Exploring Where to Build in Earthquake Country: A Study of Liquefaction in Sand, Rock, and Soil

Research Questions:

• What is liquefaction?
• In what soil types is liquefaction most likely to occur?
• How stable are buildings constructed on clay soil during an earthquake?
• How stable are buildings constructed on sandy soil during an earthquake?
• How stable are buildings constructed on gravel during an earthquake?
• How stable are buildings constructed on loamy soil during an earthquake?

During the 1989 Loma Prieta earthquake, many people were surprised that some of the worst damage to buildings occurred far from the epicenter of the quake. This damage occurred because of liquefaction. Liquefaction refers to the process by which a solid appears to turn into a liquid. While most students are familiar with walking on sand at the shore and seeing their footprints fill with water, they are less familiar with the stability of building in earthquake country as a function of liquefaction.

Materials:

• One 4’ x 4’ board
• Five tennis balls
• A deep plastic basin such as used for washing dishes
• Rags
• A bucket of sand
• A bucket of gravel
• A bucket of clay soil
• A bucket of loamy soil (optional)
• Sufficient Legos to build a 1’ tall structure
• Stopwatch
• Two ¾” screws and washers
• Drill

Many of the materials are readily available at home. If Legos are not available, other building kits such as Lincoln Logs or Erector Set structures can be substituted. Soils can be obtained by a quick trip to the sand box, beach, local creek and back yard.

Experimental Procedure

1. Drill two 3/8” holes into the middle of a dish basin 6 inches apart.
2. Affix the dish basin to the center of the 4’ X 4’ board with ¾” inch long screws, first inserting the screws through an appropriately-sized washer and then into the screw holes of the basin
3. Balance the 4’x4’ board, basin side up on at least 5 tennis balls. You should be able to easily shake the board back and forth on the floor by rolling it on the tennis balls.
4. Build a tall, relatively narrow Lego structure.
5. Fill the basin to the top with sand. Place the Lego structure into the sand, burying approximately 1 to 2” of the base of the structure in the sand.
6. Start the stopwatch.
7. Begin to vigorously shake your shake table. Short vigorous shakes mimic earthquakes more realistically than longer ones.
8. How long can you shake the table before the structure topples? What does the soil around structure look like? Is it watery? Take pictures with your camera. Write up your observations.
9. Repeat steps 5 to 8 using clay soil, gravel and clay, and loamy soil (optional). Which supports the building best? Which is worst at supporting the building?

Terms/Concepts: Shake table, Liquefaction, Earthquake

References:

Websites: Wikipedia: Soil Liquefaction http://en.wikipedia.org/wiki/Earthquake_liquefaction

University of Washington: What is Soil Liquefaction? http://www.ce.washington.edu/~liquefaction/html/what/what1.html

Books:

Iacopi, Robert L and Fred W. Fisher. Earthquake Country. Sunset Books, 1996