Shaky Sediments

1. Pour sand and water together into your basin. There should be enough sand to make a layer several centimeters thick, and just enough water to soak through the sand. Mix thoroughly and adjust your mixture so that all your sand is damp with no visible puddles of water.
2. On one side of the pan, use your spoon to scoop out a hole in the damp sand. Then place the ping-pong ball inside the hole. This will represent an underground storage tank.
3. Cover the ping-pong ball, flattening and smoothing out the damp sand to make sure the ping-pong ball is not visible.
4. On the other side of the pan, wiggle the skinny end of the brick into the sand so it stands up like a building would (see photo below).
   
5. If desired, spread a little dry sand over the surface to give the appearance of dry ground.

Gently and repeatedly tap the side of the pan with the mallet. Notice what happens to the sand, the brick, and the ping-pong ball.

Did your brick “building” topple over? Did your ping-pong ball rise to the surface? Did the sands flow like a liquid?

Sands feel solid because grains touch and support each other. Between the sand grains are pores—empty spaces that make up to 50 percent of the volume of the sand. Often, these spaces are filled with water, called groundwater.

When loose or unconsolidated sediments are shaken, they try to settle into new positions. However, when seismic waves from an earthquake hit an area, the sand and water are rapidly compressed. This can cause the water pressure in the ground to go up significantly. Ground failure happens when this high-pressure water causes a reduction of friction between sand grains. When grain-to-grain contact is lost, sediments can flow like liquid. This phenomenon is called liquefaction.

In the case of the brick building, liquefaction causes uneven support of the brick’s base, so it topples over. As for the ping-pong ball “storage tank,” its density is less than that of the surrounding sediments. It’s held underground by the weight of the solid sediments above—until an earthquake takes place. If the sediment undergoes liquefaction, the buoyancy of the ping-pong ball causes it to float up and through the temporarily liquid sediment.

To reset this Snack and do it again, make sure you “fluff-up,” or stir your sand with your spoon before replacing the brick or ping-pong ball. This will help loosen any sediments that have become compacted.

Another way to shake things up is to use a power tool, such as a battery-powered orbital sander or drill, which can simulate shaking at a much higher frequency. To use a battery-powered drill, find an object that fits safely and tightly into the chuck of the drill whose mass is not centered along the rotational axis of the drill, such as a PVC elbow. This offset weight will cause your drill to vibrate. Make sure everything is secure and won’t fly off while spinning. Press the battery pack of the power tool against the pan and see what happens (see photos below).

Do you live in an area susceptible to liquefaction? Do some research to find the geologic hazards in your neighborhood.

Entire towns, roads, buildings, cars, and people have been “swallowed” by jiggling, liquefied, uncompact sediments. Expensive underground infrastructure that is less dense than the surrounding soils, such as utilities and sewer lines, as well as storage tanks, can rise well above ground level.

Use liquefaction as an engineering challenge to see how to mitigate this potentially devastating phenomenon. Can you keep a model building upright? Can you keep an underground object underground? Is there something you can do to the sediments themselves? These are the kinds of problems civil and geotechnical engineers try to solve. See the photos below for examples of what can happened if buildings aren’t properly engineered to withstand a geologic event.

Liquefaction-related damage after the 1906 earthquake in San Francisco. source: USGS

Effects of liquefaction after a 1964 earthquake in Niigata, Japan. source: USGS

The United States Geologic Survey has great resources on geologic hazards. Visit online for more information.