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."

The solution settled upon involves taking nothing less than 18km (11 miles-plus) of the bypass underground, with twin tunnels running three lanes abreast in each direction. Comprising three tunnel sections in total, the winning design means that the E4 bypass avoids six nature reserves or places of cultural significance, including the grounds of Drottningholm Palace—a UNESCO world heritage site on the island of Lovö—and the need for bridges over the crystal waters of the Mälaren strait. Not for nothing either, E4 will be one of the largest underground highways in the world, with its longest tunnel section spanning 16km (nearly 10 miles) from the bypass' southernmost point. At its deepest, the highway will bottom out at 80m (close to 90 yards) below the surface of the Mälaren.

"It's a standard drill-and-blast approach," explained Brantmark, describing the slow-but-steady method of tunneling that has been underway since winter of 2014. The procedure—of drilling, injecting grout, blasting, clearing, and advancing—will eventually result in some 20 million tons of rock being removed. So far, having proceeded from three entry points in parallel, only around 20 percent of planned route has been excavated.

"The harder and stronger the rock is, the quicker we advance. Drilling and blasting isn't so time-consuming, but sealing the rock from water leakage is. If the rock is very fractured, we need a lot of concrete sealant or reinforcement columns to ensure integrity," said Brantmark, describing the work of the three Atlas Copco Drill machines in use.

The sealing process requires drilling horizontal boreholes ahead of the tunnel face to a depth of between 25 to 30m (~30 yards) and injecting concrete. After it's set, two or three rounds of blasting take place—advancing about 5m with each. If rock cover overhead becomes too shallow, another jet-grouting technique is employed: drilling down from the surface and injecting grout to replace till and clay with concrete columns providing additional reinforcement. Thanks to the condition of rock afforded by the region, however, this hasn't been necessary for all but a few hundred meters' worth of tunnels.

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

Mikael Ullén

One can imagine that even some ways below the surface, blasting isn't the most subtle of affairs. In order to limit disruption, the project has involved upgrading thousands of windows of local homeowners and businesses with noise-insulation measures.

Care for the environment on the project budgeted at $4.1 billion runs far deeper than the tunnels, though. Thomas Holmström, environmental manager for the project, explained: "We're aiming to deliver the whole project with a carbon footprint 10 percent lower than what'd normally be expected."

"That means reducing energy consumption and greenhouse gas emissions throughout the construction process and in terms of lifecycle of materials—it means new solutions for new considerations."

While cement actually absorbs CO2 across its lifetime, conventional cement manufacturing techniques are often flagged for contributing to greenhouse emissions. Responding to this, the E4 project is utilizing steel, concrete, and other materials produced using low-carbon methods.

Another example of clean thinking in action is evident in the use of those archipelagic waterways for transport of excavated rock away from the island of Lovö. Rather than transporting by truck, the project is establishing an elaborate series of conveyor belts, the longest of which is 1.7km (~1 mile), and three temporary harbours to take rock from tunnels to barges and onward for delivery at a local industry group.

Still ramping up in capacity since commencing operations in fall 2017, Holmström said this is one of the first uses of the method in Sweden, but so far it's working well: "One barge equals about 80 to 100 truck-loads, and the plan is for two to be loaded up each day per harbour—so it's reducing our carbon footprint significantly as well as avoiding major environmental disruption around Drottningholm Palace."

With traffic relegated below ground, all road tunnels ought to be carefully designed to combat vehicle emissions building up. With tunnel sections as long as those of the E4, this is especially true. The solution is of course ventilation. Altogether the E4 tunnels will host some 240 fans embedded in its roof to keep air circulating, and three air exchanger stations along the tunnel in each direction will replace polluted air with fresh air. At tunnel mouths, four exhaust air stations will be built in order to vent air at height instead of allowing it to concentrate at ground level.

"Our calculations show we won't have any trouble meeting regulations outside the tunnel, but there's no guidelines or regulations for inside the tunnel, so we're still refining how our ventilation systems will operate, but we're confident we'll deliver a safe environment," said Holmström. He noted that the E4 design team is recruiting help from researchers at Umeå University to investigate long and short-term health effects of the fifteen-minute drive that passing through the E4 bypass would take.

At three lanes and 16m wide, the tunnels certainly aren't narrow, but they're designed as much for today's Stockholm as for tomorrow's. Once completed in 2026, the E4 bypass will absorb a large amount of traffic currently congesting the inner city. (Stockholm's inner ring road, for instance, currently suffers twice the traffic it was designed for.) And by 2035, some 140,000 vehicles are expected to travel the new highway every day. At the same time, the highway will function to connect northern and southern regions of the city—a development potentially yielding great socio-economic benefits to a population expected to grow from two million today to two and a half million by 2030.

Listing image by Mikael Ullén