Driven piles 1

[QSE]

Hi geotech experts

I need to estimate uplift capacities of existing steel driven pipe piles in dense till. Piles are 16" in diameter, pre-bored and driven to refusal.

SPT for till is 50+ and water is at the surface. Pile reached refusal at about 30 ft and were driven with opened ends.

Thanks

QSE

 

[Focht3]

You won't like my answer...

Because the piles were prebored, they may not have much uplift capacity. And it will vary a lot from pile to pile; perhaps by a factor of 5 or more. Uplift capacities vary a lot anyway; the installation technique can add additional variability. In my experience, only jetting adds more variability than preboring.

The fact that the piles were driven to refusal is of no practical use in evaluating uplift; it does help explain why preboring was necessary. Since the majority (90%+ ?) of the compressive capacity of the piles is probably derived from tip resistance, the compressive capacity offers no real assistance in evaluating the available skin friction.

I think that you are stuck with only a few options: 1) run 5 or more uplift tests to derive a nominal uplift capacity - and hope the existing piles will provide enough resistance, 2) plan to grout alongside the piles to augment the existing friction capacity, or 3) add additional drilled piers as additional tensile capacity. The first two options are risky, the second two are expensive. (Option #2 is both risky and expensive.)

I told you that you wouldn't like my answer!

 

[QSE]

Thanks Focht3, you have confirmed my suspition.

QSE

 

[QSE]

A new twist to the original question

What if the pre-bore hole are smaller in diameter before the pipe is driven to refusal?

Thanks

qse

 

[Focht3]

How much smaller? Full depth? How stable was the pre-bored hole?

In general, a pre-bore should be limited to 2/3 of the foundation's width/diameter and shorter than the installation depth - but always at least 6 inches less than the foundation's width/diameter. (I have gone to 75% in some circumstances.) The pre-bore depth is subject to discussion; no good rule of thumb that I have seen/heard. I usually stop at least 5 feet above the final bearing level (if bearing capacity is included in the pile design.)

 

[Qshake]

Focht3,

Is there a reference on pre-bore that you've found helpful. We often specify prebore for fills that are in place less than five years and more than five feet in height. We almost always pre-bore to natural ground line, even though the pile may be driven further to obtain refusal.

Also, please give us your comments on Dynamic Pile analysis vs the old ENR formulas.

One last note on the uplift, we often discount the upper 10-15 feet when evaluating insitu pile for capacity. In many cases, our clients specify, for new design, that the pile will not be in uplift for any condition. May seem a bit of overkill, but in general, I've not found the foundations (pile caps) to be gigantic, just big.

Thanks
Qshake

 

[Focht3]

You're gonna make me work! Not sure I can pull the references - but I'll look for them...

I have generally used preboring when the piles need to achieve a predetermined penetration that is not directly related to capacity i.e. to get enough capacity below a likely scour depth. These are almost always in natural ground - fill is seldom the issue. But those are the conditions that I typically encounter -

There really isn't an "ENR formula" - it really should be called the "Engineering News formula." The EN formula is so bad that you'd be better pulling numbers from a hat...if you don't believe me, read what Ralph Peck said about it. The EN formula doesn't belong in the same sentence with PDAs - dynamic analyses are far superior to the EN formula.

Well, dead weight is effective against uplift forces...

 

[PEinc]

Design of Pile Foundations, Chapter 5, Section 5-2.3.c, from ASCE as adapted from USA Corps of Engineers states that "When preboring is permitted, the hole diameter should not be greater than two-thirds the diameter or width of the pile and not extend more than three-fourths the length of the pile. Oversizing the hole will result in a loss of skin friction and a reduction in the axial capacity and lateral support, thereby necessitating reevaluation of the pile foundation. When extensive preboring is needed, consideration should be given to using a drilled-shaft system rather than a driven-pile system." The same section also says, "Preboring through cohesionless soils is not recommended ...."

 

[PEinc]

P.S. In reference to the dynamic pile testing and PDA's, I agree with Focht3. However, over the course of many, many, many years many, many, many structures have been founded on piles installed using driving formulae such as the Engineering News Formula. Generally, these pile foundations have performed and continue to perform as intended. For some smaller pile jobs, temporary piles, or jobs with relatively low pile design loads, it may be appropriate to use a driving formula. Don't throw away the old formulae yet. They still have a place in pile installation. Just my opinion.

 

[Qshake]

Thank you for your replies.

They shed some new light on an old subject that otherwise was simply a detail.

As for the pile driving formula and analysis. I recently worked on a job where there some was some difficulty in acheiving bearing on steel H pile. We resorted to PDA rather than the blow count and also performed some insitu pile testing. The results were startling. The pile were shown as damaged - significantly. Since the client didn't beleive us we pulled a pile. Man was it ever. The end was folded over, the pile point was long gone and there was fracture at the welded splices.

Other pile were driven with the PDA and results were compared to the EN formula. According to the results the pile acheived bearing well before they would have with the EN formula. The additonal beating/driving, to me, just served to damage the pile.

Thats why I was curious.

Thanks again.

 

[VAD]

Gentlemen, having read your responses I would like to make some comments.
PEinc's comments on the prebore is certainly the way most prebores are done. Do not forget that even if the diameter is a tad larger there is always swell back of the soils and this would lead to return of pile soil contact. Judgement is applied to such situations which may vary from practitioner to practitioner. I do not have a problem with accrediting a fair portion of resistance to the prebored section. There is also significant resistance to uplift in the portion of a pile near to its tip.

With respect to the PDA test, I have found that it can be quite useless as well for a number of reasons. Remember also not to discard the conventional static analysis which is the basis for the PDA comparison regarding capacity. I have done enough comparative evaluations using the PDA and debated many a finding with the "experts". It is a nice gizmo but really not necessary on most jobs.

The PDA results are many times quite controversial especially if you do not have the correct size of hammer to move the pile when the test is being undertaken. So at times you probably obtain only a fraction of what the pile can take as determined from your static analysis.

With respect to Qshake's observation of the damaged pile. I am curious on the number of blows that the pile was subjected to. Many drive piles to refusal as they believe that if this occurs then the pile is going to be good. In my opinion using the static analysis and being able to get the pile to the depth that is required by the analysis is all that one needs to produce a suitable pile foundation in over 90% of the cases. How you determine whether the pile will reach to your predetrmined depth depends on your understanding of the soil, hammer energy and efficiency, experience, and judgement.

The experience is really gained from past jobs through a review of the depth of embedment from the production piling and review of the pile type and soil conditions. After a while, one can predict whether a pile can be taken down with ease and to predict the depth of embedment within a few feet of what was desired. Please note this is perhaps only good for the conditions you have been accustomed to re soil conditions and hammer types - local conditions and contractors etc

Whether a pile can reach a desired depth dictated by capacity considerations or by scour etc is perhaps the first few things one looks at when the boring log is obtained. This takes a brief look. The rest follows as the design is usually done with a few scratches.

I hope that I have not touched any funny bones. These are my opinions based on my experience and judgement which I have used sucessfully in relation to piled foundations.

 

[Focht3]

PEinc:
Thanks for saving me from looking for that reference!

Hmmm,

I still agree with Ralph Peck about the EN formula. The Engineering News formula isn't just junk - it's dangerous. A comparative study of pile driving formulae was done in the late 1970's or early 1980's. I think the authors were Roy Olson and Norm Dennis, but I won't swear to that. The study used published driving records and site-specific information, as well as static load tests, to compare and contrast the various driving formulae.

The study's conclusions? The EN formula is terrible at predicting capacity; and the Gates formula did a commendable job. (I'm not anti- "driving formula"; I'm just anti-EN formula!)


I'm not sure who did the PDA work on VAD's sites, or what the circumstances were. I have had phenomenal luck with PDAs on a small number of projects; but perhaps that has more to do with the operators (I don't pretend to know how to use them) than with the technique. All of my experience has been with Larry Olson in Denver or the M[sup]c[/sup]Clelland Engineer's offshore group in Houston.

PDA saved my "bacon" in about 1988. I designed steel piles to support a spiral pipe fabrication machine for a site in northwest Houston. The owner had a lot of 10 inch pipe in inventory, and wanted to use that instead of concrete or larger pipe - things the owner would have to purchase. (We did cast a large RC cap to tie them together.) The pipe was to be installed to about 25 feet; the installer used a Delmag D30. Can you say, "Sledgehammer hits finishing nail" ?! Aside from having trouble getting the hammer to start, the piles were only taking 1-3 bpf at grade. The owner's rep was very nervous; it didn't matter that the hammer was way too big for the piles and site. We had to demonstrate the piles were adequate.

Retapping the piles after a 72 hour "set" - and PDA analysis - convinced the owner and installer that everything was fine. And the completed foundation performed as expected.

 

[Qshake]

This has been a great discussion for an old guy like me. I wonder why the EN formula remains so prevalent?

Thanks for the all the responses.

 

[PEinc]

The formula remains in use because it is cheap (costs nothing to use), because "old timers" do not need to use a computer to use it, and because it has been used for a looooong time.

 

[Focht3]

And it's very, very simple.