Replacing wood fence posts at the same location 5

[miamicuse]

Location is Miami, Florida and the water table is about 24" below existing grade.

I have a 6' tall wood fence with 4X4 posts spaced at 4' interval (hurricane code). I also have a 10' wide double gate (each gate 5' wide) and the posts are 6X6 pressure treated lumber.

Posts are excavated with a post hole digger down to 24", then the post inserted, concrete mix are poured into the holes till level or slightly above grade.

Over the years the wood posts would rot (from moisture, from termites) and need replacement. The regular fence posts one can dig a hole say 2' over and install all new posts shifted and the old posts cut off at grade. However the gate posts cannot shift.

The old gate posts was removed by excavating around the concrete anchor, thus making a wider and bigger hole, then the post and concrete extracted, new posts inserted into this much bigger hole and more concrete poured into this hole to make a bigger wider anchor.

After three replacements over the past 15-20 years the hole that was excavated for the gate post went from 8" diameter to 14" diameter to 20" diameter. It is getting too wide and too heavy to attempt another iteration. If I am able to pull out the current posts and huge concrete anchor, leaving a 20"+ wide crater, how can I install a new post at the same location?

If I backfill soil/sand into the large hole, and compact the soil as much as possible, then RE-excavate a 10" diameter hole from the backfilled hole, then insert the new post and pour concrete into the new hole, I assume the post anchors will not have the same strength as the backfilled soil will have a lesser bearing capacity than the original undisturbed soil. I wonder how much weaker it will be.

I know another thing I need to tackle is to stop the posts from rotting at the base. I planned to wrap a piece of ice&shield barrier around the post where it meets grade, and build up the concrete anchor into a pyramid to allow water to run off, so hopefully that will make the posts last longer.

 

[bimr]

Use a 12-Inch Sonotube and backfill. A mix of gravel and soil, tamped down in 6-Inch lifts as you backfill will generally provide good support. In unstable soils, post depth needs to be greater. Crushed limestone is also a good backfill material.

Have you considered using a plastic or composite fence?

 

[miamicuse]

Thank you.

If I use a sonotube to "hold" the space for the concrete anchor, then backfill and compact around it, when fully compacted, install the post and fill with concrete? So the sonotube will be left in place?

I wonder if during the compaction I would end up damaging the tube if I tamper down real hard. The tube will be wet from the soil moisture.

Past excavations showed water starting to pool at 22-24 inches deep, after that the post hole digger will bring up mushy sand that when I dig deeper the sides will cave back. I am not sure I can practically do more than 24" of depth.

I live in a historic district and the historic preservation board has to approve all improvement on the exterior. Fences made by not natural materials such as PVC, composite, galvanized chain linked are not allowed. Wrought iron fence with over 80% transparency (see through visibility) are sometimes subjectively allowed depending on the Historic Preservation Board member makeup on the night of the permit approval hearing and the tone of the owner pitching the improvement. So wood is kind of the only choice.

 

[LittleInch]

Ones I've done if rotten I just dig out the rotten timbers in the concrete hole, use a wet and dry hoover to suck out all the bits and once you get a bit out the rest follows quite easily, sand down the new post and then hit it at the top with a big hammer....

Or you can get spikes like this

https://www.screwfix.com/p/sabrefix-fence-repair-spike-75-x-75mm-2-pack/23093

which you insert in and then bolt on a new post.

Tend to be a little less stable but you can pack them up a bit before you bolt them in

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[miamicuse]

These are 6X6 posts with 5' gates hanging on them so stability is important to prevent leaning gates.

I have in the past tried digging the wood rot out, in my case it didn't work because it actually got a lot harder once the loose pieces are out. The post is only rotted at the point where the post meets the gate, about 4 to 6 inches below. After that the wood is solid and sound to the bottom. So it's not like you can just unwedge the 20" or so piece of 6x6 wood that's been wedged in for years. I have actually tried using a 18" long auger bit to drill dozens of holes down in an attempt to break the wood up so I can free pieces of it with a digging bar, with no success. The bottom line is, for this to work you would have to really go at it with a heavy bar and drill pieces out bit by bit, and by the time you get half way more than likely the concrete would have been cracked from the prying of the wood and you have to remove the concrete anyways.

I did some research two years back and it seems the reason the rot is always at the soil interface line is because the bacteria that causes the rotting lives in the first few inches of soil, so further down there is no such bacteria so no breakdown of the wood. The remedy is to make sure no contact of soil with the post (by using concrete and build up the anchor to slope away from the post) and put a barrier between the post and the first few inches of soil which some use a peel N seal wrap, some used roofing tar and painted the "foot" of each post,

 

[bimr]

Does the Sonotube form have to be stripped from the column after the concrete has cured?

Removal of the form is general practice for exposed column surfaces. In applications below grade, there is no specific need to strip the form unless it is required by local building code. The Sonotube Round form contains no hazardous chemicals and is biodegradable except for the 1-2 mil thick plastic liner on the inside of the form.

Link

If the sonotube is loose in the hole, be sure that you brace the sonotube prior to placing the concrete.

 

[SlideRuleEra]

Location is Miami, Florida...
So wood is kind of the only choice.

If you want to end this rat-race once and for all, quit buying typical 6x6 "pressure treated" 6x6 from places like "big-box" stores.

Instead, go to a marine timber dealer (there should be plenty around the Miami area coastline), and purchase 6x6 treated with 2.5 lb/ft³ retention CCA (Chromated Copper Arsenate wood preservative at a concentration for full seawater immersion).
Typical supplier: Link

You won't have to replace these fence posts again because of rot.

 

[hokie66]

SRE has the idea for the posts. Or better still, a galvanized steel post.

But you are right, stability is the issue for gate posts. I doubt any of the options other than full removal of the existing concrete and replacement with concrete will work. The bigger the hole, the better. And yes, building up a shallow truncated pyramid around the base will help.

If you don't have a jackhammer, either buy a cheap one or rent one.

 

[phamENG]

What about setting a galvanized steel post for the gate and wrapping it in treated lumber? Use PAFs or other fasteners to attach it. When it rots, take it off an attach a new one. A lot less work than digging up the post and starting over. And it likely satisfies the historic review board since it'll still "look" authentic.

 

[miamicuse]

If you want to end this rat-race once and for all, quit buying typical 6x6 "pressure treated" 6x6 from places like "big-box" stores.

Instead, go to a marine timber dealer (there should be plenty around the Miami area coastline), and purchase 6x6 treated with 2.5 lb/ft3 retention CCA (Chromated Copper Arsenate wood preservative at a concentration for full seawater immersion).
Typical supplier: Link

You won't have to replace these fence posts again because of rot.

I have been using the ground contact pressure treated wood which I understand is one step better than the regular above ground PT lumber. I did not know about the marine grade lumber. I will check into it. Thank you.

 

[miamicuse]

But you are right, stability is the issue for gate posts. I doubt any of the options other than full removal of the existing concrete and replacement with concrete will work. The bigger the hole, the better. And yes, building up a shallow truncated pyramid around the base will help.

If you don't have a jackhammer, either buy a cheap one or rent one.

The issue is not the actual removal. I can get help to tease out the anchor and post, even pull it out with a truck, or whatever.

The problem is the current anchor is already 18" to 20" in diameter. It is simply too big. These are gate posts, so in between them is a driveway, I would have to remove part of the pavers on the driveway to remove this concrete. I want to go back to a smaller anchor but with a large crater like hole. If that's possible.

 

[phamENG]

For the foundation, if you're looking to be able to repeat this again in 10 years, best bet is probably to rip out what's there and back fill in shallow lifts with structural fill and compact it. Once that work is done, you'll be about as close as you can come to imitating 'undisturbed' soil. Use an auger to drill your new post foundation.

 

[CarlB]

And as a variant to phamENG's idea, instead of backfilling hole in layers, which sounds labor intensive, fill the hole with CLSM. Should be able to auger through this to set new posts.

 

[phamENG]

CarlB - for a standard fence post, sure. But for a post supporting an 8'+ wide vehicle gate...I'm not sure I'd want to rely on non-compacted, low strength fill.

EDIT: My mistake - see post below from dauwerda for what CarlB is really referring to.

 

[miamicuse]

But for a post supporting an 8'+ wide vehicle gate...

Let me clarify, the post is not supporting a gate 8'+ wide. It is a 10' wide double gate, so two 5' gate, each mounted on their respective 6X6 post.

 

[hokie66]

Do the gates sag now? If so, you don't want to make the problem worse.

 

[miamicuse]

Do the gates sag now? If so, you don't want to make the problem worse.

I am not sure I follow the question. The gate posts are rotted at the base where they meet ground, I would say probably 50% of the materials are gone.

The gates are currently not operational, because they are sitting on concrete blocks so they stay level but unmovable. I have several 2X4s propping up the gate posts to keep it up for now. The posts are going to be removed, the large chunk of concrete anchors along with it (in one piece or broken in chunks), the issue I have is once that's done, how would I deal with a 24" diameter crater, I want to avoid filling two big large holes of concrete.

I think the suggestion of using marine grade timber with a higher concentration of chemicals will help a lot.

I am also considering the option of just backfilling the holes with sand and dirt, and compact as I go to get it back to something close to undisturbed soil as possible before re-excavating a 12" hole to set a new post. From my experience this newly compacted soil will be kind of weak. Over the weekend I did a test. I excavated am 8" hole with a post hole digger down to 24" deep. I used a 2X4 to just pound on the ground say 4" or so from the edge of the hole, and the hole kept it's shape. I then widened the digging to 12" to 14" diameter, to a bigger hole same depth. After that I backfilled the hole completely, every 4" or so, I compacted with the 2X4, then wet the soil with some water as well. Once backfilled and compacted, I let it sit for 2 days. Then I used the post hole digger to dig the same 8" hole in the same spot. That hole was not as clean as the original hole. With a 2X4 knocking the ground a few inches outside of the hole, the wall of the hole started to fall in, not completely collapsing but definitely feel much looser,

May be I will try the 12" sonotube approach, set it into the large hole, insert the post, then need some 2X4s to brace the post so it stays level. Then backfill the soil around the sonotube and compact as much as possible without deforming the sonotube. Then pour concrete into the sonotube. Once the concrete hardens, compact the soil around the sonotube a second time, as much as possible.

 

[dauwerda]

phamENG, CLSM is going to be way stronger than any traditional structural fill. I believe 50-100 psi CLSM is usually specified when it needs to be excavated in the future. So, 50 psi would equate to a bearing pressure of 7200 psf, right?

https://www.concrete.org/tools/frequentlyaskedquestions.aspx?faqid=745

 

[phamENG]

phamENG, CLSM is going to be way stronger than any traditional structural fill. I believe 50-100 psi CLSM is usually specified when it needs to be excavated in the future. So, 50 psi would equate to a bearing pressure of 7200 psf, right?

hmmm....find me a hat and I'll eat it. For some reason I was thinking of it as a class of soil for back fill. Pretty sure that's what I was taught when I was intern, and I haven't really dealt with much earth work since then. Clearly I was mistaken. Thanks for that.

miamicuse - what kind of engineering do you typically practice? Compacting with a 2x4? I'm not surprised it didn't work.

 

[miamicuse]

miamicuse - what kind of engineering do you typically practice? Compacting with a 2x4? I'm not surprised it didn't work.

I do mostly drainage design.

I have hand steel tampers in 8X8 sizes. But yes I prefer to use a piece of 2X4 because it seems to be more effective when the hole size is irregular shaped and small, because it can be used to pound hard at various angles to get things as tightly packed as possible. The soil where we are (in Miami) is very sandy. Watering it actually do a pretty good job compacting as well.

Unless you think some sort of compacting machine will be useful for a hole 18, 20, 22, 24 inches in diameter?