Cut and Cover Technique
Here's how the cut and cover method works:
Cut and cover is a fast, cheap, and easy way to build a shallow tunnel in soft soil, but it's not always practical. Tunnel diggers must gouge a massive hole in the earth to make room for the tunnel. This is a real problem in cities, where skyscrapers, roads, and railroad tracks crisscross the surface of the ground. Since you can't easily move buildings and roads out of the way, cut and cover isn't always the best solution.