Like benevolent, mechanical Tremors worms, a team of eight Tunnel Boring Machines are inching their way across London and leaving neatly packed railway tunnels in their wake. Instead of, you know, all that death, mayhem and Kevin Bacon.
London's been kicking around the idea of a central East-West underground railway route since the end of World War II, though it wasn't until the 1970s that proposals were formally submitted to Parliament. In 2007, a plan cleared the legislative body, Royal Assent to the project was granted in 2008 and actual construction began the following year. The underground portion will pass under central London and connect the Great Western Main Line near Paddington with the Great Eastern Main Line near Stratford.
The project employs a set of eight 150m long, 7m wide, 900-tonne Tunnel Boring Machines (TBM) to chew through the necessary 117km of dirt and rock. Each machine costs roughly £10 million ($15 million).
The project will actually employ two different types of TBMs depending on the makeup of the soil it will be moving through. Cutting through the clay that constitutes the Thames line will require a slurry-type TBM, while sand and gravel will necessitate Earth Pressure Balance Machines (EPBM). The project's pair of slurry machines differ from the other six EPBM machines in that they separate the cutter head from the rest of the machine and pump in Bentonite, a mixture of clay and water, to assist in the excavation. They pump out the bored material as well rather than use conveyor belts.
The EPBMs work in a similar manner but without all the liquid lubrication. The foremost rotating cutter head is plated in Trimay/Hardox and its cutting implements are tipped with tungsten carbide -- its not quite as tough as Diamondillium, but it's close. It loosens soil as a screw conveyor moves the dirt past the cutter head and onto a conveyor belt that deposits the dirt at the rear of the vehicle so that it can be evacuated to the surface for disposal. These machines are so huge they include an on-board bathroom and kitchen.
While that's happening, a rotating arm positions pre-fab concrete segments that will make up the tunnel walls, with eight segments comprising each 20-tonne ring. Over a quarter of a million segments will be used throughout the project. Every time a new ring is complete, a set of massive hydraulics push the machine forward with as much as 58,000kN -- enough to lift 2900 London cabs.
To make sure that the tunnels curve, rise and fall to avoid sewer lines and other underground obstacles, these machines sport a variety of sensors to ensure they stay on course. Pressure sensors monitor the volume and density of the soil being cut to ensure equal amounts are being removed -- hence Earth Pressure Balance -- weight scales on the removal belt, laser scanners and density meters also report in real time to the operators. The machines' progress are monitored on the surface via GPS.
Drilling underground doesn't stop just because the sun goes down. These machines work 24 hours a day and require a rotating crew of 20. Each machine covers about 100m a week, which means the entire project should take about three years to complete. The TBMs will have to navigate London's existing subterranean infrastructure, including sewers and other obstacles, by diving as deep as 40m below the surface.
The rail line is expected to come online in 2018 and carry about 200 million people each year -- about 1500 passengers simultaneously during peak hours -- thereby increasing the city's rail capacity by about 10 per cent.
In the end, the TBMs will have removed five million tonnes of earth -- enough to fill Wembly Stadium three times over. And unlike New York's similar dig project, these machines will be resold to the manufacturer for refurbishment and resale for other projects. [BBC News - Crossrail.pdf - Crossrail Wiki]