Leachate implies that we're dealing with a landfill, which have regulations defining what is required. Most leachate is stored within a tank that has a secondary containment system (e.g., dual walled tank, lined tank vault). If we're talking about the leachate collection system in the landfill, at the minimum would be the regulations.
Typically from top to bottom is some type of dual liner system with a leak detection system:
top
1) 1-2 feet Drainage layer (Note: can be replaced by a geonet) (this is where the leachate pipes are located)
2) 60 mil HDPE (plastic liner)
3) 2 feet of clay with a maximum hydraulic conductivity of 1 x 10^-6 cm/s (Note: this may be replaced by a GCL liner if clay is not available)
4) 2 feet of drainage layer (Note: can be replaced by a geonet) (This is where you have a leak detection system)
5) 60 mil HDPE
6) 2 feet of clay with a maximum hydraulic conductivity of 1 x 10^-6 cm/s (Note: this may be replaced by a GCL liner if clay is not available)
7) Compacted soil
Bottom
The purpose of the dual liner system is that if the top liner system (Items 2 & 3) leak, there's a secondary system (5 & 6) to minimize the potential of releases from the landfill.