Critical Geosynthetics Interface

Q: I have a question regarding critical geosynthetics interface for a final cover system. The cross section from top to bottom is as follows:

• 18-inch cover soil
• Drainage geocomposite made up of a geonet thermally bonded to NPNW geotextiles on either side 
• Double-sided textured geomembrane
• 500 g/m² NPNW geotextile gas vent
• Soil
• MSW waste

I didn’t expect the most critical interface to be internal to the drainage geocomposite. When I spoke to the lab that conducted the direct shear tests, they told me that historically, the most critical interface in final cover system failures across the U.S. has been with the GC (i.e., GT to GN). Is this true?

A: Unfortunately, this is true and has to do with geometry and the poor ply adhesion strength of the geocomposite. Note that the textured geomembrane has nearly 100% contact surface area with the NPNW GT of the GC and often has a Velcro®-like effect at the interface. The net, on the other hand, is only bonded to the GN in about 50% of the surface area. In addition, if the ply adhesion strength of the GT to the GN is low or nonexistent, the friction angle between them will be around 18 degrees (3:1 slope). Often, this is the weak link in the system. I strongly recommend reading Richard Thiel’s keynote Rowe lecture from the 12th International Conference on Geosynthetics, May 26, 2023.