On Building Sandcastles

Environmental & Science Education, STEM, Nature of Science

Ed Hessler

The basic recipe for a sandcastle seems simple enough - sand, water, air, mixed right of course - so why would a civil and environmental engineer spend time studying them as well as teaching prospective engineers about the "recipe"?

In a short article in The Conversation, geotechnical engineer Joseph Scalia, Colorado State University tells us what he has learned. Scalia uses "sandcastles in the classroom to explain how interactions of soil, water and air make it possible to rebuild landscapes after mining metals critical to the energy transition."

Without over describing his work there is sand and then there is sand. The best sands are not beach sands but those originating in mountains, e.g., river sands. There is a reason for this and perhaps you can think of it..  Compaction also increases the strength of the sculptures.

The key, Scalia writes, is water. The moisture range is quite wide. There is some physics here and he describes the behavior of water in a glass you might have observed or missed. Water is "sticky" -enough so that it exhibits some creep (small) up the sides of the glass.  You can also make some comparisons between the creep in a straw and in a glass. You will recall some terms from science classes (explained) such as cohesiveness and capillarity.

However, one term I'd not heard of, particular to engineering, was "suction stress."  The space between sand grains can be thought of as a tiny straw which allows the formation of tiny bridges. This tension draws grains closer and closer together. Something I'd never thought of is that as salt water dries in sculptures it forms salt crystals, becoming a glue, although one that is brittle and sensitive to destruction by touch.

This lovely, informative article is linked in the right places and is an example of using a common phenomenon to open the door to understanding the complexity of re-constructing a landscape after a disturbance to remove what is underneath the ground.

By the way, Scalia provides a "general rule of thumb for building great sandcastles": one part of water for every eight parts dry sand. That ratio changes under the controlled conditions of the laboratory to "one part water for every one hundred parts dry sand." 

Whether you have been to a beach this summer or are bound for one or just want to think about the science of the ordinary, I hope you will read it.