Life's Little Mysteries

How do sandcastles stay together?

A large sandcastle on a beach with the sea in the background
Some sandcastles can be huge, but how do the grains of sand stick together? (Image credit: Laser143 via Getty Images)

Despite sand's crumbly nature, the world's tallest sandcastle to date — built in Blokhus, Denmark, in 2021 — stood a staggering 69 feet, 5 inches (21.16 meters) high, according to Guinness World Records

But how do sandcastles stay together? It requires the right mix of sand, water and air.

Sand is made of particles of minerals ranging in diameter from 0.003 to 0.187 inch (0.075 to 4.75 millimeters), according to Joseph Scalia, a geotechnical engineer at Colorado State University. Particles smaller than that are known as clay or silt, while larger particles are classified as gravel.

Although sand gives a sandcastle its structure, water holds it together.

"If you add a little water to dry sand, the water adheres to the surface of the grains, and forms small bridges of water where the grains make contact," Jim Best, a sedimentologist at the University of Illinois Urbana-Champaign, told Live Science.

Related: Why is it so hard to walk on sand?

The strength of these bridges is governed by the surface tension of water — the same property that compels water droplets to bead up into the most compact shapes possible. Molecules in liquids can stick to one another more tightly than they do to other things, making the surfaces of these liquids act like flexible membranes that can be surprisingly strong.

When there is enough space between the grains for air, water bridges can form in these air-filled pores to hold the sand together with suction. "The sand grains are essentially pulled together by the influence of surface tension," Best said.

If there's too little water, the sandcastle will crumble, because it lacks enough bridges to hold the grains together. However, with too much water, the pores — and thus the bridges — will vanish, and the sand will become a slurry, Best said.

Why stop at sandcastles? Here, a man sculpts sand into the Millennium Falcon next to Shrek at the Sand Sculptures Festival on July 15, 2004 in Zeebrugge, Belgium. The festival's theme was "Hollywood." (Image credit: Mark Renders/Getty Images)

The perfect sandcastle has a specific recipe: One part water for every eight parts dry sand, according to experiments that Matthew Bennett, a sedimentologist at Bournemouth University in England, and his colleagues performed in 2004. At the beach, sand with the right amount of moisture is usually near the high-tide line, often marked by a line of seaweed and flotsam, when the tide is low and the sand is still visibly wet, Bennett he noted in The Conversation.

Another way to help sandcastles survive is to use angular grains with sharp corners. "Imagine if the sand grains were all perfectly spherical — small, sand-sized balls," Best said. "They wouldn't be able to arrange themselves so close together and would be less stable than grains that are more angular."

The close fit between angular sand grains increases the impact of the water surface tension. In addition, more-angular grains interlock more, and the resulting resistance from friction helps keep sandcastles from collapsing, Best said.

This means the best sand for sandcastles is not usually found on coastal beaches, Scalia wrote in The Conversation. More-angular grains may be found in river sand that washed down from mountains, as they have not yet had their corners eroded by wind and water.

In addition, "professional sandcastle builders will add a small fraction of clay to the sand so the sandcastles will last for a long time," Xiong Zhang, a geotechnical engineer at the Missouri University of Science and Technology, told Live Science. 

With clay particles that are smaller than sand grains, the smaller ones can fit within the pores between the larger ones. This leads to more water bridges for sturdier construction, Scalia explained.

This is also why compressing wet sand together makes sandcastles sturdier, Scalia added. Compaction also helps angular grains interlock to increase the strength of the sand as a whole, he noted.

Eventually, sandcastles will dry out, but salt from seawater can help keep them standing, Best said. As seawater evaporates, the salt can form crystals that can stabilize the structures. However, they will be fragile. These sands will crack easily," Best said.

Charles Q. Choi
Live Science Contributor
Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.
  • SandcastleAndy
    Interesting article - and based on the science previously known - intersticial bridging and the like but there are a few faults with some of the calculations being presented - (well known if you actually work on the beach - which I do.
    The experiments quoted by scientists make assumptions based on knowledge discovered in a lab experiment under controlled conditions
    The Beach is not a controlled space and conditions vary a lot!
    it depends on whether you are building a castle that weighs over 50 lbs
    controlled experiments using a wet granular system - what sandcastles are made of - always leaves out some fundamentals
    It is a DRAINING wet granular system - there is little research on this - in fact, none
    This pile of sand is also affected by
    1. external air temperatures drying the sand out
    2 the position and flow of the tide - sucking water away from your high tide line beach position or pushing back toward it.
    3. whether you are containing the sand and water mix in a "form" or Bucket style form
    4. What type of sand you have - the published assumption - saying basically all sand is the same in the way it will act when water is added is absolute nonsense and misleading.
    5. The perfect mix ratio is 8:1 - where? There is no ratio on a beach - it has to be molten and contained to get consistent, repeatable results, as it is draining and you are against one thing - time
    6. The only way to form consistent bonding is to make the sand anoxic in other words - use a form and mix the sand and water into sloppy wet sand and add sand to water once the base is solid.
    7. Vibration on the container will give you the strongest bond possible unless there is a fractured surface to the crystals in which case artificial compaction would be helpful - but if the sand is right and contains as correctly stated, a great mix of fine granules, silt, and a little clay(natural not added) extra compaction is not needed
    8. the article says that the best sand for sandcastles would be found in rivers and not on the beach as it has not eroded yet and is still sharp - there are beaches where the sand shuffles onto the beach - retaining its "sharp" qualities and other beach models the sand is rolled by very different water and tide models and corners are eroded badly hence their use for sandcastle building is diminished or not even possible - it depends on the sands journey to form the beach and the wave action on geological composition once it has formed the beach - not on how early - you find it on its long journey! - otherwise, there would be sandcastles in front of mountains! - right? (on my bucket list)
    There is a common misconception that anyone can build a big sandcastle anywhere once you have squeezed some damp sand in their hand. This works to a point but for what we build- it is nowhere near accurate.

    To finish - Unfortunately - most science articles on this subject have just regurgitated other articles and picked out what they think makes sense - they have never built a big sandcastle.
    it is a shame that no-one wants to collaborate on a project where different beaches produce completely different results for an industry/business/location that at least 10 million people a year attend in the USA alone - THE BEACH
    MORE INFO - https://www.youtube.com/@AndyHancock - real consistant science!
    https://www.facebook.com/sandcastlelessons/photos
    I would have included photos but site doesn't allow uploads and image linking does not work
    enjoy the beach
    and remember 3 basics - Inly use wet sloppy sand - shape it like a pyramid - and work from the top down
    Reply