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\C~adian Foundation Engn~eringM~nual Jo,.

is calibration to field measurements, the analysis can only be used to provide generalguidance.20.2.4.4 Dynamic FormulaeTheassumptions made in the dynamic formulae (which ate also discussed inChapter21) are oversimplified, and the resulta eannot always be related toaetual pilecapacity. One reason is that the dynamic formulae input ~mmer-rated energy and notthe aetually deUvered energy, whid1 as ind1cated above will result in considerable error.Nevertheless, when use

q" = U'tllwhere

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\ Geotechnical Design of Deep Foundations'- .-

Altematively, elastic methods can be used~ These methods suggest how downdragrelates to settlement (for example Poulos and Davis (1972, and provide a means ofestimating the maximum downdrag force and its development with time. Various .theoretical solutions are avaUable for single pUes (Poulos and Davis, 1980).The design must consider the structural capadty, the settlement and the geotechnicalcapadty of fue pile.,20.2.5.1 Design Considering DowndragThe design has to consider fue structural capadty, fue settlement, and fue geotechnicalcapadty of the pile. It is. important to realize that drag load and live load do notcombine, and fuat two separate loading cases must be considered: permanent load plusdrag load, but no live load; and permanent load and live load, but no drag loadoFurthermore, a rigid, strong pUe will have a large drag load, but small settlement,whereas a less rigid and less strong pile will have a smaller drag load, but largersettlement. AIso, no pUe subjected to downdrag condition will settle more fuan theground surface nearest the pile.

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As a first step in the design of the pile, fue neutral plane must be determined. Theneutral plane is located where the negative skin metion changes over to positive shaft.. resistance. It is determined by the requirement that the sum of the applied dead loadplus the drag load is in equilibrium with the sum of the positive shaft resistance and thetoe resistance of the pile. The location of the neutral plane goveros both the maximumload iri the pile and the settlement of the pile.20.2.5.1 (1) Neutral plane,The neutral plane is found as the inlersection of two CUIVeS.First, as illustrated inFigure 20.3, a load distribution curve is drawn from the pUe head and down with theload value starting with the applied dead load and increasing with the load due tonegative skin mction'crcting along the entire length of the pile~ Second, a resistancedistribution curve is drawn from the pile toe and up, starting with the value ofultimatetoe resistance and increasing with the .positive shaft resistance. '"The determination of the load distribution in a pile is subject to large ~certainty. Todetermine the distribution requires reliable information on the soUstrength. Thetheoretical analysis according to the method in Subsection 20.2.1~1 isrecommended. Theanalysis should be'supplemented with information from penetrometer tests, such as theSPT and the static cone penetrometer. For driven piles, the analysis should be combinedwith results from analysis of dynamic monitoring data (~ubsection 21.1.3).20.2.5.1 (2) Structural CapacityThe structural capacity of the pUe is govemed by the structural strength of the pUe atthe neutral plane, when subjected to the permanent load plus the drag load - live loadis not to be inc1uded. (At or below the pile cap, the structural strength of the embedded

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.

PILE TOE

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IR1"'",~, NEUTRALPLANE/'

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a) b)

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Figure 20.3: Calcula~on of the location of. the neutral planeand the ~ttlement of.apile or a pUe group (after FeUenius, 1984a).At the neutral plane, the pUe is confined, anQthe maximwn combined load may bedetermined by applying a safety factor of 1.5 on the pilematerial strength (steel yieldand/or concrete 284aY strength and long..term c:rushing skength of wood).

r lf both the negative skin.friction and the positiveshaft resistance as well as the toeresistance values are determined, assuming soll-$trength -vatues 'etring"on the strongside, the calcu1atedmaximum load in the pile will be on the conservative $ide.20.2.5.1(3) SettlementAs illustrated iI;\Figure 2O.3b, the settlement of the pUe head is foundby means ofdrawing a horizontal line from the neutral plane,as determined according to theforegoing method, to intersect with the curve representing the settlement distributionin the son surroundingthe pUe. The settlement of"the pileheadis'1Kp1a1 to the

. settlement ol fue soUat fue elevation o the neutr-alplane plus fue elasticcompressionof fue pile due to the applied dead load U\dthe drag Io4d (FeUenius, 1984a).

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")'

,-(\ Geotechnical Design of Deep FOundatign!

"One condition for the analysis iSthatthe movement at the pe toe must be equal to OI.exceed the movement required to mobilize the ultilate toe resistnce oi the piJe. Inmost soils, this required mo~ement is equal to about 1% to 2%oi thepUe toe diameter. oE driven piles anc.\.about 5% to 10% oi the toe diameter for bored pUes. lE tl:temovement is smallet than this required magnitude, the neutral plane will move higherup in the settlement diagram and the settlement will maease correspondingly. If so,however, the magnitude oi the settlement will normally be negUgible and correspondto the elasticcompressionof the pile. .The settlementcalculation should be carried out according to conventional methods (seeChapter 12) for the effective stress inaease caused by dead load on the pUe(s),surcharge, groundwater lowering, and/orany other aspect influencing the stress in thesoil. The dead load applied to the pile cap should be assumed to act at an equivalentfooting located at the level ofthe neutral plane and the load distributed &om this plane.The sett1ement oftbe pUecap is,the sum oi the settlement of the equivalent footing andthe compression' oi thepUes above the neutral planeo Note that Figure 20.3 does notshow the settlement due to the dead load acting on the equivalent footing at the neutral~M. . ~The reliability of the calculation of the distribution 0&,sett1ement depends on the ,reliability oi theinput data, which in tum depend onthe completeness oi the siteinvestigation programme. It is imperative that representative samples be obtained froma11 soil layers, including those below the pUe toe, and that the strength andcompressibility propertes of the soil.be determined in the la1?oratory. In-situ testingmetbods,. such as vane tests and static cone-penetrom.eter tests, will enhanee thelaboratory testing. .- .For the case in which the structure is builtbefore the pote, pressures induced by the pUejpstallation have dissipated, it is necessary to estin\ate the additional settlement causedby the pore pressure dissipation. . .20.2.5.1(4) Ge.o,technicalCapacity

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The last part ofthe design is t check the safety against plunging faUure oi the ple. Inthis case, the pUemoves down along its entire lengthand the downdrag is eliminated.TI'\erefore, the load is the combinatin oi the dead load and the live load - no drag 10a