Whilst transportation tunnels are vital pieces of infrastructure, they also need to be mindful of the impact they have on the environment around them.

Part of this involves careful surveying of the stone and ground conditions to ensure that the most suitable and efficient equipment, boring machinery and conveyor belts are used to ensure a sustainable project completed on time and within budget.

Another aspect is to ensure that a tunnel not only functions highly effectively but also has as minimal an impact on the surrounding area as possible, and this has led to the rise of so-called green tunnels.

With the topic of green tunnels reentering the discourse thanks to the completion of the Chipping Warden tunnel as part of High Speed 2, here is a quick explanation of the green tunnel concept, when it is used and why it matters to an overall construction project.

What Is A Green Tunnel?

A green tunnel is any tunnel where the visual impact of any visible infrastructure is offset by planting grass, trees, shrubs and other greenery on top.

In most cases, this is used for the entrances and exits of tunnels not directly cut into mountains, but it can be used for more substantial distances if the tunnel is particularly shallow.

A green tunnel does not necessarily have to run the entire length of the tunnel itself; there are cases where the entrance of the tunnel is relatively shallow, but due to serious changes in elevation, it ends up intersecting with a mountain.

When Is A Green Tunnel Used?

A tunnel project often requires significant amounts of earth to be moved, but the goal is to have the visible impact be as minimal as possible.

Sometimes this is possible through the use of tunnel boring machines that efficiently cut through the earth at a level deep enough to leave a minimal aesthetic and environmental impact. 

A good example of this would be the Woodhead Tunnels which used to provide a route through the Pennines; because it was bored through a mountain range, the only impact on each side was a pair of minimally intrusive entrances.

However, with some tunnels that need to be built somewhat shallowly, especially if they are part of longer railway projects and elevation changes need to be kept to a minimum and within acceptable tolerances.

These tunnels generally cannot be constructed with TBMs, so they use the relatively simpler cut-and-cover construction method of excavating to the depth necessary and then building a cover on top of it.

This can be a more economical method and is all but required for shallow tunnels, but it has a strong visual and environmental impact, particularly if the tunnel is built near woodland or other major wildlife biomes.

A green tunnel operates as a type of green bridge, allowing animals to travel across them without putting themselves in harm’s way.

Green tunnels tend to be the best way to mitigate the impact of a cut-and-cover tunnel that is built over what would otherwise have been greenbelt land, and in some cases, become a necessary component to getting approval for its construction.

Why Are Green Tunnels Necessary?

Major infrastructure projects can be rather contentious for the communities close to where they are being constructed.

They bring with them a lot of advantages once completed, but they also bring noise, traffic, vibrations, disruption and can affect the existing natural beauty of the area, which itself can lead to serious objections, letters to political figures, which in turn can affect the potential for regulatory approval.

Part of the reason why there are five green tunnels for HS2 is precisely due to objections regarding noise, environmental and visual impact.

The visual impact is resolved through designs that harmonise as closely with the existing environment as possible, ultimately resembling a small, relatively uniform grassy hill or knoll outside of the entrances, where the impact can be minimised but not removed entirely.

Replanting trees, shrubs, and wild plants allows animals to inhabit the ground above the train and avoids the potential issues of splitting existing wildlife territories with a high-speed railway line that could be potentially harmful for the animals.

Finally, the earth, soil and plant life provide natural sound-dampening and soundproofing properties that stop the normal, everyday functioning of the train from disrupting people’s lives.

As some of the tunnel locations are based in agricultural land, the green tunnels could even be reseeded and reused as such, or they can be rewilded to help plant and animal life.