Up close and personal with Crossrail’s monster tunnelling machines

Digstarter

Work on Crossrail has been informed by London’s last engineering megaproject, the Olympics – another multi-billion build in which large numbers of different contractors worked across many different sites. The construction industry is fiercely competitive and contractors are traditionally reluctant to exchange ideas, but Crossrail has pioneered a crowdsourcing project model. Ideas from any contractor are suggested, funded from a common pot of seed money and trialled on small areas of the project.

If they work, they’re then developed into more widespread use. These can be simple things like printing safety advice on workers’ gloves, or more technical innovations like lightweight aggregates. One particularly bright idea is to create tunnels that take the excess heat from the trains (and passengers) and turn it into electricity to feed back into the grid.

Tunnel vision

At this stage, though, for all its digital wizardry and forward-thinking crowdsourced ideas, Crossrail is still very much a dig. The ‘fit-out’ stage is yet to happen; there are no rails, cables or drainage in the new Farringdon station as we walk down the enormous tube that links the east and west platforms. The walls in the deeper tunnels still show a stubble of the stiff steel wires that are mixed into spray-on concrete to give it strength.

This fibre-reinforced concrete (or ‘shotcrete’, as it’s known to anyone with a rolled-up copy of the Screwfix catalogue in their back pocket) is hosed onto the walls at high velocity in several coats, reaching a thickness of half a metre on average. On a regular building site, the damp concrete is vibrated using power tools; in the Crossrail tunnel, a metal structure the size of a road bridge is pressed against the domed ceiling and thrummed with ultrasonic ‘limpet’ vibrators. Once dry, it’s tested for strength using a bolt-firing gun that they’re only too happy to demonstrate, but which journalists are sadly not allowed to have a go on.

Now that’s what I call boring

Heading deeper still, we reach the end of the tunnel, where a digger is tearing out sofa-sized scoops of sub-London gubbins: the behemoth tunnel boring machine has finished its work here, but the end of each tunnel is domed for extra strength. The digger’s work doesn’t exactly look subtle, but above the face of smashed rock and scooped clay, an engineer’s spot laser tracks the concrete dome that’s forming, matching it up to the digital model that lives on Crossrail’s servers.

At this stage, though, it’s still the TBMs that get all the headlines, and rightly so. Like mechanoid versions of the giant sand-worms in Frank Herbert’s Dune, these 980-tonne beasts chew through the earth using a cutting head seven metres across, laying preformed concrete tunnel sections behind them as they crawl along their routes. The ground beneath London is already a tangled mass of tubes, sewers and holes and the TBMs have to thread carefully through it, sometimes missing a Royal Mail underground line or even a Tube station by as little as a foot.

There isn’t even a fixed definition of what constitutes surface level in London – Farringdon, for example, was ‘built on stilts’ by the Victorians to avoid flooding, so the real ground level is a couple of basements below Clerkenwell Road. Speaking as someone who’s had trouble parking a Nissan Micra in broad daylight, the idea of performing such delicate manoeuvres in a 150m-long, 1000-tonne vehicle, seven floors down, sounds tricky. I think I’ll wait for the train.