Eli5: How do they make road tunnels safely?

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How do they make tunnels, how do they keep themselves safe and how do they make sure that its safe for cars to drive through with rocks loosening and falling down in mind for example

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There are many techniques, depending on the geology, but one of the most popular is the Tunnel Boring Machine. This is a giant snake-like machine that grinds a round hole in the rock, moving the ground up rock out the end of the machine. It has shields that press against the rock, to keep it from falling in on the machine. After the grinding part of the machine is a casing machine that pumps a ring of concrete against all the sides of the hole, casting in place a concrete pipe that permanently supports the rock.

>How do they make tunnels, how do they keep themselves safe and how do they make sure that its safe for cars to drive through with rocks loosening and falling down in mind for example

A road tunnel is not a cave. There is no rock that can loosen and fall from the ceiling. A road tunnel is a *building* inside of an artificial cave. The cave is dug with [heavy-duty boring equipment](https://www.youtube.com/watch?v=YqG9_S2XSws). Modern equipment usually also builds the tunnel right after drilling each bit of the cave so that exposed rock and soil only ever exist at the very tip of the drill, never within the tunnel.

It depends on the geology. During construction it is possible to freeze the ground using liquid nitrogen so you can safely drill through this frozen mud. In other places it is possible to use a movable shield to protect men and equipment at the head of the tunnel. As you dig further you can move the shield forward. Openings in the rear of the shield gives access to install more permanent securing. It have also become quite common to use remote controlled equipment at the head of the tunnel. Operators can stand back where the tunnel is safe and control the wheel loaders, trucks and drillers using remote controls.

For permanently secure the tunnel a common technique is to drill long holes out from the tunnel and install bolts in them. When the bolts are put in tension the rocks are put in compression where they are strong. The rocks lock together and can not fall out as easily. It is also possible to inject concrete into the rocks which hardens to cement and acts as a glue to glue the rocks together preventing them from falling. In addition to these techniques it is also common to line the tunnel with a metal mesh to catch any stones that fall down, and then possibly reinforce this mesh with concrete to create a strong smooth lining of the tunnel where vehicles are safe to travel in.

In many places several of these techniques are combined if not all. For example a tunnel boring machine is remotely operated with a control room near the rear of the machine and incorporates a shield to protect the machinery and men. In the rear of the machine pre-fabricated reinforced concrete elements in installed to line the tunnel. These have holes in them to allow bolts to be drilled through and inserted into the rock as well. Concrete can also be injected through holes in these elements to further secure the rocks behind the concrete lining.