In reply to alps_p:
The TNF bib has a reinforced knee and seat in 80D, the Arc'teryx Beta AR pant has exactly the same setup. I've used those for 3 years, on ice, mixed and skiing without any worries for durability. In fact I have been incredibly impressed by how well the material has lasted, and touch wood... not a single hole anywhere... yet!
As for layering, I use nothing more than a merino base-layer under my trousers, have never needed any more even when climbing in temps down to -25 and ski-touring in -38. Though it depends on how you feel the cold. But the fit may not allow for heavy layering if they're athletic cut!