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Playing with Sand Helps Scientists Study Earthquakes

Next time you're at the beach, learn some basic engineering: follow these easy experiments with sand and water to learn how geologists and house builders think about the beach's building blocks.

Sand is composed of tiny grains that react differently depending on the types of physical forces acting on them. If the grains are squeezed, stretched, shaken, or mixed with water, they respond very differently. Dry sand doesn't stick together very well, and can't make steep sandcastle walls. Adding water to sand makes it much easier to build a sandcastle. Water molecules stick together with a kind of molecular glue, called polarity, that also helps the sand grains stick.

But adding vibration changes the sandcastle's sturdiness. Try this yourself: Mix some water and sand on the beach and place a seashell or rock on top of it. Next, pound on the beach about six inches away to simulate an earthquake. If you watch the shell or rock closely, you'll see it slowly sink.

Mixing sand and water together creates what's called a thixotropic mixture: while you mix it, it acts like a fluid. In fact, the more you mix it, the runnier it seems. "When the mixture is sitting still, the combination of friction and water holds everything in place," says Robert Krampf, a geology science educator. "When you vibrate the mixture, the grains move and you lose the friction." Then, the water acts as a lubricant instead of glue, causing the mixture to flow easily. Other thixotropic mixtures include ketchup and quicksand.

You can also dig for water on the beach. "When the level of the water in the sand is equal to the ocean and you dig a hole, you will strike water," says Krampf. Try this yourself: the further from the water's edge you are, the deeper you have to dig to reach water. Engineers have to know how far down water is located in order to dig a well.

During an earthquake, the sand and water in the soil are mixed together and compressed. As the water pressure inside the mixture rises, the ground becomes unstable. "During an earthquake, the soil, sand, and water underneath an above-ground swimming pool could turn from a solid, stable base to a heavy, thick liquid in a process called liquefaction," says Stein Sture, a professor of engineering at the University of Colorado-Boulder. "Liquefaction is strong enough to move a swimming pool."

"By knowing more about the fundamental process of earthquakes, building designs can be improved and existing structures can be made more stable," says Sture. "We now know more about the buildup of the water pressure in the ground than we knew before." Important knowledge like this begins with building castles in the sand.

--Inside Science News Service


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