There's a reason why more record-breaking tunnels have been constructed since the year 2000 than in the previous 150 years.

By Laurie Winkless

In 1843, it was called the Eighth Wonder of the World, attracting 50,000 visitors on its first day. But this wasn’t an exhibition or a must-see show, it was London’s Thames Tunnel, which connected the neighborhoods of Rotherhithe and Wapping. Built by Marc Isambard Brunel and his son Isambard Kingdom Brunel, the Thames Tunnel was an engineering masterpiece, the first ever tunnel to be constructed under a busy, navigable river. As with all world firsts, the project faced many challenges, the geology key among them.

The north side of London is dominated by a solid clay, but to the south-east (where the Thames Tunnel lies), water-drenched sands and gravel are the norm. Early attempts to dig a tunnel through these sands were disastrous, with hundreds of men made sick by exposure to raw sewage from the river, and numerous floods claiming lives. It took the invention of the tunneling shield to make working conditions a little more bearable. In its simplest form, it was an iron frame, split into 36 cells, each large enough for a worker to stand in. The frame supported the tunnel as it was being excavated, which gave bricklayers time to shore up the walls. Using this process didn’t entirely solve the issue of flooding, but it did allow the Thames Tunnel to progress slowly, but surely. In the end, construction of the 1332 foot-long tunnel took upwards of 16 years. Despite its impressive age, it remains in use.

These days, tunnels just aren’t considered to be a ‘big deal’ by most people. They are very much seen as a normal part of our modern landscape, and between transport and water supply, most of us rely on them every single day. But it’s in relatively recent years that this has been the case. More record-breaking tunnels have been constructed since the year 2000 than in the previous 150 years. In fact, of the 50 longest tunnels on Earth, 30 came into being since the turn of the millennium. And the sector is continuing to grow, with at least 20 rail, road, water or waste tunnels longer than 9 miles currently under construction, with another 10 to 15 at the advanced planning stage. One thing that’s driving this growth is cost – on average, the cost of tunnel construction has been dropping by 4% every year. In built-up areas like cities, tunnels are often cheaper to construct (around £50 million per kilometer in the UK) than equivalent surface structures.

The cost of tunnel construction has been dropping by 4% every year. In cities, tunnels are often cheaper to construct than equivalent surface structures.

While the Brunel shield made tunneling practical, it is the tunnel boring machine, or TBM, that heralded the current golden age of tunneling, allowing us to dream bigger and to dig further and faster ever before. TBMs are giant, high-tech earthworms that use a rotating cutter face covered in hard, tungsten carbide teeth to munch whatever is in its way. The ground-up rock is mixed with other compounds inside the TBM, to produce a material with the consistency of toothpaste that can be reused to build wetland nature reserves, among other things. While the face is busy digging, the TBM’s hydraulic arm lifts curved sections of reinforced concrete into place, building the tunnel like a circular jigsaw. Europe’s most famous TBMs can be found on London’s Crossrail project. Weighing in at 1,100 tons and measuring 470 feet long, these behemoths can each construct 4.3 miles of tunnels a year. That’s 190 times faster than Brunel could manage!

The world’s largest TBM was launched in June 2015, and will create an undersea link between mainland China and Hong Kong International Airport. With a diameter of 57 feet, it’s slightly larger than Seattle’s TBM, Bertha. She recently restarted excavating a new aqueduct under the city, after a delay of almost three years, caused by metal piping found in her path. And this brings us back to reality. As tunnel projects become more complex and the demands more extreme, delays in projects may well become more prevalent.

To my mind, excavating tunnels suitable to house Hyperloop One’s vacuum trains will push existing tunneling technology to its limits, and it’s partly thanks to the enormous lengths and short timescales involved. Earlier this year, Gotthard Base Tunnel, the longest and deepest rail tunnel in the world opened in the Swiss Alps. The 35 mile (57 km) long twin-bore tunnels are predicted to remove a million heavy-duty trucks from the roads, but they took almost 20 years to construct. For Hyperloop One, this is a snail’s pace. “One of our major challenges is the economy of building a super-high-speed system quickly”, said Hyperloop One Senior Geotechnical Engineer, Brandon Kluzniak. “TBMs are pretty cool, but they still build tunnels slowly. We want to go much, much faster than today’s systems can manage. This shouldn’t mean that we’ll need to completely rethink how we build tunnels, but it’s likely that our ideal solution will use a combination of technologies.”

New York City's East Side Access Tunnel

So how big is the challenge exactly? Well, in July, FS Links published a preliminary feasibility study for a Hyperloop One route that would run between Stockholm and Helsinki, a distance of more than 310 miles. With just 84.5 miles of track in Sweden and 98 miles in Finland, it’s clear that most of this route would be undersea. If the Hyperloop One team achieve this engineering feat, it will dwarf the current record holder – the 33.5 mile-long Seikan Tunnel in Japan.

Hyperloop One isn’t looking at tunnels just yet though. Its DevLoop test track, currently under construction, will use sections of steel tube roughly 11 feet in diameter, elevated several yards above the ground. But, longer term, their high-speed freight and (eventually) passenger system is likely to move – at least partly –underground. “Tunnels let you optimize routes so there’s the least amount of disturbance to existing landscapes. And, without tunnels, it would hard to connect urban centers. Both are fundamentally important factors for the Hyperloop system,” says Brandon.

There may well be lessons to be learned from elsewhere in the construction industry. Norwegian engineers have a rather special tunnel in mind -- one that floats. (Read this blog post on subsea tunnels from Hyperloop One marine engineer, Blake Cole). Officially called an Archimedes Bridge, it has been proposed as a way to cross one of Norway’s many fjords. It would use a combination of floats and tethers to hold the structure stable, making it indistinguishable from a traditional tunnel for those driving through it. With Hyperloop’s ambitions targeting a European route, it couldn’t hurt to look towards innovative tunneling approaches like this.

Laurie Winkless is a physicist-tuned science writer, currently based in London. Her first book, Science And The City: The Mechanics Behind The Metropolis, offers a whistle-stop tour of how cities work, and how they’ll change in future. It is already available in Europe, and will hit US bookshelves on 25 October (Published by Bloomsbury, pre-order it here).



