19/12/2025
When we think of great engineering works, we often imagine colossal bridges, endless highways, or monumental dams. However, there are infrastructures that, although hidden underground, are just as essential for the operation of our cities and territories: the tunnels.
These underground structures allow for the connection of regions, transport water, generate energy, and facilitate the modern lives of millions of people. Its construction, far from being simple, involves a combination of science, technology, and experience accumulated over centuries.
Why are tunnels so important?
The tunnels fulfill key functions in three major areas: transport, water management and energy production.
1. Transport
In mountainous areas, urban areas, or coastal areas, tunnels allow for crossing natural or artificial obstacles without altering the surface environment. Thanks to them, high-speed trains can maintain straight paths, roads avoid detours, and urban meters connect neighborhoods without congesting the streets.
2. Water
Hydraulic tunnels are essential for transporting water from collection areas to consumption areas
They are also used for sanitation, allowing the evacuation of wastewater safely and efficiently, especially in large cities.
3. Energy
In the energy field, tunnels are essential in hydroelectric power plants, where they channel water to the turbines. They are also used in energy storage projects, such as reversible pumping systems, and in geothermal installations.
The evolution of construction methods: from manual methods to tunnel boring machines
The first constancy of an underground excavation carried out by man dates back to present-day Esuatini, in southern Africa, where man Neandertal extracted hematites for rituals, using rudimentary tools such as sharpened stones.
Since ancient times we find notable examples of underground engineering: the Babylon Tunnel (approx. 2200 BC), the Persian qanats (10th century BC), the Eupalinos Tunnel in Greece (530 BC) and numerous tunnels built during the Roman Empire, between the 1st century B.C. and the 4th century d.C.
Building a tunnel is always an engineering challenge. Requires knowledge of the terrain, designing appropriate support and applying techniques that guarantee the safety of workers and the environment.
In the past, tunnels were excavated manually, with basic tools, a huge human effort, and many risks that ended with the loss of human life on many occasions.
Over time, methods such as NATM (Austrian Tunnel Construction Method) were developed, which take advantage ofthe natural strength of the ground to stabilize the tunnel, instead of relying on heavy structures from the outset. It is based on phased excavating and quickly applying light primary support —such as projected concrete, bolts, and metal mesh— while monitoring the deformations and stresses to adapt the design in real time. This flexible approach reduces costs and improves safety, as it makes the terrain itself part of the support system, later complemented with a definitive coating.
The other great progress in tunnel construction comes from the mechanization of its execution with TBM tunnel boring machines (Tunnel Boring Machines ).
TBMs are equipment designed to excavate tunnels continuously and safely. Its operation is based on a large rotating head equipped with cutting disks that, by combining rotation and thrust, fragment the rocky mason at the front of progress. The interesting thing is that these machines not only drill, but also integrate auxiliary systems that allow for the placement of provisional support —such as dovelas rings— and even the definitive lining as they advance. Thanks to this automation, a faster, more stable, and secure process is achieved, reducing risks and optimizing times compared to traditional methods.
Although the first TBM that could fit its current definition was designed and developed by Charles Wilson in 1851, the breakthrough in the development and application of this technology occurred in the 1950s with James S. Robbins, who developed a model for the Ohade Presa project. He managed to excavate 49 meters in 24 hours, a true milestone. Currently, the types of TBMs are adapted to the characteristics of the terrain, with specific models for rock, granular soils or cohesive soils, the presence of the water level, or mixed ground conditions.
Although there are other methodologies, nowadays most tunnels are executed either by the Austrian Method or by Tuneladoras (TBM) when the length and value of the project justify their use.
Key aspects when analyzing these projects
From MAPFRE Global Risks, when assessing the risks of the projects in tunnels, we evaluate the following aspects.
1. Geology and geotechnics of the terrain: in tunnels, the land-structure interaction is the key aspect, which is why a good geotechnical study with an appropriate campaign and, beforehand, a correct interpretation of the geological environment are essential. Heterogeneous or complex terrain, presence of faults and high water pressures must be anticipated in the project, which is why a good geotechnical campaign is necessary.
2. Experience of the team: Unlike other construction projects, where the project can be completely defined before starting to build, in tunnels, it is the conditions of the terrain themselves, which often differ from those initially anticipated, due to poor geotechnical campaigns and the impossibility of investigating the entirety of the trace of a tunnel, that must be analyzed by the designers. These must make significant adjustments in each section of the tunnel to withstand the efforts induced by the ground, so the experience of designers when interpreting the conditions of the terrain becomes one more key point.
The builder’s team, for its part, must also have extensive experience in underground works. These engineering teams are multidisciplinary, including geologists, surveyors, and prevention technicians.
3. Monitoring and Auscultation: the tunnel works as well as the environment must be monitored. This is done using instruments that detect deformations, movements, filtrations, or vibrations. Monitoring is essential to corroborate that the terrain and environment are behaving according to the previous estimates; if not, and if any threshold is exceeded, reinforcement measures must be taken to ensure the safety and stability of the tunnel and the environment.
Conclusions
Tunnels are critical infrastructures that, although invisible, support a large part of modern development by facilitating transportation, water management, and energy generation. Its construction involves complex technical challenges that require deep geological knowledge, advanced construction methods, an expert design and construction team, and strict exhaustive monitoring to ensure safety and efficiency.
The evolution, from manual excavations to the use of tunnel boring machines and techniques such as NATM, reflects how engineering has been able to adapt to optimize costs and reduce risks. Even so, we must not lose sight or neglect any of its essential aspects since tunnel claims can unfortunately lead to human losses and significant economic losses.
Author of the article:
Senior Risk Engineer at MAPFRE Global Risks
