pile foundation uplift resistance

[Lucian09]

Hi guys,
Concrete pile-type footings (for individual metal posts of a structure), 16" dia cylindrical, 36" below grade (6" above grade). Soil is clayey silts. Is there a simplified way to estimate the uplift resistance strictly due to soil's lateral pressure and cohesion to the concrete (besides the dead weight of the footing)? There's no collar at the bottom of the footing. Thanks.

 

[HTURKAK]

The common practice , calculate the compression capacity of the pile , half of the compression capacity is assumed to be tension capacity.
The best method, conduct tension test..

 

[geotechguy1]

Ask a geotechnical engineer

 

[phamENG]

At 36" below grade I'd put it at zero. Probably a regional thing, but with those dimensions I would call that an augered pier or a "caisson" type footing, not a pile foundation. I don't start thinking of it as a deep foundation element until you get to at least 10 feet of embedment, and then only rarely (in my area, if you need piles 25 friction piles are the shortest that make sense for anything bigger than a kid's playhouse).

 

[jayrod12]

At 36" below grade I'd put it at zero.

Agreed. Around here just to counteract the potential for frost jacking of the pile our minimum length is 25 feet as well unless you do something else to mitigate frost.

 

[Lucian09]

Zero uplift resistance from soil cohesion and pressure - hmm... by that thinking most decks, prefabricated sheds, and similar structures, would be flying with the wind. By that thinking you'd just pick up a 4x4 post fence out of the ground, while the reality is that you could not unearth it without applying an uplift force 2x or 3x its weight, or more if soil's compacted around it.
There are some methods that go by the weight of the soil of an inverted truncated cone shape based at the bottom of the footing, and others that go by soil-to-concrete cohesion and soil shear value. Does anyone know where to find some numbers for the latter please (perhaps as a fraction of the soils's compression value)?

 

[Retrograde]

Do you have any SPT or CPT data for the soil?

 

[phamENG]

Lucian - I didn't mean that it doesn't exist. I meant that I wouldn't count on it in design. It depends on so many variables that change frequently. Especially in clayey silts. Ground water tends to fluctuate seasonally, and it will have an important impact on the capacity you're looking for.

These are the sorts of things that I like to rely on "just in case". I know there's friction between the rafter and the top plate of the wall, but I don't consider it when I determine the number of nails required. I know that a steel shear tab is going to behave more like a moment connection under service loads, but I don't consider it fixed when I do deflection calculations. I put what you're talking about in the same boat, until it reaches a level that is consistent and reliable. Like what you find in deep pile foundations.

For what's worth - I've seen quite a few decks, sheds, and similar structures that have flown away in the wind, because they were not adequately anchored. I'd like to see a hurricane try to take my kid's playset...

 

[Lucian09]

"Retrograde" - just assuming 1,500 psf clayey soil
"phamENG" - thanks, but I'm afraid I disagree. With a factor of safety to account for irregularities, soil weight and lateral pressure is taken into account for uplift resistance. The entire industry of prefab small buildings relies on that.

I was just looking for a simplified method to factor it in.

Assuming good cohesion between the pier and the soil (which should be the case as the piers were poured straight in the ground holes), the soil weight of the truncated cone at 45 deg. seems like a reasonable addition to the uplift resistance, but I wonder if the soil shear around the cone rupture surface shouldn't be added too. Again, all with the factor of safety.

 

[Lomarandil]

If you're reliably above the water table, I'm happy to use 50% of the (downward) side capacity (e.g. excluding end bearing) for uplift.

If you have a periodically high water table, self weight (minus buoyancy) only.

----
just call me Lo.

 

[Retrograde]

For that type of clay I would use a value of about 400psf (hopefully that's about right - I normally work in metric and I find the American unit system confusing).

Having said that, I agree with phamENG. In normal pile design I would neglect the top 1m of the pile's resistance due to uncertainties around soil shrinking, disturbance etc.

 

[r13]

In pier/pile design, we usually strip the topmost 1' layer as ineffective. It leaves 24" for a pole foundation...may tip over by itself.

 

[Lucian09]

I saw soil shear figures varying from 10% to 50% of compression strength. 10% makes sense to me, so I'm using 150 psf here. The structure is rigid - think of it as a universal design of a table-like structure sitting on 4 legs, on piers embedded 3' into the clayey ground. Subject to straight vertical uplift, and uplift from overturning. I'd think it's safe to design for half of the total wind uplift to apply to one corner/leg only.

Soil is above the water table and stays mostly dry. Worst case scenario, I'll take only the bottom 2' for soil rupture shear resistance and I'll consider it a straight (vertical) rupture closely along the rough surface of the pier. Thus no weight of a displaced cone of soil is considered. Given the 16" pier diameter, does 1.2 kips soil shear resistance per pier sound reasonable to you guys?

 

[r13]

Without going to textbooks, I suggest to design the pier as indicated in the graph below. If you insist to relying on the friction for uplift, I think you may apply U=c*A_o for clayey soil, and U=P_p*µ_c for sandy soil. I would provide a safety factor at least equal to 2.

 

[oldestguy]

Not knowing soil conditions or water table, I'd not go over about 250 psf. Don''t forget to allow for sauration of the soil meaning that part below grade does not weigh 150 pcf, but more like 85.