Side Friction on Caisson

[CivilStruc86]

Hi everyone, this is a relatively basic question but i am not specialising in geotechical issues. The question is:

You are using a cylindrical caisson for a footing to a column and you want to take the load both in bearing on the end of the caisson and friction on the sides of the caisson. The allowable end bearing on the founcation material is 500kPa, but in the absence of any further information what value would you use for side friction in your preliminary design?

Thanks everyone..

 

[jdonville]

All,

See the FHWA drilled shaft design and construction manual. The strain-capacity curves shown indicate that side friction mobilizes faster (at less strain) than end bearing. By the time that end bearing is fully mobilized, the mobilized side friction is very significantly reduced as a percentage of ultimate capacity.

Naturally, the rates of capacity mobilization will vary depending on the size of the foundation element and the soils involved.

This makes the design of very slender elements such as ACIP piles problematic - the strain-capacity curves for side friction do not extend into the range of strain experienced by these elements. For example, my firm installs ACIP piles up to 24 inches in nominal diameter. If you assume a 1-inch allowable settlement - by no means unusual - then the strain over diameter is 1/24 = 4%. These curves go up to only 2% for side friction. Yet the most recent design guidance for ACIP piles (FHWA GEC No. 8) recommends using the drilled shaft design procedure for preliminary sizing!

Jeff

 

[Mickney]

I enjoy these "open" discussions that very often stray from the original question. Very thoughtful stuff in this thread. Also, to add additional mud to the waters, method of drilled pier construction (i.e. open hole, casing, slurry, etc.) will also play a role in the skin friction value.

Back to the original question: if end bearing equals 500 kPA, what would be the skin friction for preliminary design. From an EXAM standpoint and NOT from a CONSULTING standpoint,

Here is my answer: Q=Qend + Qskin
given Qend = 500 kPA = 10.4 KSF. I would assume soil is very firm to dense SAND. I would assume that the soil is constant from the ground surface to the bearing level. From Terzaghi and Peck, observed skin friction for caissons in a dense sand is between .7 KSF and 1.4 KSF. I would select lower value and go to the next problem. Feels good to be back in class.