It may look like "terra firma" to the naked eye, but the true integrity of loosely deposited granular soils can be deceiving. Just ask the folks in San Francisco, Mexico City and Kobe, Japan, where major earthquakes in recent years instantly transformed solid ground into quicksand-like, lethal-to-structures soups--a process called liquefaction.
Researchers at the National Institute of Standards and Technology and the University of Texas at Austin are making significant improvements to the method used for predicting soil liquefaction in earthquake-prone areas. Their measurement tool--known as the spectral-analysis-of-surface-waves test--uses a truck-mounted vibrator to shoot waves of different frequencies through the soil and two or more motion sensors to record the speed of the waves generated. From the recordings and subsequent computer modeling, estimates of the varying soil strengths in the area are derived. These are compared with data from sites that have and have not liquefied during earthquakes. The resulting profile reveals where the soil is dangerously susceptible to liquefaction.
The SASW test's major advantage is that it does not require drilling holes to do subsurface measurements, making it quick, portable, usable in hard-to-sample or sensitive areas, and cost effective. It already has been employed at two construction sites--in Taiwan and Charleston, S.C.-- to evaluate the effectiveness of techniques used to prevent potential earthquake liquefaction damage.