Beach Bum Science: Compression of Wet Sand
AbstractDid you ever notice the cool patterns around your footprints when you take a walk in the wet sand at the beach? The pressure of your feet has effects far outside your footprints. Here's a project that uses a simple experimental apparatus to investigate how the volume of wet sand changes under pressure.
Andrew Olson, Ph.D., Science Buddies
This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award:
- Rosenberg, M.F., 2007. The Effect of Compression on Granular Media. Retrieved June 18, 2007.
The goal of this project is to investigate what happens to the volume of wet sand under compression.
Have you ever taken a barefoot stroll on the beach? Right next to the water, the sand feels cool under your toes, and it's packed nice and firm so you can get good traction. You can hear the waves, over and over. Sometimes the water runs over your feet. If you look down at the wet sand, you can see that the pressure from your feet has effects over several times the area of your footprint. Marci Rosenberg noticed that, and wondered what was going on. Her investigation turned into a science fair project that brought her all the way to the California State Science Fair where she won a Clever Scientist award from Science Buddies (Rosenberg, 2007).
Marci used a simple apparatus made from a balloon and a drinking straw to investigate what happens to wet sand when it is compressed. She filled the balloon with wet sand using a funnel, then she attached a straw inside the neck of the balloon with a rubber band. Next, she taped a ruler to the straw, and filled the straw part-way with water. Her apparatus looked something like the drawing in Figure 1, below. By squeezing on the balloon with a C-clamp, she could apply pressure to the wet sand (like your foot does when you walk on the beach). By measuring the change in water level in the straw, she could observe what happened to the volume of the wet sand in the balloon as the pressure increased.
What do you think will happen to the water level in the straw when the balloon is squeezed? Your intuition probably tells you that the water level will rise. You may have some experience with water balloons, and this is probably what you would expect to happen if the balloon was filled only with water. In general, fluids have a property called "incompressibility." This means that if you squeeze a fluid, you generally cannot reduce the volume. For example, if you squeeze a water balloon, the balloon doesn't get smaller, the water just moves someplace else. If you apply enough pressure, the water will stretch the rubber side of the balloon, perhaps even breaking it.
Figure 2. Schematic diagram of Marci's balloon and straw apparatus. The balloon is filled with wet sand. A straw goes inside the neck of the balloon, and is attached tightly with a rubber band. A ruler is taped to the straw, and the straw is filled part-way with water.
The principle of incompressibility of fluids has applications in everyday life (in addition to water balloons, that is!). Hydraulic pistons are used in the brake systems and shock absorbers in cars, for lifting barber chairs, and for moving the heavy arms of construction equipment (see Figure 2). See the references in the Bibliography section to learn the details of how hydraulic pistons work (Brain, 2007; Rosignol, 2007).
Figure 2. Hydraulic pistons at work. The arm and bucket of this excavator can be maneuvered with hydraulic pistons. The diesel engine powers a hydraulic pump which is connected to each of the cylinders via control valves. (© Parrus)
Unlike liquids, gases can be squeezed into smaller spaces (if you have a container that will withstand the increased pressure). As a result, air bubbles cause problems in a hydraulic system. When there is an air bubble in the hydraulic fluid in a cylinder, instead of moving the piston the air bubble is compressed first, and only then does the piston start to move.
The Procedure section will show you how to use the balloon and straw apparatus to find out if wet sand behaves like an incompressible fluid. Find out if your intuition is correct or not!
Terms and Concepts
To do this project, you should do research that enables you to understand the following terms and concepts:
- Incompressibility of fluids
- Hydraulic pistons
- Structure of sand
- What will happen to the water level in the straw when the balloon is filled with tap water and then squeezed?
- What will happen to the water level in the straw when the balloon is filled with wet sand and then squeezed?
- What happens to a hydraulic system if there is an air bubble trapped inside?
- For information on how hydraulic systems work, see these webpages:
- Astronauts bringing sand into space? Here is a cool article from NASA on the properties of sand under pressure:
Barry, P.L. (2002). The Physics of Sandcastles. National Aeronautics and Space Administration. Retrieved June 18, 2007.
- Here's some good background information on the structure of sand:
Armstrong, W.P. (2000). Sand Grains: Chips Off the Old Rock. Wayne's Word, An Online Textbook of Natural History. Retrieved June 18, 2007.
- This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award:
Rosenberg, M.F. (2007). The Effect of Compression on Granular Media. Retrieved June 18, 2007.
Materials and Equipment
- Package of round balloons, all the same size
- Rubber band
- Metric ruler
- Cardboard box (e.g., corrugated cardboard shoe box)
- Utility knife
- C-clamp; available from local hardware stores or online suppliers such as Carolina Biological Supply Company
- Materials for filling balloons, for example:
- Wet sand (try different grain sizes, different amounts of water added to the sand)
- Plain tap water
- Salt water
- Salt slurry (saturated salt solution with additional, undissolved salt)
- Fine aquarium gravel and water
- Clay (very fine grains) and water
- Different types of soil and water
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- Do your background research so that you are familiar with the terms, concepts, and questions in the Background section.
In addition to the balloon and straw described in the Introduction, you'll need a C-clamp to squeeze the balloon. To support the C-clamp at the correct height, prepare a sturdy cardboard box (like a corrugated cardboard shoe box) as shown in the diagram below:
Figure 3. Shoebox support for C-clamp. The balloon is supported from above by the straw, and hangs inside the hole cut in the box. Adjust the height so that the jaws of the clamp are in the middle of the balloon.
- Cut a hole in the box at one end, with a diameter just slightly larger than the diameter of your balloons.
- The C-clamp will rest on top of the box.
- You'll attach the straw (e.g., tape it to a kitchen cabinet) so that the balloon hangs down into the hole.
- The middle of the balloon should be even with the jaws of the C-clamp.
- Place the box on the kitchen counter, beneath the cabinet that you will use for supporting the balloon and straw apparatus.
- Put the C-clamp on top of the box, surrounding the hole. With the C-clamp in place, you'll be able to see the correct height for hanging the balloon.
- Use the funnel to fill the balloon with the substance to be tested. Be consistent: always add the same amount (by volume).
- Insert a straw far enough into the neck of the balloon so that the end of the straw is in the wet sand. Attach the straw tightly in place with a rubber band around the neck of the balloon. The balloon will be hanging from the straw, so make sure it is on tight!
Use (removeable!) tape to attach the straw to a kitchen cabinet.
- Get permission from Mom first! If you mar the cabinets right after a kitchen remodel she's not going to be too happy!
- The balloon will hang down from the straw.
- Height-wise, you want the mid-point of the balloon to be even with the jaws of the clamp.
- Add some water to the straw to fill it part-way (no more than half) with water.
- Using the ruler taped alongside the straw, record the intial height of the water in the straw, in centimeters (cm). This will be your "zero" point.
- Adjust the C-clamp until it just "grabs" the sides of the balloon. Record the height of the water in the straw in cm.
- Tighten the C-clamp one full turn. Record the height of the water in the straw in cm.
- Repeat the previous step-several times, if possible. Stop if the water is about to overflow the straw.
- Do at least three separate trials (more is better) for each material.
- Subtract the "zero" level from each measurement. This will give you a measure of the amount of water that was displaced during each trial.
- For each substance tested, calculate the average amount of water displaced over all trials.
- More advanced students should also calculate the standard deviation.
- Make a graph of the amount of water displaced (in cm, y-axis) vs. the increase in pressure (number of turns of the C-clamp, x-axis).
- Is the shape of the graph similar for all of the substances?
- Can you explain the results?
Ask an Expert
- For a related experiment that might help explain your results, see the Science Buddies project Sand Structure: Measuring Density and Porosity of Sand.
- For a project with a real "twist" on the concept of a sandbox this Science Buddies experiment shows you how you can build a sandbox with transparent sides and a moveable end to see what happens to layers of sand under compression. You can see how layers are deformed and compare what happens with varying grain sizes, varying amounts of water, etc. Sound interesting? See the Science Buddies project Under Pressure: Sand Under Lateral Compression for more information.
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