Athens Metro: Take a walk on the underground side
We went into the construction site of the new metro line that is being built from the Athens International Airport to the Port of Pireaus. We saw what it is like to work so many meters under the surface of the earth.
Photos: Dimitris Koulelis / PopagandaGR
It looks a little bit like those weird illustrations in schoolbooks that show the underground, rambling paths of ants. A labyrinth beneath our feet, where a way longer “insect” goes through than an ant, along with many orange safety jackets. We are talking about the metro and the tunnel boring machine. We wore two of the orange jackets ourselves and followed Kostas Kitsos and Giorgos Giannaras to see how the metro pierces the ground.
We are standing at St. Nicolas Square at Nikaia and looking diagonally at the five levels of the station being built. Those are the level of the railway track, the platform, the ticket checking, the semi-open spaces and, lastly, the roof level. This will be one of the stops of the line that connects Piraeus to the Airport in just 45 minutes. When you get off the plane, you can embark on the next ferry to your dream destination island.
The spectacle is imposing. You feel something very important is happening 25 meters below ground. We went down to the track level and traveled 800 meters to the TBM machine that builds the underground tunnels.
The machine otherwise known as “Hippodamus” has traveled approximately 4,250 meters from Agia Varvara, where it was assembled. It passed Korydallos, Nikaia and now approaches Maniatika, western neighborhoods of Athens. It has carried out 65 percent of the total tunnel length and needs about 75 meters more to reach the next station, Maniatika. Every 24 hours, over three shifts, working through slate, 12 meters are opened on average. With a maximum output of 22 meters per day, 16 concrete rings are created -the ones you still see out of a metro window- and if you put them side by side, you get the tunnel. Now the digging is through limestone soil so the production is slower, which means 8 meters in 24 hours, because the material is harder and requires longer time to dig.
“Hippodamus”, moving at 25 mm/min with a length of 140 meters, is named after the ancient town planner of Piraeus, since this metro line will eventually reach the harbor. For every excavation, 8km of grout is needed, which is pressed from the pump to the “shield”, filling uniformly the ring. At this point, the train comes, brings the pieces of the ring and the versatile crane rotates and places them in the proper position, creating the tunnel. The shield has a length of 10 meters and consists of three parts: the front where the chamber and the cutting head are, the middle with the pumps and mechanical systems that drive the head to rotate and advance, and the back where the last ring is located.
The pilot of the machine stands a little further in, but we cannot reach him. Through screens, the pilot sees all the systems, and controls at what pressure and temperature the pumps and the segment erector work. With his view of the dug-out shaft and with the aid of a laser, he directs its course and corrects it if needed. He also checks the conveyors, their speed and coordination. When the metro project in Athens launched 25 years ago, foreigners had come to train the first pilots and operators. The first batch learned from them, and continues to train the younger generations.
How do we ensure to the fullest extent that there will not be any subsidence, that is, a gradual caving in of the tunnel? With a mixture of water, foam and air. This pressure is created by the soil material. But the mixture prevents anything from falling onto people’s heads or causing subsidence at the surface of the ground. In the urban environment, it is important not to exceed the limits of a few millimeters, so as not to have an alarming situation.
When an operation to the drill head is needed, for example change of the cutting blade, the workers enter the hyperbaric chamber, after certain medical examinations, because they have to stand 2–3 bars of pressure and adapt to the pressure changes. Once you enter the chamber, the pressure begins to rise slowly and in the presence of a medical team, they enter into the head and check the grinder. When finished, they get back into the chamber and follow the opposite tactic of decompression and then take a day off.
If you are wondering how a tunnel first opens up in the bare ground, the answer is, by more conventional methods. That is, with hoes and shovels, exactly as they also do the final part of the tunnel. The big machines come after the required space is first created.
The tunnel boring machine, which started its way from the starting platform in Agia Varvara, quietly and without any disruption to the rest of the city, continues the construction of the tunnel to Piraeus. Today, more than 700 people of various specialties work in the stations under construction at Agia Varvara, Korydallos, Nikaia, Maniatika, Piraeus and the Municipal Theater. The construction is advancing and it is expected by its completion that approximately 2,000 workers will have worked on this project, and the metro connection to Piraeus will be completed in 2020.
This article was originally published in Greek on PopagandaGr
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