Q: In a geosynthetic clay liner-geomembrane (GCL-GM) geocomposite, does it make any difference which side of the geocomposite is up: GM or clay?
A: This is a very detailed question that is application specific. The following design details all need to be considered: Is the GCL-GM geocomposite used for a liner or a cover, is the project in a seismically active zone, what is the steepness of the slope, where is the water coming from, what is the filling (construction) sequence, etc.? One could teach a course on this topic. I would strongly suggest you consult with the manufacturer and follow its recommendation for your application, realizing that most have a diverse product line.
With the above written, the geomembrane components of most GCL-GM geocomposites are generally designed and installed with the geomembrane side in direct contact with the leachate or waste.