连接日内瓦与洛桑的水下隧道An underwater tunnel connecting Geneva and Lausanne
Summer series. Students' projects (5/9) - Drawing inspiration from Elon Musk’s Hyperloop and the Swissmetro initiative, a civil engineering Master’s student has looked into the possibility of building an underwater high-speed train route through Lake Geneva.
One day as you reach Geneva’s main train station to start your daily commute you notice a new sign showing some kind of underwater train. At first, you focus on the ripples marking the surface of the water. But as you approach and look more closely at the sign, you notice something strange about the rails – the train has no wheels. It seems to be suspended in mid-air.
You follow the arrow, which leads you to an elevator a few meters further on. The destination is indicated above the door: Lausanne. That’s where you’re going. So you get on the elevator, and it takes you deep below the ground. Your fellow passengers look as confused as you are. The elevator doors finally open and you step into a brand-new station already packed with commuters. You get on board. As the train pulls out of the station you realize that it’s not running on the rails but ‘levitating’ above them. You get to Lausanne in around ten minutes. You’re lost for words when it dawns on you that your journey has taken you under the waters of Lake Geneva. Another elevator is waiting for you as you get off the train. It takes you back up to the surface into the familiar bustle of Lausanne’s train station.
Following in the footsteps of Swissmetro
Elia Notari has just obtained his Master’s in civil engineering from EPFL. He came up with this visionary train line for his Master’s project, which was supervised by Aurelio Muttoni, a full professor and director of EPFL’s Structural Concrete Laboratory (IBETON). Those with a sharp eye will no doubt notice some similarities with the Swissmetro project, an idea for a futuristic metro system developed at EPFL in the 1990s. Swissmetro was intended to link the main towns in Switzerland at a speed of 500 km/h. Notari’s Master’s project is in fact a variation of the Swissmetro design, which put the trains in underground tunnels rather than through Lake Geneva.
Elia Notari刚刚从洛桑联邦理工学院（EPFL）获得土木工程硕士学位。他为自己的硕士毕业设计了这条富有远见的列车线路。他的导师Aurelio Muttoni是洛桑联邦理工学院的全职教授兼结构混凝土实验室（IBETON）主任。那些目光敏锐的人无疑会注意到该毕业设计与瑞士城铁项目的若干相似之处。瑞士城铁项目是上世纪90年代由洛桑联邦理工学院研究的一套城铁系统，极富未来主义感。按照计划，瑞士城铁将以500km/h的速度连接瑞士的主要城镇。Notari的硕士毕业设计实际上是瑞士城铁设计的一个变体，将原本于地下隧道穿行的列车改为在日内瓦湖湖底穿行。
But is it really possible to build a metro through Lake Geneva? What some consider sacrilegious is a fascinating technical challenge for others – one that draws on multiple disciplines, covering everything from structural design and hydraulics to geotechnics and the environmental impact.
A submerged bridge
Notari focused on designing and calculating the dimensions of a double-track tunnel made out of reinforced concrete: an ‘underwater bridge’ – or to be more specific, a ‘floating underwater tunnel.’ Covering a distance of around 55 kilometers, the tracks would use magnetic levitation – or maglev – to get the train to travel at high speed. With this type of system, one set of magnets is placed on the tracks; a second set on the bottom of the train is repelled by the first, which pushes the train up off the track. In some cases, the repulsive force of the magnet is controlled by a current running along the track.
Fig.1: The double-track tunnel and its compartments containing ballast. © EPFL 2018 / E. Notari
图1：双线隧道及其包含压载物的隔间。© 洛桑联邦理工学院，2018年/拍摄者：E. Notari
This kind of system is not yet used in transportation, but several countries are now testing prototypes and plan to start building soon. In 2015, the Shinkansen Maglev prototype developed by Japanese company JR Central set the world record when it reached a speed of 603 km/h. For his work, Notari drew on policy guidelines developed in Germany, which are better suited to short distances, like that between Geneva and Lausanne.
Several options to choose from
There are several options available to an engineer looking to build a train that can run under water. The tunnel can, for example, be set under the ground, as is the case with Eurostar, which was opened in 1994 across the Channel and connects London with several cities in mainland Europe.
Norway is looking into another option to link its fjords with its cities – an underwater tunnel suspended at a series of floats that can be seen on the surface. Yet another solution involves putting metal rods in the ground to anchor down the tunnel. However, this will only work if the tunnel itself is buoyant enough.
Yet none of these options was quite right for Lake Geneva. This was mainly because its clear waters make it popular among sports enthusiasts and esthetes, which meant the student had to design a tunnel that was at least 20 meters deep and not visible from the surface.
30 meters underwater
Notari therefore went with a bridge located 30 meters below the surface and supported by piers made of reinforced concrete. This solution was the most rigid, which is a crucial factor for a high-speed train. It was also the most secure in the event of internal flooding. Other advantages also guided his choice. With this solution, the temperature would be constant throughout the entire structure, which would prevent any thermal deformation as a result of the changing seasons. Waves are also at their weakest at that kind of depth, and the bridge would not get in the way of any existing structures.
What’s more, the hydrostatic pressure would be a bonus in terms of the solidity of the 199 modules making up the concrete tunnels. The engineer’s tunnel measures 14.5 meters in diameter and is designed to withstand earthquakes, internal and external explosions, flooding, tsunamis and landslides. The structure would include compartments containing ballast to make it possible to stabilize the tunnel at any time based on Archimedes’ principle. To adjust to the lake’s varying depth, the height of the pillars, which are 6 meters in diameter, would range from 7.5 to 45 meters. The bridge’s route would follow the lakeshore (see Fig.2. on the left). And the two ends of the line would be connected to the Geneva and Lausanne train stations via elevators.
Too technically feasible to be utopian
The idea of an underwater bridge in Lake Geneva doesn’t seem so utopian to the freshly graduated engineer. “The concept of an underwater bridge has been around for a century,” says Notari. “Our know-how has advanced significantly thanks to projects like oil platforms and offshore wind farms. What’s missing is a pioneering investor who’s ready to put up the money to fund the construction of a prototype.”
For Aurelio Muttoni, high-speed transportation projects have great potential:“The solution that Elia studied is technically realistic. I myself came up with a similar solution in the 1990s when studying the feasibility of crossing Lake Lugano as part of the southern extension of the Gotthard high-speed train line.” Muttoni adds that he was inspired by Hyperloop, Elon Musk’s ultra-fast train, when suggesting the topic to his student. (See the EPFL news article from 23 July 2018.)
在Aurelio Muttoni看来，高速交通项目具有巨大的潜力：“Elia研究的解决方案在技术方面是很现实的。上世纪90年代，我自己也提出了类似的解决方案，当时我在研究Gotthard高速铁路线南段部分穿越卢加诺湖（Lake Lugano）的可行性。”Muttoni补充说，当他向Elia Notari提出这个题目时，他的学生已经从伊隆·马斯克的超高速列车“超回路列车”中获得了灵感。（参见2018年7月23日的洛桑联邦理工学院新闻文章。）
But given the complexity of the situation, the environmental challenges and the high costs of this kind of project, Muttoni is convinced that it will be many years before Lake Geneva gets an underwater tunnel.
Elia Notari, Conception et dimensionnement d’un pont submergé dans le lac Léman, Master’s project supervised by Aurelio Muttoni, Structural Concrete Laboratory (IBETON), 2018.
《Conception et dimensionnement d’un pont submergé dans le lac Léman》。作者：Elia Notari；毕业设计导师：Aurelio Muttoni；结构混凝土实验室（IBETON），2018年。