I'm installing a wooden fence around my back yard using prebuilt cedar panels and 10 ft (3+ m) 4x4 pressure treated posts buried 3 1/2 to 4ft (1+ m) deep. The posts are packed in with pea gravel to allow for drainage.
Most of the post holes I've dug go through 2 ft (60 cm) or so of hard clay before hitting a softer sandy soil. On one corner of the lot however, the hard clay continues down the full 4 ft. I placed a few feet of water in a hole to try to loosen the soil for digging (which helped greatly), but after 48 hrs the water level had only gone down a few inches.
What can I do to get these post holes to drain properly?
I have heavy clay that won’t drain. I dipped the end of my pressure treated posts in liquid asphalt (roofing material) and let them dry before cementing them in. 20 years, and they are still in pretty good shape.
Dig them deeper, perhaps 6ft or even 8 and fill up to level with the pea gravel.
That should provide sufficient volume to drain the water from the posts. However there may be other water seeping in.
One solution I saw was to drill into the post from the bottom and meet that centre hole with a small hole from one face. The bottom hole is capped and then a waterproofing / preservative is injected through the side hole. Retreat annually or as needed.
An old claim staking trick is to first dig the hole to the depth you want. Put the post (claim corner) in the hole. Pack 1 inch (large) gravel only in the hole. Wiggle the post to pack the gravel around the post. The pebbles wedge between each other, the post, and the side of the hole. Work the gravel into the hole until the post stops wiggling.
An option I hadn't originally mentioned would be to trench the length of the post holes and add a drain. Trenching the length of the fenceline would have been a lot of work and likely destroyed a handful of productive fruit trees.
After conversing with my neighbor it turns out he has a drain pipe 6 inches (15 cm) into his property line used for his rain gutters which drains to the municipal storm drain and happens to be buried 3 1/2 feet (1+ m). I was able to access and drill into his drain pipe (with permission) with only a little extra digging. The posts don't quite go as far down as I had planned but I think this is the best solution.
I will be accepting Anthony Stevens's answer because it is what I would have tried if this option did not become available. I added this answer in case someone else runs into a similar problem where trenching is an option.