional support. “That’s how we’re able to access every part of the mine,” says James. He describes salt as ‘a good rock. We don’t have problems, which is helped by the fact the mine is only 550ft deep – relatively shallow in mining terms’.

Getting to the mine workings is much easier now than it was in the 19th century. Shafts 1 and 2 were used for almost 130 years until they were sealed in the ‘70s. Originally miners were lowered into these shafts by bucket. Over the years shafts 1 and 2 were replaced by three more shafts. No 3 Shaft is 3.05m in diameter and 150m deep, was installed in 1941 and was used as a production shaft until the ‘70s. Today it has a self-service lift and is used for personnel access.

No 4 Shaft is 4.88m in diameter and 189m deep, is the down cast shaft. This means it’s used to draw air into the mine. It’s about a mile from No 3 shaft and was installed in 1963 with a two-tier lift consisting of a cargo compartment and a personnel carrier.

The cargo compartment is 7m high, 2.4m wide and 4m deep and brings all the materials and engineering equipment underground. It can carry 15 tonnes. The bigger mechanical items are dismantled to fit into the lift and, once reassembled underground, never leave the mine.

No 5 Shaft is 4.88m in diameter and 164.5m deep, was the last shaft to be installed in 1973. It’s used to bring the finished rock salt to the surface, which it does in a series of 9t skips. It takes just over a minute to get one skip of rock salt from the mine to the surface.

“All the air is brought down Number 4 shaft, through so-called de-watering tunnels,” James explains. “Salt is hydroscopic. In dry periods it adds moisture to the air, during wet periods it takes the water out – it’s a natural process. When it comes out of the dewatering tunnels, the air temperature is always 14 degrees Celsius, so regardless of what's happening on the surface, we’re always 14C with the humidity at about 60%. It’s a good environment in which to store documents.”

In each area of DeepStore, the air is ‘managed’, meaning it goes through an air-handling unit where James’s team has the ability to reduce it further if needed. “We can, for instance, cut the humidity to 15%, ideal for storing documents,” says James.

“We mine about 1m tonnes a year, using specially designed machines called continuous miners, which are manufactured by a US company, Joy. Essentially, there’s a large drum on the front with picks on it which cut the salt from the working face. . Until now we’ve run only one at a time, but from this year we’ll be running two.”

Because of the size of the Continuous miners, they – along with other large mechanical items - are dismantled before being transported brought to the mine in pieces, then rebuilt by Compass Minerals employees in collaboration with Joy contractors to ensure they're correctly assembled.

The sheer scale of machinery used by Compass Minerals means the business isn't particularly employee-heavy. “We don’t have many people – about 150 – because of the efficiency and scale of the machinery. We have guys here who’ve been here more than 40 years.”

The Joy continuous miner product line has been developed to meet the high productivity requirements of the underground mining industry. “Each machine employs Joy Global's Joy multi-motor concept with outboard access to motors, gear cases, controllers and other major components,” Joy states on its website.

The philosophy calls for the isolation of major components for easier troubleshooting and maintenance. “The continuous miners use individual motors with direct drive transmissions to power the cutter, traction, gathering and hydraulic systems. This permits service or repair quickly and easily, thus reducing downtime and maintenance costs. The continuous miner product range is segmented by mining height requirements into three machine classes for low-, medium- and high-seam applications.”

During the tour, James points out a monster of a machine which, he tells me, is a WA800 Komatsu front-end loader used for moving rock around the mine. The bucket on it has a 25t load capacity, and is used for moving large volumes of rock. It’s one of the biggest front-loaders in the UK. “It was brought down the No 4 Shaft and rebuilt underground before being commissioned and put into service. It’s due to be replaced. We also have a Caterpillar 990 of similar size, and two Caterpillar 988s which aren’t as big but also front-end loaders. In total we have five or six front-end loaders. The WA800 is by far the biggest.”

Vehicles used underground need to have special features to drive in a mining environment, James explains. “Each vehicle has an isolator. That means when the vehicle is parked up and not being used, all power including the electrics are isolated. All the vehicles have a fire-suppression system fitted; on the smaller vehicles it’s a fire-trace system. The larger vehicles such as the Komatsu WA800 will have a Kidde thornesystem fitted. All are designed to flood the engine which will extinguish any fires should that be a problem. Anybody driving the vehicles has to do a pre-shift or pre-use inspection to ensure its fit for use before they drive.”

According to James, the continuous miner at Winsford is the biggest machine manufactured by Joy. It gives Compass Minerals the ability to cut to a height of 6m. “The miners are operated by a guy with a remote-control. It runs off electricity so it doesn’t burn diesel, which would taint the atmosphere. The salt is then dropped off onto a conveyor belt – a walking tail-end. The belt follows the machine up as it’s moving forward, and is then taken back to a crushing plant where it’s broken into small pieces; that’s the bit you get on the road in winter.”

It’s then taken on conveyors, up the shaft where a small amount of anti-caking agent is applied to stop it sticking together. “It comes out on the surface, then goes to our customers, so we don’t add anything to it other than anti-caking agent. What goes out on the road is essentially what we extract from the mine.”

James reports to the mine manager who in turn is answerable to the MD, but because Compass is a US company, James checks in with the US as well. He has four people in his team including a safety officer, a training officer, and a contractor coordinator.

Compass Minerals mines under two rivers as well as mining under a local canal. In addition, the West Coast mainline that runs from London to Scotland, runs above the mine workings.. HS2 will also run across the top of Compass Minerals property because, as James says, ‘the underground workings are so competent and safe’.

James says contractors perform onsite maintenance on the large front-loaders (CAT and Komatsu are the other manufacturers used by Compass Minerals “We’ll have the contractors onsite to service our machines – it is part of being proactive rather than reacting only when something goes wrong.”

An example of that is the fact that Compass Minerals has changed the winder (the implement used to run the cars up-and-down the shaft) on No 5 Shaft that’s been in use since 1975. “If you look at the number of years it’s been there, it’s probably dealt with 40m tonnes of rock. Now we’ve used contractors to replace it. A lot of planning went into that.”

With respect to the environment James says the company works hard to reduce its environmental impact. “One reason we’ve opted for electrical-driven rather than diesel-powered machinery is to reduce the amount of diesel we put into the atmosphere. Anything that goes into the mine will, of course, come out the mine through one of the shafts. We’re trying to move towards electrical machinery. We’re changing our power supply this year, which will mean it’s much more efficient in the way it comes to the mine.”

Given the underground distances, I ask James about mobile working at those depths. “Everyone has mobile phones,” he says. “They are also part of our emergency systems. Our phones can work on Wi-Fi underground.

The DeepStore storage units have been designed to a high specification, each has 2 or 4 hour fire walls and a state of the art fire detection system called VESDA “We have carried out a detailed risk assessment so that the possibility of fire is kept to an absolute minimum”

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