A Sweet Earthquake Shake on Landfill
Have you ever thought about what type of ground buildings are constructed on? Rock, gravel, sand, soil and many others—there are lots of different types of "ground." And this issue becomes especially important in areas that are likely to get earthquakes. In this activity you will build a sweet building on a homemade shake table and find out how an earthquake impacts buildings constructed on landfill. How will your structure perform in a pretend earthquake?
- Butcher paper or newspaper
- Wax paper
- Damp cloth
- Starburst® candy or sugar cubes (40 or more)
- No-stir peanut butter or a substitute, such as soy butter or sunflower seed butter
- Spoon or butter knife
- Measuring cup
- Shallow box
- Marbles or any small balls identical in size (1 to 2 dozen)
- Shallow tray that is a few inches smaller than your shallow box. (Aluminum cake-trays work well.)
- Optional: Yellow food coloring
- Protect your work area with butcher paper or newspaper. This will speed up cleanup time. Have a wet cloth ready to wipe off your hands when they get too sticky.
- Place a piece of wax paper five inches square or larger in front of you. The wax paper makes it easier to place the house on the test soil, and to remove the house after testing.
- Unwrap the Starburst candies, and open the peanut butter jar. The candies will be the bricks of your structure; the peanut butter will serve as the mortar that keeps the bricks together.
- Scoop up a little peanut butter with a spoon, a butter knife or a finger. Apply it to the side of a candy and stick another candy against it. Repeat this process to build a building. You can build any structure you like, but it needs to fit well inside your tray.
- Cut the wax paper so there is about one to two inches of paper left on each side of the building. Set the building aside on the wax paper until you are ready to test it.
- To prepare the mixture that will mimic the reaction of landfill during an earthquake, scoop three cups of cornstarch into the shallow tray, and add one and a half cups of water. Add a few drops of yellow food coloring if available. Use a fork or your hands to mix it well. Getting the right consistency of the cornstarch mixture is important. It should be firm enough to support your test building—but still liquid enough that it slowly oozes when you scoop up a piece. Add more cornstarch if a candy placed on the mixture slowly sinks in, or add more water if it does not ooze. Set aside.
- To assemble your shake table, place the shallow box in front of you. Put the marbles into the box, and set the tray with the cornstarch mixture so that it is resting on top of the marbles. When you shake the box the tray should sway back and forth or side to side as it rolls over the marbles. The way the tray shakes mimics the side to side shaking seen in earthquakes.
- Practice shaking your shake table. Shake it quickly but gently enough and with the right-sized motions so that the tray does not hit the sides of the box.
- Use the wax paper to carefully place your building on top of the cornstarch mixture. Look closely. Your building should sit on the cornstarch with only the wax paper in between, but it should not sink in. It is fine if your building makes a print in the mixture—similar to a footprint in wet sand. If it slowly sinks all the way down, however, your mixture is too wet. Remove your building on the wax paper, mix in more cornstarch, and try again.What do you think will happen to your structure when you shake the box quickly?
- Memorize how the house stands on top of the mixture. Take a picture if you have a camera available.
- Shake the box fast enough so the tray with the cornstarch mixture sways back and forth but does not hit the side of the box. As you shake the box count to 20 before you take a break to observe.Has the building changed in any way? Can you see signs of cracks? Did the building sink deeper into the mixture? What do you think will happen if you shake with more force—or for a longer time?
- Try it out! Gradually increase the force with which you shake the box, making sure not to let the tray hit the side of the box. Sing the ABC song twice while you shake (or shake for about 20 to 30 seconds). Look at the cornstarch mixture while you shake.Can you see the cornstarch mixture moving almost like a very thick liquid?
- Assess the damage.Is your structure still standing?
- Look closer.If the building is still standing, is it standing on top of the mixture, or did it sink deeper into the mixture? If the building broke, are the pieces laying on top of the mixture, or did they sink into the mixture? Why would this happen?
- Use the wax paper to remove the house. You can reuse the mixture to test another house.
- Push a finger into the mixture.Does the mixture feel firm?
- Keep pushing while you shake the box vigorously with your other hand.Does your finger sink in the mixture? Does it feel like being in a very thick liquid? Why would that be?
- Throw away your cornstarch-and-water mixture with the trash. Do not pour it down the drain as it will clog the pipes.
Did you see or feel how the cornstarch mixture became a thick liquid when being vigorously shaken?
The cornstarch mixture acts like a solid when it is left alone, but it behaves more like a thick liquid when vigorously shaken. You could see and feel that it was like a solid at first, as it could hold the building, and you could feel the mixture resist your finger when you pushed on it. Shaking changed that picture. The building or its pieces probably sank at least a little into the mixture while you shook the box—just like it would sink in thick mud. You could likely also feel the mixture giving way for your finger when you shook the box while pushing on the mixture—just like it would in a very thick liquid.
Landfill and sand act in a similar way. At rest, they behave like a solid, but when shaken forcefully they act more like a thick liquid. Scientists call this the liquefaction process, and earthquakes are known to set this process in action.
The earth's outer layer is like a puzzle made up of huge pieces of land. These pieces are called the tectonic plates, and they slide around slowly on the surface of the globe; they push against and slide alongside one another. Sometimes this friction generates sudden movements, which can trigger earthquakes. Earthquakes release a lot of energy, which is radiated out in the form of waves called seismic waves. One type of wave pushes and pulls the material it moves through, and people feel it as a back-and-forth or side-to-side motion. This type of wave is called the primary or P-wave. A P-wave can move through solid and liquid material. The shake table in this activity recreates the side-to-side or back-and-forth movement created by a P-wave during an earthquake. But a real seismic wave generates more powerful movement.
The type of ground you stand on as seismic waves pass by also affects what you feel. Soft soils such as sand and landfill tend to amplify the shaking compared with hard soils such as bedrock. In addition to being soft, sand and landfill can also undergo a "liquefaction process" during an earthquake. This means the landfill can change from behaving like a solid to behaving like a thick liquid, which further amplifies shaking.
In this activity you constructed a building and placed it on material that mimics landfill. You tested it on a homemade shake table that generates movement resembling the pushing and pulling of a P-wave. How well did your building withstand the quake?
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For Further Exploration
- Use different materials to mimic different soil types, and use your shake table to see which soil types hold up a building best. Play-Doh® can model bedrock, Grape-Nuts™ cereal can reproduce gravel, and a mixture of two to three parts of Grape-Nuts cereal with one part of water works well to mimic loose soil. Repeat the procedure with each of these soil types. Test identical buildings, and keep the strength and duration of your shaking consistent. Would some soils make the shaking generated by a P-wave more intense than others? Based on what you tested, what type of ground would you build a structure on in an earthquake-prone area? In most areas where earthquakes are common, engineers use many other strategies to help buildings withstand shaking, which you can learn more about in the additional resources section.
- Test different designs of structures. Keep the strength and duration of your shaking the same, and do not change the soil type, but try an alternative building design. Would a tall building be more vulnerable? Would it be better to have a large surface area, or would smaller buildings withstand the shaking generated by a P-wave better?
- MS-ESS3-2. Analyze and interpret data on natural hazards to forecast future catastrophic events and inform the development of technologies to mitigate their effects.
- MS-ETS1-2. Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.