A couple of weeks ago my son in law and I put in five 4x4x8 fence posts so he could attach lattice to provide some privacy for their pool. I went over there this past weekend while he was attaching the lattice and I noticed the posts had a little bit of movement in them at ground level. Each hole took 60-70lbs of concrete. Any ideas what caused this? The holes were about eight inches wide and posts set 24” deep.

Were they new pressure treated post? Seems kind of quick but perhaps they are drying out and shrinking a bit.

Is the ground soft.
I have had the best luck Just packing the post in dry concrete and letting them set over time.

The diameter of the hole sounds a little small and the depth of the hole is minimal at best given the small diameter hole. For 4x4 posts, I've generally made the holes about 12" at the top with a slight taper at the bottom. An 8 footer would usually be sunk around 3 feet deep. That was for actual wooden fencing though, not lattice.

Was the concrete old? Maybe sitting around for a year or two?

I was surprised one time when I sank a 4x4 post and used bagged concrete that was in the garage for maybe a year or two. It didn't cure properly. It didn't set. I have since found expiration dates on bagged concrete. I had no idea.

are the poles loose in the concrete, or is the concrete loose in the earth? I guess either way I'm voting moisture reduction.

They may "water in" over time. Most importantly they need to be kept plumb. We try to set posts to a minimum of 30", they will be a little shakey at first but a year later things will firm up.

Graboids.

quote:

Originally posted by RichardC:
Graboids.

I’ll get the elephant gun.

You picked the wrong rec room to break in to....

We killed it, you got that!? We killed that motherhumper, come back!

quote:

Originally posted by old rugged cross:
Is the ground soft.
I have had the best luck Just packing the post in dry concrete and letting them set over time.

That's not luck, that's the right way to do it. Wet concrete in the hole leads to the "tootsie pop" syndrome after it sets where the moment arm on the post gives the concrete ball on the end of the stick a huge mechanical advantage to displace soil and lets the whole assembly wiggle. So much better to account for settling and movement with a less rigid and monolithic structure.

With pressure treated lumber, it is OK to dispense with the dry pack and just set in naked soil, in which case a vigorous application of the butt end of the digging bar to compact the fill is recommended. A couple of double handfuls of coarse gravel or river stone in the bottom of the hole, under the post and not around it to aid in drainage is a good idea, but not essential.

I've seen this with my own posts, mine was different soil for that particular post, combined with insuffient depth, too much water in soil, insufficient amount of concrete, etc. But mostly the soil, as other posts with the same amount of concrete and depth were solid but in "better" soil, whatever that is, I'm not a soil person. But I've got loamy soil, clay, sandy soil, depending on location, dry soil, and exceedinly wet soil, it's all over the map.

If you soil is clay, you'll need to water them when the soil dries. Clay shrinks. In my last house - terrible clay soil - I could actually pull out my round fence poles in July.

I don’t claim any particular expertise, but the following has always worked well for me:

Dig the hole three feet deep. Typically 9-12” wide at the bottom. Set post and start filling with lightly moist soil, roughly 4” at a time. After putting in the lift, tamp all the way around with the flat round end of a digging bar. By the time you’ve got three lifts in, slapping the posts will cause it to just vibrate while still staying plumb and tight. Now put in the concrete. I’ve had good success with both wet and dry application, but I like the dry better as I don’t have to mix it. If I’m particularly worried about the load on the post, I’ll go wider with the top part of the hole, even as much as a couple feet if it is an extreme load on a steel post. Depending on the aesthetic requirements, I either bring the concrete right up to grade, or stop a couple inches short so the concrete is covered by a light layer of dirt.