How do retaining walls hold back soil?

1. A flat path has been cut into a hill. But soil cannot stand in a steep bank for long. Bit by bit it crumbles and slides, until it rests in a gentle slope across the path.
2. A retaining wall holds the soil back. The soil pushes sideways against it. Deep down, the soil is squeezed by everything above, so the push is strongest at the bottom of the wall.
3. A thin wall tips over. A thick, heavy wall with a wide base stays put. An L-shaped wall is cleverer: its long foot sits under the soil, so the soil's own weight holds the wall down.
4. Now it rains. Water soaks into the soil and collects behind the wall. The trapped water pushes as well, and the push on the wall can more than double. The wall begins to lean.
5. The fix is drainage. A layer of gravel behind the wall lets water sink down easily. A pipe at the bottom carries it away, and small weep holes let it drip out the front.
6. Steel bars inside the concrete stop the wall from cracking where it bends. Layers of strong mesh can tie the soil itself together. The hillside is safe, with a garden on top.

Tap each part of the wall and the ground behind it.

• Stem. The stem is the upright part you can see. The soil leans on it, pushing hardest near the bottom.
• Footing. The footing is the wall's foot, hidden underground. Most of it sits under the soil, so the soil's weight presses it down and stops the wall from tipping.
• Steel bars. Concrete is strong when squeezed but cracks when it is stretched. Steel bars inside take the stretch, so the wall can bend a tiny bit without breaking.
• Gravel layer. Behind the wall is a layer of gravel. Water runs down through the gaps between the stones instead of collecting in the soil.
• Drain pipe. A pipe with small holes lies at the bottom of the gravel. Water seeps in and flows away along the wall.
• Weep hole. Weep holes are small openings through the wall. After rain you can see water dripping out of them. That means the drain is working.
• Soil mesh. Sheets of strong plastic mesh are laid in the soil as it is filled in, layer by layer. They tie the soil together, so less of it pushes on the wall.

Choose a wall shape and a height. Make it rain, add drains and steel bars, and park a truck on top. Watch the push arrows and the safety meter.

Experiment: It rains hard and the wall has no drain. What happens to the push on the wall?

1. Choose the thick wall and look at the push arrows.
2. Make it rain with the drains switched off. Watch the arrows and the wall.
3. Switch the drains on and watch again.

What we found: With no drain, rainwater collects behind the wall and pushes as well as the soil, so the total push more than doubles and the wall slides or tips. With gravel, a pipe and weep holes the water runs out, and the wall only has to hold the soil.

Turn on JavaScript to try it yourself.

Design a wall for a bank 3 meters high. It must survive the soil, a storm and a truck parked on top. Spend as few coins as you can.

Tip out a bucket of dry sand and it never makes a tower. It spreads into a cone with gently sloping sides. Soil is the same. A steep bank of soil wants to slump into a gentle slope, so anything that holds it up gets pushed sideways.

The push is not the same all the way down. Soil near the bottom of the wall carries all the soil above it, so it is squeezed harder and pushes harder. Make a wall twice as high and the push becomes about four times as big.

Water makes it far worse. When rain soaks in and cannot get out, it fills the gaps in the soil and pushes on the wall like water behind a dam. That is why good walls have gravel behind them, a drain pipe at the bottom and weep holes through the front.

A wall can fail in three ways: it can tip over, slide forward or crack. A wide base stops tipping, weight and friction stop sliding, and steel bars stop cracking. The door comparison stops working in one way: a person gets tired of pushing, but soil never does, so a wall must hold for a hundred years.

Key words

Retaining wall

A wall built to hold back soil, so that the ground can be at two different levels.

Earth pressure

The sideways push of soil on a wall. It is strongest at the bottom.

Footing

The wide base of a wall, usually buried, that spreads its load and resists tipping.

Weep hole

A small opening through a wall that lets trapped water drain out.

Safety factor

How many times stronger a wall is than it needs to be.

Geogrid

A strong plastic mesh laid in layers of soil to tie it together.

Check your thinking

1. Where does the soil push hardest on a retaining wall?

   Show the answer

   Near the bottom. Soil deep down is squeezed by everything above it, so it pushes harder.

2. Why do retaining walls have weep holes and gravel behind them?

   Show the answer

   So water can escape instead of building up and pushing on the wall. Trapped water can more than double the push. Drains let it out.

3. A retaining wall 2 meters high is rebuilt 4 meters high in the same soil. About how much bigger is the soil's push?

   Show the answer

   Four times as big. The push depends on the height squared: 2 times the height gives 2 times 2, which is 4 times the push.

4. Why does an L-shaped wall need less concrete than a thick, heavy wall?

   Show the answer

   The soil standing on its foot helps to hold it down. The foot sits under the soil, so the soil's own weight presses the wall down.

5. Why are steel bars put inside a concrete wall?

   Show the answer

   Concrete cracks when it is stretched, and steel takes the stretch. The soil's push bends the wall. Steel bars hold the stretched side together.

6. A wall is leaning forward after weeks of rain. What is the most likely cause?

   Show the answer

   Its drains are blocked, so water is trapped behind it. Trapped water adds a big extra push. Blocked weep holes are a warning sign.