Calling Pelican Civil (and un-civil) Engineers

[widebody911]

Last summer I put up shade sails in my back yard. There's 4 4" steel posts each set about 4' in concrete. There are 2 3/8" galvanized cable loops running lengtwise, spanning ~23' each, with turnbuckles on one end of each loop. I attach the sails to the cables.

Over the winter I had taken everything down and de-tensioned the cables. I put everything back up last weekend, but I noticed I wasn't getting any tension when I tightened the cables, Upon further investigation, I found that poles one side of the yard - the side on which the turnbuckles are installed - where starting to pull, so there is now about 2" of free space on the back side of the concrete anchor. The other end of the yard is fine.

So, apparently the ground around these wasn't tight enough. This side gets the runoff from my carport, which is the only thing I can think of that's different between the side that's pulling out and the side that isn't.

So - how to fix it? My idea is to remove the sails and cables, and excavate around the existing anchor, then pour in more concrete. I can probably also rig a come-along to pull the pole back into position.

Thoughts?

[javadog]

I would pull the poles out, beat the concrete off of them and drill larger diameter and deeper holes. Set them in new concrete. Wait a week, then put it all back together.

[Alan A]

I looked at an in ground basketball hoop last night. It required a 2’ diameter sonotube set 3’ deep. Surprised the hell out of me.

I can’t imagine that that giant sail requires a lot less.

[911boost]

Here’s a cement cat, since I know you like them so much.

Not that it helps.....

I would re-excavate and pour bigger holes. Can you change the run off?

[URY914]

More mass.

[masraum]

Interesting. Bigger around means more surface area, but deeper means more leverage. I wouldn't think that you'd need deeper than 4'. That seems pretty deep, so i guess bigger around. My advice on this subject is not worth much more than you're paying for it.

Good luck, and wow.

[dad911]

Or triangulate. There is a large moment at the base of the pole, as it is acting as a long lever.

can you run a guy wire (to an anchor, I would try a 'screw in anchor') like they do with utility poles?

[javadog]

I hope you did not use sonotubes when you set these posts.

[id10t]

Not any kind of an E ...

but I'd dig a deeper (slightly - 4" maybe, just so you can have the same level but have a few inches of new concrete over the old) and wider hole, and scrape out parts at the bottom so you have an over hang. Think of what the inside of a 22 case at the rim would look like kinda. Put your pole and plug back in, pour new concrete around and over to totally encase and hopefully get the "rim" part you dug out filled with concrete, forming a mechanical lock. Drilling holes, chipping pieces, etc. on the cement plug will help the old form a mechanical bond with the new as well.

I'd also bet that part of the ground holds more moisture due to the extra run off you mentioned and therefore there was more effect from the freeze/thaw cycle as water turned into ice and expanded, etc.

[911boost]

It’s CA, I doubt he has much freeze thaw, or is there?

[id10t]

Originally Posted by 911boost

It’s CA, I doubt he has much freeze thaw, or is there?

Dunno, why take it down for the winter then?

[widebody911]

Originally Posted by javadog

I hope you did not use sonotubes when you set these posts.

I did - why is that bad? I only used it for the top 2', so I could have the concrete above grade slightly.

[Hugh R]

I looked into something similar at my old house, because I was in a canyon with winds up to 60mph, the contractor wanted to put in 3' diameter piers, five feet deep.

[masraum]

Originally Posted by widebody911

I did - why is that bad? I only used it for the top 2', so I could have the concrete above grade slightly.

Right, enquiring minds want to know!

[Superman]

I've got a lot of respect for civil engineers. And wind. If I offered suggestions about diameter and depth, I would be talking out my @ss.

That said, if you could make foundations for 'guy wires,' I suspect engineers might smile upon that. That's a lot of stress at the ends of long posts. If you place enough concrete for the guy wire anchors, then I suppose you could attach them high-ish on your posts.

As I say, I got great respect for civil engineers. Mother Nature is going to destroy whatever you build. It's just a matter of time. Better to overbuild when dealing with her.

[eastbay]

Originally Posted by widebody911

I did - why is that bad? I only used it for the top 2', so I could have the concrete above grade slightly.

The top 2' is where the majority of the lateral holding power will be. That is why you should not have used the tube, you want the concrete to physically touch the dirt to avoid any airspace or voids. You can use the tube from ground level up if you need the form.

Before ripping it out I would look into the foam products for post settings (check YouTube), you may be able to push the post straight and then foam the gap.

[ckissick]

I'm a soils engineer so I might be of some help. The post moved because of a lack of passive resistance in the soil to counteract the moment arm caused by the tension in the wires at the top of the post. It doesn't look like there's room for a guy wire.

You just need deeper piers. They need to be deep enough to mobilize passive resistance in the soil. The upper 12" or so of soil is typically ignored since the soil deforms upward due to a lack of confining pressure from above. The depth depends on the strength of the soil (which translates to passive resistance) and the force from the wires. Wind load would be low since the tarp is horizontal.

How deep are the piers right now? I bet if you go 5 feet deep that will be enough. 16" inch diameter should be enough too.

[javadog]

Originally Posted by widebody911

I did - why is that bad? I only used it for the top 2', so I could have the concrete above grade slightly.

As someone else previously noted, you never do this for anything that needs to resist forces in a lateral direction. The reason is simple, you cannot get a tight fit between the sonotube and the hole, which means the portion of the pier that is encased in the sonotube is contributing nothing to the lateral resistance. This is probably your entire problem.

You need to redo these piers. Make the diameter large enough that you are bearing against undisturbed soils. You might have a little difficulty getting the old ones out of the ground without damaging they soil around the holes. Personally, I would probably rent a boom truck and pull them straight out vertically. If you want to use a short piece of sonotube above grade to form a round shape, that would be fine. I would also put rebar cages in the holes, I’ve never done a pier without steel in it.

[KFC911]

Originally Posted by Alan A

I looked at an in ground basketball hoop last night. It required a 2’ diameter sonotube set 3’ deep. Surprised the hell out of me.

I can’t imagine that that giant sail requires a lot less.

Only if Zion Williamson is gonna use it....the goal & metal pole....(damn straight it hurt ) my dad erected 5 decades ago still awaits my return....but not until I can dunk (again)....or not

Mebbe a playground "dunkin' goal" for the brothers....I still remember a 7' Tarheel shattering my hs gym's backboard....ended the "fun" game....I still remember the force though....can't relate...but saw it happen.

[CurtEgerer]

Lots of civil engineers on the board, apparently. I'm civil/structural. Given enough bandwidth, I'm sure we could collectively come up with a design and calculations to rival the moon landing

But basically, this is a simple flag pole type foundation. As someone mentioned, they are much larger in diameter than you might first think. Your's are too small unless, possibly, they are set in stiff clay. Since we have no idea of the soil type here (sand, clay, loose gravel??), assume it only has a 'lateral' strength of about 30 PSF (pounds per square foot). That's not much. Think about it. If you pushed as hard as you could on the top of these tall poles/levers (they appear to be 8-10 feet tall), 30PSF resistance against the side of the foundation isn't much to overcome. So, as several have mentioned, you need larger diameter piers. I don't think depth is an issue. 4 feet is plenty - that's nearly half the pole height! But I would go with minimum 18-inch diameter hand-dug piers (and maybe 24 if you have loose soil). And yes, lose the Sonotubes.

If you really want to do it right, use a corrugated steel ground sleeve (with a plate welded to the bottom) in the middle of the hole before pouring the concrete. Pour the concrete around the sleeve, leaving the sleeve hollow/open. Then you set the pole inside the sleeve and tamp sand down around it. This makes it easy to center, plumb, and adjust the pole. At the top, you take a short piece of Sonotube and set it so it's a couple of inches above ground, with the top sloped for drainage. This gives a finished appearance and keeps water away from the pole. Nearly all flag poles are done this way.