Underreamed (Belled) Piers and Skin Friction
Underreamed (Belled) Piers and Skin Friction
(OP)
When I have designed straight shaft piers in the past, I've added what is basically uplift using the compression SF to the allowable load obtained from Ag*σallow since that compression skin friction is adding to the pier's resistance to applied loads. (i.e. Pallow = Ag*σallow*(Π*D*(penetration into bearing stratum - top of skin friction layer)*SFcompression)
In the case of an underreamed pier, does all of allowable load resistance now come from the end bearing (Abell*σallow), or is that wall skin friction I typically add still an additive force? And of course, if you think I am way off in any aspect, please elaborate.
In the case of an underreamed pier, does all of allowable load resistance now come from the end bearing (Abell*σallow), or is that wall skin friction I typically add still an additive force? And of course, if you think I am way off in any aspect, please elaborate.






RE: Underreamed (Belled) Piers and Skin Friction
RE: Underreamed (Belled) Piers and Skin Friction
There is also the concern that the excavation of the bell might disturb the soils directly above the bell so some length of shaft directly above the bell (if you were to add in skin friction) would be ignored.
Some of our geotech posters here might chime in but that is what I've seen.
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RE: Underreamed (Belled) Piers and Skin Friction
Mike Lambert
RE: Underreamed (Belled) Piers and Skin Friction
RE: Underreamed (Belled) Piers and Skin Friction
I agree that skin friction in compression being neglected in terms of resisting applied axial loads is a good practice.
What about those that use the skin friction in tension to resist the uplift forces due to soil swelling? Does anyone agree that the amount of pier below the required embedment into the bearing strata/stable zone can be used in conjunction with the skin friction in tension to resist the applied uplift force? Furthermore, can the amount of bell outside of the shaft diameter (i.e. Π/4[db2-d2) in conjunction with the skin friction in tension be added (or is there some geometry regarding the angle of the bell ceiling involved)?
RE: Underreamed (Belled) Piers and Skin Friction
RE: Underreamed (Belled) Piers and Skin Friction
It's worth asking the question.