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There’s something very summery about building a sandcastle. From my first attempts with a simple bucket, and my Dad hydro-engineering sea-filled moats, I was hooked and these days no trip to the beach is complete without some sort of sand sculpture. From those early days and experiments I learnt how the sand had to be wet, but not too wet. Goldilocks sandcastle sand is wet enough to hold the grains of sand together but not so wet as to cause the walls to become unstable, crumbling to the ground and leaving you with a sand ruin. But how was I to know you could pursue a scientific career in sandcastle science?
Daniel Boon from the University of Amsterdam in the Netherlands, and colleagues, has shown that the Goldilocks recipes for sandcastle uses just 1% water (however, he investigated this with beach sand and deionized water, I do wonder if the saltiness of sea water makes a noticeable difference). This is enough water to form the capillary bridges between grains of sand, pulling them together and making wet sand such a good building material for complex structures.
As Boon notes in his paper (Scientific Reports, DOI: 10.1038/srep00549), the literature has previously claimed that sandcastles can only be built to around 20 cm, due to the capillary rise of the sand, but you can get much bigger sand structures than that so what’s going on? Well, more painstaking research building sandcastles, or rather sandcylinders, showed that sand buckles under its own weight at a critical height which is proportional to the cylinder’s radius to the power of two thirds. Using his calculations, with the best sand-water mix, a castle with a base with a radius of 20 cm should be as tall as 2.5 m. Compacting the mix (or tapping the sand with the back of the spade, as my Dad calls it) also helps, according to Boon.
Of course, these findings can be used by civil engineers and people working in soil mechanics, but will you be taking your calculator to the beach this summer? I think I’ll just stick with guesstimates, but might see if a drier sand than I expect will give better results.