Geotechnical Engineering Circular

8/10/2019 Geotechnical Engineering Circular

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Geotechnical Engineering Circular (GEC) No. 8Design And Construction Of Continuous Flight Auger PilesFinalApril 2!

Chapter 6: Recommended Design Method

The purpose of this chapter is to present a step-by-step generalized method for theselection and design of CFA piles.The process consists of the following design steps:

"tep #$ %nitial Design Considerations

"tep 2$ Co&parison and "election of Deep Foundation Alternati'es

"tep $ "election of Pile ength and Assess&ent of Pile Perfor&ance under"pecified oads$

o Calculation of Pile Length

o erification of Capacity and Performance for A!ial and Lateral Loads

o erification of Pile "roup Capacity and "roup #ettlement Calculations

o Pile #tructural $esign

o %iscellaneous Considerations

"tep *$ +e'ie, of Constructa-ilit

"tep /$ Preparation of Plans and Construction "pecifications0 "et 1C1A andoad 3esting +e4uire&ents

The remainder of this chapter presents an outline of each of the design steps listed abo&eand presents preliminary discussions of the most salient aspects of the design.

5.# "tep #$ %nitial Design Considerations

The initial design considerations include a re&iew of structure-specific and site-specific

conditions for the pro'ect that are necessary for any foundation design. (n this chapter)attention is focused specifically on those items that establish or preclude suitability of CFApiles for the pro'ect. #tep * is subdi&ided into se&eral components as described below.

6.1.1 General Structural Foundation Requirements

The first step in the entire process is to determine the general structural re+uirements forthe foundation. #ome important considerations include the following:

Pro'ect type: new bridge) replacement bridge) bridge widening) retaining wall) noisewall) sign or light standard. CFA piles may be considered for any of the abo&e.


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Construction se+uencing: phased construction or all at once. ,either conditioneither precludes or fa&ors CFA piles.

"eneral structure layout and approach grades.

#urficial site characteristics. A stable woring platform is re+uired for CFA pileconstruction.

#pecial design e&ents such as seismic) scour) &essel impact) etc. These factors

should be considered in planning the site in&estigation and can ha&e a significanteffect on the selection of CFA piles.

Possible modifications to the structure that may be desirable for the site underconsideration.

Appro!imate foundation loads and limitations on deformation.

6.1.2 Site Geology and Susur!ace Conditions

A comprehensi&e re&iew of this component of #tep * is the sub'ect of other F/Apublications 0e.g.) 1#ubsurface (n&estigations)1 document F/A-(-23-45*) and ,ationaighway (nstitute 6,(7 /orshop on #oils and Foundations) document F/A-,(-88-49) and ";C ,o. authored by #abatini et al. 654457? and will not be repeated here in detail. (n general) thecomponents of the site characterization include a re&iew of the site geology andfoundation e!perience in the area) followed by a carefully planned and e!ecutedsubsurface e!ploration program. (n general) the consideration of CFA piles does notre+uire specialized in&estigation techni+ues differing from those used for dri&en pilefoundations. (mportant considerations for CFA piles include the general site stratigraphyand soil classification) the depth and characteristics of the most liely bearing formation)

groundwater conditions) &ariability) and the presence and e!tent of unusual geologicfeatures such as solution ca&ities) boulders) lenses) or layers of hard roc.The use of cone penetration testing 0CPT? is generally considered to be particularly wellsuited for design procedures used for CFA piles) but especially for drilled displacement0$$? piles. CPT soundings pro&ide a continuous record of a strength measurement thatcorrelates well with CFA and $$ pile performance) CPT soundings can generally beperformed in soils where CFA or $$ piles are to be considered. (t is also a &ery cost-effecti&e tool compared to con&entional borings for sounding a large area. /hereconditions are such that CFA piles may be considered as a &iable foundation alternates)the use of CPT soundings is recommended and encouraged as a part of the e!ploration

program.

5.2 "tep 2$ Co&parison and "election of Deep Foundation Alternati'es

The information from #tep * must be e&aluated and a foundation system selected.Alternati&es to deep foundations may be considered) including shallow foundationsystems and the potential use of ground impro&ement techni+ues to allow the use ofshallow foundations. /here deep foundations are re+uired) alternati&es include dri&enpiles) drilled shafts) micropiles) and CFA piles including both con&entional CFA piles and$$ piles. The selection of the optimum deep foundation system for a gi&en pro'ectincludes consideration of multiple factors and re+uires e!perience and 'udgment on the


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part of the designer. Table 8.* outlines many of the considerations in&ol&ed in thefoundation selection process with respect to CFA piles.

3a-le 5.#$ Design Consideration for Foundation "election of CFA and DD Piles

#tratigraphic Fa&orable "round Conditions@ Type of Profile "/T Location ther Factors

Predominantly Clays: fa&orCFA

ery soft surface may beunfa&orable due to poor

woring platform

(f below e!istinggrade) not much of a

factor in clays.

$epth to competentmaterial B 4 m 0*44

ft?: not fa&orable

#ands: fa&or $$ niform or similar strata:fa&orable

"/T depth B m 0*4ft?: fa&orable

Doulders) roc layersor lenses: not

fa&orable

ighly &ariable drillingresistance: not fa&orable

Artesian conditions:not fa&orable

"ood woringplatform is especially

important for $$because $$ rigs areusually hea&ier than

CFA

Cemented soils) wea roc:fa&or CFA

ighly &ariable drillingresistance: not fa&orable

Artesian conditions:not fa&orable

Doulders) roc layersor lenses: not

fa&orable

=oc: not fa&orable for CFA or$$

-- -- --

#tructural Loading Conditions ther Conditions

Appro!imate ma!imum ultimate lateralloads per &erticalpile 0ip? &s. recommended diameter 0in.?

Low headroom

re+uirements: may fa&orCFA piles

Close pro!imity to e!isting

structures: not fa&orable toCFA or $$ due to potentialground mo&ements duringconstruction

,oise and &ibrationconsiderations: may fa&orCFA or $$ piles &s. dri&enpiles

Potential obstructions below

grade: not fa&orable to CFAor $$ piles

soft or loose soil:

*5 ip - *91

dia.

54 ip - 5>1

dia.

4 ip - 41dia.

>< ip - 81

dia.

#tiff or dense soil:

54 ip -

*91 dia.

< ip -

5>1 dia.


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Factors A&ailable@

,ecessary for CFA or $$ Large numbers of piles:fa&orable to CFA or $$

#mall pro'ect with few piles and manymo&es: may not fa&or CFA or $$

The subse+uent steps are presented for CFA piles and $$ piles. Alternati&e deepfoundation types are described in e!isting F/A design manuals for drilled shaftfoundations 0e.g.) F/A =eport ,o. (F-22-45< by ,eill and =eese 6*2227?) for dri&enpile foundations 0e.g.) Publication for ,( Course F/A-,(-*545* by annigan et al.654487?) and for micropile foundations 0e.g.) Publication ,(-4


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in simple spans and 4.44> in continuous spans. (t should be noted that only post-construction settlements affect ser&iceability of the bridge structure. Tolerablemo&ement criteria for lateral displacement should be de&eloped considering theeffects of combined lateral and &ertical displacements on the structure. AA#T05445? #ection >.>.3.5.< re+uires that horizontal mo&ements be limited to 5< mm 0*in.? where combined horizontal and &ertical mo&ements are possible) and be limitedto 9 mm 0*.< in.? when &ertical mo&ements are small relati&e to horizontalmo&ements.

. #tructural ltimate Limit #tate 0#L#?. The pile must ha&e sufficient structuralcapacity when the pile is sub'ected to combined a!ial and fle!ural loads such thatstructural yielding of the pile is a&oided. The #L# pro&ides a second definition offoundation strength.

Figure 8.*: "L# and #hort-Term #L# for A!ial Load on a #ingle CFA Pile

%ost engineers using A#$ methods are familiar with the concept of a factor of safety)which di&ides the resistance at the "L# to determine an allowable load per pile. Theser&ice 0unfactored design? loads are compared against the allowable loads per pile toe&aluate strength and pro&ide the margin of safety against the limit-state condition 0oftenreferred to as 1failure1?.For design of CFA piles using A#$) the resistance computed at the "L# is comparedwith ser&ice loads such that:

0;+uation 8.*? Qi R / SF

/here:

Qi H &alue of ser&ice 0unfactored? load of type i

R H computed resistance at "L#

SFH factor of safety

CFA piles should generally be designed ha&ing a factor of safety of at least 5.-9*? isperformed at the site) to a load e!ceeding the computed ultimate by


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first. %ultiple load tests are re+uired where the site e!tends for a great distance ofroadway structure or if) in the 'udgment of the pro'ect geotechnical engineer) thesite spans across significant geologic or stratigraphic differences in site conditions.$ynamic or rapid load tests) as described in Chapter 3) may be substituted forsome of the con&entional static load tests and their results can be correlated tostatic test or local conditions.

5. Automated monitoring systems are used on production piles to &erify thatproduction piles are constructed in a similar way and to achie&e similar

performance in the test pile0s?.

. The site geology) stratigraphy) and soil properties are not highly &ariable.;ngineering 'udgment must apply in this instance) and higher factors of safety fordesign are warranted for conditions with unusual &ariability in soil properties orgeologic conditions. (t is acceptable to use a higher factor of safety for someportion of the computed resistance. For e!ample) base resistance or the resistancewithin some deeper strong layer may be considered as more &ariable than otherportions of the profile that contribute to resistance and therefore) an engineer mayapply 'udgment to assign a higher factor of safety to this portion of the resistance.

>. The site conditions do not pose difficult construction conditions for CFA piles.

,ote that load testing and monitoring as described abo&e should be re+uired on alltransportation pro'ects constructed using CFA or $$ piles. ;!ceptions may includesoundwalls) sign foundations) or similar structures for which the design is not controlled ba!ial resistance.(t is also important to note that the foundation design engineer should consider the factorsof safety cited abo&e as minimum &alues) and should use a higher factor of safety in anycircumstance where there e!ists concerns for site &ariability) lac of redundancy) orpotentially difficult construction conditions. An especially critical structure such as a lifelinestructure for hurricane e&acuation or seismic safety concerns may also warrant a highersafety factor. The &alues suggested abo&e are for typical conditions and routine pro'ects.

6.".2 Design $rocedures

To complete a design) it is necessary to perform static analyses and estimate pile lengthsand pile diameters necessary to pro&ide the re+uired compression) lateral) and uplift loadcapacity. For most routine pro'ects) it is anticipated that the ultimate limit state 0capacity?conditions usually control the design of indi&idual CFA piles rather than ser&iceability0deflection? re+uirements) therefore) the most efficient approach for design is to chec firs

ultimate limit state conditions and then ser&iceability. ,ote that some instances ofser&iceability may control) such as with large pile groups o&er a deeper compressiblestratum.(n general) smaller diameter and greater length piles tend to be more economical thanlarger diameter) shorter length piles with similar a!ial resistance. owe&er) pile diameter ioften controlled by lateral shear and bending moment considerations. Lateral loadconsiderations almost ne&er control the length of CFA piles because the soil resistancemobilized to resist lateral loads tends to be within the shallow strata e!tending to a depthno more than about *4-pile diameters. (f significant lateral loads 0per pile? are anticipatedthe design is accomplished by first performing an initial analysis of lateral loading to definthe re+uired diameter and also the depth to the point where calculated bending moments


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are negligible. This lateral chec is followed by static analysis of a!ial load capacity todefine the pile length re+uirements.,ote that a range of diameters and lengths may be considered for groups of piles) as it isfeasible to consider foundations with larger numbers of smaller capacity piles &s. fewernumbers of larger capacity piles. At this stage in the design) it is appropriate to consider arange of pile capacities for possible different group configurations. The proceduresdescribed in Chapter < are used to perform the analyses associated with determining a!iaand lateral resistance of &arious design alternati&es.The recommended procedure for performing a foundation design of CFA or $$ piles is

outlined below. This outline is de&eloped for the design of a foundation for a structure sucas a bridge. A similar procedure for sign foundations or wall components may differ inspecifics of some components) but will follow a similar general outline.

A. De'elop %deali6ed Profile.sing the borings and geologic descriptions from thesite in&estigation program) group the borings into zones for foundation designaccording to similarities in the soil profile and properties) and establish idealizedgeotechnical design profiles for each representati&e zone at the site. The differingzones should ade+uately co&er the range of conditions at the site) and thedesigners may de&elop multiple profiles for each zone to e&aluate the possiblerange of geotechnical 0and groundwater? conditions within each identified zone. (t

may also be necessary to consider se&eral cases of scour associated with differentloading conditions. The layers and characterization of soils should be consistentwith the methods for estimating a!ial and lateral load transfer as outlined inChapter < of this document.

D. De'elop Geotechnical Design Para&eters.For each stratum defined in theidealized profiles) e&aluate the geotechnical strength and stiffness parameters toestablish design parameters for each layer. These may include:

a. #oil strength parameters such as undrained shear strength anddrained friction angle or other measurements) such as #PT-N60&aluesand cone tip resistance. These strength parameters will be used eitherdirectly or through correlations to estimate unit side-shear and end-base resistances and to de&elop p-y cur&es for lateral loading.

b. #oil stiffness or modulus and other parameters related to deformationcharacteristics for use in de&eloping p-y cur&es for lateral loading andperforming settlement analyses of pile groups.

c. ther soil properties that may be needed for design) such as unitweights and inde! tests for classification.

,ote that the actual &alues used for design are typically based on 'udgment ande!perience along with an understanding of the site geology and potential &ariability0#abatini et al.) 5445?.

C. O-tain the oadings for the Foundation.The design loadings will liely includese&eral cases of both a!ial and lateral loads. %any different load combinationse!ist of dead loads) traffic loads) wind loads) etc. #ome cases may be combinedwith scour and some may include e!treme e&ent loadings. (f downdrag or uplift dueto e!pansi&e soils is to be considered) these should be noted at this time forinclusion where appropriate into subse+uent analysis and design steps. Analyses


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of the load combinations will re&eal the ma!imum a!ial and lateral loads imposed tothe pile and represent the critical design cases.

$. "afet Factor(s) for Design.The safety factors cited in the pre&ious section aresuggested for general use in design for strength) and differ depending on the le&elof site-specific testing and +uality control. Large &alues may be applied in caseswhere unusual &ariability in subsurface conditions e!ist) difficult constructionconditions are liely) or if other considerations for the structure dictate. Fore!ample) where base resistance contributes a large portion of the a!ial resistance

and the properties of the bearing stratum are +uite &ariable) it may be appropriateto use a significantly greater safety factor on base resistance than side-shear e&enwhere a load test is performed.

;. "elect a 3rial Design Pile Group to Esta-lish %ndi'idual Pile oads.Thegeometry and layout of the pile group) along with the number of piles used tosupport the foundation loadings will determine indi&idual loads per pile. At this pointin the process) e!perience andIor some preliminary estimates should suggestsome reasonable &alues of nominal a!ial and lateral resistance for single piles sothat an efficient layout can be de&eloped. ;ngineers are encouraged to e&aluate

numerous alternati&es in the preliminary design stage.

F. "elect a 3rial Design for %ndi'idual CFA or Drilled Displace&ent Piles.

a. $esign for Lateral Loading. Lateral analyses may only be needed iflateral loads are significant. As a guide) lateral shear forces on &erticalpiles that are less than about 2 to 55 , 05 to < ips? per pile for >84to 2*< mm 0*9 to 8-in.? diameter piles) would probably not 'ustifylateral analysis at this point in the design process and the designercould sip the lateral analyses and mo&e on to design for a!ial load. (fbattered piles are used to resist lateral forces these guidelines applyto the resulting forces trans&erse to the pile a!is and the longitudinalcomponent of the force on the battered pile is considered in design fora!ial loading.

#elect a diameter that is sufficient to pro&ide the necessary nominallateral load resistance and ser&ice load re+uirements for deflection.,ote that when lateral loads are significant) the final design of CFApiles for lateral loading is typically controlled by structural designconsiderations and the necessary fle!ural strength and reinforcement.

At this point in the design process) it is prudent to consider lateral

loading because it may control the pile diameter 0but usually not thelength?. ,ote that high bending stresses combined with unsupportedlength due to scour or li+uefaction may preclude the use of CFA pilesaltogether.

A preliminary lateral analysis using a computer code as described inChapter < is warranted at this step in the process. The relati&esignificance of lateral load magnitude is certainly dependent upon soilconditions) and wea surficial soils tend to result in more significantbending moments for a gi&en lateral shear force. Datter piles may alsobe considered for large lateral forces if downdrag or constructability

considerations do not preclude their use) and groups of piles including


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batter piles are best analyzed using -$ group analysis programssuch as "=P 0;nsoft) 5448? or FD-Pier 0D#() 544?.The lateral load analysis of a trial pile design should proceed asfollows:

i. #elect a trial pile diameter and length 0although thedesign is generally not sensiti&e to length for CFA pilesha&ing lengthIdiameter ratios of 54 or more?. #elect atrial longitudinal reinforcement. A longitudinalreinforcement with a cross-sectional area of around *G

of the pile cross-sectional area is typically a good initial&alue to consider. Construct a computer model with p-ycur&es for the load conditions liely to be most critical forlateral load considerations.

ii. /ith the pile modeled as a linear elastic beam) e&aluatefoundation strength conditions by computing thefoundation response of the pile due to ser&ice loadsmultiplied by a factor of at least 5.< to ensure that the pileembedment into the soil has ade+uate reser&e capacity.

#er&ice load deflections or structural strengthre+uirements generally control design) but this 1push-o&er1 type analysis is performed to ensure that ade+uatereser&e strength e!ists with respect to the soil resistance.

At the same time) ser&ice load and factored load casescan be computed to pro&ide design information that willbe used in subse+uent steps. ,ote that there may bese&eral different load combinations to be e&aluated)although there is usually a clearly dominant lateralloading case. This chec is to ensure that push-o&erconditions do not control the design. As mentionedearlier) normally ser&ice deflections or structural limitstate will control lateral load design. The strength checensures that the a&ailable soil resistance e!ceeds thestructural capacity of the pile in fle!ure and thus thefoundation should ha&e ductility. ,ote that for analysis ofa single pile that will be representati&e of a group ofpiles) it is appropriate to include a p-multiplier 0less than*.4? which is e+ual to the a&erage p-multiplier for thegroup. The p-multiplier concept is described in Chapter 4.3 3> - 94 35 >.3

3otal$

Add to the consolidation settlement abo&e the theoretical elastic shortening 0Sel? ofthe piles acting as a column abo&e the e+ui&alent footing. These piles support atotal load of 2< ips with an area e+ual to that of < pilesK therefore) using anestimated elastic modulus for the piles of )444 si) it results

SelH 0*I5? M 0#L/%(.H *I5 M 602< ip M in.

Long-term settlements under dead load for this foundation are thereby estimated atno more than 4.8 4.4> H 4.>4 in.) or about 4.< in. (t should be noted that aportion of the dead load is applied onto the piles before the pier cap and girderbearing plates are finalized) and the portion of the settlement that occurs during thisperiod will not affect the bridge structure. Therefore) if the long-term settlementsdue to total dead load appear to be too high) it would be prudent for the designengineer to re-e&aluate the settlements to estimate the actual magnitude of thepost-construction portion of the total settlement.

8.8.>.( Pile "roup Lateral Deha&ior. Lateral deflections ha&e already been

estimated in the pre&ious step 08.8.>.F.a? at less than 4.5 in. for the design lateralloads using appropriate P-multipliers to account for group effects. A full -$


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computer model of the proposed foundation may be analyzed using "=P0;nsoft) 5448? or FD-Pier 0D#() 544? or similar software. The use of thesesophisticated programs is not necessary for this simple problem but may be acon&enience for users who are familiar and efficient in using such software.

8.8.>.J Finalize #tructural $esign. The design satisfies geotechnical limit states andser&iceability limit states. The final sub-step in designing the piles is to finalize thestructural design of the pile and select reinforcement. #tructural design follows theprocedures outlined in #ection .

The depth re+uired for the full cage section must first be determined. (t was seen inFigure 8.8 that the pile clearly e!hibited 1long pile1beha&ior) and the depth to thecounter-fle!ure point in the displacement profile 0second point of zero displacementwith depth? was appro!imately 5< ft. The cage may thus terminate at this depthbecause the piles are not bearing in wea roc or intermediate geotechnicalmaterial 0("%? socets.Longitudinal reinforcement was chosen as the recommended minimum of *G) ashas been determined pre&iously in computations of lateral resistance to pro&ideade+uate lateral structural resistance. Combined a!ial force and fle!ure was

considered in the analysis presented in Figure 8.9.For the ma!imum computed shear force in any pile) chec to determine if theconcrete section has ade+uate shear capacity without shear reinforcement0Chapter


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Finally) a brief chec on constructability issues is pro&ided in a +uestion and answerformat:

a. Are pile length and diameters appropriate@ 9es- 3: in) dia$eter and less t;an uire !asting t;e pile to t;e surfa!e

and !;ipping don after t;e grout or !on!rete ;as set) ?;ese piles $a 'e for$edto t;e surfa!e and !ut don later) @f t;e !ontra!tor !;ooses to dip t;e pile don;ile still fluid- t;ere $ust 'e a te$porar for$ provided to prevent !aveins fro$!onta$inating t;e top of t;e pile)

g. (s construction se+uence feasible@ 9es) Consider e&isting stru!tures- o'stru!tions-pile !ap footprint) %void installing CF% or DD piles over ater) % sta'le oringplatfor$ is ne!essar- espe!iall for DD piles- ;i!; re>uire larger- ;eavier rigst;an !onventional CF% piles) ?;e oring pad $a re>uire sta'ili,ationA t;is isort; of a note on t;e plans t;at t;e surfa!e $a 'e soft)

h. Are low headroom conditions re+uired@ No.

i. (s there a plan for resol&ing construction +uestions prior to production@ 9es)@n!lude pre!onstru!tion $eeting dis!ussion)

6.6.6 Step +: $repare $lans and Construction Speci!ications- Set Field C/& and #oa0esting Requirements

These items follow the general wor for de&eloping plans and specifications fortransportation pro'ects in&ol&ing deep foundation wor. The following two chapters are

focused on the aspects of inspection) testing) and specifications for CFA pro'ects. ,otethat for this e!ample) a load testing program is re+uired because the design relied on theuse of a lower factor of safety and because of the increased reliability of a design thatmust be &alidated by site-specific load test data.

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