If you live near the sea, chances are high that your home is built over sandy soil. And if an earthquake strikes, deep and sandy soils can turn to liquid, with some disastrous consequences for the buildings sitting on them. But now, U.S. researchers have found a way to use bacteria to steady buildings against earthquakes by turning these sandy soils into rocks. Today, it is possible to inject chemicals in the ground to reinforce it, but this can have toxic effects on soil and water. On the contrary, this use of common bacteria to 'cement' sands has no harmful effects on the environment. But so far, this method is limited to labs and the researchers are working on scaling their technique.
This process has been partially developed by Jason DeJong, an assistant professor of civil and environmental engineering at UC Davis. DeJong worked with Michael Fritzges, a senior engineer at Langan Engineering, Philadelphia, Klaus Nüsslein, associate professor of microbiology at the University of Massachusetts, Amherst, and the members of his lab.
Below is one of these pictures showing how unstable ground can aggravate the consequences of an earthquake. This one was taken after the 1989 Loma Prieta earthquake in San Francisco. "An automobile lies crushed under the third story of this apartment building in the Marina District. The ground levels are no longer visible because of structural failure and sinking due to liquefaction" (Credit: USGS). Here is a link to a larger version.
So what will be this new process useful for?
The new process, so far tested only at a laboratory scale, takes advantage of a natural soil bacterium, Bacillus pasteurii. The microbe causes calcite (calcium carbonate) to be deposited around sand grains, cementing them together. By injecting bacterial cultures, additional nutrients and oxygen, DeJong and his colleagues found that they could turn loose, liquefiable sand into a solid cylinder.
The new method has several advantages, DeJong said. There are no toxicity problems, compared with chemical methods. The treatment could be done after construction or on an existing building, and the structure of the soil is not changed -- some of the void spaces between grains are just filled in.
This research work has been published by the Journal of Geotechnical and Geoenvironmental Engineering under the name "Microbially Induced Cementation to Control Sand Response to Undrained Shear" (Volume 132, Issue 11, pp. 1381-1392, November 2006). Here are some excerpts from the abstract.
This paper presents the results of a study in which natural microbial biological processes were used to engineer a cemented soil matrix within initially loose, collapsible sand. Microbially induced calcite precipitation (MICP) was achieved using the microorganism Bacillus pasteurii, an aerobic bacterium pervasive in natural soil deposits.
The microbes were introduced to the sand specimens in a liquid growth medium amended with urea and a dissolved calcium source. Subsequent cementation treatments were passed through the specimen to increase the cementation level of the sand particle matrix.
And what were the results of these experiments?
SEM microscopy verified formation of a cemented sand matrix with a concentration of precipitated calcite forming bonds at particle-particle contacts. X-ray compositional mapping confirmed that the observed cement bonds were comprised of calcite.
Now, the researchers have to prove that their method can be used outside the labs. And they're working on scaling their technique by testing it in the earthquake-simulating centrifuge at UC Davis' Center for Geotechnical Modeling.Sources: University of California-Davis news release, February 21, 2007; and various other websites
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