The L9, the eternal work that already accumulates 6,000 million in cost overrun (and a sinkhole)

BarcelonaIt is intended to be the metro work with the longest tunnel in Europe. But the L9 works have already earned the title of "historic" and "controversial" for years, both for the delays the project has accumulated since the first plans and for the 6,000 million euros of cost overrun, according to the Court of Auditors. And since this Tuesday also for the 8-meter sinkhole that has forced the evacuation of six buildings in the Putxet neighborhood of Barcelona. But, at what point is this work? Where is the tunnel boring machine going? How does this large machine work and how is the tunnel built? We explain it to you in six keys:

What is the L9 tunnel boring machine and how does it bore under Barcelona?

The tunnel boring machine is a large machine that is about 12 meters in diameter and about 100 meters long. It was introduced underground piece by piece, with a large crane, through the so-called attack shaft, the tunnel that connects the underground gallery with the surface. Once assembled underground, the tunnel boring machine works like a large iron worm. At the very front it has the main part, a large cutting wheel that grinds away at the terrain without rest. When it has bored 1.8 meters of earth, everything stops and the machine installs the voussoirs, large prefabricated concrete structures — 40 centimeters thick each — that form the final lining, that is, the tunnel walls. In parallel, a system of conveyor belts removes the earth through the central axis of the tunnel boring machine to the outside. Once the walls are installed and secured, the hydraulic pistons or jacks of the tunnel boring machine shrink and gather the large machine, as if it were a large worm, to then extend forward and continue advancing.

Inside, teams of about 25 people work in three shifts, ensuring that the tunnel boring machine never stops: it works 24 hours a day and 7 days a week.

Where is the tunnel boring machine currently?

At 40 metres below ground, just beneath the Putxet neighbourhood. Last autumn it was restarted after a year's pause waiting for the replacement of the cutting wheel. In March of this year it reached the level of Sant Gervasi de Cassoles street. This Tuesday, after the scare of the Putxet sinkhole, the Generalitat has confirmed that the tunnel boring machine has now surpassed by about 10 metres the section where the hole occurred. The speed of the tunnel boring machine depends on the terrain it drills through: on average it excavates about 15 metres daily, although there are days when it can only do 6 and others, reach 24.

How does it build the tunnel?

This large boring machine not only excavates the ground but at the same time it excavates it assembles the final walls of the tunnel, the so-called voussoirs. Every time the machine stops excavating to assemble a new tunnel ring, it installs a total of seven voussoirs or concrete pieces. One of the curiosities is that these pieces are not all the same, each ring has a specific shape and it is thanks to this that engineers can design straight sections or others in a curved shape to follow the designed route.

This system allows, according to the technicians of the Generalitat, that the excavation is immediately stabilized. The voussoirs incorporate neoprene joints, to guarantee that no leaks will occur and, once each ring is finished, mortar is also injected between the ground and the lining to reinforce it.

How many years have the L9 works lasted?

The works officially began in 2002. This means that the project has been underway for 24 years. However, it should be noted that during this time there have been major stoppages and very diverse modifications that have left the project frozen for years. A large part of the material was buried for a decade in the Barcelona subsoil, which increased the cost of the work, which has accumulated an overrun of about 6,000 million euros, according to the Court of Auditors.

Where does it go and what section of tunnel still needs to be built?

The line, once completed, will be the longest in Europe, with 50 stations connecting Prat Airport with Santa Coloma de Gramenet (from T1 to Can Zam). Currently, both ends of the line are already in service. The central section of the tunnel, which connects the future stations of Mandri and Lesseps, approximately 1.6 kilometers of gallery, still needs to be excavated. It is the last section necessary for the L9/L10 tunnel to be completed and connect the two ends of the line.

When are the works expected to finish?

Although part of the installation is already operational, there is no official date currently scheduled for the complete end of the works and the full inauguration. It is foreseen, however, that it will not be until after 2031. The last forecast was that the tunnel boring machine would reach Lesseps this very 2026 and that in 2027 four more stations on the line would be put into operation.