Before I moved to Oregon, I had never seen a warning sign saying “sunken grade”. In fact, the first few times I saw one I wasn’t exactly sure what it meant. (Trust me, those grades weren’t very sunken, otherwise I would have figured it out much sooner.) Even Yahoo Answers doesn’t have the exact right definition of a sunken grade.
A sunken grade is a place where the road literally sunk down, either into some sort of sink hole or because the underlying ground has shifted. Around here, we call them “blue clay slips”. It’s where the roadbed has a layer of blue clay under it and when the clay gets waterlogged it turns liquid, and slips … often taking all or part of the road with it. That’s what we had ….









