Microbes Tapped to Harden Shaky Ground

Feb. 27, 2007 — A microbe that turns soil into stone may be able to stiffen up the ground under buildings and make them safer during earthquakes.

New laboratory tests of Bacillus pasteurii injected into wet soils show that they alter the chemistry between soil grains to form calcium carbonate — a cement form — making the ground more like sandstone.

One of the uses of the technique would be to improve the ground under existing buildings in low-lying, soft-sediment urban areas that are especially prone to deadly "liquefaction" of soils that turns some solid ground into liquid during major earthquakes.

"There are a lot of places where they are becoming aware of the earthquake hazards after building," said engineer Jason DeJong of the University of California at Davis.

In California, the most notorious of these unstable areas are in the San Francisco Bay, where several cities have built out into the bay using loose fill material. These saturated soils are especially vulnerable to liquefaction.

Other parts of the country can benefit as well, DeJong said. Large, but infrequent quakes are known to happen throughout the Midwest and Eastern Seaboard.

To establish whether the bacteria could really help, DeJong and his colleagues experimented with sterilized sand and bacteria in 55-gallon drums. They were able to control and monitor nutrients, oxygen levels and other variables within the drums to determine exactly what made the bacteria harden the ground.

The reason they selected this particular species of bacteria, out of the huge number that occur naturally in soils, is that Bacillus pasteurii is already known for being particularly good at making the waters in soils more alkaline. This forces calcium and carbonate dissolved in the water to combine and form crystals of calcium carbonate, the same natural cement that binds together sandstone, as well as manmade concrete.

These calcium carbonate crystals fill up the gaps between sand grains, locking them together.

Although the technique has only been proven in the laboratory, DeJong and his colleagues have lots of ideas on how to use it in the real world. Besides stiffening ground under buildings, it could also be used to strengthen levees and in stabilizing tunnels.

"My colleagues in the natural sciences have a few other ideas," said DeJong, whose paper on the laboratory work appears in the latest issue of Geophysical Research Letters. These include creating impermeable barriers in soils to control the spread of all sorts of contaminants without any digging or expensive building materials.

The use of bacteria to engineer soils is right in line with other advances in engineering, said engineering professor Carlos Santamarina of the Georgia Institute of Technology in Atlanta. He also works on bio-alteration of soils.

"If you look back at the 20th century, it started with mechanical engineering, then chemical engineering," said Santamarina of the ways people dealt with soils. "Towards the end of the century we started seeing that the biosciences could also play a role."

After all, it only makes sense, says Santamarina. There are about a thousand trillion microbes in a cubic meter of soil.

"If you can feed them and organize them in some way, they can do something useful," Santamarina said.