Bell Pile (Pier) Design

[JohnnnyBoy]

I'm a junior engineer looking for a little guidance since I am getting some different information for a few people on how to properly calculate capacity of a bell pile.

From my understanding a bell pile's capacity should be derived from solely End Bearing, although from this thread (

http://www.eng-tips.com/viewthread.cfm?qid=23972)

he seems to be accounting for skin friction as well and have seen skin friction be accounted for before.

If we were to calculate the force of the bell from the volume of the pile and the difference in density compared to the removed soil added to the needed capacity of the pile. We could also assume the soil at the depth of the pile could bridge and not bear on the bell then our capacity would be simply calculated as the bearing resistance minus the added weight of the pile. Although if we were to add in the resistance of skin friction then from my understanding this completely disallows the bridging of the soil near the bottom section of the pile or no skin friction were to occur at that location. I'm confused a little and would appreciate any help in this matter.

 

[JAE]

Our usual method is to take the bearing capacity of the soil for end bearing at the bottom of the bell and not account for any skin friction.
The soil directly above the belled section may sometimes become less consistent due to the excavation of the bell shape and so the soil around the length of shaft directly above the bell should be discounted.

The soil further up the shaft technically could be utilized but I typically don't.

As far as including, or not including, the net difference between the pier concrete weight and the soil weight displaced by the pier - my "elder-mentors" in south Texas (where drilled/belled piers were developed) never did that - they were conservative and just included the ADDED weight of the pier and never bothered to delete the displaced soil weight.

My original boss down there in San Antonio worked under the son of our founder who was a personal friend of Terzaghi....I know - a long drawn out relationship with fame!!!

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[dik]

The reason generally given is that the weight of the concrete is approximately equivalent to the weight of the displaced soil.

Dik

 

[JohnnnyBoy]

Thanks guys appreciate the feedback

 

[MotorCity]

We also do not rely on skin friction for belled piers. When you provide a bell, the idea is to size the bell such that it is the primary mechanism for resistance.

 

[fattdad]

I never knew belled shafts were developed in Texas!

We are quite familiar with them in Richmond, Virginia. We have a thick marine clay that is widely used for drilled shaft support. We also rely on the shaft friction to support the weight of the concrete and use the end bearing for structural loads. We are also mindful of strain compatibility between shaft friction and end bearing.

If you really want to get some serious frictional resistance though, try a double-belled shaft! That'll do it! The cylinder that's developed between the upper and lower shafts really offers some resistance!

f-d

p.s., Leo and Arthur Cassagrande did a lot of foundation design in Richmond in the '40s/'50s and these early publications are quite fun to read. They'd take the train down from Boston and work on foundation design.

ípapß gordo ainÆt no madre flaca!

 

[JAE]

fattdad,
See this link:

http://www.dawsonfoundationrepair.com/bell-bottom-piers/

also here:

http://www.dawsonfoundationrepair.com/engineering-documents/Willard-Simpson.pdf

http://www.seaot.org/honor/willard_simpson_05.pdf

http://www.cc-foundationrepair.com/bell-bottom-pier/

Willard Simpson Jr. (the son) was president of our firm in SA years ago.



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[BUGGAR]

And the clay can cause additional gravity drag loads.