Skip to main content

The “moles” digging the M6 metro: how a TBM builds the tunnel, ring by ring

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.

From unpredictable traffic to predictable time: what Metro Line 6 will bring

How many times have you set off for the airport without knowing how long it would take? The distance from Gara de Nord to Henri Coandă International Airport is about 17 kilometres. The same journey takes, depending on the hour and the day: 20–25 minutes on a Sunday morning and 60–90 minutes on a Thursday at 17:00. This unpredictability is not a minor inconvenience — it is an invisible tax on your time.

The metro works differently. There are no jams and no unpredictability: the train leaves at regular intervals and covers the route in the same time — the same at 7:00 in the morning as at 18:30 in the evening. Predictability is, in itself, a resource.

The gain compared with the rush hour is easy to calculate: Metro Line 6 will cover the route from end to end in 22–25 minutes. Compared with 60–90 minutes by car — according to the TomTom Traffic Index 2025, Bucharest — a saving of 50 minutes per journey means: 50 minutes × 2 journeys a day × 250 working days a year ≈ 25,000 minutes ≈ 416 hours ≈ 17 days and 8 hours recovered every year. This is not a statistical average — it is real time, your time, which you currently spend in traffic.

And for the 16 million passengers who pass through Henri Coandă Airport every year — according to Bucharest Airports statistics — predictable access to the airport also means something else: being able to plan precisely, without the margin of error imposed by traffic. Metro Line 6 helps you save time.

Sources: TomTom Traffic Index 2025 – Bucharest | Bucharest Airports – Traffic statistics | GD 930/2016 – Technical and economic indicators, Metro Line 6

How Metro Line 6 can ease the pressure on Bucharest traffic

According to the TomTom Traffic Index 2025 (data for 2024), Bucharest ranks 3rd in Europe and 22nd in the world for traffic congestion. The average travel speed in the city is 13.3 km/h, and a driver loses an average of 171 hours a year because of traffic — the equivalent of 7 days and 3 hours.

According to the technical and economic indicators approved by Government Decision 930/2016, Metro Line 6 will have a transport capacity of 50,000 passengers per hour, per direction. A study published in Nature Cities (2025) confirms that metro systems significantly reduce car use in European cities. Based on the experience of similar projects, between 15 and 25 out of every 100 passengers on a new metro line are former drivers who choose to leave the car at home — which, for Metro Line 6, could mean thousands of fewer vehicles every day on the capital’s northern corridor.

On saturated arteries, a reduction of 15–20% in traffic volume increases the average travel speed by 20–35% for everyone — including those who continue to use the car. (Transportation Research Board – Highway Capacity Manual, 7th Edition, 2022).

European precedents confirm the trend: after the M2 line opened in Warsaw (2015), traffic on the parallel corridors fell significantly, according to Warsaw Metro. Cities with extensive metro networks, such as Vienna or Stockholm, consistently record some of the lowest congestion indices in Europe.

Less stress in traffic is not a slogan. It is a mathematical consequence of Metro Line 6.

Sources: TomTom Traffic Index 2025 – Bucharest | GD 930/2016 – Technical and economic indicators, Metro Line 6 | Nature Cities (2025) – Prieto-Curiel et al., modal shift metro | Transportation Research Board – HCM 7th Ed. (2022) | Warsaw Metro – M2 Line

Metro Line 6 and the reduction of urban noise: the impact of a fully underground route

Urban noise is not merely an everyday nuisance. At European level it is treated as an environmental and public health issue. Directive 2002/49/EC on the assessment and management of environmental noise establishes a common framework for identifying, preventing and reducing the harmful effects of noise exposure, particularly in large urban agglomerations and in areas affected by road, rail or air traffic.

In this context, investment in electric public transport becomes essential. Metro Line 6, which will provide the direct link between Gara de Nord and Henri Coandă International Airport – Otopeni, is one such project: underground infrastructure with a direct impact on mobility and with the potential to ease the pressure of surface traffic on the capital’s northern corridor.

European data show that the noise problem is not an abstract one. According to the European Environment Agency report Environmental noise in Europe 2025, long-term exposure to traffic noise is associated with significant effects on health and quality of life. The report estimates that it contributes each year, across Europe, to numerous health problems, including cardiovascular disease and sleep disturbance, as well as to significant economic costs. Millions of Europeans are also affected by severe sleep disturbance associated with traffic noise, and the economic impact of noise pollution from transport is estimated at over 100 billion euros a year. The World Health Organization also draws attention to this phenomenon. The WHO Environmental Noise Guidelines for the European Region, published in 2018, recommend a limit of 53 dB Lden for road traffic, stricter than the European threshold of 55 dB. The reason is simple: constant exposure to noise can affect sleep, cardiovascular health and, ultimately, quality of life.

For Bucharest and the Otopeni area, the stakes are concrete. According to the technical and economic indicators approved by Government Decision 930/2016, Metro Line 6 will have a maximum transport capacity of 50,000 passengers per hour, per direction. This capacity can absorb a significant flow of passengers who today use road transport, helping to reduce the number of cars on the DN1 and on the adjacent arteries.

The more cars there are in traffic, the higher the noise level. Conversely, every journey shifted from the car to electric public transport means less pressure on the city. Metro Line 6 works in two directions: it carries people underground, with no noise at the surface along the Gara de Nord – Aeroport Otopeni route, and it helps reduce road traffic on one of the busiest mobility corridors in the Bucharest area.

The benefits will be felt both by residents of the areas along the alignment and by those who continue to use the DN1. Smoother traffic implicitly means less noise from engines, braking, acceleration and horns.

Quiet is rarely the first argument when we talk about infrastructure. More often, we discuss speed, capacity, connectivity or safety. But in the case of Metro Line 6, acoustic comfort becomes part of the wider change the project brings to the city: mobility that is more efficient, cleaner and friendlier to people.

Sources: EEA – Environmental noise in Europe 2025 | WHO – Environmental Noise Guidelines for the European Region 2018 | Directive 2002/49/EC | GD 930/2016 – Technical and economic indicators, Metro Line 6

Metro Line 6: fewer emissions on the route between Bucharest and Otopeni Airport

Transport is one of the sectors in which Romania must reduce the pressure on the climate and on air quality. According to Romania’s Long-Term Strategy for reducing greenhouse gas emissions (GD 1215/2023), between 1990 and 2019 emissions from the transport sector rose by 41%, reaching 18,937 kt CO₂ equivalent in 2019, with road transport as the dominant source — around 96% of the sector total.

source: https://sgg.gov.ro/1/wp-content/uploads/2023/11/ANEXA-1-21.pdf

On the DN1 – Otopeni – Bucharest corridor, this reality is visible day after day: the link between the capital, the northern area and Henri Coandă International Airport depends largely on cars, taxis and other road vehicles. Metro Line 6 changes that calculation, offering a public transport alternative that is fast, predictable and with far lower operational emissions per passenger.

Car versus metro
Here is a comparison, taking as a reference the road distance of roughly 16–17 km between the Gara de Nord area and Henri Coandă International Airport / Otopeni. Our World in Data indicates, on the basis of UK government reporting factors, around 170 g CO₂e/km¹ for a petrol car and around 35 g CO₂e per passenger-km for national rail transport.
For urban electric systems of the light rail / metro type, the 2024 UK government methodology (DESNZ – GHG Conversion Factors) indicates an average of 26.16 g CO₂e per passenger-km, and for the Glasgow Underground a value of 34.70 g CO₂e per passenger-km. These values are international benchmarks for comparison, not official factors specific to Metro Line 6 or to Romania.
Applying these benchmarks to a distance of 16–17 km, a car journey with a single occupant generates approximately 2.7–2.9 kg CO₂e one way, or 5.4–5.8 kg CO₂e for a return trip. For urban electric transport (26–35 g CO₂e per passenger-km), the same journey would generate approximately 0.42–0.60 kg CO₂e one way, or 0.83–1.19 kg CO₂e return.
The difference is significant: for every passenger who chooses the metro instead of a private car, the estimated saving is around 2.1–2.5 kg CO₂e one way, or 4.2–5.0 kg CO₂e for a return journey. Assuming a car with a single occupant, the metro can have a carbon footprint some 5 to 6.5 times smaller per passenger-km, depending on the emission factor used and on how heavily the system is loaded.

The other emissions matter too
CO₂ is not the only relevant pollutant. Road traffic generates nitrogen oxides, carbon monoxide and particulate matter, PM10 and PM2.5. According to the European Environment Agency (EEA) – Emissions of air pollutants from transport, the pollutants emitted by transport contribute to the deterioration of air quality and put pressure on human health. Although exhaust emissions have fallen in recent decades, particles from the wear of brakes, tyres and road surfaces remain an important component of the pollution generated by road transport.
The electric metro produces no exhaust emissions along its route. Therefore, for journeys transferred from car or taxi to the metro, Metro Line 6 will help reduce local emissions in the areas it crosses and ease the pressure created by road traffic on the Otopeni – Bucharest corridor. The benefit is not only climatic but also urban: more public transport means more predictable mobility, a more efficient link with the airport and a real alternative for the residents, employees and visitors of the northern area of the capital.

Metro Line 6 is a public utility project of national interest, which provides the link between the metro network and Henri Coandă International Airport and contributes to the development of the trans-European transport network TEN-T. The project is not only an investment in infrastructure, but also an urban mobility measure with direct effects on emissions, air quality and connectivity between Bucharest and the airport.

¹CO₂e/km = carbon dioxide equivalent per kilometre. CO₂e (CO₂ equivalent) is a unit of measurement that includes not only carbon dioxide (CO₂) but also the other greenhouse gases, expressed as CO₂ equivalent. Cars also emit methane (CH₄) and nitrous oxide (N₂O), not just CO₂. CO₂e adds them all into a single figure, converting them according to their global warming potential. In practice, for a car the difference from pure CO₂ is small, so when the article says “170 g CO₂e/km”, it can be read as “roughly 170 g of greenhouse gases per km, expressed as CO₂”. Per km means per kilometre travelled, with a single passenger in the car (or per passenger-km for public transport, where the emissions are divided by the number of people in the vehicle).

Sources:
Romania’s Long-Term Strategy (GD 1215/2023) | Our World in Data – Travel Carbon Footprint | UK Gov GHG Conversion Factors 2024 | CNAB – Statistical data 2024 | EEA – GHG emissions from transport | EEA – Pollutants from transport

U.T.C.B. students visit the Metro Line 6 construction site

Engineering is one of the fields in which theory and practice complement each other in an essential way, and direct contact with the construction site becomes an important stage in the training of future specialists.

In this context, fourth-year students from the Technical University of Civil Engineering Bucharest – Faculty of Railways, Roads and Bridges took part in a study visit organised by the Ministry of Transport and Infrastructure together with Metrorex, in order to observe the works on Metro Line 6 at close quarters.

During the visit they were accompanied by Mrs Mariana Ioniță, Secretary General within the ministry, who gave them information about the progress of the project and encouraged them to keep developing their knowledge by working with specialists in the field.

The visit took place on the site of the future Gara Băneasa metro station, where representatives of the contractor presented technical aspects and construction processes to the students. They also had the opportunity to observe the work of the TBMs “Sfânta Maria” and “Sfânta Ana”, which are building the tunnels on the SOUTH Section.

The experience offered a valuable practical perspective on a major infrastructure project and strengthened the students’ confidence that the profession they are training for plays an essential role in the development of Romania.

Spring has arrived on the metro: flowers, mărțișoare and good news

Spring has made itself felt on the Bucharest metro network too. Metrorex representatives welcomed women passengers with flowers and mărțișoare — the traditional spring tokens — offering their spring wishes and bringing extra colour and cheer to the metro stations.

The initiative is part of the tradition of celebrating the arrival of spring and sending good wishes to all passengers.

Passengers arriving at Piața Unirii 1 station can stop for a spring photograph. A special “instagrammable” board has been set up there for the season.

📍 Location: Metrorex Central Control Room, Piața Unirii 1 station, next to the Pupils and Students ticket office.

Metrorex and the M6 project team wish all passengers a beautiful spring and many pleasant journeys by metro. 🌷

M6 Christmas campaign – 2023

CORPORATE COMPONENT:

· paper bag with M6 card insert;

· cardboard box with a green ribbon;

· fir wreath, M6 branded;

· illuminated bauble, M6 branded, added to the fir wreath;

· Christmas card.

Bilingual card

Beneficiary – 218 kits; subsequently supplemented with a further 20 kits;

Supervisor – 45 kits, of which 30 in Romanian and 15 in English;

Contractor – 77 kits;

COMPONENT FOR THE GENERAL PUBLIC:

· Christmas hat, M6 branded;

· Christmas card attached to the hat;

· distribution through sampling on the metro, on the platform and on board trains.

550 hats with an attached card.

These were distributed on the metro (on board the train at Dristor station and on the platform at Victoriei station) on 20 December.

The event was photographed and filmed in compliance with GDPR requirements. The photographs and the video material were sent to the Supervisor and the Beneficiary on the day of the event for publication on the Beneficiary’s Facebook page. The images can be viewed via the links below:

Foto: https://1drv.ms/f/s!Ah1JvsExuf0bgb5Ngg116BFo5eOl8w?e=BbrIDY 

Video: https://1drv.ms/v/s!Ah1JvsExuf0bgb9fxLJ4tqsx2-WfFg?e=2pVHjV

video for the Metrorex TV screen network: the campaign for the public was also completed with a video that ran on the screens of the metro TV network over the holiday period.

1–8 MARCH 2023 “M6 – A project as green as spring”

Between 1 and 8 March 2023, the Contractor ran the PR campaign “M6 – A project as green as spring”, dedicated to this period, in accordance with the Beneficiary’s requirements, split into two components: Corporate direct mailing, to partners, and the “Travelling Mărțișor” sampling campaign, for the general public.

1.1 CORPORATE PR component – direct mailing

The Contractor distributed to the Beneficiary, the Supervisor and the contacts in its own database kits bearing the M6 logo and containing a chocolate clover, a daffodil bulb in a pot wrapped in a jute pouch tied with a cotton mărțișor thread, a flower-shaped card suspended on a wooden stick, personalised with the headline “M6 – A project as green as spring”, and an M6 mărțișor with lichen and a craft card personalised with the wish “A beautiful spring”.

The Contractor delivered the M6 mărțișoare for the Beneficiary’s list on 1 March, and the rest of the kit on 6 March.

Total number of recipients in the Beneficiary’s database, including the press 169

Total number of recipients in the Supervisor’s database 30

Total number of recipients in the Contractor’s database 101

Total number of recipients 300

1.2 SAMPLING component

The Contractor prepared 550 mărțișoare, entitled “Travelling Mărțișor”, personalised with the logo, the M6 map and the headline “Metro Line 6, a project as green as spring”, and made them available to the Beneficiary on the morning of 1 March. The Contractor, together with PR representatives of the Supervisor and of the Beneficiary, ran the sampling campaign for the travelling public.

The campaign took place on the metro route between Dristor 2 station and Gara de Nord 2 station.

The mărțișoare were distributed partly in the carriages of a metro train that set off from Dristor 2 station. The campaign was photographed and filmed by the Contractor. The remaining mărțișoare were handed out by the Beneficiary’s representatives on the platform of Gara de Nord 2 station.

During the campaign from 1 to 8 March, the Beneficiary ran on its own network of advertising screens a video prepared by the Contractor to promote the M6 project as a major and highly topical part of the Beneficiary’s activity.

The photographs from the event were made available to the Beneficiary and the Supervisor on the same day. Some of them were placed by the Contractor and the Beneficiary into templates produced by the Contractor for the Beneficiary’s Facebook page. The Beneficiary posted them the same day. The footage from the event was edited into a video based on a model sent by the Beneficiary to the Contractor and was sent to the Beneficiary and the Supervisor on the day of the event, to be used in a post on the Beneficiary’s Facebook page.

2022 – ECO Christmas

Between 10 and 28 December 2022, the Contractor ran the “ECO Christmas” campaign.

The creative concept of the campaign: “Every day we build a sustainable future. Now we are building the future.”

Approach: the importance of the M6 project, a major urban infrastructure project, was underlined through suggestive, symbolic means, drawing on a strategy based on the power of example. Rather than emphasising the glitter of the holidays, the campaign set out to highlight the benefits that well-made things bring to people’s lives. We chose a simple and memorable gift, one that can be brought to life. In the form of a direct mailing campaign, the project was communicated to stakeholders, partners and the press, conveying a long-term message and also an invitation to get directly involved. In this way, we set out to emphasise nature, continuity and perspective, the lasting value of professional and human relationships, and the importance of things achieved through working together.

Campaign elements: a fir planting kit and a holiday message.

Kit contents: a bio box, a biodegradable pot, a small bottle of Abies Alba fir seeds, fertile sowing soil, planting instructions, a craft bag, a label tied with string, a fir sprig, a card and an envelope.

The mailing was segmented bilingually (Romanian and English), according to the recipients in the database built by the Contractor, the Supervisor and the Beneficiary.

Total number of recipients in the Beneficiary’s database 218

Total number of recipients in the Supervisor’s database 27

Total number of recipients in the Contractor’s database 135

Total number of recipients 380

Campaign approval date: 12 December;

Approval date for the printed materials: 13 December;

Approval date for the print runs: 14 December;

Production period: 16–19 December;

Delivery date: 20 December.