CFA Piles

[mfio]

Hello, just for my own information, I am trying to learn more about CFA.

Is there any minimum requirements for depth of reinforcing cage for 16" diameter CFA piles approximately 35 feet long? Assuming lateral analyses are not performed because of small lateral loads (according to Section 6.3.2F of Geotech 8, lateral load analyses may not be required for lateral loads less than 2 to 5 kips)it is possible to have the reinforcing cage installed at a depth of 10 to 12 feet, with the understanding that 1 rebar goes down to full depth? Any thought? Thanks

 

[FixedEarth]

Few things to consider. Check the structural capacity of the unreinforced pier section say at a depth of 15 ft and satisfy that phiMn > Mu and phiPn > Pu. A 5 kip lateral load is not minor for a 16 inch diameter pier. A 30 ft embedment will put your pier as having L/B greater than 10B (short or rigid drilled pier), so some analysis methods like Broms & Czerniak will no longer be applicable.

In summary, you need to satisfy the geotechnical capacity of downward axial, uplift, and lateral loads. You then need to satisfy that the moment capacity exceeds factored moment. You also need to check for downward settlement & lateral deflection. You need to do all this analysis even though your loadings are light.

 

[jgailla]

From a construction point of view, a 12 foot cage should not be a problem.
I have seen cages up to 25 feet long in a 16" augercast pile. (same as CFA, no?)
At that depth, however, there were frequent problems with pushing down the cage.
We had three pile contractots on site. Two had many problems, while the other (Berkel) seemed to get the cage pushed down by hand almost every time.

 

[mfio]

Thanks for your help.
I have an internal discussion with one of my coworker.
Section 6.3.2F of FHA Geotech Circular #8, states that "lateral analyses may only be needed if lateral loads are significant. As a guide, lateral shear forces on vertical piles that are less than about 2 to 5 kips per pile and 18 to 36-inc dimater piles, would probably not justify lateral analysis".
How do you guys interpret this: can I make the argument that for a 16-inch pile diameter subject to a 4 kips shear force laterl analyses are not necessary?
Thanks for any suggestion you may have.

 

[molerat2210]

We have not answered mfio's question yet. What is minimum requirement for the rebar cage?

As geotechnical engineer's we are typically provided the lateral load by the project structural engineer. I believe there is a minimum lateral for which all deep foundations should be designed to accomodate in the absence of greater lateral loads. There are various building codes to govern this ASCE 7, AASHTO, AWWA. I seem to remember a lateral load of 5% of the axial load as a minimum requirement. The structural guys can probably elaborate on this.

Anyways, once the lateral load is specified, we geotechs usually provide information about the pile response to the lateral load such as moment, deflection, shear. L-pile can be used to this purpose. This part of your pile design would usually be done after sizing the pile for axial loads as Fixed Earth explained above.

You are correct that CFA piles usually get a full length bar or dwyidag bar (centralizer at bottom). Aside from uplift loads, my understanding is that the purpose of the bar is to hold the pile together if the pile is cracked along its length. The center bar may also help as a guide for the rebar cage.

The installation of the cage is usually not a problem if there is 3 inches of cover.

Keep in mind that GEC 8 is a design guide and not a building code. Find out the building code requirements and let the structural engineers determine the appropriate load combinations.