Auger CIP Pile Design 3

[engr567]

I need to design an auger CIP pile (12" diameter per geotechnical report), and I have never designed one before. Is there a design reference book/manual available - per ACI guidelines? How do I start?

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[hokie66]

Where I am, piles are normally designed by the engineer for the piling company. The building design engineer specifies the required loads and any special conditions, and then reviews the design by the specialist engineer. Just an aside...there are several types of CIP piles which use an auger to remove the soil.

 

[ron9876]

You design them like a concrete column with the geotech providing moments and the point of fixity for a given horizontal load.

If they have only axial load there isn't much to it. What is your loading condition?

 

[engr567]

hokie66, I am talking about particular Auger-grout or concrete-injected piles (per ACI definition and types). Geotechnical engineer has specified 12" diameter Auger CIP pile to develope an allowable capacity of 100 tons per pile.

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[dcarr82775]

I designed them as a reinforced column. The tricky part is getting the cage in after the hole is drilled and filled with concrete. A lot of designs use a full cage over some distance near the top and then a single bar down the center over the full length. FHWA has a free publication on Augercast piers that helps a little.

I wouldn't even consider an allowable load of 200kips on a 12" pier. Use a bigger pier.

 

[Ron]

You must have some competent bearing material and short piles! 100 tons is a high load for a 12" augered concrete pile. Typically that would require a 16" to 18" pile.

Here's a link to a publication produced by the Federal Highway Administration on design of augered concrete piles. Not a bad start for you.


Augered Cast-in-Place Concrete Piles

 

[PMR06]

I agree with others, it's basically a concrete column (unless laterally loaded).

100 tons sounds extreme for 12" pile. You should definitely require a load test!

 

[engr567]

thanks all.

Every Problem is an Opportunity for Engineers!!!!

 

[spats]

The only technical reference I have for augercast piles is Transportation Research Record No. 1447, published by the Transportation Research Board of the National Research Council (National Academy Press, Washington, D.C. 1994). It is a compilation of articles on the subject.

Augercast piles derive most if not all of their resistance through skin friction, not end bearing. They don't need to be designed as columns. Quoting from the Record No. 1447 article "Managing the Installation of Augered Cast-In-Place Piles" regarding reinforcing steel:

"ACIP piles can be reinforced to resist lateral forces and to increase axial compression or tension capacity. The most common reinforcement is a single steel bar introduced into the pile immediately after casting and penetrates to the bottom of the pile. Centralizers are required to be certain that the bar remains within the pile section. Less common is introducing the bar through the hollow stem as grouting begins. A bar that is at or neqar the center of the pile has only a modest effect on the bending resistance but can provide significant tension capacity if adequate bond strength is available.

Bending strength can be provided readily by introducing a reinforcing cage or a steel member like a lightweight H section or a pipe into the pile just after grouting has been completed and before the grout has set. These sections also require centralizers to ensure they remain within the grout section as they are pushed into place. In general, there has been limited success pushing steel sections or cages to depths greater than 20 ft (6 m). Ordinarily this depth is sufficient to reinforce the pile against horizontal forces applied at the ground surface, but a shoirter depth limits the usefulness of the ACIP pile for use in cantilevered retaining structures that are more than 10 ft (3 m)to 15 ft (4.5 m) high."

I've used 12" augercast piles here in Florida, using a single #8 bar as the central reinforcment. The centralizer referenced in the article is commonly known as a "football". I would consult your geotechnical engineer regarding lateral bending, in order to determine the size of your cage or steel member (I use cages). Based on your required lateral capacity, he can run LPILE for you to determine bending moments and the required depth of reinforcement.

I agree with the others that 12" is too small for 100 tons. My guess is more like 18" to 20" diameter.

 

[engr567]

Thanks spats. The information is very helpful.

Every Problem is an Opportunity for Engineers!!!!

 

[Brad805]

If you do have any lateral forces keep in mind the verts are normally wet set, and their location is not as predictable as in a column. The allowance to reduce your vertical rebar less than minimum is helpful for piles since soil strength normally governs the pile design.

Unless you are bearing on rock at the base, 12" seems doubtful. Even then I would think you would need a bell.

Brad

 

[amec2004]

Waht you are talking about is actually micropile, with single or bundled DYWIDAG Bar at center of pile.

If no lateral load, there is only axial compression or tension, pile shaft has no bending moment.

If there is lateral load, pile shaft has bending moment, you need a *permanent* pipe sleeve for pile shaft moment resistance. Without pipe sleeve, the pile with center DYWIDAG Bar has zero moment resistance.



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[Teguci]

Here is a detail I put together for an auger cast pile. Feel free to adapt from it.

 

[engr567]

Teguci, thanks again.

Every Problem is an Opportunity for Engineers!!!!

 

[JAE]

If you calculate a 12" dia. "column" with f'c = 4ksi and with a set of 4 #5 bars in the center, you get an axial capacity of about 237 kips (factored). That is 118.5 tons (factored) and probably about 79 tons based on a safety factor of 1.5.

 

[DST148]

@engr567: Ref ACI 318-05 commentary R22.2.3 : Structural elements such as CIP concrete piles and piers in ground are not covered by this code(ACI 318). Such elements are covered by the general building code.
As a starting point, you may refer to IBC. IBC 2006, section 1810.3.1 restricts allowable design stress in concrete of augered uncased piles to 33 percent of 28-day specified comp. strength, the allowable comp. stress in reinforcement to 40 percent of fy.