recommend from bottom to top
1) gravel base layer
2) sodium bentonite GCL liner, 1 x 10 -6 cm/s
3) 60 mil HDPE geomembrane secondary liner
4) geonet collection layer
5) 60 mil HDPE geomembrane primary liner

need leak detection and collection system in layer 4

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.

clay does not protect ground water. it just slows down the rate of infiltration so that the rate of contamination is extended into the future. Better or thicker clay just slows it down further. the only way to prevent leachate from reaching the water table is to collect it and remove it. So, not only is leak detection necessary, collection and removal is also required when the primary liner leaks. that means a manifold of pipes leading to a sump pump.

placing thick layers of clay is very costly unless you have a stockpile of clay within a close haul distance from your project, so GCL is usually far more economical. clay or low permeabilty materials which contain clay can be used for top cover and drainage layer over the landfill to reduce the infiltration and limit the amount of leachate produced.

the permeability of compacted clay can vary widely depending on the compaction moisture content. If you are compacting dry of the, "line of optimums" you will get a much greater permeability value then if you compact wet of the line of optimums. Compacted clay liners should really be specified for placement, based on level of saturation. We used to specify greater than 90 percent saturation. We would also prepare our compacted clay samples using 90 percent saturation.

f-d

p.s., I'm guessing your permeability in the OP is meters/second, which would relate to 10-7 cm/sec. That's very typical in the US. An earlier post suggested 10-6 cm/sec. Local environmental agency may have requirements of one value or the other.

ípapß gordo ainÆt no madre flaca!