Drilled Pier L/d ratio

[geodr]

In drilled pier design for compression loading, and in fairly uniform alluvial soils, I typically follow the FHWA recommendations for limiting the length:diameter ratio to be less than 30, adopting the rationale that load transfer below this depth attenuates.

Beyond the 1999 FHWA Drilled Shafts Manual statement that "A good rule of thumb is that the ratio of length (L) to diameter (B) of a drilled shaft should be 30 or less", does anyone have any references or experience in adopting OR exceeding this design length limitation?

Thanks

 

[msucog]

check IBC ch 18. it doesn't necessarily discuss all technicalities but does discuss what the requirements are to exceed l/d of 30.

 

[Zambo]

for 1.5m diameter piles lengths of 60 - 100m are not unusual.

sorry no references.

 

[BigH]

Do a search for Bengt Fellenius. He has written a few opinions regarding the critical depth - taken by some as 20:1.

 

[geodr]

msucog

I am aware of the IBC Ch 18 note which states something to the effect that piers with an l/d ratio > 30 can be installed under the supervision of a geotechnical professional. I take this to be addressing constructability from the standpoint that the installation of deep slender piers (perhaps exceeding l/d = 30 to reach end-bearing) needs special QAQC attention. Or do you think it is simply intended to allow design of friction piers with l/d > 30?

 

[msucog]

i see it as trying to keep Johnny Jackleg Contractor from somehow being able to use a small diameter pile to great depths in order to support something that it should not (building cheap for a big profit). in other words, it's going to make sure an engineer has gone through the "appropriate" evaluation to design the foundation system. i don't recall it discussing all the specifics of how to go through the actual design for every scenario.
i'd try to find references for your particular part of the country and/or geology if possible.