Risk-based Design in Geosynthetics | Part 1 | AGRU America

Risk-based Design in Geosynthetics | Part 1

Announcer: Welcome to the AGRU America podcast. Your source for the latest on geosynthetics, geotextiles, and thermoplastics solutions. Every month, we meet with one of the plastics experts from AGRU to learn how geosynthetics are changing the world around us. To learn more about AGRU, visit AGRUAmerica.com. Now welcome your host, Yuse Lajiminmuhip.

Yuse Lajiminmuhip: Welcome to the podcast. As mentioned, my name is Yuse and I am a technical copywriter at AGRU. I love getting to think through and discuss the issues that engineers face, and how AGRU often creates unique solutions for them. Today, I’m very excited to introduce Chris Richgels, a civil engineer with over 24 years of experience in solid waste engineering. We’ll be discussing what’s been termed “Risk-based design of geosynthetics”. If you don’t mind me jumping right in, Chris, could you tell us what triggers land slope failure? Such as the one in Mozambique which severely disrupted the community following a landslide that resulted from poor slope design.

Chris Richgels: That land slope failure in Mozambique was associated with some really heavy rainfall. Some of that rainfall, it infiltrated into the waste mass, and basically turned it into a soup. Technically, it’s called loss of effective stress. It kind of flowed like mud off of that side slope; it’s what’s called a global failure. Water content within the waste mass would be a prime contributor to that type of failure.

Yuse Lajiminmuhip: So, in these types of situations, the waste mass acts like fluid?

Chris Richgels: It’s fluid-like. I mean, not completely or homogenously. But enough in, you know, selected areas where all the effective stress is gone. And parts of it will just start to move. And, you know, it can come out of it as an entire block that’s sitting on top of a liquefied layer, if you will, for lack of a better word. And it just slides out all at once.

Yuse Lajiminmuhip: And how can engineers mitigate the risk of slope failure?

Chris Richgels: Well, in that type of slope failure — number one — it is to prevent water from getting into the mass. That would be with some sort of cover on the top deck. And then a system of channels and pipes and ditches to carry the water off the side slope down to the toe of the landfill. And then, you know, distribute it in some receiving water body somewhere.

Yuse Lajiminmuhip: And what role do liners have in these designs?

Chris Richgels: Oh — number one — it is, you know, it would be a cap on the top deck such that a rainfall would not infiltrate into the waste mass below. It would just flow off of that cap. Liners can also be used in the ditch’s channels along the side slope to carry the water down to the bottom of the landfill.

Yuse Lajiminmuhip: So, we know in Mozambique there was very significant rainfall that year and that’s what caused the failure. How can you design for changing variables such as these extreme weather conditions?

Chris Richgels: Well, I can’t speak for Mozambique, but here in the States we have historical weather records that go back close to over 100 years in some locations in some parts of the country. So, you use those to determine what’s the worst possible storm. Or say, you know, a storm that has a 1 percent chance of hitting in every single year. That’s otherwise known as a 100-year storm. Take that, what that rainfall might be from that type of storm, and then determine what your system would have to be sized to accommodate that type of rainfall. Both the flow and the ditches and how long the top deck can be before water turns from like sheet flow on the surface to actual concentrated flow which can start to erode the surface. First off, you got to know well what’s the probability that you’re going to get that heavy a storm in any given year.

Yuse Lajiminmuhip: Well, that’s very interesting. So, I know that there’s closure solutions and solutions that exist that help engineers come up with more options when it comes to designing a landfill cap. Can you tell me more about ClosureTurf® specifically?

Chris Richgels: Well, ClosureTurf is a purely geosynthetic final cover system. There’s no additional soil placed on top of the system. The only natural material is a sand ballast that is placed on the artificial turf on top of a geomembrane. And that’s either like AGRU’s Super Gripnet or MicroSpike geomembrane, depending on its location. Either, you know, the top deck with MicroSpike or Super Gripnet on the side slopes. What happens is rain hits the sand ballast and turf and it’ll percolate to the top of the geomembrane. And, in the case of the top deck, it’ll just flow along the surface of the artificial turf until you get along to a ditch or perimeter ditch or a channel or an over side drain to carry that water back to the toe of the landfill. In side slopes, water will percolate through the sand and artificial turf into what we call the Integrated Drainage System. And the water will flow through that area, integrated drainage system, much like it would like a geonet, until it flows down to some catchment at the toe of the slope or a mid-slope bench.

Yuse Lajiminmuhip: Well, that’s interesting. I know ClosureTurf has a synthetic turf on the top of it that is kind of resistant to wind and other weathering conditions. Can you tell me some anecdotal evidence how it’s performed?

Chris Richgels: First off, it was tested in the laboratories for wind tunnel testing for what level of wind it could withstand. It tested up to a Category 3 hurricane wind speed. So, that’s a lot of wind. And then there’s another location north of Phoenix, Arizona where it’s being tested for longevity — for how long it can last under, you know, the hot sun. As far as like some case histories, there’s 2 on the east coast.

Chris Richgels: There was one in the Florida Panhandle where there was just a really severe storm that hit right after a site had been closed with closure turf. It was anywhere between, you know, what we would call a 100-year or a 200-year storm. And a 200-year storm hit this area. And there were parts of Pensacola that were prettily heavily damaged. Once people were able to get to the ClosureTurf site, it looked like nothing had really happened. It wasn’t really touched. Another was a hurricane that had skirted the east coast of the United States in South Carolina. And there was a site there in Berkeley County. Very heavy winds and a lot of rain. Damage in the area and, again, in the area of that landfill that was closed with ClosureTurf, you couldn’t even tell anything had happened. It just withstands weather events like that with minimal effect. Just both with what we’ve seen in the laboratory and now what’s being seen out in nature.

Yuse Lajiminmuhip: Really great to hear. So, from this podcast and this interview, I hope the listener could understand that there’s sort of two approaches that you could take with risk based design. First is, of course, the design itself. And then the second component is what products you use. Is there anything else that you could think of when it comes to risk based design that you would want to talk about in the second podcast?

Chris Richgels: You’ve got to understand the risk. The nature of the risk is, well, it’s not linear. If you get enough data collected, you can start making what I call confidence intervals on that data that a future event will fall in-between those confidence intervals. So, you can use that for your risk bases, and know what is the low confidence interval and what is the high confidence interval. That’s the other component of what I call risk-based design, you know, is you may not have a total linear understanding of the risk, but you’ve got ranges that you can work within.

Yuse Lajiminmuhip: So, you work with probabilities?

Chris Richgels: Yeah, exactly.

Yuse Lajiminmuhip: Well, that’s great. Well, thank you so much Chris for your time and I hope to catch you next time.

Chris Richgels: Alright.

Yuse Lajiminmuhip: Thank you so much Chris.

Chris Richgels: Thank you.

Yuse Lajiminmuhip: You know, landfills exposed to heavy weather are prone to slope failure if left unattended. AGRU is a world leader in geosynthetics and has a rich case history of successful landfill closures and temporarily covered installations across the United States. Beyond manufacturing high quality products, AGRU is committed to excellence in customer service. AGRU has product representatives in every region, a dedicated on-site quality assurance engineering team, and offers detailed drop-in specifications for each product. If you’re interested in learning more about our products, feel free to give us a call. From all of us here at AGRU, thanks for tuning in with us today. Join in next month for part two on risk-based design where we look forward to another insightful interview. with Chris Richgels.