When a powerful earthquake struck Sulawesi, Indonesia last year, something terrifying happened near the coastal city of Palu: an entire chunk of the ground turned to liquid, swallowing roads, cars, and hundreds to thousands of homes.
This phenomenon, known as liquefaction, occurs when wet, loosely-packed soil loses its structural integrity due to a sudden shock like an earthquake. Engineers typically rely on brute-force methods to mitigate its effects—fortifying structures with stone columns and grout; banging the ground with heavy weights. But now, a Portland State University-led team of scientists wants to re-engineer soils at the molecular level to prevent them from ever liquefying in the first place.
To do so, they’re enlisting the help of soil microbes.
“What they are proposing is a very elegant method should it work,” said Yumei Wang, a resilience engineer at Oregon’s Department of Geology and Mineral Industries. “It would be very green compared with traditional methods,” which include pounding the ground over and over to compress it and physically strengthening building foundations.
The method Wang refers to, known as microbially-induced de-saturation, is getting first-of-its-kind field test at two locations in Portland this summer. For the past five weeks, engineers have been fertilizing a small patch of ground at each site by injecting nutrients through a well down to 10 to 20 feet below the surface. As those nutrients seep into the soil, they’re stimulating the growth of “denitrifying” bacteria—native communities of microbes that produce two environmentally-benign gases, nitrogen and carbon dioxide, during the course of their metabolism. Like yeast in bread dough, this process creates air pockets in the soil, which prevents it from becoming saturated with water—a precondition for liquefaction.
At least, that’s the idea. According to Ed Kavazanjian, the director of the Center for Bio-mediated and Bio-inspired Geotechnics (CBBG) at Arizona State University and a collaborator on the project, researchers tested a variation on the technique in Toronto last year. In that case, scientists were fertilizing soils with the goal of stimulating the formation of calcium carbonate, a mineral that acts like a natural cement. But while the treatment ultimately wasn’t too successful at cementing soil grains, it did boost microbial gas production enough that soils became aerated, or de-saturated.
“Somewhere in there we thought, maybe we don’t need to precipitate calcium carbonate at all,” Kavazanjian said. “De-saturation is about 20 times cheaper, and there was a growing body of evidence showing de-saturation was persistent.”
This summer’s field trials in Portland are an attempt to prove that.
The first step was to collect some baseline data on the soil, said Diane Moug, a civil engineer at Portland State University and principal investigator on the experiment. In June, the researchers rolled out specialized truck developed by collaborators at the University of Texas Austin Natural Hazards Engineering Research Infrastructure program. This so-called T-rex shaker jiggles the ground to mimic a small earthquake. Then, researchers use a variety of sensors to measure the amount of shaking in the ground as well as the soil’s internal pressure, also known as pore pressure. In liquefaction-prone soils, pore pressure rises following shaking, as was the case when the researchers shook their test soils in June.
Last Friday, following over a month of nutrient treatments to boost microbial activity, the researchers went to shake the ground once again, and compare the results with their June baseline. If the speed of the compressional waves, or P-waves, associated with the shaking is lower this time around, that suggests the treatment has been a success. But the real “proof in the pudding” as Kavazanjian put it, would be if pore pressure no longer goes up.
“Liquefaction occurs because you shake and the internal pressure of the soil rises, and that causes it to lose its strength,” Kavazanjian said. “So if we go back and shake and there’s no pore pressure increase, we’ve been successful.”
If the results from this summer’s pilot tests are promising, the researchers are hoping to continue long-term monitoring at one of the sites which they have access to for the next five years. That will be critical for determining how long the treatment persists and how frequently it needs to be re-applied, explained Leon van Paasen, a senior investigator at the CBBG and collaborator on the project.
“They now have monitoring tools to show how long it’ll stay in place,” van Paasen said. “If the gas is not so durable, one idea is you might be able to treat it again.”
The researchers already have a potential application in mind, too: Portland’s Critical Energy Infrastructure (CEI) hub, a series of aging oil and natural gas storage facilities that hold approximately 90 percent of the state’s liquid fuel reserves. The CEI hub sits along the western bank of the Willamette river, on what Wang described as “loose, saturated soils highly prone to liquefaction.”
Should the Pacific Northwest experience its worst-case seismic scenario—a so-called megathrust earthquake on the Cascadia Subduction Zone—these soils could liquefy, the buildings could collapse, the tanks could rupture, and their contents could spill into the river. “There’s also the possibility of a fire,” Wang said.
To round out this dual geological and environmental nightmare, the destruction of the CEI would also obliterate Oregon’s backup fuel supply at the very moment that it’s likely to be needed most for earthquake recovery, according to a recent report. This vulnerability is exactly why Wang is watching the work of Moug and her colleagues with great interest.
“As a government official, if there’s a method people are happy to use and we improve the safety of our infrastructure, I think that is a huge win,” Wang said.
Because of the technique’s non-invasive nature, the researchers ultimately are hoping their microbial technique could be applied to liquefaction-prone landscapes around the world—particularly those where infrastructure is already in place and cannot be easily fortified without digging it up.
“There are trillions of dollars of infrastructure around the world at risk,” Kavazanjian said when asked where else it might be useful. “My answer would be everywhere.”