3.2.1.3. Tunnels: Below ground or under water

Tunnels are underground or under water, enclosed passages constructed by digging through rock, soil and earth, for movement of vehicles.

Tunnels serve as pathways beneath the earth or water, allowing vehicles to traverse areas that might be challenging or impossible to cross otherwise. They are crucial for efficient transportation as they can minimize travel time and circumvent geographical obstacles. Essentially, tunnels create a direct connection between two points, enhancing the transport network.

Consider a tunnel like a straw that connects two parts of a drink—without the straw, you might have to tilt the cup or spill the drink to get to the other side. Tunnels streamline complex journeys into simpler routes.

With the massive excavation needed, the integrity of the passage is subject to the loading and pressure of the surrounding earth and/or water, construction of tunnels is highly specialised. There are various methods of tunnel construction on land, such as, Cut and cover method where a trench is cut and covered by some support; Bored method where boring machines are used; Clay kicking method common from the time of World War I, where clayey soil is literally kicked to create a tunnel; Shaft method is used for reaching great depths from the ground level and pre-cast shaft structures with concrete walls are lined and connected; Pipe jacking method where pipes are driven underground using hydraulic jacks, usually under existing infrastructures like roads, railways, etc.; and Box jacking method where boxes instead of pipes are driven in. For underwater tunnel construction usually calls for immersing tubes or boring tunnels through rock.

Building a tunnel is not simple. Different methods are employed based on terrain and requirements. For example:

• Cut and cover involves digging a trench, building the tunnel walls, and covering it back up.
• Bored method uses large machines to create the tunnel without disrupting too much above-ground activity.
• Clay kicking involves pushing clay out of the way for smaller tunnels.
• Pipe jacking and Box jacking are methods to install pipes or boxes below ground level, useful when avoiding disruption to current structures. Understanding these methods is crucial since construction engineers must choose the best technique depending on the environment.

Imagine building a treehouse. Depending on where you build it (ground, on another tree), you might dig a hole, use a ladder, or even lift a platform up to the trees. Each technique has to consider the surrounding things—just like building a tunnel does.

The world’s longest road tunnel is the Lærdalstunnelen, of 24.5 Km linking Aurland and Lærdal, and offering a ferry free connection built in 2000, followed by the Yamate tunnel in Tokyo, Japan measuring 18.2Km built in 2015 and the Zhongnanshan in Shaanxi, China measuring 18.04Km built in 2017. The Eisenhower Road tunnel in Colorado is one of the world’s highest tunnels at a height of 3,401m above sea level, on the Rocky Mountains. The world’s longest and deepest train tunnel is the Gotthard Base Tunnel in Switzerland, running 57 Km in length and at a depth of 2300m under the Swiss Alps, significantly reduced train travel time connecting Zurich to Milan. Previously this distinction was held by the Seikan railway tunnel, spanning the Tsugaru strait connecting Honshu and Hokkaido, running 53 Kms at 140m depth. Another phenomenal feat is the Channel tunnel across the English Channel between England and France, spanning 50Km, having two rail and one service tunnel for passenger and freight.

Some remarkable tunnels showcase what's possible through engineering. For example, the Gotthard Base Tunnel in Switzerland is particularly notable. It's not only the longest train tunnel but also one of the deepest, significantly reducing travel time between major cities by allowing high-speed rail access through the Alps. Each tunnel serves important functions, such as easing traffic bottlenecks or allowing for transportation under natural barriers like water bodies or mountains.

Think of these tunnels like superhighways for trains or cars that cut through mountains instead of going over them. Just as a bridge helps cars cross a river quickly, tunnels allow vehicles to ‘dive’ underground, speeding up journeys.

A piece of modern engineering is the SMART (Stormwater Management and Road Tunnel) Tunnel in Kuala Lumpur, Malaysia, designed keeping in mind the flash flood situation that the city faces and operates in three ways: as a road tunnel when there is no flood, the upper level operates for traffic while the lower channel allows water diversion when there are medium floods; and closes for traffic but allows floodwater to flow through via a holding pond, bypass tunnels and a storage reservoir during heavy floods.

The SMART Tunnel is a remarkable solution tailored to the challenges posed by heavy rains and flooding in Kuala Lumpur. Its versatility means it can serve multiple purposes: it functions as a road tunnel for everyday use, but also channels water during rainy periods. By managing stormwater, it prevents flooding in the urban area, showing how engineering solutions can adapt to environmental needs.

Envision a sponge that absorbs water: when it rains, the sponge expands accommodating excess water, but when it's dry, it serves as a regular, useful sponge. The SMART Tunnel mimics this function, adapting its role based on local conditions.

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