Fenceposts in clay soil
November 1, 2016 5:15 PM   Subscribe

Hello! I am looking to install some fenceposts in soil that is very clay-heavy, and wondering what I should set them in. Details (WITH SCIENTIFIC DIAGRAMS) inside!

I'm building a privacy fence, and have dug out the holes for the fenceposts: 3 feet deep, about 12" wide, for posts that will be 4x4 pressure-treated ground-contact-rated wood. Frost line's about 24" deep, record one-day rainfall was about 5.5", set last year (though usual rainfall's closer to 5" per month).

In many guides to fencepost installation, I've seen the setup in Figure 1: just tamped gravel, so that rain can drizzle right down it and get soaked into the soil below.

In some guides, I've seen Figure 2: a "cuff" of poured concrete around the post, with the bottom of the post open onto a base layer of gravel; again, rain can soak through that base and disappear into the soil. If rain somehow gets between the post and the concrete, no worries: gravity pulls it right down into the gravel, the post stays dry, all is well.

MY PROBLEM IS: down about 8 inches into our soil, it turns from a nice loamy soaky veggie-growing kind of texture to almost pure clay. Like a plow pan or something, it's almost dang solid clay, like I was pulling up chunks I could throw on a wheel and make into a vase while getting frisky with a spectral Patrick Swayze.

Because the clay isn't great at absorbing water (it just hangs out there, like in a cup), my worry is that any water draining to the bottom will just hang out for days in the gravel layer (Figure 3), eventually rotting the wood, and softening the clay until the gravel shifts under the weight of the fencepost and the fencepost goes all slanty.

I wonder if I might try the setup in Figure 4: solid concrete in a kind of "sock" all the way around the fencepost (maybe with a caulked silicone line sealing the top of the concrete to the post). The downside is that there'd be no drainage from the wood down into the soil; the upside is that there'd be no room for standing water.

Does anyone have experience with setting posts in clay? If so, WHAT THINK YOU? What are my best practices here, ye fence-builders?
posted by Greg Nog to Home & Garden (9 answers total) 4 users marked this as a favorite
Concrete is not impervious to water. So if the soil down in the bottom of your figure 4 is very wet, the concrete will be wet as well and you will get rot. Sealing around the top won't help (and probably won't work) because the concrete will just absorb moisture from the surrounding soil.

Gravel is best unless you need additional structural support from, in which case you want figure 2. Best is to have a few inches of gravel in the bottom of the hole (where water can sit and slowly be absorbed into the surrounding clay), then the post above surrounded by gravel. If this is strong enough for your application, and it very likely will be as long as you compact the gravel around the post, then concrete isn't going to gain you anything.

Just as another idea, the metal spikes that you pound into the ground to hold a 4x4 work pretty well if you bury them down into your hole.
posted by ssg at 5:34 PM on November 1, 2016 [4 favorites]

Most of your rot concern will be taken care of by using pressure treated posts. Posts set in concrete is common enough, but calking them is going overboard, IMO.

Can you punch through the clay with your planned 2' hole? I guess not.

If it were me, I'd probably dig a hole 1/3 the length of the exposed pole plus 4". 10' pole with 2' in the ground. Then fill the hole with 4" of pea gravel. Set the pole and fill with 2-4" of pea gravel surrounding the pole. Tamp well in lifts.
posted by humboldt32 at 5:37 PM on November 1, 2016 [2 favorites]

I just did this a few weeks ago. Our soil is pretty much pure clay. I talked to the guys at the hardware shop and they sold me some bitumen paint for sealing the bottom few inches of the wood, and recommended setting the sealed wood in concrete all around. We also put a square metal base at the bottom of the hole first (I used termite shields, but more for the waterproofness rather than pest control). I don't know if it's perfect, but the hardware guys seemed to think it would be fine.
posted by lollusc at 5:39 PM on November 1, 2016 [2 favorites]

First of all, congratulations on taking the most scientific approach to fencepost-planting in all of recorded history. But so far, no fence post has lasted more than a hundred years or so, unless made of stone.

You do not want to go with Figure 4 and encase your fencepost in concrete all the way around. The postulated silicone sealing will almost certainly break down and permit water in, and then you're screwed.

Unless the water table is above the bottom of the fencepost, drainage is your friend. So I second ssg, use plenty of gravel.
posted by beagle at 5:42 PM on November 1, 2016

Ah, my boyhood fencing dad's farm pays off. Based on generations of experience, dad and our neighbors built usually with all of the above and more, like your figure 4, with pressure treated posts often slathered with tar/bitumin to above the concrete cap. For posts at the bottom of a hill, we spiked barbs of old cultivator blades upside down to prevent the post from being pulled by the wire fence out of its concrete sock. (Barbs shouldn't be needed for your situation.) The post hole was bigger at the bottom than at the top, with perhaps an inch or two gap around the post at the top. We first splashed in a few inches of concrete, dropped in the post. We filled in the rest of the concrete mounded to an inch or two above the surface so that rain would run off. Later, we'd take old cans or scrap sheet metal and after applying a bit of tar, nail on a metal cap to prevent water from entering on the top of the post.

When we dug post holes at the bottom of a hill and didn't set the post the same day, overnight the water table would flood the hole. Unavoidable. Just bail it out. Posts at the top of a hill did not have such problems with subsoil water, and might not get the concrete treatment. Even when we used steel instead of wood posts, they got the barbed anchor, tar, and conical concrete sock if they were at the bottom of a hill. Even at the top of a hill, corner posts got the anchor, tar, and concrete and diagonal braces. As I recall, rotting was never a problem even when the concrete sock cracked. Water will come, just try to divert what you can from prolonged contact. I suspect that with a concrete cap like figure 2, worse than rotting, in dry times the post would loosen from lack of water.

A privacy fence won't have tight-strung wire yanking posts up, but even on level ground there will be significant lateral stress from winds. Winds tend to come with rain. Tree roots can push a fence up or sideways. In my new suburban spot, I set a fence with the above precautions. A neighbor had a wooden fence installed with no concrete. Alas, his fence fell down with the first big storm, mine is still standing thirty years later.
posted by gregoreo at 7:04 PM on November 1, 2016 [5 favorites]

I would skip the wood fence posts entirely, in favor of steel set in concrete. These things are what the pros generally use to connect fence panels together nowadays, because they are just so easy and will last forever. There are also no call-backs because one of the posts dried weird and has giant bow in it. They are about 3-4 times as expensive as wooden posts, but make up for it because they are faster for the initial install, and they also make your fence last basically forever. By keeping the wood off the ground, and protected from a regular source of moisture, your fence should pretty much last forever if you keep it sealed. They are definitely a do-it-once-do-it-right kind of product though, due to the cost, which is not insubstantial if you have a large yard.

I personally hate removing old rotten fence posts sitting in concrete (in fact, I have avoided several for an entire summer!) so it is worth it to me to take this approach for next summer's big fence project.
posted by rockindata at 7:05 PM on November 1, 2016 [1 favorite]

Finished this barn last winter. All posts were treated 6"x6" set in concrete for stability and to displace water in the soggy spots. https://www.instagram.com/p/BD3uCmVhe70/
Same principle as setting fence posts.

Concrete is a hassle, but it's the way, the truth, and the life when you can't have shifting. Stone gravel can work if you can't do concrete but avoid round "pea" stone - use angular gravel (1") so it can nestle and lock.
Good luck.
posted by greenskpr at 7:30 PM on November 1, 2016 [2 favorites]

I'm going to assume the clay is too deep to punch a hole down to the next layer of dirt and just fill the hole up with gravel. Honestly, I'ld start with half a foot or so tamped gravel under the post, then tamp gravel up to two or three fingers below the top of the clay bed, then fill the rest of the hole with clay. Or go with a steel post in concrete.

If it's one of those things that's likely to nag at you and you want wood, primer and paint the wood, double the gravel under the post, pour concrete from four inches below the clay's top to at least two inches above the soil, trowel a slight slope from the post to the edge. Who ever has to take the posts out will be cussing you. gregoreo's answer is pretty much definitive, I just wouldn't want to work that hard on a fence that's not for keeping livestock in.
posted by ridgerunner at 8:07 PM on November 1, 2016 [1 favorite]

This is not be the recommended way to do this so take it for what it's worth.

In 1996 in almost identical clay soil conditions my father and I set about 20 treated 4x4s to build a pergola over a walkway. This was our process:
1. Dug a 3 foot hole.
2. Set and leveled the posts
3. Dumped a bag of dry quickcrete in the hole
4. Filled the hole with water

Fast Forward 20 years later:

This summer we pulled out about half of the posts to do some new landscaping. There was zero rot and the bottoms were encased in concrete which had set just fine. The most damage to the poles was 20 years worth of weedeating against them. I plan to cut the posts down and use them for supports for an above ground pool deck next summer. I'll probably set them the same way.
posted by jmsta at 5:15 AM on November 2, 2016 [6 favorites]

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