New privacy fence bowed down from wind

[kco]

Like Mace I also worked in the power industry for several years, much of it designing wood pole power lines. Depending on the soil types and conditions, we often had to do soil probes to determine the soil bearing strengths at various depths for setting poles normally from 6' to 8' deep. In normal solid soils we found the critical condition exceeded on a pole set 6' deep was the pressure about 2' from ground level. Although the pressure is much less than at the bottom of the pole, the soil is less dense and resists far less pounds per square foot. Simply setting the pole deeper was often the most cost-effective solution. But if that was not sufficient, another solution was to set a treated log "key" on each side of the pole about 2'below surface.

The problem may be at the soil surface--not the bottom of the hole. Depending on your soil, a possible solution might be to remove the soil near the top and backfill with gravel to about 6" below the surface, or place rocks or treated planks on each side near the top to act as a "key". Can you tell if the fence is leaning because it failed at the surface, or did the bottom shift?

 

[Creamer]

I have to agree with Coyote - if the wind pushed the posts over and the posts are not damaged if you could gently push them back and brace them and let the ground settle you should be fine - unless of course the soil type and location is one that will always get soggy and never will be stable against the winds even a year from now. If that is the case I would still push it back straight and then only dig out the top of the post above the concrete and fill that in to give a larger surface area to push over and reduce the leverage the wind has on the concrete in the bottom of the hole. This should not be a total redo.

Also, keep in mind that a round object (think concrete around post) will push through the soil easier than a square one so if you dig the top of the holes out square them off. Since this is probably shovel work anyway square would be easier than round.

Good Luck! I would try Coyote's suggestion first though.

 

[ray66v]

29" is simply not deep enough for a fence that size, in ground that gets that wet.

Unless you put a yard of concrete on each pole, they are going to move.

When you use gravel, if you don't use a gravel sand mix, it holds a lot of water, and that compounds the problem with the soft ground.

 

[AchingBack]

Where did you get that recommendation from? I'm a Journeyman Lineman having worked on both distribution and transmission crews for 2 decades for the provincial electrical utility and have set more 35 to 85 foot poles than I care to remember and I've never heard of that.

Power poles and fence posts are different animals.
Fence Post Hole Digging
Outdoor Structures (Home Repair and Improvement, Updated Series): Time Life Books: 9780783539034: Amazon.com: Books

 

[lzicc]

I pulled some of the post back and braced them for now so the wind doesn't do anymore damage. I think what I am going to do is dig down about 1' on each post and fill the top with another back of concrete mix. I think having the bottom already in concrete and then pouring a bag on top, that should stead this from happening again. Trying to dig the hole completely back out is going to be too hard and possibly overkill. I have 20 post to dig out and I just got back from Lowe's and got 20 - 80lb bags of concrete mix.

I did think about just bracing it for the winter and let the dirt settle which may work, but I would feel better putting concrete in the top part of the hole.

We never had wind like this before. Crazy.

 

[mbohuntr]

I built a solid fence 9' tall, and 80' long out of rough sawn pine about 8 years ago. I went down between 3'-4' until the auger hit hard clay it wouldn't cut through. I then put in PT 4 x 4s and concrete. Then I installed angle iron knee braces and embedded their bases in more concrete.
That fence has withstood some pretty strong winds without bowing at all. The new owners ought to be thankful.... :confused3:

 

[Mace Canute]

Power poles and fence posts are different animals.
Fence Post Hole Digging
Outdoor Structures (Home Repair and Improvement, Updated Series): Time Life Books: 9780783539034: Amazon.com: Books

The mechanism responsible for "frost jacking" power poles and fence posts is the same.

From Fence Post Hole Digging

Frost starts to heave your concrete upwards
using the tops of those V-shaped concrete footings to grip them (since the tops form a lip).

Dirt then slips in beneath the concrete. Your fencing is raised as a result. The process is then repeated next winter, raising your fencing a bit further. And so on.

I've seen lots of smooth poles "frost jacked" that were backfilled with just dirt and I've seen lots of steel posts that had the same "V-shaped concrete footing" that author speaks of that were never "frost jacked". An engineer who worked for us (provincial electrical utility) said he had seen a leg on a steel lattice work tower pulled in half underground because of "frost jacking". Those towers sit on a large square steel plate to keep them from sinking and the plate is well below the frost line so it was just the dirt frozen to the galvanized steel angle iron leg above the base that supplied all the gripping force and as the frost line moves downward the moisture in the dirt turns to ice and expands inexorably raising everything above it.

I would say the moisture content in the soil has the most influence on how much "frost jacking" is possible.

 

[AchingBack]

I just installed about 300' of 6' vinyl, solid privacy fence last week. I augered down about 3', put the vinyl post in the ground and pour about a bag of concrete in each post, I did mix the concrete first, then filled the rest of the hole in with dirt. Today I came home and allot of the fence is bowed down now. It's was extremely windy today. I can't believe that actually happened though. Is this normal to happen? I am going to have to dig each post out and fill the hole with concrete to fix it.

In case you haven't had a chance to go the link I referred to, here is the page:

Fence Post Hole Digging
Valley Fence LLC Contractor located in Spokane Valley Greenacres, Washington

Fence Post Hole Digging
Tips on digging your own fence post holes
The trouble begins when
Frost starts to heave your concrete upwards
using the tops of those V-shaped concrete footings to grip them (since the tops form a lip).

Dirt then slips in beneath the concrete. Your fencing is raised as a result. The process is then repeated next winter, raising your fencing a bit further. And so on.

Eventually, your concrete footings will be significantly elevated beyond where they should be, resulting in instability.

Of course, such movement will never occur evenly along the whole course of your fencing, resulting in yet another problem: your fencing will no longer be level.


If you're still intent on setting fence posts yourself, rather than having a professional do it,
The key is in filling the hole only two thirds up with concrete
Even though it seems this would result in less stability, this is not the case.

Instead, you're depriving Jack Frost of a lip he can use to get a tighter grip on the concrete footing.

After the concrete sets, fill up the rest of your hole with your excavated soil (or gravel in damp areas), and tamp it down firmly.

Setting And Concreting fence posts
Leveling and evenly spaced is only part of the challenge.

The problem begins with the shape our holes normally take. The top of the average hole that you dig is bound to be wider than the bottom, since you have easier access to the top portion. The result is a V-shaped hole.

Novices anticipate no problem in setting fence posts in such holes, then filling the holes up with concrete.

But when you fill those V-shaped holes with concrete, you end up with V-shaped hunks of concrete. And in areas subject to frost, this can be an invitation to trouble down the road.

The best looking one is a to have a
Bell shaped hole
in other words larger at the bottom then the top, these type of holes are very difficult if not impossible for frost to heave upward, thus making your fence look nice after many winters.

 

[Hiltz]

Anyone try the the 2 part liquid that turns into a hard foam for setting fence posts? It looks promising but if I remember correctly it was a little pricey. I also saw a video where a crew was using something similar to close up old verticle mine shafts. Maybe a solution to your problem.

 

[skyhook]

I would also put "T" bar in the hole, about 4 to 5 feet deep and up against the post, then fill with cement, this will give it a little more strength, if possible, tie them to the post. Thats quite a long fence and since there's no room for air/wind to pass through, you may still have problems, I would have staggered the fence, leaving room for wind to pass. Hope it works out.