Q: We are designing a composite liner system for a coal ash landfill. From the bottom-up, the cross section is subgrade soil, geosynthetic clay liner (GCL), geomembrane, composite drainage layer with nonwoven geotextiles on both sides and then ash from coal combustion residuals (CCR). Do you have an opinion on the design, particularly on 3:1 slopes in a seismically active zone?
A: Wow, this a big question for the GMA Techline. I think you need to get a professional engineer involved. That written, there are considerations to this design that have presented themselves in the past that I can offer. Three points of interest are outlined below.
You might want to consider a single-sided textured geomembrane with textured side down. This will ensure no tension in the geomembrane. This is a result of the friction on top being less than that on the bottom of the geomembrane.
One is going to need to do a good job with the filter design of the geotextile for the drainage geocomposite. If fines from the CCR (particularly if it includes fly ash) pipe through the geotextile of the drainage geocomposite, this will cause trouble. There may be a chance that the friction on a contaminated interface becomes greater than the bottom-side textured geomembrane to the GCL. If this is the case, the geomembrane will be in tension, which will be problematic for long-term stability.
Depending on how the CCR is placed, the range of shear strain or displacement needed to trigger the residual strength of these interfaces is very low. It is probably in the 3%–5% strain range. Therefore, the interface will slide suddenly with little warning as opposed to other more elastic interfaces in your cross section.
Please be very careful with this. These points deserve much more discussion beyond casual questions and answers in the GMA Techline. You need a professional engineer for this design and an accredited lab to run direct shear tests to establish real answers for such a challenging application.