Tunnel construction is complex and challenging. Tunneling has evolved over the centuries. The first tunnels used for transporting goods were created in Persia as early as 1000 BCE, where tunnels were dug to transport water from mountain aquifers to farmlands below. In ancient Greece and Rome, tunnels were used to transport water, goods, and sometimes troops. Over time, construction techniques evolved, and tunnels got bigger, longer, and more advanced.
A key part of our project in Northern Colombia is an 80-mile tunnel. Different circumstances require different techniques to create a tunnel. Below are seven modern tunneling techniques, briefly describing their use, advantages, and disadvantages. Many factors influence the choice of technique used, like the area’s geology, the tunnel’s purpose, and budget constraints. We will use a combination of several techniques mentioned below.
Six different tunneling techniques
1. Cut-and-Cover Tunneling: This is one of the simplest tunnel construction methods. A trench is excavated and then covered over. It is mainly used for shallow tunnels, mostly in urban areas. The method is relatively simple and cost-effective. The biggest disadvantage is that it is disruptive to surface activities.
The Paris Métro in France has largely been constructed using this technique.
2. Bored Tunneling: A tunnel boring machine (TBM) excavates tunnels without disturbing the ground above. The technique is used for deeper tunnels and can handle various ground conditions. TBMs are expensive and require significant logistics support.
The most famous TBM company is Elon Musk’s Boring Company, which creates a tunnel in Las Vegas using a new and innovative tunnel-boring technique. The Boring Company method combines automation and boring, installing tunnel sections simultaneously. Another famous example is the 25-mile-long Gotthard Tunnel in Switzerland.
3. Drill-and-Blast Method: This method is used when tunnels go through hard rock. Explosives are used to break the rock, which is then removed by machinery. This method is commonly used in hard rock conditions and mountainous terrains. With hard rock, it can be more efficient than using TBMs. The downside of the method is that it can cause vibrations that can affect the surroundings.
The Laerdal Tunnel in Norway has been constructed using this technique.
4. Immersed Tube Tunneling Method: Prefabricated tunnel sections are constructed and then floated to the tunnel location, where they are sunk into place and joined. This method is used for underwater crossings such as rivers, estuaries, and sometimes larger bodies of water. It can be more cost-effective than boring under a body of water but requires significant work underwater and can have environmental impacts.
A great example of the Immersed Tube Tunneling method is the Fehmarn Belt Tunnel, the new Mega Tunnel being built in Europe, connecting Germany and Denmark.
5. New Austrian Method (NATM), Sequential Excavation Method (SEM), or Sprayed Concrete Lining Method (SCL/Shotcrete): This method evolves the excavation of small sections of the tunnel, supported immediately with sprayed concrete walls or other temporary materials. The technique is very suitable for situations with challenging ground conditions or when there are many curves in the tunnel. The biggest advantage of this method is that it is very adaptable to varying ground conditions. The downside is the speed, or lack thereof, as this method can be slower than other methods.
The 15-mile Guadarrama Tunnel in Spain was constructed using this method.
6. Shield Tunnelling Method: A shield is pushed forward in the ground or underwater while the tunnel is being constructed. This method is used for soft ground and underwater conditions. While it provides good protection against collapses, it is more labor-intensive than modern TBM methods. Large parts of London’s Underground were constructed using this method.