Tunnel boring machines are colossal engineering systems that have transformed underground infrastructure construction. From the heart of dense cities to the most remote locations on earth, their ability to excavate safely, precisely and with minimal environmental impact has permanently changed the way we build. They are far more than machines: they are proof that human ingenuity can open paths where once there were only limits.
A tunnel boring machine (or TBM) is a masterpiece of modern engineering. Built to tackle both hard rock and soft ground, it is a system capable of excavating tunnels and underground passages with millimetre-level precision, protecting the surrounding environment and ensuring worker safety throughout.
Some TBMs exceed 100 metres in length, reach 18 metres in diameter and weigh up to 2,800 tonnes, equivalent to nine city buses end to end, a five-storey building in height and fifteen Boeing 747s in weight.
In terms of output, ACCIONA’s TBMs can advance over 1,000 metres of excavated tunnel per month.
A TBM is a highly integrated system in which every mechanism is designed to function as a unified whole: sensing, boring, building and protecting —metre by metre.
- Cutterhead: The front-facing rotating component that breaks through the ground. Fitted with cutting tools —disc cutters or scrapers— adapted to the specific soil or rock type.
- Shield: A steel cylinder that protects the excavation face and provides structural support to the tunnel as the machine advances.
- Thrust system: A series of hydraulic cylinders that generate the pushing force required to drive the TBM through the ground.
- Segment erector: Places the precast concrete ring segments —known as tunnel liners— to reinforce the tunnel walls and ensure structural stability as the machine progresses.
- Muck removal system: Extracts and transports excavated material —rock and soil— out of the tunnel continuously.
- Ventilation and climate control: Maintains safe working conditions for all personnel inside the machine and the tunnel.
- Control cabin: The operational nerve centre, from which all TBM activity is monitored and managed in real time.
No two ground conditions are the same. That is why different TBM types exist, each engineered to address specific geological challenges:
From the nineteenth century to today, the evolution of tunnel boring machines reflects humanity’s determination to advance —even when the path ahead cannot be seen.
Marc Isambard Brunel designs a mechanical shield to excavate the Thames Tunnel in London. James Henry Greathead improves on it decades later with a more stable cylindrical system. Two engineers, two decisive advances that lay the foundations of the modern tunnel boring machine and open a new era in underground infrastructure construction.
The first TBM with a rotating cutterhead appears at the turn of the century. A technical leap that, between 1950 and 1970, is consolidated with the introduction of hydraulic technologies, mechanical ground support systems and increasingly precise ground control. The TBM moves from promise to the go-to tool for large-scale civil engineering projects.
The earth pressure balance TBM —EPB— solves one of the biggest challenges in urban construction: excavating through soft ground and mixed soils while maintaining constant face pressure to prevent collapse and minimise surface disruption. In the 2000s, digital control becomes part of the process: data analysis starts to drive how each machine is operated.
The undergrounding of Madrid's M-30 motorway puts large-diameter TBMs to the test in one of Europe's most demanding urban environments. ACCIONA begins equipping each machine with 200 to 300 sensors, tracking thrust, rotation, energy consumption and early fault detection in real time. Between 2017 and 2022, four TBMs work in parallel on Norway's Follo Line. A step change in how tunnelling projects are planned, coordinated and delivered at scale.
ACCIONA launches its TBM Control Centre, bringing together over 65,000 sensors per machine and 34 million data points tracked every day. Real-time data feeds predictive maintenance, advance modelling and faster, better-informed decisions on site. In 2024, the company begins trialling immersive reality monitoring, giving engineers full visibility of every machine parameter from anywhere in the world, and laying the groundwork for remote TBM operation.
Rail infrastructure
TBMs are essential for the construction of metro lines and high-speed rail corridors, enabling long tunnels to be driven without disrupting the urban surface above. ACCIONA has deployed this technology on the São Paulo Metro in Brazil and the Follo Line railway tunnels in Norway, both examples of precision delivery in complex urban and geological environments.
Road tunnels
TBMs make it possible to create underground motorways in congested cities or through mountain ranges, reducing landscape impact and improving traffic flow. A defining example is the burial of Madrid’s M-30, one of the largest underground urban infrastructure projects ever undertaken in Europe.
Hydraulic works
In water transfer, sewerage and sanitation projects, TBMs allow tunnels to cross rivers, protected areas or dense urban cores without interrupting life at the surface.
Energy infrastructure
TBMs enable the safe, low-impact installation of power lines, gas pipelines and district heating networks underground. At the Alto Tâmega dam in Portugal, ACCIONA built the associated tunnels for a hydroelectric power plant, integrating energy infrastructure below ground.
Complex urban environments
In densely populated cities with existing infrastructure already in place, TBMs are the preferred solution for minimising surface impact. This was the case on the Quito Metro in Ecuador, where ACCIONA delivered the city’s first metro line.
ACCIONA delivers every tunnel project with a focus on minimising surface disruption, reducing emissions and making the most efficient use of resources.
ACCIONA has built over 800 km of tunnels worldwide —more than 400 of them using TBMs— adapting every machine to the specific conditions of each project through its SPO (Self-Performing Office). The company operates its own TBM Control Centre, which analyses over 34 million data points per machine every day: cutting force, advance speed, energy consumption and more. Each TBM is equipped with up to 65,000 sensors and generates approximately 45 terabytes of data per year, enabling predictive maintenance and advance simulation.
Up to 1,000 metres per month, depending on ground conditions and the type of machine.
TBMs are used for metro and rail tunnels, road tunnels, hydraulic works, energy installations and complex urban projects where surface disruption must be minimised.
TBMs are equipped with ventilation, climate control and continuous monitoring systems managed from the control cabin. ACCIONA also enforces strict safety protocols and provides ongoing training for all personnel on site.
TBMs significantly reduce surface and environmental disruption. Optimised processes and sustainable technologies minimise emissions and waste throughout the excavation cycle.
Dimensions vary by project, but some machines exceed 100 metres in length and reach diameters of up to 18 metres.
Advanced sensor arrays, real-time data analytics, automation and predictive systems are now standard in modern TBM operations, enabling early detection of potential issues and continuous process optimisation. ACCIONA is also trialling immersive reality systems that allow engineers to access all machine data within a virtual environment.