Manchester’s cool new winter wonderland
Get your bets on now. If you live near junction 10 of the M60 in Greater Manchester you are guaranteed a white Christmas.
Junction 10 is not just home to the 21,000 sq m of granite flooring and two million facing bricks that make up the Trafford Centre. It now features Chill Factore, an airtight ski box complete with its own luge and ‘alpine village’, as well as 2,000 tonnes of snow.
It is huge. There are already four indoor snow centres in the UK, but this one has the longest ski slope so far, at 180 m. It is an L-shape - 40 m wide at the top and 100 m at the base - and features a main leg some 35 m above ground level. The slope starts at 15 deg and then shallows out to 10 deg towards the bottom.
But the site itself represented a test of skill and nerve.
During the 1880s spoil from the excavation of the Manchester Ship Canal had been spread on the slope’s site, overlaying the alluvial deposits of the diverted River Irwell with between 1.8 and 5.4 m of weak fill. 7,500 cu m of spoil had been deposited in heaps.
Preparing the ground
David Firth, project manager at Sir Robert McAlpine, explains: “It is silt so it is very weak sub-soil. The cement stabilisation is formed in two 300 mm layers with three per cent and 30 per cent CBR load bearing test values respectively. It is topped with 150 mm of imported stone as the running surface.”
Mr Firth’s team re-used what was on site rather than digging and removing material to dump.
The ground had to be able to support the huge 51 tonne side trusses as well as two 400 tonne cranes and two smaller 50 tonne cranes. And with this year’s poor weather the six-week ground preparation process proved to be a shrewd move.
“The exceptionally wet summer would otherwise have converted the site into an enormous bog,” says Mr Firth.
Gantries for the ski box and alpine village are made of a steel framework which was craned into position. Prefabricated in sections and then sent to the refrigeration contractors, the gantries were fitted out with blast chillers and trace heating control panels.
“The sections of pipework were all pre-fixed onto those gantries and then lifted into place, so we eliminated the need to work at height by doing that very early on in the build process,” he says.
Actually getting up to the sloping ski run was a challenge.
“The biggest problem was accessing the ski box and working on a slope. We’re used to building level structures. We had to come up with a method of working on a 15 degree slope at height. So we had a number of spider cherry pickers operating within the ski-slope,” says Mr Firth.
Adding the snow
And then the snow machines were switched on. “There are 14 blast chillers with associated snow guns which make the snow and top it up every night. During the initial snow-making period we were producing 80 tonnes of snow every day,” he says.
The frame of the building is made from the S355J0 grade of steel, which has been tested at low temperatures. It needs to be able to withstand huge variances as the steel passes through the ski chamber and beyond. A minimum temperature of -16.5 deg C will further test the steel’s properties.
David Sterland, managing director of Extreme Cool and the man behind the similar Xscape snow centre at Milton Keynes, says: “Where the steel penetrates the inside the ski box we have to use a mixture of trace heating and insulation. We’ve got six columns which come up inside the ski box and traditionally you’d use trace heating. But we’ve actually insulated the columns two metres high.
“You only need to insulate to this height because that is where the cold creeps up to.
“The air temperature is about -1 deg C and we drop it to about minus six when we make snow overnight,” says Mr Sterland.
The ski box is enveloped by two layers of insulating composite panels. Along with the blast coolers, ice rink technology is used underneath the snow. “There is a glycol circuit which cools the snow. It is underneath the slope at -16.5 degrees C,” says Mr Sterland.
The cooling pipes sit on top of a sheet of steel. Under that are two layers of insulation and then the concrete base.
It is also crucial to keep the box airtight. “If you let the moist outside air in, then that condenses on the surfaces. Because it’s sub-zero it freezes and then you get icicles, which can fall down on people’s heads,” says Steve Clark, construction manager at Sir Robert McAlpine.
The actual air leakage test results show 0.23 m3/hr/m2 for the ski box, far better than the 10 m3/hr/m2 required under building regulations.
This has been achieved by vapour sealing joints in the insulated panels around the box. “It’s about the level of detail. Every single joint is sealed,” Mr Firth adds.
The 180 m aluminium roof was rolled in 35 m sections through an on-site roll press and then each sheet craned into position and site-welded.
Despite a couple of last minute additions in the building’s mall - which meant that two columns had to be moved - construction was completed on time.
“It was a pretty complex operation and we had to jack up the existing steel structure and replace beams and two columns,” says Mr Firth.
Time was tight.
“The programme was squeezed towards the end but we reprogrammed and rescheduled our finishing trades,” he adds.
Would he change anything? “We’d have more storage space if we did it again. And perhaps I’d try and make the architects put some windows on the outside of the building,” says Mr Sterland.
At least one more centre is in the pipeline, so Mr Sterland’s wish may come true. But for now there is time to sample the delights of Manchester’s apres-ski scene.
Constructing the UK’s first indoor luge
“There was a six metre gap [on the beginners’ slope] and I asked if we could put a toboggan track there,” explains Extreme Cool operations director Andrew Lockerbie.
“Everyone looked at me and said: ‘How do we do that then?’.”
The luge was added to the design in January 2007 and the glycol cooling under-floor pipes had to be laid before the concrete to support the luge’s legs.
Waterslide specialist Design and Display Structures designed it. It is the first frozen ‘flume’ of its kind in the UK. It is 60 m long and slopes between seven and 12 degrees. “We knew so little about riding on ice – we were scratching our heads at the earliest stages,” says Mr Lockerbie.
“We went to Dubai’s snow centre, which helped. Ice is a bit of a different animal. In a waterslide you’re normally in an enclosed tube and you can have bigger drops.
“But they wanted an open flume so that you could feel like you were part of the snow experience,” says Design and Display Structures’ designer Richard Kiss.
“You’ve got to get the right combination. You can’t do anything too wild at the end of it. At every drop you increase your speed as you go round centrifugally on a surface.
“We spent a long time on the construction of the drops and curves so it would be both exciting and safe, which is a difficult combination,” he says.
The slide was built with 50 sections of glass-reinforced plastic. “It was done offsite and so was possible to fit it into the build timetable. Because its strength-to-weight ratio is very significant it means you can put together strong units that are easily handled,” says Mr Kiss.
Once it was assembled, the refrigeration contractor Criotec fitted mats of glycol pipes to the GRP slide’s curves.
Two thousand tonnes of snow sit on the slopes.
Sixteen tonnes – or a 5 mm layer - are produced every night to top this up and keep the slope looking clean.
Used or dirty snow is piped into an external 4 cu m concrete pit and then warm water is sprayed on to melt it.
Site area 4.3 ha
Building footprint 15,602 sq m
Topsoil strip 8,000 cu m
Concrete precast piles 1,001
Pile depth 21 m max
Steel frame 2,700 tonnes
Depth of snow 400 mm
Contract duration 63 weeks