To understand the future of tunnels, we must look to the past

Tunnels have formed a major part of the world's infrastructure for many years. Have they changed much? Do we still use the same techniques to build tunnels? Do we even still need tunnels, or will other forms of infrastructure be more widely adopted for future civil engineers? Our resident expert, Tommy Olsen, tells us that to understand the future of tunnels, we must look to the past.

What is a tunnel?

In its most basic form - a tunnel is a structure that creates an underground space, for example, for access to natural resources, for storage, or as a passage under an obstruction (mountain/hill/city etc.), below the ground’s surface, or under a body of water. In contrast to bridges, they are not described by their structural form and function but by the construction method; mined or excavated, drill & blast, cut & cover, bored and immersed (check out the fact box below for more information).

Society always needed tunnels

Did you know that the first recorded tunnel in the world is the Euphrates tunnel in Mesopotamia (present-day Iraq)? Built around 2170BC, the tunnel – waterproofed by asphalt – connected the two sides of the city of Babylon under the river Euphrates. The remains of the subsea tunnel have never been found, but Greek historians have described it in ancient chronicles.

Early tunnels - to the wider benefit of society - were excavated for mining purposes, for example, for copper or other metals, but not always recorded.

When modern sewage systems were introduced in major European cities some 200 years ago, the sanitary condition and health of the population increased significantly. Viral and bacterial diseases didn't spread so easily, despite people being closer together in urban areas due to the industrial revolution and subsequent population growth. Moving more of the sewage underground was a way to protect people, allowing them to live closer together.

With the rate of urbanisation today, tunnels are still needed – and not only for sewage systems!
The Thames Tunnel under the River Thames in London was built in 1843. It was the first subsea tunnel and is still in operation today! The construction method used was the so-called Shield Method (by I.K Brunel), with a frame to support the excavation as the tunnel face moved onwards while the excavation progressed. The Shield Method was invented to improve workers' safety and to increase production rates. This method was used for most of the UK's long railway tunnels during the Victorian era and paved the way for the modern Tunnel Boring Machines (TBMs)

When people connect, ideas evolve; knowledge is shared; markets thrive; technologies develop; opportunities for people grow; and consequently, the economy improves. A growing urban economy needs more energy, more water, more food, and a better supply of products. Developed economies very often support investments in innovation and improvements to the environment when constructing major infrastructure projects. Road infrastructure, public mass transportation systems, water supply, sewage, power plants and powerlines etc. are needed in both developing and developed economies. And tunnels often provide the solution to how these 'needs' can be met, with respect to the planet and to future generations.

In order to reduce the demand for natural resources, existing infrastructures must serve their function for longer before new infrastructure is constructed.

We're going to focus more on transportation tunnels for the purposes of this article…

Transport tunnels to protect space on the surface

The first underground metros were constructed in major cities such as London, Paris, Budapest, and New York over 150 years ago and are still in use today. The use of underground tunnels is a particularly effective mode of transportation with expanding underground travel network systems around the world. They do not influence or block existing activities on the surface and are particularly attractive where there is a growing demand for space in cities due to the fast pace of urbanisation and land scarcity. Also, the traffic in the tunnel is not affected by congestion within the surface traffic systems.

And it's not just in cities that tunnels are considered for transportation purposes - rural areas have a growing demand to preserve the natural environment. With a high demand for transportation of people and goods (usually requiring longer roads and longer rail lines), sometimes passing difficult obstacles like mountains and bodies of water, it is often a requirement that infrastructure takes up less surface space, both in urban and in a rural setting. And let's not forget that it is often preferable for infrastructure to be 'invisible'…

Why choose a tunnel? Why not a bridge?

Societies invest in infrastructure for the common good. Infrastructure should not only support the economy in the region, but also protect social groups, the environment and the climate. Simultaneously!

For major infrastructure projects, there is often a balance to strike between the planned construction cost and the identified benefits (or drawbacks). Other solutions constructed at ground level, such as a bridge, could often serve the same function and are typically less costly. However, tunnels are often selected for the long-term benefit of freeing up space at the surface.

When planning new infrastructure, solutions where the most benefits are identified at the lowest possible cost, MUST be considered. However, the solution with the lowest cost is not always the solution with the most benefits. Therefore, logical and systematic decision making is required to identify a sustainable solution with an acceptable balance of pros and cons. The detailed solutions will differ depending on several factors - the location, for instance.

Subsea tunnels are constructed as fixed links across rivers, inlets, straits, fjords and harbours, where tunnels are sometimes an excellent alternative to a bridge. For example, in a harbour where a tunnel could have less impact on shipping traffic. Or where a tall bridge could require long approach spans or influence the flight paths to and from a nearby airport. In general, where there is an opportunity to protect the existing natural landscape, or maintain the identity of a place, a tunnel may be an excellent alternative to a bridge.

Why does location matter?

Tunnels are constructed to pass natural barriers like mountains or water or manmade obstacles in rural areas; sometimes, the ground condition itself drives the shape of the infrastructure – for instance, in excavated tunnels where favourable ground conditions will often define tunnel alignment. The most important factors to bear in mind when considering the benefit and construction of a tunnel are the people getting the benefit from the tunnel and the natural settings - geology, environment, climate, vegetation, access to resources, energy, and water.

Societies that would benefit from a tunnel would still depend on the natural settings they live in and settings they have nearby. One should ask, how good is access to natural resources? What are the local needs for energy? What are the existing physical connections and barriers between places, and how will this new infrastructure impact the people living there? Are these connections and barriers governed by mountains, extensive forests, urban settings, water, or deserts?

The natural setting shaped how people lived in the past - how they got their resources, connected with their neighbours, who they traded with, and how technology was introduced in new locations. It is seen in the historical development of a given society. Tunnel construction is highly influenced by the local natural settings. But we also know that there are many elements to tunnel design that can be used universally around the world when the local conditions are understood. This is the case, especially for unique and record-breaking tunnelling projects.

As society and technology changes, the demand for underground space and tunnels also changes. Mines used for exploring natural resources in the past can, under the right conditions, be used for storage, industrial complex or housing today. Some tunnels were built as shelters in times of war; the function of flexible and durable underground space, therefore, can change many times during the lifetime of a structure.

How locations and natural settings have made an impact on tunnel technology

Rock tunnel technologies developed in the mining industry have been scaled and are now widely used for mountain and urban tunnels for road and public mass transport. Technologies originally developed for buildings, bridges and marine construction are used for deep excavations for underground space and fixed links.

The ground conditions have a direct and significant influence on the construction cost. There is, thus, a preference to locate the tunnels in competent ground (like in rock or stiff clay) as it imposes less construction cost and risk. In cities with highly variable ground levels, it is often a challenge to locate the alignment of a metro to fit the surface; hence some metro stations need to be constructed very deep underground to accommodate the topography and maze of existing urban developments.

The location and access to natural resources, defined by geology, often determine the cost of construction materials and the preferred construction methods. In locations with competent rock, tunnels are constructed within the rock itself with limited material required to support the rock mass and, if possible, ideally utilising the rock's ability to self-support. In areas of highly fractured and weak rock, sand or gravel, the tunnel needs to be supported by concrete and steel. Access to construction material governs the construction cost and the preferred construction method for a given location.

It all starts with natural resources, energy and water

Coal has been an essential source of energy since the Industrial Age. It has provided fuel to power transport solutions like trains and ships, used in factories to cut wood for furniture, make fabrics for cloth, make iron and steel etc, and is still burned to produce electricity. In many places of the world, coal has been replaced with oil and gas over the last 50 years and will, in the coming years, be replaced with other sustainable technologies such as hydropower - where the power of moving water is used to generate electricity.

But what has this got to do with tunnels?

Well… hydropower operations are usually located in mountainous regions, with tunnels often used to direct water from a higher elevation to a power generator at a lower height, which then powers a turbine to generate electricity for transportation, industry and households. Tunnels can also be used to pump water back and forth in a hydroelectric pumped storage plant - a type of hydroelectric energy storage used by electric power systems for load balancing. The method stores energy in the form of the gravitational potential energy of water pumped from a lower elevation reservoir to a higher elevation. This technology is used in mountainous regions all around the world.

Hydroelectric energy storage systems can be connected to other types of renewable energy sources, namely wind power or solar. Energy generated from windfarms (on-shore and off-shore depending on location and space) needs to be transferred to the users or temporary storage when the energy production of the windfarm is high and energy demand is low. Infrastructure for power cables can be placed in tunnels, where space on the surface is too valuable to have powerlines in high voltage masts or where for example, the distance for suspended power lines across a river is too long.

Energy is used not only for transportation and heating, but energy (and lots of it) is needed in industries to produce construction materials for tunnels (concrete and steel). Between 50% to 75% of the carbon footprint from tunnel construction comes from concrete and steel, and most of that carbon footprint comes from heating in the process of producing the material.

Like tunnels for hydropower, similar types of tunnels are also constructed to move water from a source, say in a mountainous area, to the end user, say in a dry, dessert-like city or region.
Desert-like major cities exist around the Mediterranean Sea, in the Middle-East, the South-West of the US, Mexico, in some regions of India, China and in the Andes mountain South America.                                                                                        Without tunnels, people living in these areas would only have access to water by open canals with needs for protection!

What does the future look like?

Equipment and materials for tunnelling in the future will be more effective, quicker, more accurate, more reliable, bigger, less polluting, less noisy, create a smaller carbon footprint and run on electricity. Longer service life for non-replaceable components, more flexibility in function over the service life, more and more technology in installations with replacements during service life, and more data collection for performance and asset management. More pre-fabrication, less waste, more recycling, more automation, and more digital processes to connect the supply chain from source to user. Performance based designs, optimised design for construction and operation, minimising the use of natural resources (incl. digital processes in the supply chain). The future is about "sustainable processes and outcome". As seen in the past, for example, when reinforcement was introduced to immersed tunnels (in a period with shortage of steel), the most adaptable solutions, suited for demand will evolve as the preferred technology until another technology takes over.

So there definitely are benefits to tunnels

In summary, tunnels allow for improved living conditions in many different aspects!

They support society by maintaining sanitary conditions through sewage and flood protection stormwater systems.

Mining tunnels provide natural resources for a growing population in urban areas and economies, with an ever-increasing focus on sustainable use of mining. Minimising and re-using construction materials ("urban mining") reduces the need for mining the limited amount of natural resources.

Tunnels are used to harness hydropower and house underground utilities such as water and power supply.

Transportation systems connect people and support businesses in delivering goods and services.

Tunnels are valuable in societies with high economic output, in communities where people demand that infrastructure is not to be seen. Still, where users or neighbours experience the impact of infrastructure, it must be flawless and tailored to those communities' needs, and not the other way around.

Tunnels are needed where the space on the surface is occupied by others.

Tunnels often have a higher cost than other types of infrastructure serving a similar function, for a road at grade vs a road in a tunnel. Tunnels can provide more comprehensive benefits to society, which may outweigh a tunnel's additional cost compared to a less costly alternative. Benefits are often the creation of valuable space at the surface.

To identify sustainable infrastructure solutions, it is always recommended to consider several alternatives, sometimes including tunnels. Do not select the first alternative that serves a narrow function or has the lowest cost, but identify options that can bring long-term benefits for future generations. It shall be emphasised that sometimes a tunnel alternative is so costly that it is better for society not to construct anything, but to save the money for other infrastructure where the return of investment is higher.

Tunnels are valuable. And I am sure that as we have seen their use and technology evolve in the past, we will also see tunnels evolving in the future. Where tunnels of the past would fulfil a specific need (water or transport), tunnels of the future would still need to meet basic function, designed and constructed to provide maximum benefit to society with limited to no negative impact on the environment. It is just a matter of multiple success criteria.

Short description of tunnelling methods:

  • Mined or Excavated tunnels – Used in soft and medium hard rock, with low permeability, can be excavated with a so-called road-header operating at the tunnel front. Tunnel excavation can be supported by rock bolts, steel frames, or a concrete lining (shotcrete or cast in-situ). Excavators will be loading the spoil from the advancing excavation on conveyors and trucks, which will take the material to landfill or for other uses. This is the earliest tunnelling method used for mining coal, metals and minerals.

  • Drill and Blast tunnel – Used in medium and hard rock, with low permeability, can be constructed by drill and blast method. Dynamite (these days, more advanced explosive material is often used) is placed in pre-drilled holes at the tunnel front. After blasting, excavators load the rock onto trucks to be taken to landfill, or for re-use. The spoil from drill and blast tunnels is more likely to be re-used in road construction, as it is of higher quality than generally softer materials from mined and excavated rock. The drill and blast method is used as an alternative to mined and excavated tunnelling, where mechanical means of breaking are inefficient.

Dynamite (based on nitroglycerin) was developed and commercialised by Swedish chemist Alfred Nobel in the 1860s, after his studies in Paris. Until then drill & blast tunnels were constructed using different types of gunpowder.


  • Cut and cover (C&C) tunnels – can be used in all types of ground conditions. Excavations are carried out from the surface with retaining structure and groundwater lowering. A tunnel structure is constructed within the excavation, as a reinforced concrete structure. After the hardening of the concrete, the structure and excavation are backfilled. The C&C cover method is widely used for urban tunnels that are shallow, relatively short or have complex geometry which is not suitable for boring machines. If required, cut & cover tunnels can also be constructed at greater depth (>50m), and circular shafts allow deep excavations without large steel structures to support the retaining wall component. Approaches to immersed tunnels and large diameter bored tunnels are often cut & cover tunnels.

  • Bored tunnel – Can be used in all types of ground conditions and locations, where tunnels are long and have a continuous cross-section, where economies of scale justify the relatively high initial
    cost of a tunnel boring machine (TBM) and mobilisation. Where excavation from the surface by C&C tunnel method would disturb too much, excavations are carried out at the front of the TBM. Concrete lining segments are placed to support the excavation behind the head of the TBM. Some TBM tunnels are without lining support, in case the rock mass is good and generally self-supporting. This tunnelling method is widely used for urban, mountainous and for subsea tunnels for diameters up to 16m.

  • Immersed tunnels – Tunnel structure used for subsea crossings. Prefabricated tunnel elements are constructed at a separated construction facility and immersed in a pre-dredged trench. The tunnel is backfilled and protected by armour stone on top of the tunnel. With excavation from the surface, the construction method is very similar to the Cut & Cover tunnels, just carried out in the wet. The buoyancy of tunnel elements and the associated possibility of floating the elements allow for pre-fabrication and low-cost transportation.
The first immersed tunnels (IMT) were sewage tunnels in Boston (US) and Copenhagen (DK) around year 1900. The first IMT for transport (rail) was built in 1910 under the Detroit river tunnel in the US. Immersed tunnel technology was also used for the Paris metro line crossing the Seine at Îsle de la Cité, open 1900. The first road IMT is the 1928 Oakland-Alameda tunnel in the US, constructed with a steel shell and in-fill concrete. The first IMT in Europe and the first in the world constructed in reinforced concrete is the Mass tunnel in Rotterdam, the Netherlands, open in 1942.

Get in contact

Tommy Olsen

Tommy Olsen
Major Tunnels Market Director / Head of Sustainability Transportation International
Tunnels and Underground Infrastructure, Denmark

Tel: +45 28431017