A tunnel is an underground passage beneath the earth surface. They are built for pedestrians, vehicular traffic, Metro Rail and long distance railways to overcome congestion or shorten long and circuitous overland routes. Tunnels are also built for supply of water or transport of sewerage for treatment. In some engineering structures, tunnels may be needed for transportation of steam, chilled water, and placing of electrical or telecommunication cables. In special cases, secret tunnels could be built for military purposes, wildlife crossings etc.

Tunnel construction is expensive, and often time consuming. It requires sophisticated technology & skilled manpower. In this article, we will discuss some of the key tunneling techniques.

1. Cut and Cover Method
As the name implies, this technique includes the excavation and back filling of the soil after tunnel is built inside it. The two types of cut and cover method are discussed below.

a) Bottom-up Method
In this type, the ground surface is excavated and a base slab is built below the ground. Over this base slab, the tunnel is constructed. Finally, the soil is filled back over the tunnel and the surface is restored.

b) Top-down Method
This is a reverse of the previous method. The construction starts from the top and ends at bottom. Initially, the side support walls and beams are constructed in the ground surface and tunnel roof is constructed. Then the tunnel roof is filled back except the inlet and outlet opening of the tunnels. Then, excavation is done under this permanent tunnel roof and finally a base slab is constructed.

The major disadvantage of cut and cover method is that it creates disruption and congestion at the surface level during construction. This technique is suitable for shallow depth tunnels. It is one of the commonly used methods for tunnel construction due to its ease and cost effectiveness.

Take a look at this video to understand this method better.
https://www.youtube.com/watch?v=0O5N6jCLd_U

2. Bored Tunnel Construction
This method is a modern one which uses Tunnel Boring Machines (TBMs). This machine consists of blades specially designed to cut hard rock surfaces. TBM is a cylindrical machine which rotates and drives into the soil or rock surface automatically with hydraulic jacks. It excavates the surface and simultaneously the excavated material is collected in a dumping train through the conveyor.

This method is much faster than the previous technique, hence being widely used for applications like Metro Rail. The advantage of this method is that it doesn’t disturb the surface much during construction. It can be used in any difficult areas such as below water table, under existing structures like roadways, railways etc.

In applications where bridge construction is not possible, this method can be used. Recently, India’s first underwater tunnel was constructed under Hooghly river to provide metro connectivity between Howrah and Kolkata. It was constructed with the help of Tunnel Boring Machine (TBM).

Take a look at this video to understand this method better.
https://www.youtube.com/watch?v=MuYdnzcQXhk
https://www.youtube.com/watch?v=iVh2hl__Zzo

3. Shaft Construction Method

Initially, a well-like structure called shaft is constructed at the required depth. Entering into this shaft, the tunnels are excavated in the desired direction. Shafts are constructed at the inlet and outlet of tunnels and intermediate shafts are also built in case of longer tunnels. After the construction process, these shafts are used for ventilation purpose or as emergency exits. This method is suitable for construction of tunnels at greater depth.

Take a look at this video to understand this method better.
https://www.youtube.com/watch?v=VmX8gQd1oYo
https://www.youtube.com/watch?v=1c0cDZoQiVU

4. Box Jacking Method

This method is similar to bored tunnel construction. But instead of TBM, simple boxes are used. These boxes are precast units which has cutting edges at its front. These boxes are driven into the soil with the help of hydraulic jack, meanwhile the excavated material is removed. This technique is best suitable under railway tracks or roadways for construction of culvert, subway, etc. without disturbing the surface level. This method is fairly popular in India.

 Take a look at this video to understand this method better.
https://www.youtube.com/watch?v=ONNpZ3_tEj8

5. Drilling and Blasting Technique

In this method, holes are drilled into the rock and filled with explosives. Then these explosives are detonated at a specific time. The desired shape of the tunnel depends upon the number of holes, its position and depth. This process of drilling and blasting is repeated until the desired length of the tunnel is attained. This technique is best suitable for mining, quarrying, etc.

Take a look at this video to understand this method better.
https://www.youtube.com/watch?v=G2Hs51QDszc
https://www.youtube.com/watch?v=6XWHsVnvztk

References:
https://en.wikipedia.org/wiki/Tunnel
https://miningandblasting.wordpress.com/2011/07/27/tunnel-construction-methods-and-their-comparison/
https://www.nbmcw.com/metro-tunneling/35028-tunneling-technologies-driven-by-evolving-construction-methods.html
http://www.kmrc.in/press_brief_23jun17.pdf