Drilled Pier Uplift Resistance?

[tocoadog]

Hello everyone.

I am looking for advice on how to determine uplift resistance for drilled pier design? The pier is for a sign approaching 75 feet high.

Soils consist of medium dense to dense (N of 15 to 33 bpf) silty clayey sand underlain by stiff to very stiff lean clay (LL 49, PI32). The moisture content of the lean clay is close to the plastic limit. All we have is N values, sieve and atterberg limits for lab results. Small commercial client does not have the money for extended laboratory testing.

Anyway, hopefully someone has a literature reference you could point me to.

Thanks in advance.

 

[Focht3]

What are the soil layer thicknesses? Sign geometry? (I'm having a hard time seeing uplift forces on a sign foundation. Please help me out.)

I have a technique to use; just not sure it's appropriate -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[tocoadog]

Thanks for responding.

Thicknesses consist of 0-17 feet, SM-SC. 17-40 feet, CL.

Actually I misspoke in my original post. The sign configuration is still being designed but height limits could reach 200 feet with sign dimensions of 20 X 20 feet.

The structural engineer is the one who requested uplift data. Anyway, I think it's conservative to provide uplift data due the thickness of the underlying clay layer and lateral loads. The sign is going to be in a windy part of New Mexico. Additionally, the LL, imo, denotes some swelling potential.

Any help, insight, contradictions, you have would be great. And even if you don't agree that I need uplift data, how to find it in the future would be great too. Bottom line, I'm open to criticism on my reasoning. I'm always learning that's for sure and will always take a second (third...) opinion.

Thanks.

 

[Focht3]

Argh! I can't find the uplift reference; it's in an ASCE GT journal article sometime after 1982 (I think it was 1986 or 1987.) The paper's authors treated the problem as an upward bearing capacity problem, and showed that N_c is a bit above 9 where D/B > 4. I simplified the chart for my own use - and set

N_c = (2[×]depth/B) [≤] 8

I also use a higher factor of safety because of the consequences of failure -

But the problem you describe is a lateral load problem, not uplift. (That is, unless you are dealing with a group of drilled piers and a pile cap - which is possible with a sign that's 200 feet high.) You may need bells for the axial forces (which seems doubtful) but will need the piers to have L/d values of about 20 (guess) to limit the lateral deflections.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[JAE]

With drilled piers in buildings, we would get from our geotechnical engineer (I'm structural) a simple formula for the uplift force on a drilled pier (with bell) due to swelling of expansive clays. This would give me a force to use to determine an amount of reinforcing in the drilled pier to include to resist this uplift and avoid a horizontal crack across my pier. In south Texas, some buildings have historically been pushed up where all or many piers were cracked through.

But this is a vertical effect in a building due to clays swelling.

For your sign, I would think (as Focht3 insinuated) that there is a large lateral force on the pier, with a moment at its top as well, and this creates lateral forces on the surrounding soil - causing a vertical rotation in the pier with a pivot point somewhere near the 60% depth point.

As a structural, I'd want to know the stiffness modulus of the soil along the length of the pier (and whether it varies with depth) and an allowable bearing value along the depth of the pier.

With these parameters, I could model the pier and analyze its deflection and stress on the soil.

Uplift on a sign? Not sure how this is relevent.

 

[Ron]

I would be more concerned with overturning moment at the top of the drilled pier than for uplift. If you run through a true uplift computation for the dimensions you have given you'll find it to be small relative to the dead loads of the sign and pier. The resultant lateral load from the moment couple should be a major design consideration for the pier. Hopefully it can be socketed into competent material to develop moment capacity.

 

[MotorCity]

A few comments:

1. Although you could have uplift forces due to negative skin friction, I would not expect these forces to control the design.

2. I do not recall the equation off hand, but there is a formula to compute the resistance to uplift forces (i.e. dead load + skin friction + upward bearing capacity of a bell if there is one). You should be able to find it in almost any foundation design book. It is essentially the pull out capacity of the section.

3. Ron - Not sure if I understand your comment. How do you overturn a drilled pier?

 

[Focht3]

[blue]MotorCity[/blue]:

Think of a wind load applied to a 20 ft by 20 ft sign 200 ft above the ground; the ground line shear isn't the major load on the foundation - it's the moment. This moment is commonly referred to as the 'overturning' moment -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[MotorCity]

Focht3,

I understand what an overturning moment is, I am just having a hard time visualizing a drilled pier actually overturning. I guess I am assuming that a drilled pier, like a pile, is almost entirely encased by soil, thus preventing any sort of overturning (unlike a retaining wall that could overturn).

 

[Focht3]

If they aren't adequately embedded into the ground they will "overturn"!




Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"