How giant tunnel-boring machines dig through changing soil

HostThink about the last time you were in a subway or a tunnel under a river. You're deep underground with tons of dirt and water sitting right over your head, and it's easy to take for granted that the walls stay up. But the people building those tunnels have to deal with a big problem because the ground under our feet isn't all the same. One minute it's hard rock, and the next it's soft, wet sand that wants to rush in and fill the hole. How does a machine that's as long as two football fields handle those sudden shifts without getting stuck or causing the street above to sink?

GuestIt's a huge challenge because you're basically driving a giant, round factory through the earth. Most people think of these machines as just big drills, but a drill just makes a hole. These machines have to hold the earth up while they work. When you're in soft ground, like clay or wet sand, the machine has to act like a plug in a bottle. If it moves too fast or doesn't push back hard enough, the ground above can slump down, and you end up with a sinkhole in the middle of a city street. To stop that, the machine uses the very dirt it's digging as a shield. It chews up the soil and keeps it in a tight space right behind the cutting face. By keeping that space full of crushed-up dirt and pushing against it, the machine balances the weight of the ground outside. It's a constant game of measuring the push of the earth and matching it perfectly.

HostBut sand doesn't have any real strength. If you dig a hole in the beach, the sides just fall in. It seems like using loose dirt to hold up a whole city street would just lead to a mess.

GuestWell, it would be a mess if we just left the dirt as it is. That's where the chemistry comes in. As the big wheel at the front spins, the machine sprays things like foam, water, or a thick jelly into the dirt. It turns the raw ground into something that feels more like toothpaste or thick mud. This mud is much easier to control. It flows through the machine without wearing down the metal parts too fast, but it's also thick enough to hold back the water and the soil pressing in from the outside. The crew on the machine is constantly checking the thickness of that mud. If the ground changes from clay to sand, they have to change their recipe for the foam on the fly. If they get it wrong, the mud could become too thin, and that's when the pressure drops and things get dangerous.

HostSo the machine is basically swimming through a mud pie it made itself. But that sounds like it only works for soft stuff. What if you hit a wall of solid rock? A foam-covered mud pie isn't going to do much against a slab of granite.

GuestThat's where the design of the cutting head matters. The front of the machine is a massive spinning disc covered in different kinds of teeth. For soft ground, you use blades that look like giant cheese graters to scrape the dirt away. But for rock, you use heavy steel discs. These discs don't actually cut the rock; they press against it so hard that the rock cracks and flakes off in big chips. The real nightmare is when you hit a mix of both. We call it a mixed face. Imagine the top half of the machine is in soft, runny mud and the bottom half is in rock as hard as a sidewalk. The machine wants to tilt or bounce because it's meeting so much resistance at the bottom but none at the top. If the operators aren't careful, the whole machine can veer off course or even snap the expensive cutting tools.

HostThat sounds like you're just guessing what's in front of you. If the machine is as big as a building, how can you tell the ground has changed before you have already crashed into it?

GuestWe do a lot of homework before we even start. Engineers drill long, thin holes from the surface all along the path of the tunnel to take samples. They want to know exactly what they're going to hit. But those samples are just little snapshots. You might drill a hole every hundred feet, but a lot can happen in the space between those holes. While the machine is moving, we're also looking at the power it takes to spin the head and how fast it's moving forward. If the motor suddenly has to work twice as hard, we know we hit something tough. Some of the newest machines even use sound waves to look through the dirt a few yards ahead. It's like an ultrasound for the earth. It gives the crew a few minutes to change their plan before the teeth hit a new kind of ground.

HostSo it's a factory that builds as it goes, but what happens once the machine passes? If you're just leaving a hole behind you, the mud and foam can only hold it for so long.

GuestThe machine never leaves an open hole for more than a few minutes. As the front part moves forward, the back part of the machine is busy building the actual tunnel walls. It has a robotic arm that picks up huge, curved pieces of concrete. These pieces fit together like a 3-D puzzle to make a perfect ring. Once a ring is bolted into place, the machine actually uses it to move. It has big metal arms that push off the edge of the concrete ring to shove the whole machine forward another five feet. Then it stops, builds the next ring, and does it all over again. The machine is always inside a steel or concrete shell, so the soft earth never actually gets a chance to cave in.

HostEngineers are now trying to use sound waves to see through the dirt in front of the machine so they aren't digging blind.

HostThe city streets stay level because these giant metal worms are carefully balancing the weight of the world on a thin layer of mud.