Crossrail is Europe's biggest infrastructure project. Budgeted at £14.8 billion – £12 billion of which is going on 42km of tunnels up to 40 metres deep – the line will stretch from Royal Oak in west London to Plumstead, south-east of the capital. "The scale is immense," says Andy Mitchell, Crossrail programme director. "Over 8,000 people are working across 40 construction sites. Eight tunnel-boring machines will excavate six million tonnes of earth and use a quarter of a million concrete segments." The 1,000-tonne tunnel-boring machines (TBMs), which were made by German firm Herrenknecht in Schwanau, travelled from Rotterdam to Essex by ferry last year.
Each machine is 7.1m across and up to 150m long. Six are earth pressure balance machines for tunnelling through clay; two are slurry machines for the chalk, flint and water found under south-east London.
One of the slurry machines, TBM6, is currently on a 2.6km drive from Plumstead, passing under the Thames to North Woolwich. (Traditionally, each tunnelling machine is given a female nickname; Crossrail held a competition to suggest names: TBM6 is called Mary, after Isambard Kingdom Brunel's wife.) It operates 24 hours a day, every day, and is staffed by a 20-strong gang working 12-hour shifts. Powered by electricity from the national grid, the cutter head rotates three times per minute. The resulting earth is mixed with water to form a chalky milk, which is sent to the surface via pipes, where it is used for land restoration.
The tunnel is then lined with rings composed of seven concrete segments and a concrete keystone. To get the segments and keystones to the front of the machines, they are lowered into the shaft (from which the machine was launched) via a gantry crane, then carried on a small locomotive. The tunnel borer's vacuum-suction erector lifts the segments and keystones into place. Once a ring is installed, hydraulic rams push against it, propelling the machine forward at a rate of around ten rings per day.
Ideally they operate nonstop, but sometimes a halt is unavoidable, usually because cutting tools need changing. "You can do as much desktop analysis as you want," says Gus Scott, project manager for the Plumstead-North Woolwich section. "But until you're down there and find out what condition it's in and how many flints are in it – on big tunnelling jobs through chalk the flints are what damage the cutter heads – you can't know how it's going to go."
The front section of the tunnel borer is pressurised, forming an air bubble, which keeps soft ground and water out. Engineers enter the bubble via an airlock to repair the tools. The gang faces risks each day: falling equipment and material, flooding (there is a water alarm), collapsing ground, and fire and gas leaks. Each worker must carry a "self-contained oxygen self-rescuer", a mask and bag that recycles exhaled air.
If stoppages are kept to a minimum TBM6 will be in North Woolwich by the summer of 2014. All the tunnels are due to be completed by the end of 2014, at which point the stations, over-ground sections and fit-out will become the focus. The first trains are due to run in 2018.
Andrew Hankinson has written for The Independent and Esquire. This article was originally published in September 2016.
This article was originally published by WIRED UK
