The “moles” of Metro Line 6 dig the earth, bring it to the surface and raise the tunnel behind them, almost simultaneously. Their technical name is TBM, from Tunnel Boring Machine.
The TBMs “Sfânta Maria” and “Sfânta Ana”, which are working on the South Section of Metro Line 6, weigh around 600 tonnes and have cutterheads roughly 6.60 metres in diameter. In length, they come close to a football pitch. They enter the ground, and behind them they leave a circular, reinforced tunnel, ready for the next stages.
At the front is the cutterhead, a huge metal disc fitted with cutting tools. It rotates, breaks up the ground and pushes the material into a chamber where the pressure is controlled at all times. The pressure keeps the excavation face stable and allows the machine to advance under control. From there, the material travels onto a screw conveyor, then onto conveyor belts, and out of the tunnel. It is loaded up and taken to authorised sites.
While digging goes on at the front, the tunnel is already being assembled behind the shield.
Segments and tunnel rings
The TBM advances using hydraulic rams that push against the last ring installed. After each advance, a device called an erector lifts the segments, the precast concrete elements, and sets them side by side until a new ring is closed. A gap remains between the concrete ring and the ground, which is filled by injecting a special grout known as bentonite. The operation fixes the structure in place and helps stabilise the surrounding ground.
Then the cycle begins again: excavate, remove the material, advance the machine, install a ring, inject the grout. And so it goes, a few metres a day, month after month.
How the “mole” knows where to go
Tens of metres underground, orientation has nothing to do with instinct. The TBM has guidance systems, sensors and survey reference points tied to the project coordinates. The operators follow the alignment constantly and can correct the direction with very fine adjustments. From the control cabin they monitor, in real time, the pressure in the excavation chamber, the amount of material extracted and the position of the machine relative to the designed axis. To these are added the operating parameters of the cutterhead and of the hydraulic systems, plus the ring-building operations.
Under normal conditions, the TBMs on the South Section of Metro Line 6 in Bucharest advance on average by about 1.5 metres per hour. That sounds like very little, until you look at what happens in that hour: the machine excavates around 60 cubic metres of material and prepares the installation of the tunnel’s permanent structure.
In 24 hours, an average of 13 to 14 rings are installed. The teams work in three shifts and the activity is continuous, with the stoppages needed for checks and maintenance. The pace varies according to the ground, the recorded parameters and the maintenance work.
Romania’s “moles”: from the metro to tunnels through the mountains
TBM technology is not being used in Romania for the first time. On Metro Line 5 in Bucharest, the excavation was carried out by “Sfânta Varvara” and “Sfânta Filofteia”.
On the Brașov–Sighișoara railway line, on the Apața–Cața subsection, four TBMs are at work, “Eleni”, “Varvara”, “Alexandra” and “Konstantina”, on the Ormeniș and Homorod twin tunnels. The railway machines have a different diameter and work in different geological conditions, but the principle is the same: controlled excavation, removal of the material, immediate installation of the structure and monitoring of the parameters.
The names follow a long tradition in tunnelling projects. Saint Barbara is regarded as the protector of miners and of those who work underground. On Metro Line 6, the tradition continues with “Sfânta Maria” and “Sfânta Ana”.
What happens at the surface
Although the activity takes place underground, the operation of the TBM is backed by an extensive monitoring system. The pressure in the excavation chamber, the amount of material extracted, the route of the machine and the behaviour of the ground are checked at all times. In the areas within the zone of influence of the works, special equipment is used to monitor the ground and the buildings.
The underground route to the airport
Metro Line 6 will have 14.2 kilometres of double track and 12 stations, and will create the first direct metro connection between Gara de Nord and Henri Coandă International Airport.
The video shows how the machine advances and how the concrete segments become, ring by ring, the structure of the future tunnel.
Press “Play” and step inside a TBM for a few minutes.
