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Civil engineering 101 – How to Build A Strong Sandcastle

Updated: Dec 14, 2021


One of the favorite activities that kids like to do in the summer is to play at a beach and build sandcastles. I remember digging up sand and building various shapes of sandcastles and structures when I was on vacation with my family.

Believe it or not, we can teach structure design in civil engineering while kids are having fun playing with sand. On July 25th, 2021, HandsOnSTEM showed kids how to build amazingly strong sandcastle. You may be impressed by just looking at the picture above.

The first step involved creating the modified sandcastles, the kids put layers of carbon fiber mesh horizontally as they added sand to the container. After carefully packing the sand down and flipping them over, we tested their strength by adding bricks on top. On average, the modified sand castles held five bricks and our record of ten bricks was achieved by having 5 layers of carbon fiber mesh. Afterwards we wanted to create a control with no mesh to compare our results and it couldn’t even hold up one brick.

This remarkable increase in the strength of the structure is due to the nature of sand. A pile of sand is only held up by the force of friction in between particles. Each sand particle has infinitely many slip planes, or direction in which it can slide when force is added. Normally, sand will collapse when the force added to the structure is greater than the forces of friction holding the sand particles together along a diagonal slip plane. However, the reinforced sand has reduced and shortened slip planes. The carbon fiber mesh interrupts slip planes and makes the sand more resistant to top loading, allowing it to hold significantly more weight.

This property of stabilized sand and earth make it an attractive low-cost building material. For example, elevated approach ramps for highways are often created out of Mechanically Stabilized Earth (MSE), where layers of mats are interspersed into the section of earth holding up the ramp. The tessellated retaining walls are common indicators of the use of this technique.

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