Beneath our streets giant ‘moles’ will be munching away at the earth to carve out 26 miles of tunnels for Europe’s largest construction project worth £16billion. After being kitted out with orange clothing complete with hard hat, glasses and gloves, and sitting in on a safety briefing, I stay close to construction manager Rob Gordon as we enter a big lift lowering us 40-metres down a shaft at Stepney Green.
Above ground, the location of this Crossrail construction site could not be more peaceful with animals grazing at Stepney City Farm and children playing at nearby Sir John Cass School.
Rob is keen point out that they have only had one noise complaint from nearby residents and that all the activity is underground. Lorry movements to and from the site are restricted to outside school hours.
As we step out from the lift below we are faced with immense activity as I find myself inside one of the largest mined caverns ever constructed in Europe.
Flashing red lights warn of moving vehicles, cranes, and skips unloading cement and carrying excavated earth as we stand between the eastbound and westbound caverns. About 7,500 cubic metres of soil was dug to create the eastbound cavern and 9,000 cubic metres for the westbound cavern.
Technique
Stepney Green will not be a stop on the Crossrail network but it is unique among the construction sites as this is where the line coming from Whitechapel will branch out in two directions – one heading to Shenfield via Stratford and the other towards Abbey Wood via Canary Wharf, Custom House and Woolwich.
Tunnelling machines Elizabeth and Victoria are expected to break through here in October – one into the eastbound cavern and one into the westbound cavern.
Two other twin machines, which will be named in a competition, will be coming from Pudding Mile Lane and are expected to arrive here in August.
The caverns are five metres apart – 35 and 40 metres deep – allowing the tunnel to Stratford to run under the route to Canary Wharf as they cross each other.
The tunnels are being sealed —using a technique called sprayed concrete lining —after the tunnels were created using traditional mining methods and diggers —rather than letting the giant boring machines carve their way through.
Rob explained that it took two months to get the texture of the concrete right.
“Normal concrete is a bit like scrambled egg whereas this stuff is a bit like a souffle – it is very sensitive, you need to get all the constituencies right. It’s got additives, microsilica and steel fibres.
“Normal concrete takes longer to settle, which is fine if you have got a supported building, but we need to achieve an inner strength or the cement won’t stick to the tunnel walls.
“The mix is made off site because of the school and residential homes nearby and dispatched over. It then goes through a special pumping unit, which sprays the concrete out with an accelerator at the nozzle.
“The reasons these techniques are employed here are because of the geology of the cavern itself. It’s a bit of a trumpet that gets larger and larger. London clay is soft like cheese and ideal for tunnelling.”
While the cement is being sprayed on the walls of the westbound cavern, a waterproof lining is being fitted to the floor of the eastbound tunnel, allowing water to drain away.
The £500,000 contract to build the Crossrail caverns was awarded to a Spanish-Irish venture, Dragados/Sisk JV, and the highly skilled workforce, which includes civil engineers and miners, has come from across the world.
Rob says: “Most of the guys here have been doing this type of work for at least 20-odd years.
“They may have worked on the new stations at Tottenham Court Road, King’s Cross and Heathrow, and before that the Jubilee Line and Channel Tunnel or elsewhere in the world. Currently we have about 30 people on site but at one point we had about 40.”
The site operates 24/7 with 12-hour shift. A longer weekend allows workers from places such as Wales to return home to see their families while others have settled within commuting distance.
A £13m Tunnelling and Underground Construction Academy, part-funded by Crossrail, was set up in Ilford after it was recognised that there was a skill shortage for future projects in this country.
Once I have surfaced from underground, I’m taken with our photographer on a minibus to the nearby Pudding Mill Lane Portal.
As we are lowered in a basket, offering a view across to the Olympic Park at Stratford, a very different construction site is unveiling itself before our eyes.
Inside a box-shaped hole, 16 metres deep, one of the giant boring machines is already in place underground waiting for take-off to the cavern at Stepney Green.
Its twin machine is currently going through final factory testing in Germany before it will be dismantled and shipped over.
It is expected to arrive on site in about two months when it will be put together.
Construction manager Frank Jenkins explains that each of these beasts costs £10million.
Each machine is about 150 metres long and weighs 1,000 tonnes and on average creates 100 metres of tunnelling a week.
He said: “Whereas the construction method at the Stepney Green cavern is more traditional mining – using traditional excavators to dig away against the ground before spraying on the supportive concrete – here we use the tunnel boring machines.
“It’s basically like a huge mole. At the front of it there is a cutter disk which rotates about three times a minute and grinds away at the clay and the chalk and it gradually gets pushed forward by hydraulic rams creating rings in the earth.”
Though computerised, a driver sits inside a control panel at the front of the machines to monitor progress.
Their route has been carefully planned to avoid any obstacles such as water pipes and to stay clear of buildings that may not withstand movement in the ground.
Once the machines reach Stepney Green they will be lifted out and taken by police convoy to the Limo site at Canning Town before being put to work again as Crossrail takes shape.
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