Today’s Wonder of the Day was inspired by sophie. sophie Wonders, “What keeps tunnels from falling down?” Thanks for WONDERing with us, sophie!

London Bridge is falling down, falling down, falling down… You’ve heard the rhyme. You may have even sung the song or played the game. And it’s true! London Bridge has fallen more than once over the years. However, another London structure is much stronger. But somehow, there’s no song about it. What are we talking about? The London Underground!

Also called the Tube, the London Underground has a long history. It started with the Thames Tunnel in 1843. This was the first underwater tunnel--Londoners built and connected more tunnels. Today, the London Underground is a large system. Its tunnels are strong and safe. People even took shelter there during World War II when German dropped bombs on London!

The Tube isn’t alone. Around the world, thousands of tunnels provide safe transportation. The world’s longest road tunnel opened in 2000. It’s called Laerdal Tunnel and is found in Norway. At 15.2 miles long, it takes cars straight through a mountain range. In Turkey, the Marmaray Tunnel is 47 miles long. It connects Asia to Europe. It’s the longest underwater tunnel in the world! Another underwater tunnel, the Seikan Tunnel, runs 34 miles in Japan. It opened in 1988 and goes deeper than any other rail tunnel. Its lowest point is 800 feet below sea level.

Other tunnels are known for a number of reasons. The Tunnel of Love in Ukraine is formed by trees. The Bund Sightseeing Tunnel in China is an interesting sight. It has brightly colored lights. It also plays loud music all day long. In Colorado, USA, the Eisenhower Tunnel has an elevation of 11,013 feet. It’s the highest vehicle tunnel in the world.

We could go on and on about these cool tunnels. People travel through them every day without a second thought. For some, however, the thought of going through a tunnel causes fear. They worry that, like London Bridge, the tunnel will fall. It’s a scary thought.

Luckily, tunnels are highly unlikely to fall. Civil engineers work hard to build safe, strong tunnels. They know that tunnels have to stand up to forces of tension, torsion, compression, and shearing. Tunnels also need to carry the weight of people and vehicles traveling through them. That’s why tunnels are built with strong materials. Most tunnels use steel, iron, and concrete. The strength of these materials helps keep tunnels from falling.

Before materials are even ordered, though, engineers start planning to make sure the tunnel won’t fall. They do this by learning about the ground where they will dig and build the tunnel. Engineers start by studying the soil and rock types. This helps them decide how to dig the tunnel. They also learn about water patterns and any nearby fault lines. All of this information helps engineers dig tunnels that won’t fall.

Engineers decide on the shape of a tunnel based on everything they’ve learned. The likelihood of soil and rock moving determines a lot about how a tunnel will be built. For instance, tunnels built inside of stratified rock may have flat roofs. This is because, for that type of rock, a flat roof is less likely to fall. Tunnels shaped like horseshoes or with Gothic ceilings are also common. That’s because those shapes are more stable.

In larger tunnels, the engineers add supports to make the tunnel more stable. For instance, they use steel rock bolts to secure tunnels in rock. These drill into the rock to prevent collapse and protect the tunnel. Some types of rock need less support than others, so the supports vary by location.

Have you ever gone through a tunnel? Next time, think of all the work that goes into making it safe for you. Whether it’s above ground, underground, or underwater, the tunnel was made with your safety in mind. How can we make tunnels even stronger? Do you have any ideas?

Standards: NGSS.PS2.A, NGSS.PS2.B, ETS1. B, CCRA.L.1, CCRA.L.2, CCRA.R.1, CCRA.R.2, CCRA.R.10, CCRA.W.2, CCRA.W.3, CCRA.W.8, CCRA.W.9, CCRA.SL.1, CCRA.SL.2, CCRA.L.1, CCRA.L.2

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