Variable Depths of End Bearing Piles

[hollowstemhead]

I recently monitored pile driving for two small bridge abutments. The piles were end bearing, relatively small (steel HP 10x42) and lightly loaded (to be dynamically tested to 100 kips). Our test borings identified 10 feet of fill/organic silt over 10 feet of stiff clay over 20 feet of very dense glacial till with frequent boulders over bedrock. The structural engineering, without consulting us, identified approximate pile tip at the top of bedrock and the contractor based his pile length purchase on this. Upon dynamic testing, it became apparent capacity was easily being achieved in the glacial till layer. The remainder of the piles were driven using the set criteria established but the depths were quite variable, up to 10 feet different from one side of an abutment to the other. I believe some of the shallower piles were catching up on boulders but were still achieving final set blow counts. Of course the contractor felt burned on the actual driven depths compared to the purchased lengths. Anyone have any experience in a case like this? Should I be worried about the piles sitting on boulders even though final set was achieved? Just looking for some general comments on a situation like this.

 

[DRC1]

The boulders are probaly between the till and the clay. the problem I see is with the shallow cover, especilly for resisting lateral loads. I would imangine that you are probably driving to a set of 2-4 blows per inch for 100 kips ultimate. You probably have money in your pile driving budget. I wuold have the contractor restrike some of the piles you are concerned about to make sure the are set. Don't drive them too hard (if the Struct Eng wants 20 blows to the inch tell him forget it) But just be sure they are not moving off. Seeing how you have a lot of cut off, it would not hurt to drive a few extra piles in that area, plus it will ease some of the contractor's pain. Are you using shoes?

I have seen this a lot. More and more structural engineers are geting more involved in foundation design, which typically results in recomendations that piles be driven to rock regardless of the soil conditions above it. It is important to rember that pile driving criteria should be flexible and adjusted to meet the results of the pre production test program.

Good Luck

 

[kslee1000]

DARC1:

I fully agree with your assessment and recommendation to drive few more piles in the questionable areas. However I am puzzled on two questions:

1. While the contractor purchased piles longer than required, I can see the waste, but won't he be reimbursed for it? He is justified to feel "burned" if not. Also, wouldn't it be worse if the piles are few feet too short?

2. "Seeing how you have a lot of cut off, it would not hurt to drive a few extra piles in that area, plus it will ease some of the contractor's pain.
I don't quite get the idea where is the pain came from, and what he is going to gain by driving more piles except the original profit margin included in the unit price?

I may have misunderstand something, please advise.

 

[Ron]

In my experience with monitoring driven piles, the tip elevation can vary a lot, obviously depending on soil stratification/bearing layers/rock, etc. Apparently the structural engineer was looking at multiple parameters to set a tip elevation, with consideration to more than compression capacity. Why not ask him?

 

[BAretired]

red text

BA

 

[BAretired]

Sorry about that. I was fiddling with the HTML feature and accidentally hit 'submit post'. Ten thousand lashes with a wet noodle should be adequate punishment, okay?

BA

 

[dicksewerrat]

Was the contractor paid by the foot or by the project?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[oldestguy]

If there is some problem later on contractor's claim, who gets involved? Did your report provide data for estimating those pile lengths? Or did you show an estimated tip elevation in your report?

Usually there is more grief if lengths are under estimated than if they are over estimated. Bringing more footage on the job is usually more of a problem than having cut-offs.

The fair way to pay for the piles is to pay for the footage estimated and also then pay for footage driven. In that case contractor should come out OK.

In a case like this it might be fun to pull a few of the piles and let the powers-that-be see the tips, and at least they did not curve so bad that they emerged at the ground surface. Reinforced tips do wonders.

 

[hollowstemhead]

Thank you all for your commentary so far. Some more info and a few answers to your questions: The contractor is being paid by driven length, that is footage in place and won't be reimubursed for cut off footage. Cast driving tips were welded to the pile tips. The contractor first vibrated the piles into place and then set them with an air impact hammer. Since we had the vib. hammer onsite we actually did remove one or two piles that encountered relatively shallow refusal on probable boulders. The piles showed very minimal damage with only slight mushrooming at the tip. We also restruck several of the piles that seemed to have encountered refusal on boulders to see if they would bypass or cut through the obstruction. Several of them did not move pass the boulders, the blow counts per inch jumped rapidly and I called them good so as not too overstress.

As for my statement about the contractor feeling burned, what I meant was that he based his purchase length of piles on the structural engineers interpretation of our findings, which was that they would all go to bedrock and not achieve capacity in the 20 or so feet of till atop it. The structural engineer designated this tip elevation without our geotech consultation. Since they contractor is being paid by driven length as mentioned above, he ended up with a lot of cutoff steel with no reimbursment.

Thank you all your continued discussion and opinions.

 

[kslee1000]

Can you really do better than the structural engineer, who has acted conservatively by assuming the worst case scenario (longest pile length required). Sure, the contractor has to carry the waste if he didn't quote properly - seperate prices for pile driving and actual material delivered. Still, if it your call, how could you possibaly tell where the boulders are, and their respective depth? Quite interesting if you can predict with high accuracy.

 

[kslee1000]

Have you check to see if the cutoffs are long enough for those areas that hitting boulders in shallow depth? You may drive couple short ones around as assurance. Also, can these cutoffs be spliced and re-use in softer areas?

 

[hollowstemhead]

kslee:

My aplogies if I came across as being arrogant or sounding I could do better than the structural. What I meant is that perhaps the situation could have been discussed further with all parties involved to identify the fact that a fairly lightly loaded pile could likely achieve capacity within the 20' +/- layer of dense glacial till and perhaps estimates and pay method could be adjusted accordingly. Certainly not trying to sound superior to the structural here. Just attempting to here others similar experiences and ease my relatively inexperienced mind about the situation.

The cutoffs were generally not long enough for reuse without splicing which was not desired due to timeframe/budget.

 

[DRC1]

First off, pile programs of any quantity should have a testpile program prior to ordering production piles. Second, I am not sure why the structural engineer is decviding the lengths of the piles. That should be the geotechnical engineer's responsibility. Third I would think that since there was a 20% underun, that would constitute a cardnial change and the contractor might want to pursue getting paid for the cut off. Driving through 20 feet of till is a pretty unreasonable expectation.
To answer an earlier question about how the contractor could get hurt on unit pricing, even if the buy is seperate from the drive: Driving consists of four seperate operations, locating the crane, lofting and setting the pile, driving the pile and cutting off the pile. Only driving the pile is dependent on length. The other three are not but can take as long as the driving. Thus this time is estimated and divided into the anticipated pile length. When the piles are short to begin with but them the pay length drops in half, your unit production costs will double.

 

[hokie66]

The contractor has a legitimate reason to claim additional costs, as he was misled in the tender documents. He should at least be compensated for the value of the purchased material not used. But as DRC1 points out, he will still be shortchanged on his expectation of profit.

 

[kslee1000]

steamhead & darc1:

Thank you both for the clarification. Yes, I now would agree the structural guy has stepped over boundary, and the contractor has legit reason to feel been shortchanged. But, what was the recommendation regarding depth on the geotechnical report? Also, if use of spliced pile is possible and allowed, wouldn't the contractor be reimbursed for his extra effort, which constitutes a major change, to recoup some of the lost profit? Sure, now the owner might get upset, and ask the same question (report) above.