As part of the £14.8 billion Crossrail project, which will open in December 2018, there’s a new pair of underground tunnels spanning 26 miles. The new Elizabeth Line will operate from Reading and Heathrow to the west, through central London, all the way to Abbey Wood in the south east and Shenfield in the east. It’s the largest addition to the southeast’s rail network in half a century.

To accomplish this feat, gigantic tunnel boring machines meandered between the subterranean maze of existing underground tunnels, meticulously avoiding utilities, snaking through building foundations, sewers and navigating London’s hidden rivers.

Crossrail: The challenges

So how do you tunnel beneath a busy, heavily built-up city like London? We asked a man who knows better than anyone: Mike Black, Crossrail’s Head of Geotechnics, who led the planning for the marathon of new tunnels.

“There’s a multitude of issues you need to look at in advance,” he said in an interview with BT.com. “You need to know exactly what’s beneath the surface, where things like existing utilities and railways lines are.

“Obviously London is a very densely built up area. The modern buildings are taller, which means the foundations are deeper. It’s lot of work to understand where the obstructions and constraints are. Once we understand that, we can plan to minimise the ground movement inevitably caused by tunnelling.”

Crossrail tunnel

Most of the building records were obtained through local authorities, while a process of “back analysis” also helped to determine the depth of foundations. Further investigations into building led to more than 2,000 individual bore holes along the Crossrail route. Even with prior human intervention accounted for, Crossrail planners still had to assess the natural make-up of the ground they wished to tunnel.

“In the west of the route it tends to be clay, which is a much better medium for tunnelling,” Black says. “To the east of the route, the ground is a lot more variable, with a lot of ground water, which you don’t want to encounter.”

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All in all, it means the tunnelling is anything but A-to-B. In some places tunnels even split and rejoin to avoid building foundations. Most things can be avoided by sinking to greater depths, but there are limits there too.

“There’s a balance between going deep enough to avoid any obstructions but at the same time staying within a reasonable depth for access,” Black says. “Should an incident occur, you have to be able to evacuate it quickly.”

The deepest point reached on the new Crossrail route was 42 metres at Finsbury Circus, near Liverpool Street.

A giant underground factory

Construction workers dug the earlier London tunnels, like the Greenwich foot tunnel and the Brunnel tunnel, with pick axes. Today, underground projects like Crossrail benefit from gigantic tunnel boring machines (TBMs) that cut through the ground like the creatures from the Tremors movies.

Crossrail boaring machine

At £10 million each, these massive underground factories are the length of 14 London buses, weigh more than 1,000 tonnes, yet operate with laser-guided millimetre precision. The Crossrail project used two different kinds of machines and eight in total, moving beneath the surface in pairs.

A rotating cutting head sits at the front of the device and chews through the earth. While that excavated material is used to immediately stabilise the ground directly behind the cutting head, it is then carried away on a conveyor below. The long, 7.1m diameter TBM then takes over to prevent ground movement above and below.

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At the back end of the TMB, 150 metres away, the pre-built reinforced concrete rings are immediately installed, securing the tunnel.

“They are phenomenal machines,” says Black. “You can exert pressure on the ground as you tunnel and limit ground movement. It allows you to get the lining up behind it so you have a nice tightly packed tunnel.”

These machines averaged 38 metres of progress each day, and achieved 72 metres between Pudding Mill Lane and Stepney Green on a single day in April 2014.

Crossrail boaring machine

While earth pressure balance machines handled the majority of the route, and the softer clay ground in west London, tunnelling beneath the Thames created different challenges requiring a different type of machine. These machines are called ‘mixed shield’ TBMs and dig in a slightly different way.

“Those machines were used specifically for the Thames crossing in the east where we were tunneling though chalk and there’s quite a lot of water in the ground,” Black adds.

“It helps you excavate wetter materials by pumping bentonite [a slurry of clay and water] into the face of the machine. It sucks up the particles the machine has cut and then separates it out on the surface where it is reused again.”

With the tunnelling phase long completed, the finishing touches are now being added. Train tracks, electronics, ventilation, communications, signalling and lighting will be completed next year, while work on the 10 new stations and upgrades to 30 existing stations continues ahead of the opening in December 2018.

“Seeing everything come together is the most satisfying part,” says Black.

“We spent a long time in the planning stage and the process of designing, and we’d only seen it on paper before. Now the platforms are being constructed and the escalators are going in. What’s there now compared to what was there when the tunnel was first being dug is very impressive.”

Minimising disruption

The Crossrail team aimed to minimise impact on Londoners by keeping construction sites small and ensuring all the necessary materials were delivered in a timely fashion.

“It happens with a great deal of planning and sequencing of the work,” says Black. “We have very small construction sites considering the amount of work that’s going on underground. People are living very close to our work sites, so you have to be very tidy and make sure you’re not creating lots of noise and dust.”

Crossrail tunnel

The Elizabeth line will become fully operational in December 2019, but it may not be too long before the giant TBMs are once again meandering beneath London’s streets.

The Crossrail 2 proposal would see the addition of a north-south route and is backed by Transport Secretary Chris Grayling and London mayor Sadiq Khan. It would link central London with Broxbourne in the north and Epsom in the south.

Black adds: “We’ve learned a lot from building Crossrail. London has been built upon and tunneled through for centuries now, but it’s amazing how much knowledge you gain from every time you do one of these things; especially in terms of the geological conditions and the nature of the soil.

“You learn more for future projects like Crossrail 2, all of that knowledge makes future jobs easier and cheaper to build.”

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