Last week, a plot of land in North Portland felt a shake, but not one caused by an earthquake, but instead by a machine known as T-Rex. Researchers with Portland State University were simulating a minor quake to test a soil treatment that would fortify the ground from liquefaction. Arash Khosravifar and Diane Moug are both associate professors in Civil and Environmental Engineering at PSU. They both join us to share why their research is important and what they learned from the recent demonstration.
Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. There was something like a tiny earthquake in Northeast Portland last week. It was not caused by shifting tectonic plates. The brief trembling was created by a machine known as the T-Rex. Researchers at Portland State University were simulating a minor quake to test a soil treatment that could prevent liquefaction. That is of urgent concern in Oregon, given how much key infrastructure is on top of land that could lose its solidity when the Cascadia Subduction Zone earthquake hits.
Arash Khosravifar is an associate professor in civil and environmental engineering. So is Diane Moug. They join us now to talk about their work. It’s great to have both of you on the show.
Arash Khosravifar: Thanks for having us.
Diane Moug: Thanks for having us.
Miller: Diane, first – as I noted, you’re trying to prevent liquefaction. What exactly is that?
Moug: Liquefaction is when we have ground that’s saturated with water and it’s shaken with a strong earthquake. It causes the ground to lose its strength. The soil particles, they want to get denser when the ground is shaken, like when you’re at the beach and you stand near the water, and you jump up and down. You see water coming to the surface and you are also sinking down into that sand that’s lost its strength. That’s what happens during earthquake liquefaction. So you can imagine that ground behaves like a liquid with very little strength and then any structures or infrastructure on top of it also becomes a victim of that loss of strength.
Miller: Arash, can you give us a sense for the scale of land just in the Portland area that is prone to liquefaction?
Khosravifar: We’re particularly focused on this type of soil called low-plasticity silt, which is pretty common in Oregon and Washington, pretty much the Pacific Northwest. You find it very commonly at project sites along the Willamette and Columbia rivers. You also find it pretty much everywhere in the Willamette Valley. It’s referred to as the Missoula Flood deposits. And if we get a large magnitude earthquake on the Cascadia Subduction Zone, the intensity of shaking will be large enough that these soils are known to liquefy.
Miller: But it’s not just the overall acreage in Oregon, it’s particular areas that are important for the region that are on those soils. What kind of infrastructure is on this tenuous soil?
Khosravifar: A lot of infrastructure. You think about highway bridges, fuel tanks, buildings – all these are prone to severe damage and past earthquakes have shown that you could get severely damaged. The particular area that was a motivation for our study was the fuel tanks in the Northwest Industrial Area in Portland, commonly referred to as the CEI hub or Critical Energy Infrastructure hub. These fuel tanks were built before the engineering community knew that these soils may liquefy in an earthquake, so they’re sitting on potentially liquefiable soils.
If we get liquefaction, these tanks will tilt, settle and leak, so you could be dealing with fire, you could be dealing with potentially fuel getting into the reverse environmental impacts of it, and also just the disruption to the fuel supply. That’s something that we definitely need right after the earthquake for emergency responders. So we’re dealing with all those hazards.
Miller: Diane, we’re going to talk about the microbial application that the two of you have been working on and testing recently … we’ll talk about that in just a second. But are there other options for land that’s prone to liquefaction? I mean, what other solutions are possible?
Moug: The engineering community and geotechnical engineers, they have a lot of solutions. We call it ground improvement – to improve the ground, to make it less susceptible to liquefaction and stronger in earthquake shaking. These are solutions like compaction, which works really well for sands, like dropping a big weight onto the sand and densifying it. There’s also methods like mixing the soil with a grout or cement to really bond those soil particles together and strengthen them.
Miller: Can you do those after something’s already built on top of this soil?
Moug: That is one of the big challenges, is that those are fairly disruptive methods. You can imagine that we have these tanks that, many were built in the ‘50s or earlier, that having these methods of injection and mixing and vibration can be quite disruptive to overlying structures. So in a lot of cases, we’re challenged to find economic and effective ground improvement methods that can work for soils beneath existing structures.
Miller: Kind of a soil retrofit?
Moug: Yeah, yeah.
Miller: So let’s turn to this experimental solution that your team has been working on, Arash. It’s called microbially induced desaturation, or MID. What is it?
Khosravifar: Microbially induced desaturation is a method where we try to use nature to help us strengthen these soils. We’re feeding the microbes that are already in the soil with essentially food. They consume it and that consumption causes a chemical reaction called denitrification, and they take that food and turn it into nitrogen gas and CO2. The nitrogen gas essentially works almost like little air cushions, almost like an airbag in your car, that absorbs the energy. They do the same thing in an earthquake where you get the earthquake shaking, these bubbles absorb earthquake energy, and by doing so, they protect the soil from liquefying.
It was developed by our collaborators at Arizona State University. They tried it in the lab, they showed that it’s effective. And then we partnered with them in 2019 with funding from the National Science Foundation and later from the Foundation Institute. We tried it in the field, which I believe is the first field trial of this method in the U.S., where we wanted to see if the method that works in the lab, does it also work in the field, in the natural environment where you don’t have control on a lot of things.?
Miller: So Diana, I was fascinated to find out that you’re not actually putting in new microbes, you’re instead feeding the ones that are already there. What’s the food?
Moug: There is a source of nitrogen to create that nitrogen gas and then there’s also an organic carbon through calcium acetate. These are environmentally inert nutrients that we pump into the ground and these are denitrifying microbes that are ubiquitous. They’re found in almost all natural soils and they’re stimulated by these nutrients to perform their denitrification reaction.
Miller: So to test the effectiveness of these treatments, you have this big earth shaker, nicknamed T-Rex. Can you describe what it looks like and what it does?
Moug: T-Rex is a large shaker truck. It is transported here from the University of Texas, Austin. These are a part of large-scale experimental infrastructure that the National Science Foundation also supports. These trucks are brought here to our site. It’s just a large truck, very large, with large tires, and there’s a hydraulic plate that’s lowered down from the truck and that’s put in contact with the ground. The rest of the truck, all the wheels are lifted up off the ground. And then this plate in contact with the ground then shakes the ground, with the weight of the truck on top of it.
Miller: How far away could you be and still feel, just as a human, the vibrations or the banging made from this truck?
Moug: It’s small vibrations that you feel. It’s almost like when you are near a train that’s coming by. There’s more control over the shaking and you can sustain it for longer, but I’d say you’re probably about 50 to 100 meters away, and you can still feel a little bit of shaking. Close by, it’s a stronger vibration that is quite impressive.
Miller: Arash, what were you trying to learn from the test last week? And what did you learn?
Khosravifar: In our field trial of this method, there are things that we’ve learned and there are things that we need to learn. We know now that we can apply this method in the field, we can desaturate the soils. We’ve also been monitoring this method since 2019 and we noticed that after five years, the soil started to re-saturate. So we have some idea about the longevity of these. And this summer we re-treated the site, which means we know now that we can reapply it, and that was a big engineering practical question.
Miller: Every five years or so?
Khosravifar: Probably every five years or so. If this method scales up and becomes a viable method that contractors would use, it would probably be some sort of … you would be on a maintenance kind of schedule where every five years you would go back and use the wells that you’ve already built and installed in the ground to re-treat the soils.
What we still need to prove is if the method is effective in mitigating liquefaction. There are collaborators and we have shown in the lab that it works. We just need to show it in the field, under real field conditions, and that’s what this T-Rex machine will eventually hopefully help us to show.
Miller: Diane, we have about a minute-and-a-half left, but how much could this scale? I mean, how feasible would it be to treat just the area around Portland’s Critical Energy Infrastructure hub, just that area alone? Which is, I should say, tiny compared to liquefiable soil writ large, but it’s not small, just for our region.
Moug: I think quite feasibly this is a method that could be applied around the infrastructure at the Critical Infrastructure Energy hub. It is a method that you could use to target specific critical tanks or other infrastructure, or potentially the area along the river, and target that itself, so that maybe there’s not so much spread into the river from an earthquake and deformation of the ground.
Miller: Diane and Arash, thanks very much.
Moug / Khosravifar: Thank you.
Miller: Diane Moug and Arash Khosravifar are associate professors of civil and environmental engineering at Portland State University.
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