The following essay is covered in The Conversation, an online publication.

If you have a background in engineering, you can understand why some sandcastles are tall and have intricate structures while others are shapeless.

Sand castles are used in the classroom to explain how soil, water and air interact to make it possible to rebuild landscapes after mining metals critical to the energy transition.

The right mix of ingredients is needed to build a sandcastle. The structure is provided by the water between the sand grains that hold the sand together. The water would push the sand grains apart if there wasn't enough air.

Not just any sand

Sand grains are soil particles with a diameter of less than 0.25 inches. Half of the particles in that range are from the sands. Sand is larger than Silt or clay. Gravel is the soil with particles bigger than sand.

Sand looks and feels different depending on the size of particles. Sand grains have a texture similar to powdered sugar. Small dry lentils are similar to the size of the largest grains.

Grains of sand in several different sizes and rough edges make the best sand for a sandcastle. Smaller sand grains are able to fill the pockets between the larger sand grains. Increased sand strength is achieved.

The sharper the sand grains, the stronger the sandcastle. A pile of wooden blocks will stay in a pile, but a pile of marbles will move around.

The best sand for sandcastles is not usually found on an island or a beach. Sand that is moreangular is found closer to the mountains. Sand grains have not been rounded off yet. Sand castle builders will import river sand for their creations.

The closer the sand is to each other the stronger it will be. Increasing the effect water can have by squeezing sand grains together is accomplished by pressing wet sand together tightly. Sand strength is increased by compaction.

Water is key

Sand can form a pile without water. Sand and water are liquid. Between dry sand and saturated sand there is a wide range of levels of water that can be used to build sandcastles.

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Water sticks to water because it is cohesive. Water can stick to or climb up certain surfaces. If you look at a half-full glass of water, you will see the water going up the inside of the glass. The water is trying to get up and wet the surface despite the gravity holding the water in the glass. Sand castles are possible because of this tiny power struggle.

The air and water are close together. The air-water interface tries to hold the water together against other forces. The water is pulled together by surface tension. Sand grains are pulled together.

The water would rise and have more surface tension if the glass was skinnier. The bigger the straw, the higher the water would go. Capillarity is a phenomenon.

The water behaves the same in sand. The spaces between the sand grains are very small. The water creates tiny bridges. The grains are pulled together by the force of the water in the bridge.

Just enough water

The strength of the water bridges is determined by the amount of water in the sand. The sand grains have little bridges between them. The amount of water and the number of bridges increases the amount of sand held together. Sand is perfect for a sandcastle.

The sand is not strong enough to hold it together.

One part water for every eight parts dry sand is a general rule. One part water for every hundred parts dry sand can make a difference in a laboratory. When the tide is low, sand with the right amount of water is near the high tide line.

Sand castle stability can be improved by salt from the sea. Sand grains are held together initially by capillary forces, but capillary water will eventually evaporate. Salt is left behind when the water is dry. The salt crystallises at the points of contact that the seawater formed. Salt can keep a sandcastle standing for a long time. It is brittle and collapsible and should not be disturbed.

To build a strong sandcastle you need sand and water. I like to create a dense mound and scoop and carve to reveal the art inside. You can build from the ground up if you compact the sand. The sand must be dense and the mold must be placed on a foundation. A shovel or a seashell will allow for more precise carving than with hands. Don't be afraid to get sandy.

The conversation published this article. The original article is worth a read.