Inside: Compression of beach sand is a neat effect. Stepping on wet sand makes it drier. Try an easy experiment investigating the soil mechanics of wet beach sand.
The compression of beach sand involves some interesting physics. Click To Tweet
Most of us have walked along the shoreline of a beach. I have done a lot of shoreline walking this particular summer. Have you ever looked closely at your footprint the moment your foot lifts from the wet sand? If not, do it the next time you’re walking along a beach shoreline. What is beach sand made of? Beach sand composition has a lot to do with the physics behind the compression of beach sand. Check out this related post to get an inside view.
What would be your first guess about what you would see? Wetter sand, perhaps a puddle in my footprint would have been my guess.
Pushing the sand should squish more water to the surface – you would think. However, you’d be dead wrong. Compression of sand is also known as the wet sand effect. The wet sand effect is actually misleading. When you step on wet sand the opposite of what is expected is what you see happening. Compression of beach sand actually results in the footprint being drier than it was before you stepped on it. Weird or what, isn’t it?
The Science Behind the Compression of Wet Beach Sand
So what is the science behind the wet sand effect also known as beach sand compression? Click To Tweet The dry sand on your footprint is due to the dilatancy effect of sand under a compressive force. What does that mean you say? Dilatancy effect – it certainly is a mouthful but it is really not that hard to understand – is the tendency of compact granular material – such as sand – to expand when it is pressed on.
Grains of sand when wet and compact are interlocking and have no freedom to move. Add a compression force, like a foot with 60 pounds or more weight behind it, and each grain of sand acts like a little lever pushing against its neighbors creating little pockets of space. The water on the surface of the sand now has somewhere to move. Water sinks down due to gravity filling in the gaps between the sand grains. The sand on the footprint becomes briefly dry.