How to Manage Risk During the Design of Geosynthetic Systems

There is a saying in the industry that, «nothing is ever built the way that it was designed.» When planning projects with geosynthetics, among the greatest risks to account for is the design itself. There are often secondary systems and items that are overlooked during the design process, which can lead to cumulative impacts to system stability.

In this episode, we will be exploring the third and final part of our series on risk-based design with geosynthetics. Specifically, how to manage risk during the design of geosynthetic systems.

Transcript

Yuse Lajiminmuhip: Hello and welcome to the AGRU America podcast. My name is Yuse Lajiminmuhip and joining me today is Chris Richgels, a civil engineer with over 24 years of experience in solid-waste engineering. In this episode, we’ll be exploring the third and final part of our series on risk-based design in Geosynthetics.

Announcer: Every month, the plastics experts at AGRU explore how geosynthetics are changing the world around us. Discover more about AGRU at agruamerica.wpengine.com or by calling 1 (800) 373-2478.

Yuse Lajiminmuhip: Chris, what do engineers mean when they say «nothing is ever built the way that it was designed»?

Chris Richgels, P.E.: Well, simply because it isn’t. Once you get out in the field, you run into situations that don’t show up on paper and the AutoCAD screen, and you have to adjust and count for what you find in the field. And sometimes that requires a design change, and then sometimes it can just be a construction adaption of the design intent at the specific location of the field anomaly. The way to prevent or minimize that type of situation from occurring leads to another anagram that we call the KISS principle. That stands for «Keep it simple and stupid». Or, depending on the engineer you’re dealing with, «Keep it simple, stupid». That way, you can keep the design as simple as possible which reduces the room for error for changes in the field application of that design that can lead to issues.

Yuse Lajiminmuhip: So that means that there are inherent risks in all design. The more elements you incorporate, the more you expose yourself. What does that mean for a project like a final cover design? What are some of the component parts in this project that could increase the risk?

Yes, using final cover as an example: There’s lots of variables during construction that don’t show up as nice, neat lines on drawings. One, the cover soil thickness isn’t going to be perfectly say two-feet thick. Typical tolerance in earthwork grading is a tenth of a foot. That soil cover could be thicker than two feet by a tenth of a foot or less than two feet by a tenth of a foot. So there’s variability there. Also, slope variation. Landfill final slopes are never straight. There’s always going to be dips and concave surfaces, slope bowls etc. that may get steeper or, if it’s the reverse pattern, it may have a shallower section before it goes to that steeper section. So there’s changes that happen there. The other thing is interface friction strength can vary with materials. No one material is perfectly uniform across its entire surface. I would venture to say that AGRU MicroSpike is more uniform across its surface than others which rely on a random pattern —  more so than AGRU. For a final cover, there’s always going to be sub-drainage underneath the soil cover section, and that’s usually done by a geocomposite or, in case of AGRU product, you can use the Integrated Drainage System, which is either MicroDrain or Super Gripnet — depending on shear strange requirements on that particular slope. Now, what happens with a sub-drainage system is that soil cover above it is stabilized using vegetation. Vegetation can have, depending on the specie, differing root lengths. Some plants root down only a foot, which would be fine in a two-foot cover section. Other plants can root down much, much deeper. Once they get to that sub-drainage system, the roots will start to penetrate into that system and can clog it. You have to account for that and the transmissibility selection of your drainage system to allow some clogging to occur from the plant roots.

Yuse Lajiminmuhip: So it sounds like there’s a combination of site-specific design components and product specific components. In both cases, there are some variability involved. What are some of the variabilities inherent with the products?

Chris Richgels, P.E.: The variability in the products is going to show up in the testing. The issue there is, it’s not just the material variability but the testing variability — even if you had exactly the same surface. Like an interface friction test, a direct shear test, ASTM D5321. Even if you had the exact same surface, and it was just perfectly matched, and even if you put it in the same test apparatus at the same laboratory, and the test is run by the same lab technician on the same day under the same ambient conditions, the results you’re going to get from those two tests are not going to be exact. There will be some variability in that test method.

Yuse Lajiminmuhip: What can you do in these situations?

Chris Richgels, P.E.: You’ve got a lot of test data results, interface friction strength, for example. You can build what we’ve built here at AGRU: a database of those test results. Analyzing that database, you can make some statistical predictions as to what future test results will come in at. I use what I call a 90-percent confidence interval. What that means is, in future test results, there’s a 90-percent probability that they’re going to fall in-between those two lines. If you’re just using the lower confidence interval, then you can say that there’s a 95-percent probability that future test results are going to fall above that line, which is where you want them to be, if you’re going to use that 90-percent interval as your friction-strength design envelope. That’s one way to reduce the risk of material and testing variability once you go from design and then to construction.

Yuse Lajiminmuhip: How do the construction types and installation methods affect variability and risk?

Chris Richgels, P.E.: There’s specified ways to construct with geosynthetics — Welding with geomembranes, as an example — There are specified ways that has to be done. There’s ways that you can make sure you get a qualified contractor on-site that knows how to do that type of welding, simply by demonstrating that they’ve done X-million feet of it in the past with successful results. All of it generally boils down to construction quality assurance (CQA). CQA inspections make sure everything is being done the way it should be done. And that’s the primary way to reduce risk on the construction side of things, is make sure there’s proper inspection all along the process to make sure it’s done as close to the design as possible — that all the assembly techniques are proper.

Yuse Lajiminmuhip: I’m sure there’s a point where, as an engineer, you have to stop and say, «This is an acceptable amount of risk.» How do you get to this point, and what role does regulation have in identifying acceptable risk?

Chris Richgels, P.E.: Regulation requires construction quality assurance for something like a landfill base liner. All of that CAQ effort has to be documented and then submitted in a report to the regulatory agency demonstrating that the project was built according to design and all the testing, field testing, lab testing produced passing results. That’s in regulation for landfills, in particular. So, in that case, the regular regulatory system isn’t assessed on making sure you get a good product.