Monopole tower foundation 6

[pelelo]

Engineers,

I am involved in a small monopole tower geotechnical work.

I understand it is a very high tower, approximately 180 ft, with moments of around 100,000 in-kips, shear 75,000 lbs and vertical of 65,000 lbs.

After reading few similar geotechnical reports, I noticed it is mentioned that this type of structure can be founded either on drilled piers or mats. I have seen these structures only on drilled piers in my experience.

From the geotechnical point of view, I understand this structure needs to be founded in piles for the following reason:

If P x tan phi > Shear, then it is ok to be used in mats, otherwise drilled shafts:

65000 x tan 30 = 37,000 lbs < 75,000 lbs, therefore drilled shafts is needed.

The N values of the strata is between 20 and 50 bpf and is composed of SP, SP-SM.

Do you understand that this is a valid reason to support my decision of using drilled shafts? or do you think additional checks need to be performed.

thanks.

Please let know.

 

[fattdad]

your friction angle seems low. For N-values of 20 to 50 and for SP or SP-SM, I'd expect a larger angle of friction. Then I looked at your equation and realized it'd have to be 49 degrees to get to your 75,000 lbs, so it all may be moot?

You'll likely get a pretty good p-y from those dense sands; however!

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

I'd leave this post for viewing for several days and hopefully a poster like Ron with experience in that type of setting to advise. What is the water table situation?

 

[pelelo]

Thanks,

WT is at 3 FT.

Fd, I agree, probably something higher than 30 would make sense. I just wanted to be conservative. Still, not even having a high phi will provide something close to the 75,000 lbs of shear.

 

[BridgeSmith]

If I'm thinking about this correctly, "P" would include the weight of the mat, as well as that of the tower, in order to prevent sliding of the tower and foundation. Considering that a mat foundation big and thick enough to prevent overturning due to the applied moment would be quite a bit north of 200 kips, I don't think sliding would be an issue. The issue is how big and heavy the mat foundation would have to be to prevent overturning.

The mat foundation will require considerably more concrete, but does not require the the monster drilling rig necessary to auger the large diameter hole for the drilled pier. The more economical option will probably be determined by how far they would have to transport the drill rig (and of course the number of foundations).

 

[skeletron]

That's a big tower with huge force. The water table at 3ft seems like a decisive factor to use a pile foundation.


...but I can't recall if I have ever solved that problem yet.

 

[oldrunner]

I've got one pole foundation that's about 56 foot square and 8.5 feet thick. Had a water table issue. Pole is 426 feet high with a big gold crown.

 

[Ron]

Your base moment is all the justification you need for a drilled pier. The overturning moment will make a shallow foundation inordinately large.

180 feet is high for a monopole. As you add antennas for cell service, the overturning moment will increase. Consider a maximum antenna population for your OT moment for the foundation. Each antenna addition will require re-analysis of the structure, but there's not much you can do with the foundation after the initial construction.


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[pelelo]

Thanks a lot Engineers, your inputs were greatly appreciated.

 

[msquared48]

I agree that 180 feet is a very high monopole. This type of tower is usually around 100 feet. You might want to consider a multi leg or guyed tower.

Mike McCann, PE, SE (WA)