Setting a post in difficult ground

BarnyardEngineering · Jun 5, 2023

Replacing a rotten off 4x4 post in a privacy fence. When I originally put it in I hit a boulder right where the post needed to go. It's at least 3' in diameter, and the edge of it is right where the post needed to go!

I ended up setting the post in concrete next to the boulder, but even there I could only get down about 12" before hitting what seemed to be more solid rock.

This time I came armed with an SDS rotary hammer. Busted out all the old concrete. Rattled a bunch of rocks loose and pried them out. Got down as far as I can reach with a coffee can. Probably around 30".

Which approach should I take to burying the new post? I do NOT want to repeat this digging job again.

1. Direct bury the post in the dirt like the rest. At least when it rots out I can pull it out of the ground with a handyman jack and slip a new post in.

2. Concrete a 4x4 1/4 wall steel sleeve in against the boulder so when the post rots out I can slip the old post out and drop a new post in.

Stephen Newell · Jun 5, 2023

If your treated post rotted, it's not what you did but the wood wasn't properly treated. I have some which are around 38 years old. Some of these companies will just hose off the wood with the chemical. Then you end up with a post that is only treated about a 1/4. The center of the post then is untreated. Yellawood is the worst offender. You shouldn't have any problem with Lifewood brand however with any brand you have to insure the wood is rated for ground contact. The above ground stuff is made like Yellawood.

To properly treat wood the lumber is kiln dried to a very low moisture content and then put in a tank which is pressurized so the chemical goes through to the center.

old-9 · Jun 5, 2023

Backfill with gravel, holds like concrete but digs easy.

redforlife · Jun 5, 2023

Concreted in? For a privacy fence? Sounds
like overkill to me. I also think that
concrete around a post shortens the life of
the post and hastens the ground rot
(moisture can't get away).

But, .... might be a good idea if you are
going to do the steel sleeve thing this
time. But if you do that, I'd insert sleeve
& post into the hole and let free stand
until post is fastened securely to the
fence. Temporairily brace the fence for
linement and straightness if you have to.
Then put the concrete around the sleeve. Do
the concrete last, instead of first. Do
whatever it takes, to do the concrete last.
Take down temporary braceing once concrete
is set up.

I'd only use the concrete if doing the
sleeve. But that's just me.

Gary Mitchell · Jun 5, 2023

Along those lines, I made a 4x4 post out of pine two by fours for my mailbox ages ago and soaked the bottom in Penta over night before tamping it in. It managed to last 25 years. gm

ss55 · Jun 5, 2023

I have added an additional post a foot or two offset from the original posts that were rotting at ground level. 70 MPH winds cracked off two cedar posts in a tall wood shadow box style privacy fence. The fence and posts were still in good condition from the ground up, but all the posts were already fairly rotted at or below ground level. Installing another set of posts eliminated having to tear apart the old fence to replace the old posts.

A clam shell post digger and post hole spade worked to dig the post holes even with the edge of the fence. I used pressure treated 4x4s for the new posts, bolted the new posts to the existing fence rails, back filled the posts, trimmed off the tops and then stained the new posts to match the rest of the fence. I lucked out as the existing cedar stain was easy to match with an off the shelf stain, so only the new posts needed to be stained, using less than a quart of stain on nine new posts.

Crazy Horse · Jun 5, 2023

I worked a summer job in the late 60's for a fencing outfit. We dealt with a LOT of wood posts set in concrete that rotted off right at ground level. Before the days of preserved or pressure treated posts of course. My own experience is that a wood post set in concrete is more likely to rot off at ground level than a post set in the normal soil/clay or in a gravel/sand mixture, etc. Another factor is how old you are .... for me it wouldn't make much difference at 77.

Geo-TH,In · Jun 5, 2023

Pole barn post are the old cca.
Try to find the old posts used for
construction.
Menards sells 2x6 CCA boards.
I would make a post using CCA boards if I
couldn't buy CCA posts.

Stephen Newell · Jun 5, 2023

What is Penta? I tried looking it up and there was nothing.

HFJ · Jun 5, 2023

pentachlorophenol

shaun wallace · Jun 5, 2023

In my country if its not a T post its set in concrete whether wood or pipe. Its easy to dump a little water in hole, dump in sac-crete little more water and on to next post.

Hawkeye85 · Jun 5, 2023

Set the post using clean gravel as backfill. 30 inches May
work, depends on your frost line, but for a privacy fence I
wouldnt worry about it too much. Drainage around the
post is important, hence gravel.

Hawkeye85 · Jun 5, 2023

Agree with the gravel.

Stephen Newell · Jun 5, 2023

Looks like something the government wants to make go away. As best as I can tell from a quick search you have to have a certification to use the stuff now.

redforlife · Jun 6, 2023

I'd be curious to know what the purpose is, if your area requires it.

BAnNC · Jun 6, 2023

Lumber treaters still do some in CCA for use near the coast. See if your local guy won't try to order you some.

BarnyardEngineering · Jun 6, 2023

(quoted from post at 22:56:43 06/05/23) I'd be curious to know what the purpose is, if your area requires it.

The purpose for what, the fence itself?

It's so my neighbors don't have to look at my junk and I don't have to look at their junk.

The town limits fences to 3' high unless you have a compelling reason for a variance. This fence was there when I bought the property, so as long as I keep repairing it, it is "grandfathered in."

Tony in SD · Jun 6, 2023

If you do not want a replay of replacing a rotten post. Make sure you us a post treated for earth contact. The pack it in tightly. If it rots it will be at ground level. Wood can survive air. Wood can
survive water. But wood can not survive air & water together. That is why a tightly packed post will be perfectly good below ground but rot at ground level.

Stroby · Jun 6, 2023

Just use lime to fill the hole. Pack it as you fill. Sets up very firm.

BarnyardEngineering · Jun 6, 2023

The posts I used WERE rated for ground contact. Lasted 16 years.

Rotten all the way to the bottom. Can't pack sand. Drainage is excellent. Where this is is sand 20' deep.

This post was edited by BarnyardEngineering on 06/06/2023 at 05:02 am.

Setting a post in difficult ground

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