Why Some Tunnels Are Dug From the TOP Down, Not Bored

Not every tunnel is drilled. Some are just a big trench that gets a lid put back on top.

So when do engineers dig a giant hole from above instead of boring one deep underground?

It comes down to depth. For shallow tunnels, roughly the first thirty to forty feet, it is cheaper to dig straight down. Crews open a long trench from the surface, brace the walls, build the tunnel box inside, then bury it and rebuild the street on top. That is cut and cover. But go deeper, under rivers, hills, or a packed city, and a surface trench becomes impossible. That is when a boring machine takes over, threading a tube far below without disturbing anything on top. Depth, cost, and what sits above decide which method wins.

Here's the strange part: the low-tech method is often the smarter one. When a tunnel is shallow enough, a plain open trench beats a multi-million-dollar boring machine on both time and cost.

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