I would tend to think effectively c=0 be overly conservative. On the one hand I have seen examples in book by the head of geotechnical studides in the civil eng school in Madrid in the 30's considering some cohesion; as well, almost every book has a table for the case with cohesion, which should not be the case if proper would be never considering some to stand. I would say that for generic cases reducing cohesion to one half the engineering safe value should be enough. One can as well think that as soon as the soil gets clayey enough, permeability is greatly reduced, so the main concern is not to allow any separable (from a cohesionless inset) wedge to fail...what will be uncommon if the soil is clayey and homogeneous due to the scarce surface permeability (ensure all water will go quickly drained, if necessary by the same surface). OK, longterm all will be flate (till meteorits come)...but, what is the frequncy of a rainstrom so savage as to undermine the whole cohesion of your site? I think you can count reasonably with some cohesion standing. Of course planting will help the absorption of what rained increasing one formerly dry status for another storm.