Underground Engineering: How Subway Tunnels Are Built Beneath Busy Cities

-Karthik Gurumurthy

Ever wonder how they build those subway tunnels under busy cities? It’s actually pretty fascinating underground engineering!

First, they have to map everything out super precisely – all the underground utilities, rock conditions, foundations, and existing tunnels. Then they have two main approaches depending on what’s above.

If they’re building under an open area without many buildings, they can use what’s called the “cut-and-cover” method. Basically, they dig a massive trench right from the surface, block by block. They mark out the exact dimensions on the street, insert heavy vertical beams on each side, place steel beams across to span them, and then lay thick wooden beams or steel plates on top. This creates a temporary road platform while they dig below.

During this process, they have to divert all utilities (pipes, ducts, etc.) or support them as they dig. Any water seeping in gets channeled into sewers. They build the tunnel floor first, then the sides with supporting rods, and run electrical cables through ducts in the walls. Then they pour concrete through the formwork, construct the ceiling, install tracks and lighting, and waterproof the whole thing. Finally, they fill the trench back up to street level with compacted gravel and soil.

But what if they’re going deeper or under existing buildings? That’s where it gets even more interesting! They dig vertical shafts at regular intervals along the planned route and line them with steel to prevent collapse. These shafts serve double duty – they’re how they remove all the “muck” (dirt and rock) and pump in fresh air.

Deep underground, workers use high-powered drills and boring machines to cut through the earth. When they hit hard clay or solid rock, they drill small holes for explosives. All the debris gets carted away, and they cover the tunnel sides with a steel tube called a shield. They lower shield segments down the shaft piece by piece and use jacks to push them through the earth. The front segment always sticks out a bit to catch any falling debris.

For really wet ground, they have to get creative with air pressure. They build two parallel concrete walls within the tunnel and maintain high pressure between one wall and the tunnel face to keep water from rushing in. Workers have to stay in a compression chamber between the walls to adjust when moving between normal and high-pressure areas, passing through special doors with air locks.

Multiple crews can work in different shafts simultaneously since they’ve made precise measurements to ensure the tunnels will meet perfectly. After everything’s done, those original construction shafts become stairways, elevator shafts, and ventilation ducts.

Pretty incredible feat of engineering happening right under our feet, isn’t it?