Underground Water Tunnels

The other day, as I was riding the 2 train from Brooklyn to Manhattan, I thought: damn, it's pretty cool that I'm underwater right now. But I couldn't answer any of my ensuing questions: how exactly are underwater tunnels built? How does the pressure not cause the tunnel to cave in? How do people breathe in underwater tunnels? So, when I came home later that night, I set about answering them.

Why do underwater tunnels exist?

If you think about it, it's not clear why underwater tunnels are better than other types of transport. First, if you're travelling across a body of water, isn't it easier to just get across by ferry or bridge? Second, building a tunnel already seems complex enough. To not only build a tunnel, but to do so underwater - isn't that insane?

Turns out, there are two main reasons why underwater tunnels are pretty good modes of transport:

1. Weather: Transportation via ferry and bridges is often hampered by bad weather conditions. Both modes of transport are unshielded and completely exposed to all the elements. Ports will close when sea conditions are too dangerous, and bridges will often shut down if there are strong winds or bad storms.
2. Congestion: Bad weather conditions mean that congestion gets pretty bad during inclement weather conditions. Tunnels are also a much faster method of transport than ferries and bridges. For example, getting across the English Channel Tunnel takes ~20 minutes via train and ~75 minutes via ferry. In fact, the first underwater tunnel was created in London, where the rise of industrialism caused waterways to be extremely congested.

How are underwater tunnels built?

First, a clarification. This wasn't obvious to me, but underwater tunnels don't actually go through the water. They are built below the riverbed. This diagram of the Seikan Tunnel (the longest total underwater tunnel in the world) makes that obvious:

Concepts currently exist for submerged floating tunnels, which do go through the water. Submerged floating tunnels are basically just tunnels that are propped up by cables and float in the water, at just 25m - 50m below sea level. However, none currently exist.

With that out of the way, let's move on. There are three primary methods of building underwater tunnels.

Tunnelling Shield

The original tunnelling shield, created by British engineer Marc Brunel, is my favorite method of underwater tunnel construction because it's so simple, and yet so painstakingly complex! The original tunnelling shield he invented was a flat face composed of a grid of iron frames supported by poling boards. Each poling board was removed by the workers, who excavated a hole of a pre-determined depth behind it. They then replaced the poling board, moved onto the next one and excavated behind its surface, until they finished excavating the area behind the entire shield. They moved the shield forward in the space they just excavated, and repeated this process until the entire tunnel was completed.

Here's a diagram of Brunel's original tunnel shield, where at the bottom you can see the worker behind a poling board manually excavating the tunnel:

Brunel created this tunnelling method after noticing how shipworms burrowed through wooden ships (we truly learn the best from nature!). The Thames Tunnel - beneath the River Thames in London and completed in 1843 - was the first tunnel built in this painstaking way. Unsurprisingly, it took 18 years to complete!

The next tunnelling method, as you'll see, is a modern re-invention of Brunel's original tunnelling shield method.

Tunnel Boring Machines (TBMs)

Tunnel boring machines today retain the same basic mechanisms as the tunnelling shield method described above. The biggest improvement is that the entire method is automated and doesn't require workers to follow behind and manually excavate. Today, a tunnelling shield presses its cutting wheels against the front of the tunnel and rotates itself to excavate.

Its stablized in the back by shield jacks, which are brought in once the shield face has excavated up to a certain distance. Then it casts a ring-shaped wall behind it called a "segment", which lines the newly formed tunnel.

Tunnel boring machines were famously used to complete the English Channel Tunnel, the longest continuous underwater tunnel in the world. Its construction required 11 TBMs and only 6 years to complete.

Prefabricated Tunnels

This method of tunnel construction is amusing in its simplicity: it involves building a tunnel above ground and then later placing it underwater.

Prior to placing the tunnel inside, the underwater cavity is created by dredging and grading the trench. Then, segments of the tunnel (already pre-built above water) are sunk and placed in the dredged tunnel cavity piece by piece.

The Ted Williams Tunnel in Boston was famously constructed in this way. While the biggest advantage of this method is its cost-effectiveness, it also requires a ton of precision to get right. Even a slight misalignment can cause disastrous consequences. Point in fact: a 3 ton section of the Ted Williams Tunnel's ceiling collapsed.

How do people breathe in underwater tunnels?

Oxygen levels are much lower underground, but breathing in underwater tunnels is a solved problem. They're typically designed with ventilation systems that force fresh air through the entire length of the tunnel. The air is circulated through the tunnel by multiple exhaust fans. However, concerns with ventilation have caused the longest underwater tunnels to be electrified, so that passengers can move through them more quickly.

The end: some cool underwater tunnels

The Seikan Tunnel - the longest total underwater tunnel in the world.

Entrance to the Cross Harbour Tunnel, the most congested underwater tunnel in the world.

The English Channel Tunnel - the longest continuous underwater section of any tunnel in the world.