Bugs may help buildings stay upright
A microbe that turns soil into stone may be able to stabilise the ground under buildings and make them safer during earthquakes, scientists say.
New laboratory tests of Bacillus pasteurii injected into wet soils show that they alter the chemistry between soil grains to form calcium carbonate, an ingredient in cement.
This, the scientists say, makes 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.
This liquefaction turns some solid ground into liquid during major earthquakes.
"There are a lot of places where [people] are becoming aware of the earthquake hazards after building," says engineer Assistant Professor Jason DeJong of the University of California, 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 areas can benefit as well, DeJong says. Large, but infrequent quakes are known to happen throughout the Midwest and eastern seaboard of the US.
Could bugs be the answer?
To establish whether the bacteria could really help, DeJong and his colleagues experimented with sterilised sand and bacteria in 200 litre drums.
They controlled and monitored nutrients, oxygen levels and other variables inside the drums to determine exactly what made the bacteria harden the ground.
The reason they chose this particular species of bacteria, out of the huge number that occur naturally in soils, is that B. pasteurii is already known for being particularly good at making the water 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 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, they say it could also be used to strengthen levees and in stabilising tunnels.
"My colleagues in the natural sciences have a few other ideas," says DeJong, whose paper on the laboratory work appears in Geophysical Research Letters.
These include using soil microbes to create impermeable barriers in soils to control the spread of all sorts of contaminants without digging or using expensive building materials.
The use of bacteria to engineer soils is right in line with other advances in engineering, says engineering professor Carlos Santamarina of the Georgia Institute of Technology in Atlanta, who also works on bio-alteration of soils.
"If you look back at the 20th century, it started with mechanical engineering, then chemical engineering," says 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 metre of soil.
"If you can feed them and organise them in some way, they can do something useful," Santamarina says.