274 clarification of confusion

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Piled Foundation Design Clarification of a ConfusionBengt H. Fellenius

A frequent confusion and lack ofunderstanding exists with regard to thedesign of piles subjected to drag loads.Some will lump the drag load in withthe dead and live loads when assessingpile bearing capacity. Also common isto disregard the root of the problem:settlement of the piled foundation. Itmust be realized that: dead and liveloads applies to bearing capacity, deadload and drag load applies to structuralstrength, and downdrag is settlement.

A few weeks ago, I was onceagain asked if the allowable load for apile should be reduced whenconsidering drag load. Shortlythereafter, when I took a look at thediscussions at www.Geoforum.com, Inoticed a very similar question.Perhaps I should not be that takenaback by the lack of knowledgedisplayed by the questions. Thepersons asking may not have beentaught better. The following is a quotefrom a textbook published in 2001 and

assigned to 4th Year Civil Engineeringstudents at several North AmericanUniversities:

May 2006, Geotechnical News Magazine, (24)3 53-55 53

Piles located in settling soil layersare subjected to negative skin frictioncalled downdrag. The settlement of thesoil layer causes the friction forces toact in the same direction as the loadingon the pile. Rather than providingresistance, the negative skin frictionimposes additional loads on the pile.

The net effect is that the pile load

capacity is reduced and pile settlementincreases. The allowable load capacityis given as:

where

Qallow = Allowable load capacityQult = Load capacityFs = Factor of safetyQneg = Downdrag

First, "negative skin friction" isnot "downdrag" but defines adownward directed shear force alongthe pile, while downdrag is the term forsettlement of a pile (caused by thesettling soil 'dragging a pile along').

Second, the term "load capacity" meansdifferent things to different people and"allowable load capacity" is anabominnable concoction of words.

Third, and very important, thephrasing in the quoted paragraphconfuses cause and effect. Dragload is not downdrag, and itdoes not cause settlement, but iscaused by settlement of thesurrounding soil and ismobilized when the pile resiststhis settlement. The worst boo-boo, however, lies in the quoted

formula, which does notrecognize that the factor ofsafety and the drag load areinterconnected, i.e., changingthe factor of safety changes thedrag load. As this may not beimmediately clear to all, thefollowing example will try toclarify the interaction betweenthe pile, the factor of safety, andthe drag load.

Example

Consider the case of a 300 mmdiameter pile installed to a depth 25 mthrough a surficial 2 m thick fill placedon a 20 m thick layer of soft claydeposited on a thick sand layer. Thecase is from a recent project in the realworld. Let's assume that a staticloading test has been performed and theevaluation of test data has established

that the pile capacity is 1,400 KN. Asis visually presented in Fig. 1, applyinga factor of safety of 2.0 results in anallowable load of 700 KN (dead load600 KN and live load 100 KN).Moreover, due to the fill and a loweringof the groundwater table, an almost200 mm settlement of the groundsurface will develop after theconstruction. How should the designerassess this case? Incidentally, asseveral full-scale case histories haveshown, whether or not the soil at the

site settles 200 mm or 2 mm, or for thatmatter 2,000 mm, the magnitude of thedrag load will stay the same.

0

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0 500 1,000 1,500 2,000

LOAD (KN)

DEPTH

(m)

Neutral plane

CAPACITYDEAD

LOAD

DRAG

LOAD

LIVE

LOAD

TOE

RESISTANCE

C

L

A

Y

S

A

N

D

FILL

A B

Fig. 1 Distribution of load in the pile.

"A" is the long-term load-distribution and "B" is theresistance distribution measured atultimate resistance (capacity) in astatic loading test

!negS

ultallow Q

F

QQ =


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Comments and questions

Some practitioners believe that all iswell because the foundations includepiles with a capacity of twice thedesired allowable load. Then, there arethose who understand that, for thenumbers indicated above, the pile willbe affected by a drag load of

about 300 KN, acting at a neutral planelocated at 17 m depth (Fig. 1). A fewof these practitioners will subtract thedrag load from the pile capacity beforeapplying the factor of safety and arriveat an amended allowable loadof 550 KN (which is a violation ofprinciples as this approach in effect hasreduced the drag load by a factorof 2.0). Others will apply the quotedformula, and arrive at an allowable loadof 400 KN. Yet others will realize thatthe latter approach means that the drag

load is applied without a factor ofsafety, preferring to apply the formulawith the drag load increased by a factorof safety, say 2.0. This results in anallowable load of (1,400/2 - 2x300) =100 KN don't laugh, I have seen itdone several times and it was proposedfor this project! So, which allowableload is right? Is it 100 KN, 400 KN,550 KN, or 700 KN? (Similardiverging approaches abound in theload-and-resistance-factor-design,LRFD).

Suppose the structure supportedon the piled foundation was built beforethe drag load conditions wererecognized (no signs of distress arenoticeable). Then, what factor of safetywould a back-analysis show the piles tohave? Would it be 1,400/700 =2.0, or(1,400 - 300)/700 =1.6, or1,400/(700+300) =1.4? And, I wonderhow the fellows advocating thelaughable approach would react whenrealizing that the piles are supportingseven times more load than themaximum load their approach wouldallow as safe.

Before answering, consider thatthe magnitude of the drag load dependson the magnitude of the dead load onthe piles. Reduce the dead load and thedrag load increases, and vice versa. Forexample, after reducing the allowableload by 150 KN to arrive at a 550 KN

value (made up of a dead load of,say, 475 KN and a live load of 75 KN),the drag load is no longer 300 KN, it is400 KN! If the allowable load isreduced by an additional 150 KN, sayto 400 KN (made up of a dead load of,say, 325 KN and a live load of 75 KN),the drag load increases 500 KN!

Note, for the three values of deadload 600 KN, 475 KN, and 325 KN the neutral plane location changesfrom depths of 17.0 m to 18.0 mto 19.5 m, respectively. To simplify theexample, no change of the toeresistance is included. In reality,however, the deeper down the neutralplane lies, the smaller the enforcedpenetration of the pile toe into the sandand the smaller the mobilized toeresistance, and when the toe resistanceis reduced, the location of the neutral

planes moves upward and the drag loadchanges. Altogether, the load at theneutral plane, that is, the maximumload in the pile, is essentiallyunchanged for the three alternativevalues of allowable load. In stark andimportant contrast, for each reductionof allowable load, the projectfoundation costs increase.

Clarification

Bewildering, ain't it? Many select one

of the four approaches as the one that tobe correct, ignoring the others thusavoiding having to make the small leapof understanding of what a properdesign needs to include, as follows::

First, the drag load does not affectthe pile bearing capacity the ultimateresistance. That is, the pile capacity isthe same whatever the magnitude of theload from the structure. The factor ofsafety is applied to ensure that, shouldthe load on the pile be inadvertentlylarger than intended and should the pilecapacity be inadvertently smaller thanthought, the pile might be close tofailure, but it would not fail. Nonegative skin frictionno drag loadis present close to failure. Therefore,only the first approach, that withthe 700 KN allowable load, is correct.

Second, the drag load has to beconsidered, of course, but not in thecontext of bearing capacity. Theconcern for the drag load only affectsthe pile structural strength at thelocation of the maximum load, i.e., atthe neutral plane. For the examplecase, if the structural integrity of the

pile is safe considering the sum of deadload and drag load, that is, 900 KN forthe example case, the design for dragload is complete.

Third, with (A) the dead load pluslive load safe considering the pilecapacity, and (B) the dead load plusdrag load safe considering pilestructural strength, it remains to showthat (C) the pile will not settle morethan acceptable, that is, that downdragis kept in check.

In checking the pile for downdrag,it must be realized that there are twodifferent definitions of the neutralplane. Both give the same result, orlocation, rather. One defines theneutral plane as located at the forceequilibrium in the pile, which is wherethe shaft resistance changes fromnegative to positive direction and wherethe sum of the dead load plus drag loadis in equilibrium with the positiveforces in the pile. The second definesthe location to be where the pile and the

soil move equally. (Note, the toeresistance is only as large as is neededto establish the equilibrium betweenforces and movements. Moreover,whatever the factor of safety chosen inthe design, if the soil is settling at theneutral plane, the pile will settle too andas much as the soil settles at thatlocation). The influence of varyingdead load and toe resistance isillustrated in Fig. 2. The diagram to theleft shows the load distributions andlocations of the neutral plane for the

dead loads associated with thementioned different allowable loads onthe pile (the 100 KN case is excluded).

The diagram to the right shows thedistribution of soil settlement andlocation of neutral planes for the threeapproaches. (More explanation anddiscussion is available in Fellenius,2004).


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The biggest problem

The foregoing appears to be news tomany. It shouldn't. The long-termresponse of piles in settling soil wasmade known in several very accessiblepublications as early as some 40 yearsago. How does one get the textbookwriters and the teachers of foundationdesign to become aware of theknowledge and convey it in theteaching of future practitioners? Howdoes one get practitioners to fill thevoids in their professional educationand to keep abreast with advances inthe profession? The latter,unfortunately, may be the biggestproblem of all, but discussing it liesoutside this contribution.

References

Fellenius, B.H., 2004. Unified designof piled foundations with emphasis onsettlement analysis. "HonoringGeorge G. GobleCurrent Practice andFuture Trends in Deep Foundations"Geo-Institute Geo-TRANS Conference,Los Angeles, July 27 - 30, 2004, Editedby J.A. DiMaggio and M.H. Hussein.ASCE Geotechnical SpecialPublication, GSP 125, pp. 253 - 275.

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LOAD (KN)

DEPTH

(m)

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0 50 100 150 200

SETTLEMENT (KN)

DEPTH

(m)

Pile toe penetrations

Neutral plane

Force

equilibrium

Neutral plane

Equal settlement

Pile-head

settlements

Ground surface

settlement

Fig. 2 Distribution of load in the pile and interaction with soil settlement. The dashed curves

represent the gradual change in a transition zone from negative direction of shaft shear topositive direction.

Bengt H. Fellenius,Dr.Tech., P.EngBengt Fellenius Consultants Inc.1905 Alexander Street SECalgary, Alberta, T2G 4J3

Tel.: (403) 920-0752e-mail: Web site: [www.Fellenius.net]

274 clarification of confusion

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