Sand Bridge

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Author: Tricia Edgar

Future engineers, unite! Discover how to keep a sandcastle’s bridge from falling into the moat.


What’s the best way to design a bridge in the sand?


  • Playground sand
  • Clay
  • Spray bottle
  • Trowel
  • Cup
  • Deep container at least 2 feet by 2 feet
  • Stopwatch


  1. Get building! In this experiment, you’ll build a sandcastle with a moat, and then see how you can make a sturdy bridge across the moat.
  2. Create your sand castle, and then dig a hole around it. The hole should be at least as deep as your palm.
  3. Keep some sand to the side for your bridge.
  4. Create a hypothesis, your best guess about what will happen. What happens when you build something from dry sand? What happens when you build something from wet sand? What happens when you mix sand and clay?
  5. Now it’s time to build your bridges. Get your materials ready.
  6. Divide the sand into five piles.
  7. Spray one sand pile with the sprayer until it is damp all over.
  8. Pour water all over a second pile of sand.
  9. Keep one pile dry.
  10. Build a bridge with the dry pile, the damp pile, and the very wet pile. Does each bridge stay up?
  11. Wiggle the container. How long does each bridge stay up?
  12. Mix a small handful of clay into the remaining damp pile and the remaining dry pile.
  13. Make another bridge with each of them. Do the bridges stay up?
  14. Wiggle the container again. Does either bridge stay up?


Damp sand with a little bit of clay or silt in it will stick together better than drier, rougher sand.


If you like to build with clay, you’ll notice that it is very sticky. It even sticks to your hands! What is sand? It’s made up of tiny rocks. Think of the pebbles on the beach. The water washes them in and out, until tiny chunks break off. The chunks get smaller and smaller and become sand.

Clay is made up of even tinier particles. While sand grains look like small boulders under a microscope, clay particles are actually flat and can connect with each other to stick together. Clay is also a silicate, and water sticks to silicates. The force that makes things stick to other things is called adhesive force. If you have a lot of clay in your sand, or if you have really small sand particles called silt, then your sand bridge will stick together more easily.

The stickiness of a sand bridge also depends on how wet your sand is. A study published in the magazine Nature actually examined how to build the perfect sandcastle. The study found that while water helps stick bits of sand together, you only need 1% water to get the best stickiness. A lot of water makes the sand goopy and it falls apart. Slightly damp sand with clay in it will stick together very well, making a bridge that’s hard to beat.

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