on July 11th, 2022

A bucket and pail are essentials if you’re hitting the beach on 6 August for International Sandcastle Day.

It’s straightforward, the event is celebrated on beaches around the globe where creators, builders and artists convene to make the biggest, prettiest, most interesting sand sculptures out there.

If you’ve seen mega sand sculptures before you’d know they can get really big – and there’s simple science behind building massive castles fit for a king.

International Sandcastle Day is celebrated annually on the first Saturday of August, and although there’s no clear history on why the day started – there is a correlation that the day came to be due to competitive sandcastle competitions since the early 1980s.

Since then it has become institutionalized internationally with many nations hosting Sandcastle events at local beaches.

What holds a sandcastle together?

The mix of fine sand granules and water creates capillary bridges between sand grains are the reason for the stiffness of perfectly wetted sand.

Hard sand in a dry state cannot support its own weight, but thanks to these bridges sand can stand up during the sandcastle building process.

More specifically it’s the curvature of the liquid that interfaces to lead to capillary pressure. This causes a force of attraction between the grains of sand.

It also means the finer the beach sand is, the more “bridges” are built leading to a structure that can withstand more weight.

But, the “bridges” need to be optimized, and that is to say that in building the perfect sandcastle there cannot be too much liquid or ocean water.

It also means the finer the beach sand is, the more “bridges” are built leading to a structure that can withstand more weight. But, the “bridges” need to be optimized, and that is to say that in building the perfect sandcastle there cannot be too much liquid or ocean water.

However, sand that hasn’t been too rounded is also essential. Round grains “roll” off each other easier, while grains with jagged edges interlock much better, leading to a sturdy structure that doesn’t become as susceptible to the landslide phenomenon when sand structures implode.

It would seem that in engineering terms building sandcastles would be well studied, but a group of researchers at the University of Amsterdam discovered that it’s not the case. They took it upon themselves to pen a seminal work on a sandcastle building titled How to construct the perfect sandcastle.

There’s math that shows us how to build tall castles

The only estimate in literature, in the paper, argues that the stability of sandcastles is related to the capillary rise in the granular medium – and the maximum height of a sandcastle following this rule is a mere 20cm.

The other argument is that the stability of the structure depends on the radius of the base of the castle.

The elastic shear modulus vs. volume fraction of water (circles: experimental measurements, dashed line: theoretical prediction for the optimal strength from Eq. 3). The measurements were done on a commercial rheometer using a vane-in cup geometry where the cup was covered with sandpaper and the sand compacted as for the sandcastle experiments. Inset: picture of the grains.
Experimental data points and theoretical prediction of the maximum height of sandcastle as a function of its radius. The solid line is the theory without any adjustable parameters using G = 0.054 a−1/3E2/3γ1/3, where a = 100 µm, E = 30 GPa, γ = 70 mN/m. The density of the sand is ρ = 2.6 g/cm3. The small but systematic discrepancy between the theory and the experiments is likely to be due to perturbations that arise when we remove the PVC pipes, leading to a somewhat smaller maximum height than the theoretical one.


The truth is, from the mega sand structures that spring up annually at events like International Sandcastle Day is that these structures aren’t that limited, and as a result, the writers found that columns of sand are an effective way to show elasticity and the buckling point of upright columns of sand.

They found that the model for the strength of wet granular matter assumes when you add a volume of liquid to grains, the capillary attractive force and elastic response from the Hertz contact between two spheres will be balanced.

As two beads are always separated by at least the surface roughness, below a critical liquid volume fraction of about 0.2%, the bridges between the beads cannot form.

At higher volume fractions, the bridge force is dominated by the curvature of the meniscus and at even higher volume fractions the bridges start to merge into larger pockets of fluid.

Using typical values for sand found at beaches, a cylinder with a radius of 20cm could be as tall as about 2.5m

The equation the researchers use to show the buckling point of sand cylinders could be of particular interest for civil engineering and even soil mechanics since it offers a better understanding of partially saturated granular materials.

It also helps to explain the maximum height and a recipe to construct a mega sandcastle.

Source: Guinness World Records/ YouTube

Some Sandcastle records

  • The tallest Sandcastle ever built

At the 2019 Sand Skulptern Festival in Binz, Rügen was the construction place of the world’s tallest sandcastle.

A team of 12 sculptors and eight technicians accomplished the feat in just under four weeks. Their castle had no internal structure and there were no additives in the water to bind the 11,000 tonnes of sand that went into creating the goliath structure.

The circular base diameter of the sandcastle measures approximately 26m (85 ft 3 in) and the structure is of a real castle with a village around it (etched into the sand).

  • The longest sand sculpture ever created

The longest sand sculpture measures 27.3 km (16.96 miles). It was achieved in 2011. Special non-coastal sand called "Grubensand" was used to build the sculptures. The sand’s composition includes up to 10 per cent clay, is unwashed by the sea and has therefore a rougher surface that helps the sand grains stick together. 

  • Most Sandcastles built-in 60 minutes

In 2013 a total of 2,230 sandcastles were built in St Peter-Ording, Germany.

  • Most people build sand sculptures and castles at the same time

The most people making sand sculptures simultaneously are 396 individuals. The record was set in 2019 at an event organised by Sculpture Westende and Philippe Bourleau in Belgium. Participants were free to create what they please, but the aim of this event was to promote the sustainability of marine life and the reduction of pollution.

References:

Pakpour, Maryam & Habibi, Mehdi & Møller, P. & Bonn, Daniel. (2012). How to construct the perfect sandcastle. Scientific reports. 2. 549. 10.1038/srep00549.

Universiteit van Amsterdam (UVA). (2012, August 2). How do you make the perfect sandcastle? A little water can give you a five meter high castle. ScienceDaily. Retrieved June 23, 2022 from www.sciencedaily.com/releases/2012/08/120802101124.htm

https://www.guinnessworldrecords.com/news/commercial/2019/7/worlds-tallest-sandcastle-finally-achieved-after-two-years-of-trying

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