For any landfill slope lining system once you get steeper than a 1v:3h gradient, then the possibility that the system will fail is great - there are also practical aspects of associated with slope gradients and what type of waste id contained in the landfill itself.

Generally, landfill covers I have worked with have always been 1:7.5 to 1:10 in gradient, to enable effective earthwork activities and installation of gas collection systems.   

I am working on one that is quite steep.  It is an existing, closed landfill and I am not quite sure what the actual slopes are, however from a stormwater / erosion standpoint it is extremely unstable. I am working on a drainage repair project to reconstruct / repair / enhance some of the brow ditches and downdrains which have problems with siltation, overtopping and extreme amounts of erosion.  Maximum 10H:1V slopes would be a huge improvement!

3H:1V is typical and I'm not sure steeper would work considering interface shear.  Include in your design "let-down" ditches for surface water also.

f-d

¡papá gordo ain’t no madre flaca!

I've seen portions of old landfills that were greater than 3:1, but they had to be regraded to 3:1 or less per regulation prior to capping. Landfill slopes greater than 3:1 can be a long term maintenance problem.