How to Look at Compression of Wet Beach Sand:Easy Experiment

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.

[bctt tweet=”The compression of beach sand involves some interesting physics.”]

Summer Skyes II, CC BY 2.0, via flickr

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

[bctt tweet=”So what is the science behind the wet sand effect also known as beach sand compression? “] 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.

Wet Beach Sand:Easy Experiment

[cow_johnson general_float=”center” general_width=”500″ general_bg_type=”bgcolor” general_bgcolor=”#ffffff” general_bgcolor_to=”#ffffff” general_background_repeat=”repeat” general_color=”#ff964a” general_font=”Acme” heading_caption=”Easy Experiment” heading_tag=”h3″ heading_align=”left” border_style=”inset” border_width=”15″ border_color=”#985b3f” border_radius=”5″]Try this wet beach sand experiment with your students or kids at the beach or at home:

1. Fill some baking pans or containers big enough for a hand with sand from a beach or sand table or even purchased sand from a home supply center.
2. Add enough water so that it has the consistency of shoreline beach sand.  All the sand should be wet but there should be no puddles of water on the surface of the sand.
3. Have your students or kids predict what will happen to the sand when they press their hand into it.  You will likely get a variety of answers.  Try to redirect them if necessary to what may happen in and around the hand print itself.
4. Have them record this hypothesis or record predictions on the board.
5. Next have the students press firmly on the sand with one hand.
6. Once they remove their hand, have them observe immediately the state of the sand in the hand print. They should see the shiny wetness of the sand replaced with a dull, drier hand print.
7. Ask students for their comments on what happened to the water at the surface of the hand print.  Where did it go?  With older students their comments and questions should lead to discussions of changes in volume and perhaps sand grains as levers.[/cow_johnson]

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