Pile Size

[JedClampett]

I'm getting an enormous amount of grief for a project I'm on in Florida. I'm a structural engineer for this project and am not a pile expert. My Geotechnical Consultant gave me this design and seems to be quite confident.
The design involves 528-12 inch square prestressed concrete piles spaced at 6'-6". The piles are to be driven into a 15 foot predrilled hole until they meet refusal of 20 blows per inch. The minimum depth (beyond the predrilled hole) is 40'-0". Design load for each pile is 50 tons.
The contractor has been griping about the piles even during the pre-bid. He says he needs 14 inch piles and that's what he bid. He won't give any details on why he thinks this.
Since the pile spacing is based on the size, I'm worried I'm going to have to change my layout. Plus the larger piles will need more driving force and possibly damage adjacent structures.
Does anyone else see a problem with the 12 inch piles?

 

[kleo]

I would spec augered piles and forget the expense & unknowns w/ predrilled holes (with an auger)for driven piles. Using driven piles is usually very inefficient, not only the driving but the cutting of piles for cap placement. I work for a pile contractor and we could easily cut schedule in half using CFA piles. We often do design build & VE jobs where our focus is foundation optimization. This sounds like a perfect application.

 

[DRC1]

Van Komurka recently gave an excellent talk on selecting the most economical capacity pile.
Simply adding all the piles on the job and multiplying by the orginal capacty and dividing by a new capacity will not give you the number of piles on the job. Some locatons will require a fixed number of piles regardles of capcity.
What Van Komurka suggested, the most economical approach is to select several pile sizes, say 3-5 different piles.
Capacity can be varried by length or size.
He gave the talk at "Design of Cost Efficent Piles" sponsered by the Pile Drivers Contractors Association in New Orleans.
As for auger cast piles, I have not heard of these being used with much sucess except for low capacities. For high capacities, driven piles have much better quality control and an be installed quickly.

 

[kleo]

DRC1- some education is prudent here.
We design & install continuous flight auger (CFA) & DeWaal piles (DWP), as well as drive precast piles, micro-piles, etc. Under the right conditions, driven piles are the proper choice - we have driven thousands of them.
CFA piles have been used for design capacities up to 1500 tons (1m diameter CFA in Las Vegas - Wynn Tower).
CFA piles are quite different from conventional auger cast in place piles (ACIP) and installed with a high torque auger that minimizes soil sidewall disturbance and hence ensures intimate contact w/ the soils and high pile capacities. We routinely use 100 ton designs for 14" DWP. DWP are actually augered displacement piles which keep all soil cuttings in the hole thereby increasing the soil density and maximizing pile capacity. In most cases except for dense/hard soils DWP's will far outperform driven piles in capacity and installation time. This has been recently proven through a series of 100 pile load tests on all 3 pile types for a huge project (50,000 piles).
We are currently installing DWP's at oil refineries in LA & TX, where our production is 20+ piles per rig/day to 60-80'.
Using 3-5 different pile sizes may appear to be economical, but it often slows field production because of the equipment change.
cheers,
Rick
Morris-Shea Bridge Co.
morrisshea.com

 

[JedClampett]

In addition to kleo's points, I'm very confused on how you would write bidder's documents and a contract with multiple varying sizes of piles. Some of the "art" of design is to write documents such that you get fair bids without contractors playing games. It's difficult to do with piles as you have to allow for subsurface conditions, adjustment in pile lengths, etc. So to make sure that everyone is operating from a level playing field, you give the theoretical length of piles and specify the number.
This type of bid is just asking for some contractor to "underestimate" the amount of piles, and make it up on allowances. Theoretically it's cheaper, but how could you implement it?

 

[IFRs]

If the piles are supporting a monolithic pile cap, then increasing the pile spacing may significantly increase the pile cap cost.