[Note that this article is a transcript of the video embedded above.]

2024 marks thirty years since the opening of the channel tunnel, or chunnel, or as they say in Calais, Le tunnel sous la Manche. This underground/undersea railroad tunnel connects England with France, crossing the narrowest, but still not that narrow, section of the English Channel. The tunnel allows passengers (and, in many cases, their cars, too) to cross the channel in just over half an hour at speeds as high as 99 mph! While there are longer tunnels out there, this is the longest underwater tunnel in the world.

When it was proposed in the mid-1980s, it was set to be the most expensive construction project ever, and like so many mega projects, it went way over budget and opened a year late. But unlike many megaprojects, this one was funded entirely by private investors. That’s a good thing, too, because it hasn’t exactly been a mega-financial success. The BBC once said that, "Depending on your viewpoint, the Channel Tunnel is one of the greatest engineering feats of the 20th Century or one of the most expensive white elephants in history.”

Elephant or not, the tunnel is legendary among engineers, and in light of the 30th anniversary, I thought it was about time I dug into it. It is a challenging endeavor to put any tunnel below the sea, and this monumental project faced some monumental hurdles. From complex cretaceous geology, to managing air pressure, water pressure, and even financial pressure, there are so many technical details I think are so interesting about this project. I’m Grady, and this is Practical Engineering; today, we’re talking about the channel tunnel.

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The idea of building a permanent connection between England and France across the English Channel isn’t a new one. An engineer way back in 1802 came up with a plan for a horse-and-buggy tunnel intended to be lit with oil lamps, featuring an artificial island midway for horse changes, and some pretty scary ventilation chimneys. Needless to say, that idea didn’t get there. In 1882, another tunnel proposed got a bit further, a few kilometers further, in fact. Several thousand meters of tunnel were actually dug before political pressures regarding the fear of future potential invasions killed the project. By the 1970s, another attempt to build a tunnel broke ground, but that project fell through, too. It wasn’t until the mid-1980s that the proposal for the tunnel as we know it was accepted and work began in earnest. A handful of other proposals were also considered at the time, including an even more ambitious project featuring an absolutely enormous suspension bridge 70 meters above the sea using an exotic fiber called parafil and carrying traffic within huge concrete tubes.

Unsurprisingly, this monster bridge did not get selected. The plan for an underground electric railroad connection won, and work began on the channel tunnel. But it’s not so much a tunnel as three separate tunnels with a variety of connections between them. There are two main railway tunnels, each with one-way service across the channel, and a third service tunnel that runs between the two. The three service tunnels each began on either side of the channel and, pretty impressively, met in the middle, deep under the sea bed, with an offset of less than two feet. They were even able to incorporate some of the work of those previous failed attempts.

The accuracy of this dig is even more impressive when you consider that the tunnels aren’t level or straight. The geology of the English channel is, putting it mildly, a bit complicated. There are layers of different kinds of sedimentary formations, and the project was designed to follow the path of a layer known as chalk marl, although some geologists call it marly chalk. This layer was less permeable and had fewer cracks and fissures than the overlying material. But that doesn’t mean there were NO fissures. The marly chalk was the best option for tunneling under the channel, but it was still far from simple.

In some ways, those past proposals and attempts to build the channel tunnel failed because the technology just hadn’t reached the level to make a project like this feasible. But by the 1980s, one piece of equipment had made huge strides in efficiency and safety. With the creative flair you’d expect from any civil engineer, they are aptly named: Tunnel Boring Machines, or TBMs. Drilling is just one of the multitude of jobs that happen in a tunneling operation, and TBMs manage to combine and accomplish them all in one massive and incredibly complicated machine.

There are lots of different styles and sizes of TBM, and the channel tunnel used a total of eleven separate machines to finish the job. Most of us are familiar with the process of drilling a hole, but doing it through soil and rock, underwater, across a vast distance, as you can imagine, adds some nuance to the process. For one, there are no drill bits that extend for miles, so the whole machine has to fit inside the tunnel it’s creating. For two, there are no big hands to push at the back of the drill. Instead, tunnel boring machines grip onto the tunnel walls and use hydraulic cylinders to provide the thrust forces needed to advance forward. For three, except in the most ideal circumstances, the hole of a tunnel is always trying to collapse. TBMs use a cylindrical shield at the front to support the walls of the tunnel until they can be permanently lined with cast iron or concrete and sealed with grout for strength and water resistance.

Also, there’s pressure. The soil, rock, and water deep below the ground are under immense pressure. When you try to excavate, especially in softer soils like were experienced on the French side of the project, they have the potential to collapse or flood the operation. Many of the TBMs used in the channel tunnel project were called earth pressure balance machines. Here’s how they work: The rotating cutter head chews through rock and soil, allowing it to pass through openings into a chamber behind where it is mixed into a pliable paste. As the machine moves forward, the pressure in the excavation chamber builds to match the earth and water pressure on the tunnel face, supporting it against collapse and preventing uncontrolled inflow of water. A screw conveyor creates a controllable plug. Its speed is carefully adjusted to remove only enough of the cuttings to maintain this balance.

Even that wasn’t enough in some cases. Water flowing into the excavated tunnel was a constant problem, making it difficult to work and damaging equipment. In many cases, the crews would inject grout into the rock ahead of the machine, effectively making it stronger before drilling through it. Imagine trying to drill a hole through a big bag of rocks and water. The drill bit would be easier to push, but it sure would make a mess. Grouting the rock ahead of the operation made it more physically challenging to drill through, but it simplified the process considerably. There are so many examples like that, where tunneling knowledge and experience improved drastically, just from running into problems and using trial-and-error to solve each one.

Most TBMs come with a train of equipment to support and power the operation behind the cutter head and lining systems. Each machine is basically its own factory with a workshop, cranes, transportation facilities, and more. And like any factory, you need a way to get materials and people in and out. Workers, lining segments, equipment, and materials travel to the machine from the entrance of the tunnel, often over miles on a temporary railway. And all the excavated spoils have to travel the same distance, often on conveyor belts, in the opposite direction. On the French side of the Channel Tunnel, the spoils were pumped as a wet slurry to a nearby area known as Fond Pignon. On the British side, the spoil was used to construct an extra 111 acres of new England. Well, not New England, but a portion of England that was new. This is now the site of the UK side’s cooling plant, but also a new nature reserve called Samphire Hoe.

Keeping the tunnel headed in the right direction was another challenge. For one, they needed to stay in the right geological layer to reduce the challenges of drilling through unstable ground. Of course, engineers had mapped the geology ahead of time but only using core samples from the surface. Those cores only provide a thin, tiny snapshot of what lies below, like trying to navigate a car by looking through a paper towel tube. And for two, they were drilling from both directions with the goal of meeting in the middle. The TBMs were guided with a sophisticated laser system to keep them on track as they tunneled through the marly chalk. Without a direct line of sight to the surface, surveyors had to set benchmarks along the tunnels with extreme accuracy. Any error in the measurements would propagate, since there was no way to “close the loop.” Crews also regularly took core samples, horizontally and vertically, along the way to keep the tunnel within the target geologic layer.

One of the ingenious parts of the channel tunnel design was for the service tunnel to lead the rest of construction. In a way, this tunnel was the pilot. It was a way to explore the geology with less risk, encountering the challenges on a smaller scale before making progress on the main tunnels. It was also a way to confirm the guidance and ensure that the tunnels were aligned properly when they met in the middle, which, to the relief of many, they famously did in 1990. For the first time since the ice age, there was a dry-land route from mainland Europe to Great Britain. Several of the TBMs were left and buried underground after they finished, since the cost of getting them out was too high. Now they serve as an electrical earth connection

Connecting a hole in the ground all the way across the channel is only part of the story, though. Many more engineering challenges lay ahead. As I mentioned, there are three tunnels: two large, one-way rail tunnels with diameters of 7.6 meters (nearly 25 feet) with a 4.8 meter (16 ft) diameter service tunnel running between them. But that’s not all the tunnels. There are two enormous crossover caverns where the two rail tunnels merge. During normal operation, gigantic steel doors keep the two sides separated, but they can be opened, allowing trains to cross over from one tunnel to the other. This means the tunnel can shut down large sections without the need to fully suspend train service.

The service tunnel connects both rail tunnels every 375 meters with cross passages. These allow for emergency escape from the rail tunnels should an accident or fire occur. And they’ve been used for evacuation in several cases in the past 30 years, including fires in 1996 and 2008. The air pressure in the service tunnel is higher than that in the rail tunnels so smoke can’t travel in. There are special, rubber-tired vehicles that are kind of like miniature trains, called the Service Tunnel Transport System or STTS. Of course, passenger egress is possible with these vehicles, but they are primarily, and ideally, used for shuttling staff to various locations along the tunnel.

Another engineering problem is created by the nature of trains passing through very long tunnels. On ordinary outdoor tracks, the air in front of a train gets pushed aside fairly effortlessly by the leading face of the locomotive. In a tunnel, the train acts kind of like a big piston, driving a pressurized slug of air in front of it the whole way down the tube. The rapid fluctuations in air pressure create drag on the trains, affect passenger comfort, and mess with ventilation systems. To solve this piston effect problem, a series of 2-meter-wide connections called piston relief ducts allow for controlled passage of air from one tunnel into the other, giving that chunk of air a place to go instead of just riding in front of the locomotive the whole way. A funny part of the engineering of the tunnel was investigating whether this long tube with regularly spaced holes would function like a big flute. Thankfully, it didn't end up being an issue.

Getting fresh air along the tunnels is another concern. And here again, the service tunnel shows its value. In addition to providing access to maintenance vehicles and an evacuation route, it also acts as a duct, delivering fresh air along the length of the main tunnels, allowing the stale air to discharge at the tunnel entrances. There is also a supplementary ventilation system that can pump air directly into the rail tunnels in the event a passenger train becomes immobilized.

Along with ventilation, the tunnel also has to manage heat. The trains use electricity for traction, but some of that energy is lost as heat through inefficiencies and friction. In ordinary railroad situations, this would be no big deal since the atmosphere can easily dissipate this heat. But engineers estimated that the trains would raise the temperature in the tunnel to 122 F or 50 Celsius. So, the project also required Europe’s largest cooling system. Enormous chilling plants were built on either side of the tunnel, and miles and miles of pipes carry chilled water throughout the tunnel at a cool 95 F, 35 C. Air conditioners on the trains bring this down to something more bearable for passenger comfort, rejecting more heat that has to be managed by the tunnel cooling system.

Of course, being a rail link between the two countries, the Channel tunnel is flanked by enormous rail terminals on either side, one in Folkestone, UK, and an even larger terminal located near Calais. There’s a shuttle that allows passengers to bring their vehicles along with them, effectively connecting the highways of France and the UK at the terminals. There’s also a passenger train service that crosses through the tunnel, and with the addition of High Speed 1, or HS1, in 2007, it is now possible to take the train from London to Paris and beyond.

The ordinary shuttle trains run on a loop, meaning that at each terminal, there is a track that goes from the exit of one tunnel, loops around, and then enters the other tunnel. In order to avoid uneven wheel wear from always turning in one direction like a NASCAR race, the French side features a crossover, which makes the whole tunnel loop into a huge figure 8. People aren’t the only cargo that passes through the channel tunnel, though. Freight makes its way as well. There are services for heavy trucks that get placed on trains, and there’s even a club car for the drivers to hang out in during passage under the channel. Full-on freight trains also pass through the tunnel, with service continuing past the terminals on either side.

Clearly, the channel tunnel is a triumph of modern civil engineering, and engineers around the world study its design and construction today. It wasn’t all something to celebrate, though. Like so many mega projects, there was a human cost to building the tunnel. More than ten workers perished in the construction of the project. Of course, it is absolutely unacceptable to trade safety for construction speed, even on the biggest construction project in the world, and after multiple lawsuits and investigations, things improved, and the remainder of the project saw far fewer safety incidents. The tunnel has also played a complicated role in illegal immigration and asylum-seeking in the UK, including some tragic incidents involving migrants.

The project also went significantly over budget, which is saying something since it was already slated to be the MOST EXPENSIVE construction project in history. I have a whole video that talks about some of the reasons projects like this end up costing more than we expect, so I won’t go into all those details here. The Channel Tunnel is unique in that it was privately funded, unlike most large infrastructure projects of its kind. The vast majority of the financial burden and risk was taken by banks and individual investors, and there was even a public offering. There aren’t many infrastructure projects that you can buy a share of. Over time, the tunnel has slowly turned a profit, but it’s been less lucrative than predicted. While it may be the most epic way to cross the English channel, it certainly isn’t the ONLY way. Discount airlines in Europe are far more prevalent than they were in the 1980s, and in many cases, it is more desirable and economical for travelers to just fly, especially if their ultimate destination is not the south coast of England or the north coast of France. Plus, for thousands of years, people have crossed the channel by sea. Ferries are still a totally viable and economically competitive way to cross. It might seem a little crazy to choose a ferry over the sense of wonder and delight that comes with passage through one of the most incredible tunnels in history, but maybe some people just like boat rides.

A lot has changed over the 30 years since the Channel Tunnel was completed. Construction technologies, of course, but transportation infrastructure as a whole has evolved as well. There’s probably a lot we would change about the channel tunnel if we could go back to those days when the project was first conceived, but actually, many would argue that perhaps it shouldn’t have been built at all. Knowing what we know now about the complexity of the job in a world of cheap flights, ferries, dynamic international relations, and 21st-century financial markets, it might be a bit harder to show that the costs would be outweighed by the benefits. But that’s part of the rub with megaprojects: it’s impossible to separate their wide-ranging impacts on the world, and the benefits they provide compared to an alternative where they don’t exist. Just last year, construction finished on a high-voltage electric interconnection between the UK and France through the tunnel, a project that may not have even been considered if the tunnel wasn’t already there. It’s easy to criticize the optimism required to justify huge, expensive projects in the face of an uncertain future, but projects like the Channel Tunnel create opportunities and benefits that permeate society in unique and often intangible ways.

I’m an engineer, so I see the achievement through a technical lens. It is, without a doubt, one of the most spectacular engineering feats of history. For me, that’s worth celebrating in its own right, from the intensive geological research leading up to the project, to the massive TBMs eating through so many miles of marl, from the creative ventilation and piston relief systems, to the unsung hero of the service tunnel. Whether or not it was a strictly practical idea, I’m glad it’s there. I haven’t had the opportunity to travel from Folkestone to Calais just yet, but if and when I do, I know how I’m getting there, and it’s not a ferry.