alternative foundations for poles

[smulmi]

Hi all,
I'm looking for alternative foundation designs for 70 ft tall poles resisting axial, lateral and moment at base. The soil is medium to dense silty sand with phi of 28 to 30 degrees. Also at one of the location of the poles, peat was found in between the sand layer; this one definitely calls for piles or driven shaft foundation.

Any suggestions are warmly appreciated in advance.

Thanks.

 

[MiketheEngineer]

Do you want to build footings or are you going to "bury" the poles??

 

[smulmi]

I am looking to build foundation footings for the poles. They are actually stadium light poles.

 

[MiketheEngineer]

Sounds like BIG DEEP blocks of concrete.

There are a number of good books on this subject. Google it.

 

[smulmi]

MiketheEngineer,

I am reading quite a few literature on bored piles and driven piles as two alternatives.
The problem with piles from my preliminary calcs was the uplift force vs the uplift resistance. From the geotech report, the uplift resistance per pile is, I think, calculated as a fraction of skin resistance and the wt. of the pile. Which gives rise to a question: "Can I also incorporate lateral soil resistance to uplift due to overturning moment for both piles and driven shafts?"

 

[MiketheEngineer]

I am NOT a soils guy... Just trying to aim you in the right direction.

 

[aeoliantexan]

Not sure I understand your last question,but a pile cap on four or more piles or piers would not be too unusual for supporting a tower or chimney. Moment and uplift resistance are likely to control, so a wider base is more efficient. Allowable skin friction for uplift is often taken as 2/3rds that allowed for downloads. Be sure to check the weight of a cone or prism of soil resisting the combined uplift on two or more piles. You can analyze lateral resistance and deflection on a pile that also carries axial load using LPile.

If the sand is truly dense, you may have difficulty getting driven piles down to the penetration required for uplift.

If you use drilled piers or ACIP piles be aware that excessive soil removal can loosen the sand and reduce the capacity of adjacent previously-installed piles or piers.

Get a geotechnical engineer to help with the analyses.

 

[smulmi]

aeoliantexan,

Thanks for your response. Regarding the uplift capacity, does negative friction have any effect (positive or negative) on it? Based on the values provided from the geotech, the uplift capacity of single timber pile (FS=3) is greater than the compressive capacity (FS=2).

Thanks

 

[EBSEngineering]

Why not try a helical pier/anchor? They are good in tension and compression. You will have to sleeve it to get the lateral resisitance but you can do that with a Helical Pulldown Micropile from Chance. Used them before, works well.

 

[JedClampett]

I've had to review submittals for some poles. The foundation design is basically a deep cast in place concrete pier 3 or 4 ft. in diameter, depending on the pole diameter, and 20 ft. deep (or more). FDOT has a program that designs the poles from top to bottom, called High Mast. And if it's good enough for Florida with its high winds, it's probably good enough for you. I've linked to it below. It might not be exactly what you need, but it will give an idea of the standard procedures used.
I'm confused, though, with your uplift forces. What's causing an uplift force? Are you trying to resolve a moment using a couple? Because the approach taken in all the literature I've seen uses a couple developed horizontally, using a kind of passive pressure.

 

[smulmi]

EBSEngineering,
I am currently looking at helical piers too. Lateral loads is not a concern. The only problem is the axial capacity of piers in liquefiable soil depth. I'm assuming the piers to be unbraced on the entire portion of the liquefiable soil depth. I'm trying to find the increase in capacity by adding sleeves. Do you have any suggestions?

JedClampett,
Thanks for the info. Drilled shafts was actually the first option we've looked into. With a single drilled shaft, the concept of uplift forces is not valid, I guess. I was referring to a pile group which when subjected to moment, will produce tension in one-half and compression in other half of the pile groups.

 

[EBSEngineering]

Look at the Helical Pulldown Micropile by AB Chance. This pile uses grout around the shaft to provide the lateral support to the shaft during an event. Also, Dr. Hesham El Naggar at the University of Western Ontario has done significant research into helical piers and liquifyable soils. Look him up and contact him for more answers around this.

 

[cmrdata]

Typically, we always extend our continuous site wall footing to below the frost line in the northern climate. Very often, the clients and contractors would ask us if we can place the below-the-frost footing only at certain (say 15' or 20') interval to reduce the cost of the deeper footing. They figured we can design the site wall as a grade beam to span between the footings.
However, I'm not sure if there is any way we can quantify the force (due to heave and thaw) that we should design the wall for.
Anyone has any thoughts on this one?
Thank you.