I imagine that overbreak due to blasthole misalignment and blasting are going to make a mockery of any estimate based on thickness.
Your best bet is estimate based on 200mm -300mm thick, and then place concrete as best you can, giving just 25-50mm cover over the high spots.
Make sure it all...
Have you verified a methane problem by testing, and not just based on a bad smell. Methane is odourless but often accompanied by hydrogen sulphide (bad egg smell)
Treating the smelly part would be simpler if there was no major methane problem.
I assume below that you have detected methane...
Are you talking about
1 concrete cast on top of previously cast concrete ? in which case the surface preparation of the first layer is a factor.
2 or between a concrete block and a concrete surface ? again the finishes on both surfaces are important.
