How can underground pipes be repaired without digging up a street?

1. Deep under this street runs a pipe that carries used water away from the houses. It is very old. It has cracks, and a tree root has pushed inside. Water is leaking into the ground.
2. The old way to fix it is to dig a long trench, lift out the broken pipe and lay a new one. That closes the street for days, and it is noisy, muddy and expensive.
3. There is a smarter way, and it starts at a manhole. A small robot with a camera and a light drives through the pipe. On a screen, the crew sees every crack. Then a water jet washes the pipe clean.
4. Next comes the liner: a long, soft tube of fabric soaked in a special liquid glue called resin. A winch pulls it through the old pipe, from one manhole to the next. It lies flat and floppy.
5. Air is pumped in, and the liner swells like a long balloon until it presses tightly against the old pipe. Then a train of bright lamps rolls through. The light makes the resin set hard, like glue drying.
6. The air is let out, but the liner stays round, because now it is hard. A robot cuts a hole where the house's pipe joins. Water flows again with no leaks, and the street never closed.

Step through the repair and watch what happens inside the pipe each time.

• 1. Look inside. A camera robot drives through the pipe on little wheels. The crew watches its pictures on a screen and notes where the cracks, roots and side pipes are.
• 2. Wash it clean. A nozzle sprays powerful jets of water. They cut the tree root away and wash the grit out, so the liner can lie smoothly against the walls.
• 3. Pull the liner in. The liner is a soft fabric tube soaked in liquid resin. A winch at the far manhole pulls it through, like threading a lace through a tunnel.
• 4. Blow it up. The ends are closed and air is pumped in. The liner swells until it presses against the old pipe everywhere, covering every crack.
• 5. Make it hard. A row of very bright lamps is pulled slowly through the liner. Their light makes the resin set, turning the soft fabric into a hard shell. Hot water or steam can do the same job.
• 6. Cut the side pipe open. The liner has covered the place where the house's pipe joins. A robot with a spinning cutter drives to the right spot and cuts a neat round hole.
• 7. Let the water run. The air is out, and water runs through the smooth new pipe. It has no joints, so there is nowhere for water to leak out or roots to get in.

Pull the liner through, pump air in and switch on the curing light. Then let the air out. What happens if you let the air out too soon?

Experiment: The liner is in place and full of air, but it has not been hardened yet. What happens if you let the air out?

1. Pull the liner all the way through and pump air in.
2. Let the air out without using the light. Watch the liner.
3. Pump the air in again, switch on the light, wait for it to finish, then let the air out.

What we found: Before the light, the liner is only wet fabric, so it flops as soon as the air stops holding it up. After the light, the resin has set hard and the liner keeps its round shape by itself. The air is only a helper: it holds the shape until the resin can.

Turn on JavaScript to try it yourself.

You are in charge. Tap the six jobs in the right order to mend the pipe. Each job only works when the pipe is ready for it.

Under every town lies a hidden web of pipes. Some bring clean water, and others, called sewers, carry used water away. Many of them are older than your grandparents. Over the years the ground shifts, heavy trucks rumble overhead and tree roots search for water. Pipes crack.

Digging a pipe up is a huge job. The street has to close, the hole is deep, and other pipes and cables are in the way. So engineers found a way to mend a pipe from the inside, working through the manholes that are already there.

First a camera robot looks inside, and a water jet washes the pipe. Then the crew pulls in a liner, a soft fabric tube soaked in liquid resin. Air blows it up until it presses on the old pipe, and bright lamps or hot steam make the resin set hard. It is a little like a plaster cast: soft at first, then strong once it has set.

When the air is let out, a smooth new pipe is left inside the old one, with no joints for roots to get through. A robot cuts open the places where house pipes join. A plaster cast comes off when the arm has healed, though. The liner never comes out: it is the new pipe.

Key words

Sewer

An underground pipe that carries used water away from buildings.

Manhole

A covered shaft in the street that lets workers and tools reach the pipes below.

Liner

A soft fabric tube, soaked in resin, that becomes the new pipe.

Resin

A liquid glue that sets into a hard plastic.

Curing

Making the resin set hard, using light or heat.

Trenchless

Done without digging a long trench along the pipe.

Check your thinking

1. How do workers reach the pipe without digging up the street?

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   Through the manholes that are already there. Manholes give access to both ends of the pipe, so everything can go in from there.

2. Why is air pumped into the liner?

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   To press it against the old pipe and hold its shape until it is hard. The soft liner cannot hold itself up. Air presses it into place until the resin sets.

3. What would happen if the air were let out before the light was used?

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   The soft liner would flop down inside the pipe. Until the resin sets, the liner is only wet fabric. It needs the air to hold it up.

4. Why must the pipe be washed before the liner goes in?

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   So the liner can lie smoothly against the walls. Roots and grit left inside would make lumps and wrinkles in the new pipe.

5. A liner 30 meters long is cured by lamps moving at 0.6 meters a minute. How long does the cure take?

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   50 minutes. Time is distance divided by speed: 30 divided by 0.6 is 50 minutes.

6. After lining, why does a robot cut a hole in the new pipe?

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   To reopen the place where a house's pipe joins. The liner covers every side opening. Each one has to be cut open again from the inside.