Topics How the world runs

How do subways get built under cities?

Subways get built in one of two basic ways. In cut-and-cover, crews dig a trench down from the street, build the tunnel box inside it, and then restore the surface. In bored tunneling, a huge machine grinds through the ground far below, lining the tunnel behind it as it goes. Many projects use both: trenches for stations, boring machines for the long stretches between them.

What makes it interesting is that the hard part is rarely the digging. It is everything around it: soil that can collapse or fill with water, old pipes and cables, building foundations overhead, and a city that cannot simply stop. Engineers survey the ground with test borings first, and even then the underground can surprise them. Where the ground is uncertain, they plan for that.

An episode would walk through the choices in order: picking a route, mapping what is already down there, choosing a digging method, holding up the earth, and hauling out the spoil. It would also cover how stations and ventilation fit in. bre is a podcast app whose hosts are AI, so they can get details wrong, and you can press Talk to ask about your own city.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Picking a route and reading the groundPlanners weigh where people travel against what lies beneath. Test borings reveal rock, clay, sand, and groundwater before anyone commits to a path.
  2. Cut-and-cover: opening the streetCrews dig a trench, support its walls, build the tunnel, and cover it again. It is simpler and cheaper in some cases, but it disrupts everything on the surface.
  3. Boring machines and how they workA tunnel boring machine has a rotating cutting face and builds a ring of lining behind itself as it advances. It is a moving factory in a tube.
  4. Holding up the ground and the buildingsLinings, bracing, and careful pressure control keep soil from caving in. Engineers also monitor nearby buildings for any movement.
  5. Water, utilities, and surprisesGroundwater, old pipes, forgotten foundations, and odd geology can all slow a project. Crews pump, reroute, or freeze ground to cope.
  6. Stations, ventilation, and finishingStations are often the largest and slowest part. Then come tracks, power, signals, air shafts, and safety systems before a single passenger rides.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Here's the picture I can't shake: you're walking down a busy street, and forty feet below your shoes, a machine the size of a building is eating its way through the dirt. Nobody upstairs notices. How is that even possible?

  2. CalAI host

    Okay, that's wild. And my first worry is the obvious one. What stops the street from just dropping into the hole? I'd be sweating the whole time.

  3. breAI host

    That's the right worry, and it's most of the job. The digging is almost the easy part. Keeping the ground up, and the buildings on it, is the real work.

  4. CalAI host

    Hold on, how does that actually work? Because soil isn't solid. Sand just pours. Clay squishes. Wet ground is its own mess. Do they dig differently depending on what's down there?

  5. breAI host

    They do, and that's why they drill test holes first. The ground decides the method. Solid rock, soft clay, and water-soaked sand are three very different problems.

  6. CalAI host

    So the plan starts with a geology lesson, not a shovel. I like that, honestly. It's like checking the ingredients before you cook.

  7. breAI host

    Pretty much. And there are two big approaches. One opens the street from the top. The other tunnels in secret from below. We'll start with the street.

Questions people also ask

What is the difference between cut-and-cover and bored tunneling?
Cut-and-cover digs an open trench from the surface, builds the tunnel inside, and covers it. Bored tunneling uses a machine to dig a tube underground without opening the street. Cut-and-cover is common for shallow lines and stations, while boring suits deeper, longer stretches.
How do crews keep buildings from sinking while tunneling?
They support the tunnel as they dig, control the pressure at the digging face, and monitor nearby buildings for movement. In some cases they reinforce the soil or foundations beforehand. These methods lower the risk, but they cannot remove it entirely.
Why are subways so expensive and slow to build?
Costs come from digging in crowded, unpredictable ground, relocating utilities, building large stations, and working around a living city. Why costs differ so much between countries is debated, with factors like regulation, project management, and design all discussed.
What happens to all the dirt that gets dug out?
Excavated material, called spoil, is hauled out by conveyor, truck, or rail. Where it goes depends on what it contains. Clean soil may be reused for fill or construction, while contaminated material has to be handled under disposal rules.

Related topics

More: all 300 topics, how the world runs, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.