What is tunnelling?
In simple terms, tunnelling is the physical process of constructing an underground passageway beneath the earth’s surface, tunnels can also be created underwater. To be precise, a tunnel is a more or less a horizontal underground passageway constructed via excavation processes. Whereas a vertical underground opening is referred to as a “shaft.”
If we take a look at history, it is clear that humans have indulged in the process of hollowing out earth and rocks for centuries now. Initially, tunnelling excavations were carried out for mining purposes to reach minerals such as coal, copper, tin. But they soon gained popularity with train companies and canal builders as efficient ways to avoid obstacles. The building of the very first road tunnels is still considered to be a mammoth accomplishment in the fields of construction and civil engineering.
Even in this modern age, the significance of both underground and underwater tunnels is unparalleled. The technological advancements in recent times have allowed us to build tunnels that are longer and deeper which can be used to cross beneath seas, such as the 50-km long English Channel Tunnel and tunnels that join continents, such as the Tunnel of Mammary, in Istanbul.
What are the different types of tunnels?
The tunnel construction process is costly and time-consuming. However, transport and communication via tunnel networks is extremely efficient and comfortable. Modern machinery has made the excavation and stabilisation processes much easier and safer, which is why tunnel networks have become a more popular choice for roads, railways, and even waterways.
For our discussion, it is imperative to note that there are different tunnel construction methods that are used according to the geology, type of overburden as well as seismic and noise restrictions, as mentioned below.
- Cut and Cover method:
This method is commonly used to construct shallow tunnels. Basically, this method involves the cutting of a trench in the soil, tunnel box elements are installed after which they are covered over to reinstate the surface by a support method that is capable of bearing the load. Early underground metro rail stations in London were built using this method. Some sections of road networks are also built this way when the depth of the required tunnel is relatively shallow (The Marselis Tunnel in Aarhus and the Nordhavnsvej tunnel in Copenhagen are the first Danish cut and cover tunnels).
- Bored or Mined Tunnelling:
In this method, the tunnels are built without excavating the ground surface. This method is enabled by the use of Tunnel Boring Machines (TBM’s). These allow tunnels to be excavated in a wide variety of ground conditions and beneath towns, cities, rivers and even tall buildings. They do this by maintaining the static pressure of the existing ground to virtually eliminate settlement, thereby avoiding damage to surface buildings or structures. TBM’s are also able to install a concrete ring for permanent ground support when the tunnel is in operation. Bored or Mined tunneling can be sub-divided as follows:
- Soft Ground Tunnelling:
This would typically be described as tunnelling through non-self supporting materials such as Sand, Marine Clay, or a mixture of both, with MPa levels of around 40 or less. The Netherlands is commonly associated with soft ground tunnelling. Examples of tunnels in soft ground can be metro’s and road tunnels, such as the Rotterdamsebaan Road Tunnel in The Hague and the Westerschelde Road Tunnel in Southern Holland.
- Hard rock Tunnelling:
As the name suggests, the geology will typically be through hard to very hard rock with MPa levels of greater than 100. Excavation would be done with specially adapted TBM’s which can cope with highly abrasive conditions in largely self-supporting geology. In such geology hard rock tunnels can often remain unlined and are used for water transfer or hydro power plants. They can also be used in in mountainous regions such as Switzerland for road tunnels, The Gotthard Road Tunnel in Switzerland runs from Göschenen in the canton of Uri at its northern portal, to Airolo in Ticino to the south, and is 16.9 kilometres long.
- Microtunnelling and Pipejacking:
This method is used for relatively short and correspondingly small diameter tunnels, mainly used by the utilities construction sector. This allows for cables and pipes to cross beneath roads, rivers railway lines and even airport taxiways and runways without disrupting any surface traffic. The principal of excavation is based on “State of the Art” full face tunnel boring machines (TBM’s).
What is the main purpose of tunneling?
As discussed throughout the article, tunnels have various uses that make life easier for us. Tunnels could be used between underground railway stations for carrying freights and passengers, also as water and sewage disposal channels. This is what makes tunnels so versatile and efficient.