Traffic is just eating our brains and what is left of our patience. Amidst all the congestions, infrastructure, and traffic above the ground, there exists a critical piece of infrastructure that makes travel a busy man’s abode. Yeah, you read me. I am talking about the metro. Leave behind the impressive engines and trains, lets talk about the supporting infrastructure.
From subterranean roadways to the mass transit system, tunnels are playing a vital role. They are even used to conduct water and waste materials, transmit electric lines, and more. Isn’t the idea of tunnels – a world underground seem impressive? I already feel like an earthworm! Believe me. The complexity of the structure makes it even more impressive.
History of Tunnels
The civilizations of Mesopotamia, Egypt, Greece, and Rome constructed aqueducts both above and below the ground to cope up with irrigation needs and water supply. The famous aqueduct tunnel was built in 530 BC on the Greek island of Samos. They even transported people, animals, and goods. The Pouzzoles, built by the Romans in 37 AD measured 900m in length, 7.5m in width, and 9m in height! For more information on the history of tunnels visit planet-tp.com
A most common form of tunnel construction is the Cut and Cover method-a very disruptive one. London's Metropolitan line, the world's first underground railway route was constructed using this method. In this method, the site is excavated completely and the structure is built. Later, the backfilling and reinstating of the surface structures is done. It is mostly employed in rural projects. Although it offers safe work progress in unstable weak grounds, it requires large stretches of streets above to be closed for extended periods. It leads to congestion and sometimes requires the acquisition of the land above.
How are Tunnels constructed now?
It took engineers years to perfect the art of tunnel digging and construction. Nowadays, we create tunnels through mountains and beneath the ocean-thanks to all the advanced machinery we have got now. A stable tunnel rests on these three ideas:
We will cover these aspects in a new post:)
The tunnels are constructed in either one of the following ways:
- Cut and Cover Technique
- Tunnel Boring Machines (TBMs)
- Clay kicking
- Shaft method
- Pipe Jacking
- Underwater tunnels
The Cut and Cover method being explained, this post will cover another popular method that employs boring technique-Tunnel Construction using Tunnel Boring Machines.
These are boring machines that come in a lot of variations according to the soil type. One common machinery that all such machines have is the forward-facing rotating head which is aligned with a series of blades to cut through soils and rocks. These machines excavate and line tunnels with concrete in one continuous process and hence eliminates the need for supports. This method is highly efficient, improves safety, and is commonly employed in urban areas. Due to their large size, they are transported in pieces and then aligned at the site. With so many features, one cannot expect it to be cheaper and it’s true. It is capital intensive with high mobilization costs.
There are a lot of challenges in tunnel construction and the problems carry on during operational phases too. One of the most important issues is the provision of adequate ventilation.
Ventilation is a crucial part of both construction and operation of a tunnel facility. During construction or drilling, there might be explosions or blasts releasing fumes and dust. So, the ventilation system must go the extra mile in clearing the tunnel filled with poisonous gases and refilling fresh air. And this lifesaving task must be accomplished within the 30 minutes between the explosion and the murking process.
- Transit tunnels generally require high air quality during normal operation and smoke control in case of fire.
- Cable tunnels require cooling, smoke control, and a certain amount of air exchange.
- Mine tunnels and station tunnels also require adequate ventilation for physiological, cooling, and smoke control requirements.
The selection of a ventilation system depends on the following factors:
- Tunnel length and size
- Frequency of explosions and type of explosives
- Temperature and Humidity requirements
Why don't tunnels collapse?
Let’s brush up some fundamentals on soil mechanics.
- The weight of the soil particles above a layer exerts pressure on that layer. The layers beneath gets compressed due to the weight of the soil above them.
- So the entire soil system is in a state of compression. They behave similarly to a column in supporting the load above them. Yes! Through Compression.
- The strength of the soil is due to the friction between the particles i.e., particles pressing close against each other.
Now, imagine the effect of digging a hole in the soil. It is like losing a column in a building. The flow of compressive forces gets hindered and a zone of tensile stress gets built up on the ceiling of the hole or tunnel. The soil particles want to move apart and hence will lose strength ultimately. The ceiling will fall apart due to lack of support and the tunnel will collapse.
That’s why most of the tunnels are lined with steel or concrete even though it increases the cost of the project. And that’s how tunnels don’t collapse:)