A tunnel between Morocco and Spain: ‘It’s technically possible’

The summer holidays are just around the corner, which means that thousands of people from the Netherlands and other parts of Europe will once again be making the crossing between Europe and Morocco by car and ferry. It would save them a great deal of time if they could swap the ferry for a tunnel. It is no coincidence that Spain commissioned tunnel boring machine manufacturer Herrenknecht to investigate the technical feasibility. Their conclusion: such a tunnel is possible. How does TU Delft tunnelling expert Wout Broere (Faculty of Civil Engineering and Geosciences) view this?

“No, the King of Morocco hasn’t called TU Delft yet,” Broere laughs. “Nor has the Prime Minister of Spain. But it’s great that the idea of a tunnel between the continents is back in the spotlight. As an enthusiast of civil engineering and geo-engineering, you can only be excited by it.” But the key question remains: is a tunnel between Morocco and Spain actually feasible?

100 years in the making

“The short answer is: yes, such a tunnel is technically feasible,” Broere begins. “But that only answers a very small part of the question, because feasibility depends on far more than just technology. Think of financing, geopolitical relations, international agreements, safety, and so on. Still, the project certainly captures the imagination.”

The first plans date back more than 100 years, Broere knows. “But that’s still young compared with the Channel Tunnel between France and England, which was on the drawing board for 200 years before construction finally began. So, it’s certainly not unlikely that such a tunnel will be built one day.”

The deeper you go, the longer it gets

Back to basics: even in the least challenging studies, the tunnel would reach depths of around 450 metres below sea level. How do you deal with that, and where are the challenges? Broere explains:

“First you must take a close look at the location. At its narrowest, the strait is 900 metres deep. And that’s not all, because for a tunnel you also need sufficient cover and a solid ground layer, which at this location is only found at least another 120 metres deeper. If you need to find a truly stable layer, it could be even deeper.”

The main issue with that depth is the gradient the tunnel must have, Broere says. “Let’s assume a rail tunnel, like the Channel Tunnel, as that’s the most logical option in terms of safety and feasibility. Trains are limited to a maximum gradient of 2%. That means for every metre of depth, you need 50 horizontal metres. For a depth of one kilometre, you’re looking at 50 kilometres of length on both the Moroccan and Spanish sides. A shallower route makes the tunnel shorter and therefore cheaper, but even at a maximum depth of 450 metres, you still need many kilometres of gradient.”

Finding the right layer

At such depths, tunnelling is perfectly possible, provided the soil conditions are suitable. “The geology has already been thoroughly studied. In some places, the ground isn’t very suitable for tunnelling because the rock is highly permeable,” Broere says.

‘Additional technology or more complex drilling methods will make the tunnel significantly more expensive’

“With a water column of 450 metres above you, that guarantees substantial water inflow into the tunnel. The best solution is either to go deeper until you reach a less porous layer or to develop effective sealing and drainage systems. The tunnelling industry has plenty of experience with this, but additional technology or more complex drilling methods will make the tunnel significantly more expensive.”

Tectonic plates

Another challenge is the tectonic plates the tunnel would pass through. In the Strait of Gibraltar, the African and Eurasian plates meet, which can lead to shifts and earthquakes. “That can be managed to some extent,” Broere continues. “After all, the tunnel itself becomes part of the moving ground. By bolting tunnel segments together, you keep them intact.”

You could also use shock absorbers to accommodate movement. This is already done in the Bosphorus tunnel, which also crosses tectonic plates. With limited shifts of a few centimetres, the tunnel remains intact, even if it crosses a fault line, Broere says. “There is little you can do against movements of several metres. You would have to start drilling again. But many tunnels worldwide are already located in earthquake-prone areas, so we have plenty of experience. There are challenges, but not so great that new technology is required. We can build this tunnel with the knowledge we already have today.”

More of a design challenge

That knowledge is available, but before construction can begin, a lot must happen. Broere concludes: “The technology is ready; it’s more of a design challenge than a scientific one. What is the best solution in terms of materials, concrete, geology, construction, planning, connecting infrastructure, fire safety, logistics… the list is endless. A project like this brings together an enormous range of disciplines; it would be great for a university to be involved.”

So, if a call does come from Spain or Morocco, would Broere pick up? “Of course, gladly, although my expertise really lies in tunnelling through soft soil, while the Strait of Gibraltar mainly involves rock formations. But still, if I can contribute, I’d be more than happy to.”