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?

 

[PEinc]

What does the pile designer say? Is the 12" pile structurally designed for the design load? Is the 12" pile strong enough to be lifted from horizontal to a vertical driving position without damage? Did anyone do a WEAP analysis or a drivability study? What are the predicted pile driving stresses? Does the contractor want to use the same pile hammer that the WEAP and drivability analyses were run with? Did the contractor provide a WEAP analysis for the hammer he wants to use? Did the contractor have a problem with 12" piles in the same area?

 

[JedClampett]

The pile designer says he stands by his design.
The contractor is saying nothing except he doesn't like what we have.

 

[IFRs]

I'd sit down with them and discuss the issues professionally - maybe there is something to learn fromthem. If they are not willing, that says soething to you. Will the contractor pay for the difference in pile cost, driving cost, damage to adjacent structures, the hiring of another geotech for analysis, the overall design responsibility, the PE stamp and all other liability? Are they ready to back up their negative comments with technical documentation? If not, then either get another contractor or if that is not possible, document their objections and simply more on.

 

[PEinc]

If the pile contractor is unwilling to explain his concerns, then why should the pile designer have less confidence in or concern about these piles than other pile jobs that he designed? The contractor may be planning to use a pile hammer that is very large, larger than what was used in the engineer's WEAP analysis, and which might cause damage to a 12" pile. Question the contractor about his planned driving method and equipment. Does the contractor have a WEAP analysis for the hammer he wants to use? He may be planning to use too big of a hammer in order to increase productivity without checking for potential pile damage.

 

[JedClampett]

I'm still trying to get information on this. But I heard rumblings during the bid and now I'm hearing them again. I thought maybe there's something obvious that I'm missing.
I'm sure a meeting is in order, but I'm trying to get my thoughts in line.

 

[hokie66]

Jed,

The issue may be with splices. Many piling companies are only set up to splice certain sizes, and I think 14" is a common one.

 

[msucog]

i'm not pile expert but it seems to me that it should not be the owner's (or owner engineer's) responsibility to accommodate the contractor...if they want to provide 14" dia piles in lieu of 12" piles (assuming 12" piles are appropriate), then it should be their additional cost. it sounds like they are trying to jocky for the situation described in the previous posts. i would also be extra cautious about driving hammers, drivability, etc simply due to their unsubstantiated objections. and keep in mind that contractor's primary objective to make money...not look out for the well-being of the owner/project...especially in a tough market when they've already gotten the job!

 

[PEinc]

I agree. The engineer designs the pile and the contractor picks the appropriate hammer to drive the piles, usually using a wave equation analysis to show that the pile stresses during driving are within specified, allowable limits. Ask the contractor for his hammer size and WEAP analysis. If the hammer can drive the pile to the required capacity at or below the specified pile stresses and at a reasonable number of hammer blows per inch of penetration, then the contractor should be allowed to use that hammer. If not, he can analyze a different hammer and resubmit.

 

[DRC1]

The contractor for what ever reason is playing his cards close to his vest. I don't think this is useful to either party in the long run, but is certianly his option and I can see how some contractors can develop this viewpoint.
Pile driving,depite all the hype, is not an exact science. For 528 piles at 55 feet and lets say $40 per ft, you are over $1,000,000. Such a program should, even without contractor's questions, should have a test pile program. Drive a number of teat piles, I would say 5, with wave eqaution, PDA and at least one static load test. If you have vibration concerns, moniter structures durring driving. If the pile works, the pile works, if not,the design needs to be revised. In any event, production piles should not be ordered until the results of the test pile program are digested.

 

[hokie66]

The contractor may be playing some sort of game to try to convince Jed to reduce the number of piles, but he should stand his ground. The spacing is set, and the required capacity is 50 tons per pile, which should be easily achieved whether the contractor decides to use 12" piles or 14" piles. Damage to adjacent structures is a legitimate concern. A preconstruction condition report should be required.

 

[BigH]

If the design was for 12 inch piles, and the contractor says he bid for 14 inch piles, the price for the later is already included. You wouldn't pay more for the bigger piles once you've accepted the bid. At the time of bid, did he indicate that he was giving an alternative to the 12 inch piles at some additional cost? Was his bid competitive with others that "might" have been bidding on 12 inch piles. So long as you don't have to change the pile spacing, pile cap size, etc., why can't you accept the larger piles - you aren't going to pay more for the change - as he already gave a bid price for what he bid. Just don't "agree" to an increase of cost for bigger piles. As for the pile spacing, a 6.5 ft spacing on 1 ft piles is 6.5x s/d ratio - means the piles will act "independently" for the most part (not interact with each other). 6.5/1.25 = 5.2. Basically the same (my memory is telling me that a s/d ratio of 5 is enough not to interact.

 

[PEinc]

Yes, BigH, but someone would still have to check the design of the alternate 14" piles. Who pays for that?

 

[hokie66]

What design? If a 12" pile will do the job, surely a 14" one is just as capable of carrying the 50 ton load.

 

[escrowe]

Usually 3d minimum spacing is required.

Two possible reasons for the contractor's atitude occur to me:

First, maybe the contractor has a line on some cheap 14-inchers
left over from another project.
If so, he should share the wealth, IMO.
But maybe his proposal reflected that.

Second, perhaps the contractor has gotten 'in trouble' driving 12-inch
piles in the region. If so, it can be dangerous to discount local experience.

FYI, I believe FDOT still requires a minimum 18-inch pile cross section.
Factors such as corossion and driving stress through hard limestone might be an issue.

 

[PEinc]

What design????? Of course the pile would need to be designed. Why wouldn't it need to be designed? All concrete piles don't have the same reinforcing steel and prestressing.

 

[hokie66]

It's probably a geographical thing. Where I am, the contractor is responsible for the design and performance of his own piles, so if he wants to use a bigger one, there would be no issue.

 

[JedClampett]

The project is a low bid situation, so we have very little leverage on changing the contractor. Officially the project hasn't been given a notice to proceed, so all I'm hearing now is second hand grousing.
Once they submit their piles and start the testing program, maybe we can pin them down. As I mentioned before, the Geotechnical Engineer is responsible for the pile design, inspecting and administering the pile program. However, for now, I'm the first ear bent.
Thank you all for your input.

 

[civilperson]

Allow a substitution of 14" for 12" at a no cost change. Thus the contractor gets his way and your increase in capacity is an additional safety factor for good night's sleep! (Or decrease the required depth for equal capacity and a savings for the owner).

 

[JedClampett]

As an update to this thread, we've met with the contractor. Nothing is proven, but we think the contractor threw a bunch of misinformation out there so he could bid the project with larger piles. Larger piles equals more capacity per pile mean less piles. In this way he won the bid. The owner doesn't like the low bidder in the first place and is making them build the project as designed or back out.
Long story short, there doesn't appear to be anything wrong with the 12 inch piles, except they result in a more expensive end product.