TLDR: Tunnels are underground passages that are dug through soil or rock. They can be used for roads, trains, canals, or even to supply water or utilities. Some tunnels are secret and used for military purposes or smuggling, while others are built specifically for wildlife to cross safely. Tunnels can be connected together to form tunnel networks.
A tunnel is basically a long, narrow underground passage. It can be used for different purposes, like for cars and trucks to drive through, for trains to travel, or for boats to pass through. Tunnels are usually enclosed, except for the entrance and exit at each end.
Tunnels can be made for different types of traffic, like foot traffic or vehicles. They can also be used for things like sewers or aqueducts to supply water. Utility tunnels are used to route things like steam, water, or power cables. They can also connect buildings to make it easier for people and equipment to move around.
Some tunnels are built for secret military purposes or for smuggling things like weapons or contraband. There are also special tunnels called wildlife crossings that are built to help animals safely cross human-made barriers. And sometimes, tunnels are connected together to form tunnel networks.
There are different types of tunnels, like cut-and-cover tunnels that are built by excavating a trench and then covering it. There are also bored tunnels that are constructed in place without removing the ground above. Another type is an immersed tunnel, which is a tube that is sunk into a body of water.
When building a tunnel, engineers have to consider things like the ground conditions, the water conditions, and the logistics of supporting the excavation. They may use different methods like tunnel boring machines or sprayed concrete techniques. They also have to plan for things like ventilation and emergency exits.
Sometimes, tunnels need to be enlarged or replaced if the traffic using them outgrows them. And in some cases, temporary railways are installed during construction to remove excavated soil.
Overall, tunnels are important for transportation and infrastructure, allowing us to travel underground and connect different places.