Augercast Pile Reinforcement

[dengebre]

I am trying to resolve a debate with regards to the reinforcement of an augercast pile subjected to uplift and lateral loads. A search of previous posts seems to confirm the methodology I have always used: 1) provide a full length center bar sized to resist the uplift; and 2) provide a "column cage" to resist the bending moment (typically in the top 1/3 of the pier). My colleague is adamant that a full length "column cage" must be provided all the way to the bottom of the pier. I have never heard of this and do not believe it can actually be built that way. Any comments?

 

[Loui1]

I just did a job with nearly 700 friction augercast piles and they were all designed the way you described. What types of shear are you trying to put into the pile? The only way i'd see the need for a full depth cage is if you're designing the pile for tip bearing only and socketing it into rock to develop moment. That would be the only case you have moment in the pile all the way down to the base. Doesnt the geotechnical engineer have any suggestions? Typically that's where our pile design comes originates.

 

[dengebre]

Sandy soils with "refusal embedment" into a limerock layer. Capacity of pile is being provided almost entirely via skin friction.

 

[JAE]

I don't think the cage to the bottom is required at all. I agree with Loui1.

 

[jgailla]

I've looked at a few augercast jobs. I haven't seen a full depth cage and it seems that even a 25' cage is hard for the construction crew to get into the pile. If your pile length is over 40 feet, I would think the full depth cage is pretty much not constructable.

 

[JAE]

We just completed a project with 16" dia. shafts and a four-bar cage in the top 15 feet of the pile. Worked OK.

 

[haynewp]

I have extended the cage 1/3 the depth before. But check the seismic design category you are in. There may be special requirements for SDC D and greater in the IBC chapter on foundations, I don't have it with me now to look.

 

[haynewp]

There are some additional requirements in IBC 2K3 section 1810.1.2.2, but I don't see a mandatory cage extension to the bottom of the pile.

 

[eric1037]

A full-length cage is possible to construct by inserting it prior to grouting. With regard to the length, a lateral pile analysis using COM624P or LPILE should be performed to determine the location of the maximum moment. Your geotechnical engineer should be able to perform this type of analysis.

 

[TDAA]

And what cage layout lets you place he cage with the auger in the hole?

 

[eric1037]

TDAA:

Although the technique is predominantly used with single-bar reinforcement, it is possible to insert a reinforcing cage into the auger as long as you have sufficient clearance. It is easier with bigger auger-cast diameters. Most often greater than 24 inches. However, I have seen it done with 18-inch diameter piles.

Another technique to get a full-length cage is to vibrate it down. However, it will need to be inserted within 10 minutes of completion of grouting. Even then, it is sometimes difficult to get the cage down.

 

[luedkelincoln]

The geotechnical engineer should be able to give you a point of lateral fixity. This will be the depth at which you can stop your cage and assume a cantilever type column design. The cage should go to this point or a little past it.