The search for quicker, cheaper ways of tunnelling


A BIG hole in the car park at SpaceX’s headquarters in Los Angeles is the first visible evidence of another of Elon Musk’s ventures. Mr Musk who, besides leading SpaceX, a rocket company, also runs Tesla, a maker of electric cars, is going into the tunnelling business. The goal of the Boring Company, as he dubs his new enterprise, is to dig tunnels faster and more cheaply than is possible at the moment.
Apart from the pit in the car park, Mr Musk says he has also begun a series of test tunnels for a project that will, if it comes to fruition, carry cars under Los Angeles on high-speed sledges, in order to avoid the dreadful traffic jams above. More ambitiously, he claims to have official support for a 320km (200-mile) tunnel that would, in half an hour, whisk people between New York and Washington, DC, in magnetically propelled capsules, using a technology he has dubbed the hyperloop.

Loopy these ideas may sound, but Mr Musk is surely right about one thing—that tunnelling, which is currently slow and expensive, is a technology ripe for innovation. And he is not the only one who thinks so. In Europe, things are also stirring beneath the surface. Loopy these ideas may sound, but Mr Musk is surely right about one thing—that tunnelling, which is currently slow and expensive, is a technology ripe for innovation. And he is not the only one who thinks so. In Europe, things are also stirring beneath the surface. In January a consortium of academic and commercial researchers began work on a project called BADGER. This is intended to develop a robot tunnelling machine (albeit one for tunnels much smaller than Mr Musk has in mind) that can detect and avoid obstacles such as pipes, cables, the foundations of buildings and even buried boulders.
Existing tunnel-boring machines are, in effect, building sites on rails. At the front, a cutting wheel with a diameter a little larger than that of the final tunnel (to allow for the thickness of the lining) is pushed forward by pistons and chews away at soil and rock as it travels. The spoil from this excavation is then taken to the surface by conveyor belts. Once enough material has been cleared, the borer is stopped and the newly exposed section is lined with precast concrete sections.
Slow, this process certainly is. The boring machines employed to construct the tunnels for Crossrail, a new railway under London which should open next year, cut through the strata they were faced with at a rate of around a metre an hour—literally a snail’s pace. As to expense, Crossrail required eight boring machines, each of which cost around $15m in 2012, when tunnelling started. Each also needed to be supervised by a gang of up to a dozen people on board, adding to costs. The total bill for Crossrail’s tunnelling was £1.5bn ($2.4bn). This bought 42km of tunnels (21km each for the eastbound and westbound tracks); the longest individual tunnels (8.3km each) took 2½ years to dig.
The Boring Company thinks it can speed this sort of operation up, and also cut costs. To do so it plans to make boring machines more powerful, so that they can cut through material faster. It also wants to automate things, to reduce labour costs, and to line the tunnel as the machines progress, instead of stopping excavation when linings are added. One idea is to compact the spoil into bricks and use those as lining material. Reducing the diameter of tunnels would also help. That is part of the reasoning for putting cars on sledges. A two-lane road tunnel needs to be about 8.5 metres wide. Crossrail, at around seven metres, is slightly narrower. But a sledge tunnel could be a single lane, because the sledges can be packed close together and so do not need as much space. It could thus work with a diameter of four metres, cutting costs by as much as three-quarters.

No one could ever accuse Mr Musk of not thinking big. But tunnels do not necessarily need to be big to be useful. BADGER is being designed specifically for small-diameter tasks, such as digging conduits for cables and pipes. The initial plan, according to the project’s co-ordinator, Carlos Balaguer of Carlos III University, in Madrid, is for the machine to burrow at depths of up to four metres, at speeds of around two metres an hour. If that works, it should then be possible to increase both speed and scale.
BADGER’s face will combine a conventional rotary cutting head with an ultrasonic drill, which will pulverise rock with high-frequency sound waves. As with existing machines, the spoil will then be sent to the surface. Unlike existing machines, however, BADGER will move forward not as a rigid unit, but like a worm. The rear section will clamp itself to the wall of the newly cut tunnel and push the front section forward. The forward section will then clamp itself and pull up the rear. And so on. As it advances, BADGER will line the tunnel behind it using a 3D printer. One idea is to print the wall with plastic, so that the result resembles a conventional pipe.

BADGER will navigate using various sensors including, crucially, ground-penetrating radar. This will enable it to operate autonomously and detect potential obstacles before it reaches them, so that it can steer around them. The great benefit of BADGER is being able to excavate tunnels below busy cities without closing roads to dig trenches—thus avoiding making the traffic jams about which Mr Musk complains even worse. Whether the tunnels are straight or loopy, though, the future of tunnelling will be anything but boring.

Source: economist.com

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