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Replacing wood fence posts at the same location
5

Replacing wood fence posts at the same location

Replacing wood fence posts at the same location

(OP)
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.

RE: Replacing wood fence posts at the same location

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?

RE: Replacing wood fence posts at the same location

(OP)
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.

RE: Replacing wood fence posts at the same location

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-s... 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.

RE: Replacing wood fence posts at the same location

(OP)
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,

RE: Replacing wood fence posts at the same location

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.

RE: Replacing wood fence posts at the same location

3

Quote (miamicuse)

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.

RE: Replacing wood fence posts at the same location

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.

RE: Replacing wood fence posts at the same location

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.

RE: Replacing wood fence posts at the same location

(OP)

Quote:

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.

RE: Replacing wood fence posts at the same location

(OP)

Quote:

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.

RE: Replacing wood fence posts at the same location

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.

RE: Replacing wood fence posts at the same location

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.

RE: Replacing wood fence posts at the same location

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.

RE: Replacing wood fence posts at the same location

(OP)

Quote:

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.

RE: Replacing wood fence posts at the same location

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

RE: Replacing wood fence posts at the same location

(OP)

Quote:

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.

RE: Replacing wood fence posts at the same location

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/frequentlyaskedques...

RE: Replacing wood fence posts at the same location

Quote (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?

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.

RE: Replacing wood fence posts at the same location

(OP)

Quote:

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?

RE: Replacing wood fence posts at the same location

I'd at least try the steel. If you have trouble getting into a tight spot, use a steel rod. Home depot rents smaller vibratory compactors with a 14" plate. If you want to get it to a level of compaction that comes close to approximating undisturbed soil, you'll probably need a bit more than hand tamping.

RE: Replacing wood fence posts at the same location

Quote (miamecuse)

After that I backfilled the hole completely, every 4" or so, I compacted with the 2X4...

That is why 2x4 hand compaction did not give the desired results... proper hand compaction this way need to be performed in small lifts, say 3" thick soil at a time. Compact the entire 3" lift with the 2x4, then add another lift and repeat. Hand compaction performed this way will be superior to any machine compaction performed on a completely backfilled hole.

Note: Even machine compaction is supposed to be performed on lifts, often specified 6" thick.

For future reference, for small quantities of backfill, say, around a post in a small hole use dry bagged concrete mix (Sakrete, Quickrete, etc.). In areas with high ground water (like Miami), soil moisture will cause it to set enough to be more or less equivalent to CLSM. Like CLSM the hydrated mix will be weak enough to allow fairly easy removal in the future. You can "save" some money, if you want to by using the dry mix for only the top 6" to 12" inches of the hole. Use compacted soil backfill below the dry mix. This works almost as well as 100% dry concrete mix backfill.

Note: The dry concrete mix makes an excellent backfill immediately (while it is still dry). The optimum mix of fine and coarse aggregate makes mix, right out to bag, ideal backfill.

RE: Replacing wood fence posts at the same location

Quote (miamicuse)

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.

This is the best approach, with one caveat. Your order of operations here is set sonotube - brace sonotube - place concrete inside sonotube - let concrete cure - backfill around sonotube.

You cannot compact soil around a sonotube to the level required for real lateral support without collapsing the sonotube. Do it after you place the concrete and you can ram it in as hard as you want without issue.

Bracing the sonotube is relatively easy; this is how it's done in the commercial world:



In other words, you don't have to go crazy. Couple 2x4s, couple wood screws, couple concrete pins and you're done.

RE: Replacing wood fence posts at the same location

Would diagonal braces be allowed? You could use a brace from the top of the gate post to the bottom of the next post to control sag of the gate.

RE: Replacing wood fence posts at the same location

(OP)

Quote:

That is why 2x4 hand compaction did not give the desired results... proper hand compaction this way need to be performed in small lifts, say 3" thick soil at a time. Compact the entire 3" lift with the 2x4, then add another lift and repeat. Hand compaction performed this way will be superior to any machine compaction performed on a completely backfilled hole.

I probably worded it poorly, I did do small lifts and compact as hard as I can. I said 4" lifts but more or less I was doing two to three shovels, then compact with the 2X4 into the hole all the way around, then I ran water into the hole to further settle the soil, the repeat until the hole is completely backfilled. I have a steel tamper but I really don't think it is as effective, because the footprint is bigger so when I get close to the edge of the hole, the lifting of the tamper out of the hole and back down will actually disturb the soil near the edges. With a 2X4 8' long I can pound and pound straight down AND at an angle.

I did notice that when I finished compacting one lift, and added water, the water does not drain as quickly, and I thought it was may be the soil has much less voids with the compaction. But then when I poked at it with a stick, the wet soil looks "mucky", seems to remember back in the school days taking soil mechanics this may be a "plastic" state and this is not a good thing? Did I over compact the soil such that it gets that way?

RE: Replacing wood fence posts at the same location

You cannot 'over compact' soil by hand. You could do it with equipment, but you're not using equipment.

You can, however, add too much water. If you want 'perfect' compaction, you have to add the exact right amount of water. If you add more water than the 'perfect' amount, you get less compaction no matter what you do.

I suspect you're probably adding way too much water.

RE: Replacing wood fence posts at the same location

It sounds like it is time to buy concrete posts that have been colored to nearly match the wood
Those are what we used for gate posts at my daughter's house in Palm Beach.
The we fastened the hardware to them with Tapcons.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Replacing wood fence posts at the same location

(OP)

Quote:

It sounds like it is time to buy concrete posts that have been colored to nearly match the wood Those are what we used for gate posts at my daughter's house in Palm Beach.
The we fastened the hardware to them with Tapcons.

That will not fly where I am. I am in a historic district where materials used are very restricted. For fences it's basically wood or wrought iron, but wrought iron is not typically usable for a privacy fence because of it's transparency, and wrought iron fences of many configurations are not allowed due to it being climmable by kids if there is a pool or pond deeper than 24" on the other side of the fence (and I do have a pool and a pond). Something that looks like "wood" will not be acceptable to the historic preservation board, everyone tried to use composite sidings that looks like wood but they have not accepted one petition yet and everyone had to use real wood sidings.

RE: Replacing wood fence posts at the same location

Sounds painful.

RE: Replacing wood fence posts at the same location

Maybe hire or rent a concrete hole boring rig with a 6 inch bit and run it down until it hits dirt or bottoms out.

If it bottoms out the go down the middle of the post with a one inch bit and then out to the sides with 1 inch holes, leaving wood in the corners. Snap off the concrete chordal bits and then break off the wood in the corners and put an extension on the coring bit and repeat. The bits I see are about 14 inch internal depth.

Unless they were maniacs with the concrete that should carry most of it out after 2 shots. If not, then look at renting a drill that can take out 3 to 3.5 inch diameter of wood for as deep as the post goes. Put in a 1 inch steel pipe and loosely tamp a pile of charcoal and fit up a blower to the pipe and light up the charcoal. When the charcoal is going OK, turn on the blower. Don't go with 100 psi - 2-3 psi should be enough to get the charcoal going and burn out the remaining wood. Stay upwind. See if the fire department has any worries about underground fire, but I think flooding the hole and then sucking the water back out when the wood is gone should be enough to leave a clean hole and no fire. Remember, this is after going nearly 30 inches.

If the concrete runs out before then, then drill for and install a large lag bolt to pull the stump out with. You can use the gaps in the concrete from the hole boring as clearance to remove dirt along the sides of the post - I've seen people using vacuum cleaners to remove the loose material so it doesn't fall back and block the view. How you grip the bolt is up to you maybe weld a piece of threaded rod to it and use a plate and a nut to pull with at the top so any failure doesn't release a lot of stored energy.

RE: Replacing wood fence posts at the same location

I thought I typed "coring rig". Sigh.

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