An inside look at how Sweden is building the world’s second-longest tunnel

Nowhere to be seen: Stockholm's E4 bypass will tunnel under the straits of its archipelago. (credit: Karlheinz Woschee)

Stockholm is a city known for its waterways. Founded on an archipelago of some thirty thousand islands, bodies of water are rarely more than a stone's throw away in the Swedish capital. An appreciation for lakes, rivers, and nature in the fullest sense runs high in Swedish sentiment. It's not by chance, after all, that Stockholm is world-leading in air quality, access to green spaces, and sustainable urban planning.

That green mentality recently challenged engineers tasked with the design and construction of a new six lane bypass running 21km (more than 13 miles) north to south along the rapidly expanding city's western edge. How can you tackle a task that big whilst keeping the environmental impact to an absolute minimum?

"Typically, in an area with as low a building density as we have here, you'd take an overground route and construct bridges where necessary," Johan Brantmark, E4 project manager for the Swedish Transport Administration tells Ars. "That would be cheaper, and far easier of course. But we're motivated by other priorities, not the least of which is to preserve the surrounding environment. For that, tunnelling is the best approach."

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