I'm not talking about setting them directly into the clay/dirt. I'm going 3' deep and using gravel to set. With it being an area that will be consistently moist plus our climate, my understanding is that utilizing the drainage afforded by the gravel should keep the posts drier/allow them to dry faster than concrete would. We have 8' as code here, and the fence will only be 6'. The bottom block will be essentially buried and the second will be specifically to hold the level under the fence, not really adding height. 30 years of erosion from the yard into the easement has caused a noticeable drop off at the fence line.