Dragload on Piles

[VAD]

Are you satisfied, really comprehend, drag load and how its is to be used in pile design. In my opinion, there there are so many conflicting methods and really no strong attempt has been made to indicate why a particular approach is better than another. We still see, modern texts and publications producing information that could be conceivably be in error. What is your take on the subject above what sounds nice or good.

 

[SlideRuleEra]

VAD - I have limited knowledge of the various methods, but the procedure developed by Dames & Moore (literally Mr. Dames & Mr. Moore) in the 1940's is understandable. A copy of a four page paper outlining this method is on my web site under the title "Estimate & Eliminate Downdrag Forces On Piles".

http://www.SlideRuleEra.net

 

[VAD]

Thanks SlideRule Era:

After seeing the article, it jogged my memory that I had seen this sometime before in my domain. I retrieved it from my library along with the others you have on your site. These came from the February 1969 publication of the American Wood Preservers Institute - a copy of which I have as well.

It is of interest to take a good look at this article as with a few manipulations I believe we can see that the concept of the neutral point or neutral plane was in the making at the time. Just a observational hunch at this time.

Do you have by any chance a copy of Moore's 1949 paper quoted. I thinknthat this would be of interest as Figs 2 and 3 of Enkeboll's paper. If not I will try to get it from the Eng Library here.

The problem that I have with aspects of the geotech literature and is that it is difficult to be able to obtain all info that is relevant to a topic and as the years go by much is buried,lost or forgotten. However, when one searches and finds and one was taught to believe in certain aspects then confusion reigns.

On the other hand, I would expect that educationalists would be able to review and bring the relebvant literature to the fore when they write textbooks etc. This hardly ever happens, hence again confusion to the you graduate and pratitioner.

This article, in my opinion, is by no means outdated to our understanding of this topic.

Finally, I agree with your statement that the article is understandable and if I may add needs to be revived or used to expand on what is being projected in many books and literature today.

Finally, despite what is written today many of us realize that comfort is often obtained by reviewing Terzaghi's Teoretical Soil Mechanics and Soil Mechanics in Engineering Practice.

 

[SlideRuleEra]

VAD - Sorry, I don't have Moore's 1949 paper. My library is a mixed collection of books, papers, etc. that came from my Uncle (structural engineer), my Father (civil engineer), plus those that I have come across over time. I'm still adding to it, such as by purchasing documents on Ebay. Also happen upon obscure academic web sites that offer interesting out-of-print books (.pdf format), but downloadable only one page at a time.

I'm currently working on three such "projects":
1. Assemble several 19th century books (.pdf) on bridge design.
2. Scan an 1892 Wrought Iron / Structural Steel Handbook
3. Assemble a 20th century book on concrete slab-on-grade design.

Plan to put all of these on my website over the next couple of months and start on some others.

http://www.SlideRuleEra.net

 

[BigH]

VAD - Poulos and Davis have a nice section in their book on the downdrag. It came into the fore in Sweden/Norway, if memory serves me right - due to piles failing - but these were piles driven to end-bearing on rock - an unforgiving tip. As a result, failures did occur due to the additional load on the pile due to the settling fill. If the pile base can 'move' - i.e., depending on extent of the rigidity, the actual download forces are greatly reduced. I know Fellenius talks about this a lot - have never really gone into it in any detail. A lot of pile design is coming up with reasonable loads on the piles - usually conservative. Better than some Indonesians I saw years ago who gave allowable bearing values as 67.23 tons (and they got the friction angle from SPT) - with a Safety Factof of 3.00000 (not 2.94443)!!
I have a friend whose dad wrote a book in 1945 with a revision in 1961 on Deep Foundations and Sheet-Piling - British Publication. I'm going to research to see if we can get it "out there again" for historical purposes, etc. Anyone have Prentiss and White's book on Underpinning?
SRE - I sympathize and empathize with you on page by page downloading - I scanned a 900 pager once - took a long long time - and you now can get some publications that download chapter by chapter and WON'T let you combine them afterwards! Yeck.
Cheers to all.

 

[VAD]

Big H - I agree on what you have said about Poulus and Davis and there are many others who have provided seemingly convincing information on the subject. My concern is that we seem to discredit many of those past ideas without doing a good job of it. There is a lot of imagination that goes into soil mechanics before we decide whether to use one approach or the other. Certainly, it should not be because one is considered an expert. Of course, at the beginning we all tend to follow some book or the other.

It seems, however, that the experts are not always are not doing too good a job at explaining why new concepts are better than those around, or rather they need to do a better job. If you rummage the past or even present info on the subject you will gather what I mean.

 

[emmgjld]

I am in Seattle this weekend and have just completed a 'short course' taught by Fellenius. Very enjoyable and very interesting. For piling which are NOT principally end bearing, I think his concept is generally right. I am afraid that the lack of good observations during installation/testing and the erratic definitions of terms are much of our problem of sorting this all out.

Fellenius is obnoxious in consistent terminology and I think that he is absolutely correct. We live in a very sloppy world, considering our use of words.

For a smile, the last page of his text is:
"The contractor is not to make fun of the engineers. The bid of any contractor walking around the site with a smile on his face will be subject to review".
LOL

 

[VAD]

emmjild, if you do not mind, you can probably explain clearly why dragload is not to be added to dead load when determining the allowable load in piles that are and are not end bearing, and why drag load is not to be added to dead load for the two situations in settlement determination. Also if dragload is not to be considered as a load why are we taking it into consideration in evaluating the structural integrity of the pile. If dragload is lost in a pile in a failing situation and this gives the reason for not considering this in bearing capacity, why are we considering dead load at this point.

What portion of the drag load is transferred to the soil in any of the above processes. Is this considered

 

[vulcanhammer]

Re Bengt Fellenius: I remember he was the chairman of the 1988 Stress Wave Conference (the Third) in Ottawa. Everyone was sent a copy of his requirements for consistent terminology (which I laboured to apply in my driven pile book for Pile Buck.) My paper was one of a handful that wasn't sent back to fix problems in terminology, grammar, etc. Truly the schoolmaster of the driven pile industry. That paper is found at

http://www.vulcanhammer.net/info/zwave.php

Downdrag has been a passion of his for a long time; I also remember one of his more emphatic lectures on this subject at the Ohio Valley Soils Seminar in 1978.

 

[VAD]

There is absolutely no doubt that Bengt Fellenius has made significant efforts in the design and construction of driven pile and also in ensuring that terminology used in piling is consistent and appropriate. However, we still see publications in recent times abound with mixed terminology and some according to Fellenius that should not be used.

The paragraphs below provide some of my thoughts on terminology and the concepts that Fellenius have been advocating for years. My belief is that explanations are not stll very clear resulting why we see varying interpretations on the dragload on piles.

Lets take shaft resistance. This is preferred over skin friction and we should use for example positive shaft resistance and negative shaft resistance. Positive skin friction is caused by a swelling soil and negative skin friction by a settling soil. Negative skin friction is a directional shear force acting downwards, while positive skin friction is an upward directed shear force. However, I have noticed that Fellenius has also used the terms negative skin friction and positive shaft resistance in the same sentence. Negative skin friction should have been replaced by negative shaft resistance to be consistent. Not a big deal.

Dragload is a load transferred to the pile as a result of negative skin friction. Similarly, downdrag is downward movement of a deep foundation unit due to negative skin friction and expressed in terms of settlement. Hence, based on the above, negative skin friction is responsible for two events - downdrag, and drag load.

In determining settlement, this is composed of the settlement of the soil at the neutral plane plus the elastic compression of the pile from the sum of the desd load and drag load. Further that drag load nor live load should be included in the calculation. The dead load, however, is used at the neutral plane to spread the load through an equivalent footing in terms of a pressure at that location. Hence, this suggests that dead load is responsible for two effects, one at the pile head causing compression and one when this load is transferred to the soil at the neutral plane.

That the dragload does not cause settlement of the soil suggests that this load is not transferred to the soil, but taken up within the pile material itself as a result of the soil below the neutral plane resisting the downward movement of the pile. This is plausible since the drag load resulted from the action of the soil on the pile and therefore the action under this condition leads to compression of the pile material itself.

The question of the dragload not to be taken in consideration of the ultimate resistance of the pile can be explained that for ultimate resistance the load that the soil would provide would be determined through soil characteristics and not dictated by any superimposed load, same sort of principle we use in determining ultimate resistance of a footing. Another reason is if the system is at the ultimate state the pile is plunging and dragload would be of no consequence.

When this ultimate resistance is determined, it is divided by a factor of safety to obtain the allowable load. Some still subtract the drag load from this load. This has been demonstrated by Fellenius to be incorrect. The reason for this is that this drag load as previously mentioned is obtained from the soil and hence in effect cannot cause any further distress to the soil. Hence it is not providing anything to the soil to enhance its ultimate resistance and therefore should not be subtracted from the allowable load determined from the ultimate resistance. This is my interpretation from a lay point of view. This section causes some confusion despite the expalantions provided by Fellenius.

The concept that the drag load and dead load are to be used in determining the structural integrity of the pile is not an issue since both drag and dead loads are applied to the pile itself. The dead load being the load applied at the neutral plane.

In moving on to total stress analysis of ultimate resistance, should we not be using the terminology shaft adhesion resistance. Shaft resistance, which is comprised of adhesion and friction components of two materials with one moving in relation to the other is not identified in the terminology shaft resistance. Note that Fellenius stays clear of this term as well as cohesion another (bad word according to Schoefield) no doubt as a result of others and his preference to use the effective stress approach rather than the total stress approach and hence considers adhesion and cohesion to be non existent.

While I can agree on cohesion being eliminated, adhesion in my opinion is still valid if we are to still advocate the use of total stress approach to design. Adhesion should be left for clay soils with none or very little granular size particles, recognizing that there is no truly pure clay soil except perhaps for bentonitic clay.

For those accustomed to glacial tills-such as clay till, silt till etc, the term "shaft resistance" is valid and perhaps the effective stress approach is better suited However, other aspects of the analysis applied to such soils dominate as well, making the choice of total stress being preferred by some. We, therefore, still need to work on the terminolgies if we continue to use both total and effective stress approaches.

I think I have said enough. Excuse the length and sentence errors etc. Comments are appreciated.

 

[eric1037]

Perhaps, instead of trying to describe these concepts in words, it would be helpful to draw a free-body diagram of forces acting on a pile. I'm sure it would clear things up.

 

[emmgjld]

Sorry for the delay in posting. I returned from Seattle and my office required some attention. Those pesky clients!!

I will go over VAD's post and see if my recent 'education' is of use. I have to admit that I am VERY glad that my last 29 years of piling have almost all been end-bearing and relatively simple. Very few problems, mostly with 'out of town' engineers who had not worked with driven piling and had less than extensive understanding of drilled piers. The engineers from Denver are very familiar with expansive soils but, not much quality experience with settlement issues.

The following page on the Fellenius website has the downloadable publications.

http://www.fellenius.net/papers.html

The first paper,
274 Clarification of Confusion.pdf - 72 Kb
Clarification of confustion with regard to drag load on piles.
is relatively new and does explain the issue a little better than some of his previous.

 

[VAD]

Thanks emmgjld. I have previously read the papers from the link you have given. My interest is that despite this constant flogging of the concepts provided we still in 2006 seem to have publications that are in opposition to what Fellenius says. My objective is to try to see if we can bring out the concepts clearer for easier understanding. I am not sure that this is necessary for some but somethings do not seem to stick for a long time. I fully agree with you that the recent paper you quoted is an improvement on previous ones.

eric 1037: Yes some freebody diagrams would be of use. Some diagrams on the topic can be obtained from Fellenius's papers as given in the link by emmgjld.

I won't keep beating the subject much longer, all I was hoping for is to attempt to lessen the confusion that seems to exist and the fact that certain concepts as Fellenius has said are not taught at the Universities. He has also in the March issue of Geotech News(recent paper) looked at the practitioner perhaps being responsible for getting the ball rolling. I think the start should be made at the academic institutions as well.

Thanks and Regards

 

[emmgjld]

I think this subject does need more attention in that too many, geotechs and structurals, treat piled design as fairly simple and almost trivial. After all, if it is a steel or concrete element, set in 'bedrock' or driven to 'refusal', whats the concern? It's better than any other foundation type, isn't it?? A proper course in Piled Elements would take 2 days to work through a few common, simple examples. No wonder this is not adequately covered in normal course work.

I was considering VAD's discussion, in light with the interesting lectures and examples by Fellenius. I think much of the problem starts with consistent terminology and a lack of a Logical Process or steps of design. Eric1037 suggestion of a free body diagram, coupled with a flow chart would be helpful. A commentary alongside the flow chart would be essential. Several of the previous comment posts could easily be broken into a dozen individual commentary paragraphs. This is getting more complicated.

I have also considered the problem of the 'never ending' flow chart. Comparing the old Soil Sieve ASTM D-422 (8 pages) with the new Particle Size Distribution ASTM D-6913 (35 pages) should give one pause when trying to put down a complete set of directions.

A Logical Process gets very involved as the design must incorporate:
characteristics of Driven & Bored to Pressure Grouted elements.
Bearing Strata ranging from Compressible to Expansive to Metastable.
Other important strata may also range from Compressible to Expansive to Metastable.
End bearing only to Side bearing only to Combination bearing.

I think something needs to be done because I realize that students and practitioners, geotechs and structurals, have not realized how complicated this can be. I am not saying that all piled foundations are complicated but, this is not a matter of 'slapping in piles till they stop' or boring until the blue shale is penetrated 6' - 10' or whatever, and then assuming a simple maximum and minimum end bearing.

I have been very fortunate in that local practices and the local geology are such that dragload has been of little concern to me, except when dealing with collapsible soils & Auger Cast Piling. I have been blessed. Things are changing and I have to get up to speed.

 

[Panars]

Thanks for all the discussion. I am also very interested in downdrag and related topics, as I am trying to address how Ohio DOT will be incorporating the AASHTO LRFD code. While the new LRFD code does clarify some aspects of downdrag and how it should be addressed in design, I still suspect that there are some problems with the code or that some items are not addressed. At some time in the future I hope to sit down and figure out where the AASHTO LRFD code and Fellenius' discussions on downdrag agree or disagree.

 

[DRC1]

I recently saw Dr. Fellenius at a presentation in San Juan.
Although AASHTO references his work on pile capacity, It does not adopt his view on downdrag. Neither does the Eurocode. Utah DOT however, (The guys who built the interchange at Salt Lake in record time)is adopting his calculation methods.