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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?

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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.

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  • even just cementing them in with well compacted concrete will work well. – Jasen Jul 6 at 11:42
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    I'm trying to avoid concrete. Concrete is quite hygroscopic and hydrophilic. It is often associated with rotting the posts within 5-10 years. I see this in other fences in my neighborhood. Dipping the post into asphalt will likely help with that and may be the ticket. I wonder if the concrete would still be necessary with the asphalt or if the compacted pea gravel would suffice. – psaxton Jul 6 at 16:46
  • I don’t think the concrete is necessary, as long as the posts are buried deep enough. This is where clay is your friend! – Anthony Stevens Jul 6 at 19:37
  • Other than the weight of it, concrete doesn't really hold the posts in place, that's still the ground: clay or otherwise. I've removed posts set in concrete simply by rocking them back and forth and side to side. But if you've made them water-tight, the concrete won't wick the water to the wood either. – JimmyJames Jul 6 at 19:46
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Your other option is to fit a short concrete post, and bolt the wooden post to it. It's more expensive, but the concrete won't rot when the hole fills with water.

Around here they're often called a 'grandfather' but apparently the official name is a 'repair spur'

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    'Godfather' is what we call them.Somewhat different! Grandfathers wouldn't tie you to one and drop you in deep water... – Tim Jul 6 at 16:25
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    I've also seen solutions where a metal brace is screwed or set in concrete and the pole set on top of the brace so that the wood and concrete don't touch. – JimmyJames Jul 6 at 19:40
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    This is best practice generally when building a fence. Most professional fence-builders don't do it though, because rotting posts guarantees them repeat business! Posts will always rot off very quickly at ground level, but rotting higher up is much slower. The concrete spur holds the post in place, even after the rot at ground-level has happened. And when (eventually) the rest of it gives up, fitting a new post is a trivial job with a drill and a couple of bolts, instead of a major project excavating the old post and concrete. – Graham Jul 7 at 8:57
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    @JimmyJames Those work really well, but you need a solid base to put them on. Great for mounting posts on an existing patio or parking area. If you're putting posts in soil though, you'd need enough concrete to keep everything upright, which tends to be excessive. The repair spur is already made of the right stuff and is long enough to hold solid by leverage in the soil. – Graham Jul 7 at 9:03
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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.

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  • I always appreciate your answers. I am afraid digging deeper may be the only solution. I would like to avoid it if at all possible -- getting dirt out of a narrow hole past a few feet is a pain. If I get no better answers, I'll accept yours. – psaxton Jul 6 at 7:21
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    There are tools designed exactly for that - like two shovels bolted together, or hire an auger. – Solar Mike Jul 6 at 7:23
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    @psaxton if you're planning on going 6-8' plesae rent an auger! The few bucks on that will spare you considerable time and immense soreness... – FreeMan Jul 6 at 13:09
  • @FreeMan: Augers available for rent around me only go to 3' depths. With a bit of shoveling around the top, I can get as far as 4 ft, but that doesn't get me to the 6-8'. I have a 6' long digging bar I've been using for soil too hard for the auger to get started in. Also, pro-tip: An old shop vac removes the dirt left at the bottom of the hole quite nicely. – psaxton Jul 6 at 16:58
  • You can't get extension bars? I purchased one for the small auger I bought. Was only about 8 or 12", but they could add up... – FreeMan Jul 6 at 17:00
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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.

Works as well a concrete.

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  • Thank you for your contribution. I am actually doing something similar with crushed pea gravel. The problem I am encountering is that the soil will not allow the water to drain from the hole. I'm not sure switching to a larger aggregate would help with this problem. – psaxton Jul 8 at 20:32
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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.

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