6821982-pilefounddeschap041

8/2/2019 6821982-PileFoundDesChap041

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TECHNICALENRINEERINGNODESIGNRUIDESASADAPTEDFROMTHEUSARMYCORPSOFENGINEERS,o. 1

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DESIGNOF

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IPONTrFIC'A UNIV8RSIO~O

Ci\TOLlfA DEL PERUBIBLIOTECA

f. INGENIERIA.

Published by American Society of Civil Engineers1801 Alexander SeU DriveReston, Virginia20191-4400


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~

ABSTRACTThis handbook, Design of Pi/e Foundations, provides intormatiOn. foundationex-ploration and testing procedures, load test methods, analysis techniques, designcriteria and procedures, and construction considerations tor the selection. design,and installationot pile toundations.While the understanding01pile tound


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.,-.r.

-USIGN Of PIlE FOUNDATIONS ni

TABLEOF CONTENTS

Chapter 1. Introduction1-1 Purpose1-2 Applicability1-3 References, Bibliographical and Related Material1-4 Definitions

1112

Chapter 2. General Considerations2-1 General2-2 Structuraland Geotechnical Coordination2-3 DesignConsiderations ..2-4 Nature of loadings2-5 FoundationMaterial2-6 IdentiRcationand Evaluationof PileAlternatives2-7 Field Responsibilities for the Design Engineer2-8 Subsurface Conditions2-9 Pile Instrumentation

333455788

Chapter 3. Geotechnical Considerations3-1 Subsurface Investigations and Geology3-2 Laboratoryand Field Testing3-3 Foundation Modification3-4 Groundwater Studies3-5 DynamicConsiderations3-6 PileloadTest -3-7 Selection of Shear Strength Parameters

99910101011

.' . ... ... .-. . -o. - .. ... --. '-"'- '--' "'-"

. . Chapter- 4~.Aftalysis ande Design4-1 General4-2 DesignCriteria4-3 PileCapacity4-4 Settlement4-5 PileGroupAnalysis4-6 DesignProcedure4-7 SpecialConsiderations

13131723263134

'.';-'

-.


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IV DESIGN OF PIte FOUNDATION

Chapter 5. Engineering Considerations Pertaining.to Construction- - - -o"; -,-~5-1 General5-2 Construction Practices and Equipment5-3 Pile Driving Studies5-4 Control of PileDrivingOperations5-5 Resultsof Corps Experiences5-6 As-BuiltAnalysis5.7 Field Evaluation

38384850525354

Chapter 6. Field Pile Tests6-1 General6-2 Decision Process6-3 Axialload Test6-4 Monotonic lateralload Test

565657

Appendices ---A-l Appendix A. ReferencesB-l Appendix B. Bibliographical and Related MaterialC-l Appendix C. Case History-Pile Drivingat lockand Dam No. 1 Red RiverWaterwayD-l Appendix D. Pile Capacity Comp'utations

63646780

Index 99

.- -- 0_0_- -. -.. -.0'- 00 -- -_o- -'0 -' . -_o-. /""


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AND DESIGN 13

CFiAPTER'4ANALYSIS AND DESIGN

a state of ductile, stable equilibrium is attainableeven if individual piles wiJI be loaded to their peak,or beyond to their residual capacities. Special provi-sions (such as field instrumentation, frequent orcontinuous field monitoring of performance, engineer-ing studies and analyses, constraints on operationalor rehabilitation activities, etc.) are required toensure that the structure wi/l not catastrophica/ly foilduring or after extreme loading .conditions. Devia-- tions fromthese criterio for extreme loading condi-tionsshQuld be fOl"mulatedin consultation with and,approved by CECW-ED.

4. Foundation Properties. Determinationof foundation properties ispartia/ly dependent ontypes of loadings. Soil strength or stiffness, and there-fore piJe capacity or stiffness, maydepend onA. APPLlCABlUTYAND DEVlATIONS. whethera load is vibratory, repetitive, or static and

The design criterio set forth in this paragraph whether it isof long or short duration. Soil-pileproper-re applicable to the design and analysis of a broad ties shouJd, therefore, be determined for each type ofange of piles, soils and structures. Conditions that loading to be considered.re site-speciFicmay necessitate variations which must C. FACTOR OF. SAFETY FOR PILEe substaotiated by extensive studies and testing of CAPACITY. The ultimate axial capacity, based onoth the structural properties of the piling and the geatechnical considerations, should be divided by theeatechnical properties of the foundation. factors of safety deFined in Table 4-1 to determine theB. LOADING CONDmONS. . design pile capacity for axialloading.

1. Usual. These conditions inelude normal The minimum safety factors in the table areperating


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14

"

DESIGNOFPILEFOUNDATlONS.

Table 4-1. Factor 01~ty for pite capacity .Method of MinimumFactor of SafetyDetermining Capacity Loading Condition Compression TensionTheoretical or empiricalprediction to be verifiedby pile load test

Theoretical or empiricalprediction to be verifiedby pile driving analyzeras described inParagraph "-4A

UsualUnusualExtremeUsualUnusualExtreme

Theoretical or empiricalprediction not verifiedby load testUsualUnusualExtreme

1.5teel Piles. Allowable tension and com-pression stresses are given- for both the lower andupper regions of the pile. Since the lower region ofthe pile is subject to damage during driving, the basicallowable stress should reflect a high factor of safety.The distribution of allowable axial tension or compres-sion stress along the length of the pile is shown in Fig-ure 4-1. This factor of safety may be decreased ifmore is known about the actual driving conditions.Pile shoes should be used when driving in dense sandstrata, gravel strata, cobble-boulder zones, and when

..Mj:V

...-fW,y.......

2.01.51.152.51.91.4

3.02.251.7

2.01.51.153.02.251.7

3.02.251.7driving piles to refusal on a hard layer ofbedfock.Bending effects are usually minimal in the lowerregionof the pile.' The upper region of the pile maybe subject to the effects of bending and buckling aswell as axialload. Since damage in the upper regionis usually apparent during driving, a higher allowablestress is permitted. The upper region of the pile isactually designed as a beam-column, with due consid-eration to lateral support conditions. The allowablestresses for fully supported piles are presented inTable .4-2.

~ ~,,-.".-.- --'---"-'--.'"" ---....-

....I'U

JIU

t MOMENTDIAGItAMS,

l8J

-- ---- F.- ....

Figure 4-1. Allowable tension and compression stress for steel piles

r PU

F.- ....

l1li

AUOWAIILE AXIAL1I18JN 0It COIfN[llllOltiF. FOa IWIII

lcI


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ANDDESIGNrabie 4-2. Allowabletresses

nsionor coinp~.ssioninlower pile region'-Concentric axial tension or com- 10 kips per squarepression only 1Okips per inch (ksi)for A-36square inch (1/3 x Fyx 5/6) materialConcentric axial tension or com- 12 ksi for A-36 mate-pression onlywith driving rialshoes (l/3 x Fy)Concentric axial tension or com-pression only with drivingshoes, at least one axialloadtest and use of a pile drivinganalyzer to verifythe pilecapacity and integrity(1/2.5 x Fy)

14.5 ksi for A-36material

ombined bending and axial compression in upper pile

Ifa :t fbx :t ~F I S 1.0Fa Fb b

fa" computed axial unitstressFa" allowable axial stressFa" ~ X! Fy" ~ Fy" 18 ksi (forA-36 material)xand fby" computed unit bending stressFb" allowable bending stressFb" ~ X! Fy" ~ Fy.. 18 ksi (forA-36noncompact sections)

15

factor aqual to 2.7For.all combinations of d.~d andlive loads. To account Foraccidental eccentricities, theaxial strength oFthe pile shall be limited to 80 percentoFpure axial strength, or the pile shall be designed Fora minimumeccentricityequal to 10 percent oFthe pilewidth. Strength interactiol1 diagrams For prestressedconcrete piles may be developed using the computerprogram CPGC (Item 16). Control of cracking in pre-stressed piles is ochieved by limitingthe concrete com-pressive ond tensile stresses under service conditions tothe volues indicoted in Table .4-3. The o/lowablecompressive stresses Forhydraulic structuresare limitedto approximately 85 percent oF those recommendedby ACICommittee5.43 (Item20) Forimproved service-obility. Permissiblestresses in the prestressing steel ten-dons should be in occordance with ltem 19. A typicalinteraction diagram, depicting both strength and ser-vice lood designs, is shown in Figure 4-2. The use oFconcrete with o compressive strength exceeding7,000 psi requires CECW-E approvol. For commonuses, a minimumeffective prestress oF 700 psi com-pression is required For handling and driving purpos-es. Excessively long or short piles may necessitatedeviation from the minimumeffective prestress require-mentoThe capacity oF piles may be reduced by sien-demess effects when a portion oF the pile is Freestanding or when the soil is too weak to provide later-al support. Slenderness effects can be approximatedusing moment magnification procedures. The momentmagnification methods of ACI 318, as modified byPCI, "Recommended Practice for the Design of Pre-stressed Concrete Columns and Walls" (Item.47), arerecommended.

b. Reinforced Concrete Piles.Reinforced con-crete piles shall be designed for strength in accor-- Fb" ~x t-F-y--~ Fy-"20ksi. (forA-36compact- '-'--rable 4-3. Allowab~,co1c.ie'~~S4Ii;'~'-'._--... - -.-;s~tipns1 - .. -- - - - p~stressed- concrete. pite. (considering prutres.)For laterally unsupported piles the allowobletresses should be 5/6 of the American Institute of

teel Construction (AISC) (Item 21) values for2. Concrete Piles. Design criterio For fourpes oFconcrete piles (prestressed, reinForced, cast--place and mandrel driven) are presented irthe fol-wing paragraphs.a. PrestressedConcrete Piles. Prestressedcon-

rete piles are used frequenriy and must.be designedsatisfy both strengthand serviceability requirements.trength design should Follow the basic criterio setrth by the American Concrete !nstitute (ACI) 31 8ten119) except thesfrength feduction fador(l2J) shalle O] for ll failure modes and the load factor shalle 1.9 for both dead and live loads. The specifiedad and strength reduction factors provide a safety

-.

Uniform Axial Tension

. Bending (extreme fiber)CompressionTension

o0.40 f~O

For combined axialload and bending, the concrete stressesshould be proportioned so that:,. ,

fa+ fb + fpcS 0.40 f~fa - fb + fpc ~ Owhere

fa =.computed axial stress(tension is negative)-fb - computed bending stress (tension is negative)fpc" effective prestressF~- concrete compressive strength


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16 DESIGN Of 'ILE FOUNDATION$

dance with the general requirements of ACI 31 8(Item 19) except as modified below. Load factors pre-scribed in ACI 318 should be directly, applied tohydraulic structureswith. one alteration. The factoredload combination "un should be increased by ahydraulic load factor (HE!.This increase should lead toimproved serviceability and will yield stiffer membersthan those designed solely by ACI 318. The hydraulicload factor shall be 1.3 Jorreinforcement calculationsin flexure or compression, 1.65 for reinforcement in moments, so that compression always controls. Indirect tension, and 1.3 for reinforcement in diagonal order for a pile to qualify as confined, the steel casingtension (shear). The shear reinforcement calculation must be 14 gage (USStandard) or thicker, be seam-should deduct the shear carried by the concrete prior less or have spirally welded seams, have a minimumto application of the hydraulic load factor. As an yield strengthof 30 ksi, be 17 inches or less in diame-ahernata to the prescribed ACJ load factors, a single ter, not be exposed to a detrimental corrosive environ-load factor of 1.7 can be used. The 1.7 should then ment, and not be designed to carry a portion of thebe multiplied by Hf. The axial compression strength of working load. Items not specifically addressed in thisthe pile shall be limited to 80 percent of the uhimate paragraph shall be in accordance with ACI 543.axial strength, or the pile shall be designed for a mini- 3. Timber PUes. Representative allowablemum eccentricity equal to 10 percent of the pile stressesfor pressure-treatedround timber piles for nor-width. Strength interaction diagrams for reinforced mal load duration in hydraulic structures are definedJ:oncr~tc;Lpiles may be develo~P-lIsin.g the,.Corps. . in Table 4-5,.. , -- - .. --.--..-.computer program'CASTR {ltem'T8).-Sledernessc -~~.." ..'. ' ,- .- . '. ., '0_'. -'~""" . .'. ", --". '," -.. - a. The workang stressesfor compresslon par- ~effect~ .ca~ be ap:>roxlmateO uSlng the ACl moment allel to grain in Douglas Fir and Southern Pine ma .magnlflcahon procedures. b ' . d b O2 f h f f I YhIncrease y . percent or eac oot o engtc. Cast-in-Placeand Mandre/..Driven Piles. For frQmthe tip of the pile to the critical secton. For com-a cast-in-placep ile, the casing is top-driven without the pression perpendicular to grain, an ncrease ofaid of a mandrel, and the casing typically has a wall 2.5 psi per foot of length is recommended.thickness ranging from 9 gage to 1/4 inch. The casing b V I r S th P.' . htedfb f ffi ... . a ues ror OU ern me are welg ormust e o su clent thlcknessto wlthstand stressesdue I I f I h I bl II d h ti f t ti.h d " . d . . h . ong ea , s as , o o y an s or ea represen a vesto t e nVlngoperahon an maantaln t e cross section f .1 .f th .1 Th . h. k f d 1-eI' .o pl es '" use.o e pl e. e casing t IC ness or man re nven r . .piles is normally 14 gage. Cast-in-place and mandrel- . C. The above worklng stresses ~ave beendriven piles should be designed for serviee conditions adJustedto compensate for strength reductlons due toand stresseslimited to those values listed in Table 4-4. conditioning and treatment. For untreated piles orThe allowable compressive stressesare reduced from piles that are air-elried or kiln-elried before pressurethose recommended by ACJ 543 (Item 20), as treatment, the above working stresses should beexplained for prestressedconcrete piles. Cast-in-place increased by dividing the tabulated values by the fol-and mandrel-elriven piles shall be used only when full lowing factors:embedment and fuI! lateral support are assured and PaciRcCoast Douglas Fir:under conditions which produce zero or small end Southern Pine:

r~~-' '-: ::i-.::::.:.::::: JULTIMATe 8TREN~TH DIAGRAUzoo

J"'0""'00 . '-"o 2.7I 000 ....'0 .-'-000::. ...J

!f .. ..0-; .......9~ .~-

Figure 4-2. Typicalinteradion diog~m, 16 x16 in. square preslntssed co~te pite

Table 4-4. east-in-place and mandrel-drivenpile., allowable concrete slN.se.(Participation 01 steel casing or shell disallowed)Uniform Axiol CompressionConfined

UnconfinedUniform AxiaJ TensionBending {extreme fiber)

CompressionTension

.j10.33 f~0.27 f~O

0.40 f~OForcombined axialload and bending, the concrete stressesshould be proportioned so that:

Ifa !h.

1a + Fb ~ 1.0wherefa= computed axial stressFa= allowable axial stressfb = computed bendingstressFb= allowable bending stress

~

0.900.85


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.;.w.YSISAND DESIGN

----17

d. The allowable stresses for compressionparallel to the grain and bending, derived in accor-ance with ASTM D2899, are reduced by a safetyfactor of 1.2 in order to comply with the generalintent of Paragraph 13.1 of ASTMD2899 (Item 22).e. For hydraulic structures, the above values,except for the modulus of elasticity, have beenreduced by dividing by a factor of 1.2. Thisadditional

reduction recognizes the difference in loading effectsbetween the ASTM normal load duration and thelonger load duration typical of hydraulic structures,ond the uncertainties regarding strength reduction dueto conditioning processes prior to treatment. For com-bined axial load and bending, stresses should be soproportioned that:

I

fa fbIa + Fb ::; 1.0

wherefa" computed axial stressFa= allowableaxial stressfb= computed bending stressFb= allowablebendingstress

E. DEFORMATIONS.Horizontal and verti-cal displacements resulting from applied loads should-limitedto-nsureproperoperation anr,i1tegrity ofthe structure_-Experienceuhas shown that avertial de. .formation of 1/.4 inch and a lateral deformation of1/.4 to 1/2 inch at the pile cap are representative oflong-term movements of structures such as locks onddams. Operational requirements may dictate morerigid restrictions and deformations. For other structuressuch as piers, larger deformations may be allowed ifthe stresses in the structureand the piles are not exces-sive. Since the elastic spring constants used in the pilegroup analysis discussed later are based on a linearrload versus deformation relationship ot a specifieddeformation, it is important to keep the computeddeformations at or below the specified value. long-term lateral deformations may be larger than the com-puted values or the vatues obtained from load testsdue -to'cre~p- or plastic flow. lateral deflection mayolso increase due to cyclic loading and close spacing.These conditions should be. investigated whendetermining'the maximumpredicted displacement.

F. ALLOWABLE DRIVING STRESSES.Axial driving stresses calculated by wave equationanalysis should be limited to the values shown in Fig-ure .4-3.

G. GEOMETRICCONSTRAINTS.1.Pile Spacing. In determining the spacingof piles, consideration should be given to the charac-teristics of the soil and to the length, size, driving toler-once, batter, and shape of the piles. If piles arespaced too dosely, the bearing value and lateral-resis- -tance of each pile will be reduced, ond there is dan-ger of heaving of the foundation, and uplifting ordamaging other piles already driven. In general, it isrecommended that endbearing piles be spaced notless than three pile diameters on centers and that fric-tion piles, depending on the characteristics of the pilesand soil, be spaced a minimum of three to five pilediameters on center. Piles must be spaced to avoid tipinterference due to specified driving tolerances. See

paragraph 5-2A3 for typical tolerances. Pile layoutsshould be checked for pile interference using CPGI, aprogram which is being currently developed and is dis-cussed in paragroph 1-3C6.2. Pile Batter. Batter piles are used to sup-port structures subjected to large lateralloads, or iftheupper foundation stratum will not adequately resist lat-eral movement of vertical piles. Piles may be battered

- -in oppositedir~on~-orused'in combination wfthve...----tical piles:.Th~-cixallad 6n a batter-.piJe-shourd-hotexceed the allowable design load for a vertical pile. Itis very difficultto drive piles with a batter greater thanl' horizontal to 2 vertical. The driving efficiencyof thehommer is decreased as the batter ncreases.4-3. Pile Capacity

Pile capacities should be computed by experi-- enced designers thoroughly familiar with the varioustypes of piles, how piles behave when loaded, andthe soil conditions that exist ot the site.A. AXIAL PllE CAPACITY. The axial

capacity of a pile may be represented by the follow-ing formula: Qult - Qs + Qt

Qs'" fsAsQt- qAr

Table 4-5. Alto'W'Clblestresses for pressure-treatecl round timber piles-- Compression Compression ModulusPorallel to Bending Horizontal Perpendicular ofGrain (psil (psil Sheor to Grain Elasticity

Species Fa Fb (psil (psil (psi)PacificCaost (a)"Douglas Fir 875 1,700 95 190 1,500,000Southern Pine (a)(b)" 825 1,650 90 205 1,500,000


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18 DESIGN OF PlLEFOUNDATlONS

whereQult" ultimate pile capacityQs" shaft resistance of the pile due to skin frictionQt" tip resistance of the pile due to end bearingfs" average unit skin resistance

As" surface area of the shaft in contact with the soilq" unit tip-b!Oring capacityAt" effective (gross) area of the tip of the pile incontact with the soil

1. PUes in Cohesionless Soil.a. Skin Friction.For design purposes the skinfriction of piles in sand increase linearly to anassumed critical depth (DJ and then remain constantbelow that depth. The critical depth varies betweenExcended Dr1vin

Pile MacerialSudReiDforced ConcreceCOtIIIr..oaTeuioa

0.85t'e500 pd

Pre.cre..ed ConereceCoIa'pre..ioatea.ion

(0.85 f' - f )e pefpct1llber 3000pd

I H.

! P

.,

I P - .--.--------.'~__"d

TIIllI - '.. ':rnIII PI,. - '..IIllll'" - M, .:rnm(0.11:. '..). 14,UD 0-11 /;o

N. . COMP"." IY. I'OIIC. 'NUUC~D.Y .." -.,, (COMPII."'VII PII..,N. .n"IIL~ ,onc. 'NDue." .Y 14"""'" (111411014tLII'.. . ",.eTIV. P"IIT"." "".IRLO..". . PRIITR.U/Na1'0"0'

Figure 4-3. Prestre...d concrete pile drivingslresses

----------

lOto 20 pile aiameters or widths (B), dependng onthe relative density of the sand. The critical depth is .assumed as:De.. 10B for loose sandsDe .. 15B for medium dense sandsDe" 20B for dense sands

The unit skin friction acting on the pile shaft may bedetermined by the following equations:fs .. Kcr~ tan o

cr~ .. 1'D for D < Decr~ = 1'De for D ~ DeQs = fsA

whereK = lateral earth pressure coefficient (Ke forcompression piles and Ktfor tension piles)dv= effectiveoverburden pressureo.. angle of frictionbetween the soil and the piley' = effective unit weight of soilD = depth along the pile at which the effective over-burden pressure is calculated

.~

Values of o are given in Table 4-6.Table 4-6. Values of o

Pile MaterialSteelConcreteTimber

o0.6711'to 0.8311'0.9011' to 1.0 "'.0.8011' to 1.0 11'

Values of K for piles in cmpression (Kc)ane.'-'-"piles in tension (Kt)aregiven in Table 4-7. Table 4-6and Table 4-7 present ranges of values of o and Kbased upon experience in various soil deposits. Thesevalues should be selectd for design based upon

r

Note: The above do not apply to piles thatare prebored, jetted, or installedwitha vibra-tory hammer. Picking K values at the upperend of the above ranges should be based onlocal experience. K, 8, and Nq volues backcalculated from load tests maybe used.

Table 4-7. Value. of KSoil Type Ke KSand 1.00 to 2.00 0.50 to 0.70Silt 1.00 0.50 to 0.70Clay'- 1.00 0.70 to 1.00


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For steel H-piles~ should be taken as the area indud-ed within the block perimeter. A curve to obtain theTerzaghi-Peck(Item59) bearing capacity factor Nq(among values from other theories) is shown in Fig-ureU;-To--use'the-cl,-..ye-:-one:::mUsrobtaiiim~s:red--~- --- -- -values'of-theangle'rte;:nal~f(ic:tion (~I which repre- --- --sents the soil mass.

c. Tension Capacity. The tension capacity ofpiles in sand can be calculated as follows using the Kvalues for tension from Table .4-7:

It)tperience and pile Ioad test. It is not intendedthatthe;cMsigner:would uS8:the'minimumreduction of thefI ongle while using the upper range K values.For steel H-piles, As should be taken as theblock perimeter of the pile and 5 should be the aver-

age friction angles of steel against sand and sandogoinst sond (fll. It should be noted that Table .4-7 isgeneral guidance to be used unless the long-termengineering practice in the area indicates otherwise.Under prediction of soil strength parameters at loadtest siteshas at times produced back-calculated valuesof K that exceed the values in Table .4-7. It has alsobeen found both theoretically and at some test sitesthat the use of displacement piles produces higher val-ues of K than does the use ef nondisplacement piles.Values of K that have been used satisfactorily but withstandard seil data in some locations are as presentedin Table .4-8.

b. End8earing. Fer design purpesesthe pile-tip bearing capacity cCnbe assumed to increase lin-early to a critical depth (De) and then remainsconstant. The same critical depth relationship used forskin friction can be used fer end bearing. The unit tipbeoring capacity can be determined as follows:q = (J ~ Nq

where(J ~ - 'Y'D for D< De(J: = y' De fer D ~ De

Qult .. QSension

19

2. pjles in C~hesiv. -Soil. - - -" --a.' SlcinFrlction; Althoughcalled skin friction;the resistance is due to the cohesion or adhesion of

the day to the pile shaft.

f .. caca .. acQs .. fsAs

whereCa= adhesion between the day and the pilea = adhesion factore = undrained shear strength of the day from a QtestThe values of a as a function of the undrained shearare given in Figure .4-5A.

An alternate procedure developed by Sempleand Rigden (Item 56) to obtain values of a which isespecially applicable for very long piles is given inFigure .4-58where:

a=ala2and f .. acb. End Bearing. The pile unit-tip bearingcapacity for piles in day can be determinedfrom thefollowingequation:

q = 9c-- - q-------------Q= ~q",-- n.,-

However, the movement necessary to develop the tipresistance of piles in day soils l1)ay be several timeslarger than that required to develop the skin frictionresistance.- c. Compression Capacity. By combining the

Table 4-8. Commonvc.lues for corrected KSoilTypeSandSilt -Clay

Displacement PilesCompression Tension

2.001.25 -1.25

Nondisplacement PilesCompression Tension

0.670._500.90

0.500.350.70

- - --,_e- ,,-- -- -- -,-1.501.001.00

Note: Although these valueS'may be commonlyused in some areas they shouldnot be used without experience and testing to validate them.


12/14

20 DESIGN OF PIte FOUNDA11ONS

whereQs'" capacity due to skin resistancefs = average unit skin resistance

Qlf-=-Gs-- -- -_u --, ~~---~As-"" --stlrkIce- area.:ofthe~ pile shafti n~ontoc:rwith '';":',r, --~,_. -, .$011 --'o - - --- - - '. -.- -

e. The pile capacity in normally consolidated K= seeTable4-7clays (cohesive soils) should also be computed in the F. A 5A d Bex= see Igures anlong-term S shear strength case. That is, develop o S D = depth below ground up to limitdepth DeO= limitvalue for shaft friction angle fromTable 4-6

100'GGESTEDRANGE..

10010

lE: 10gC):: 40~): 20eC)i~ 10.:l' .l..6.Etm

o My&RHOFo nRZAGHI & PECK() V&81C OltlV&N. TOMUNSON

1O 1 10 11 20 '21 'lO " .0 41 ..- . ANQU OF INTIRNAL FRICTIOH

Figure 4-4. Bearing capacity factorskin frictioncapacity and the tip bearing capacity, theultimate compression capacity may be found as fol-lows:

Qult=Qs + Qtd. Tension Capacity. The tension capocity ofpiles in clay may be calculated as:

a:~~ 1.01&.Zoi111!e o...--- 0.21 -'... 0.71 1.0 2.0. UNDRAlNED 8HEAR STRENGTH. T8F

Figure 4-5A. Values of a versusundrained shear slrength

case shear strength trend as discussed previousiyandproceed os iHhesoil is drained.The computationalmethod is identical to that presented for piles in gran-ular $Oils,and to present the computational methodol-ogy would be redundant. It should be notad howeverthat the shear strengths in clays in the S case areassumed to be e > O and e . O . Some eommonlyused S case shear strengths in alluviolsoils are asreportedin Table 4-9.3. PUes in Silt.a. Skin Friction.The skin friction on a pile insilt is a two component resistanee to pile movementeontributed by the angle of internal friction(e) and thecohesion (e)acting along the pile shaft. That portion ofthe resistanee contributed by the angle of internal fric-tion (e) is as with the sand limitedto a critieal depth of(Del, below which the fridional portion remains con-

- stant,the limitdepthsare statedbelow.Thatportionofthe resistancecontributedby the cohesion,mayrquire /~limit if it is sufficien~ylarge, see Figures'4-5A and B.The shaft resistance may be computed as follows:

~

Ky' D tan o + (XCwhere

(DS;DelQs =Asfs

~,

r - b. End Bearing. The pile tip bearing capacityincreases linearly to a critical depth (Deland remainsconstant below that depth. The critical depths aregiven as follows:De = 10 B for loose siltsDe = 15 B for medium siltsDe =20 B for dense silts


13/14

21

,,-,)." ".,1.." ,;, ~,..,~., t""";7' - . .".~.,. . "'. - 'o';"i..." - ',,,,,r. 'p;.;.,C:..,i;, ' ...""" .;",, '"'' .' '' .

a., 0.&

IIIII---r----,IIII

G.3& o.aoS.U (ct(j';

1.0.00.7

IIL_____-,,IIIII..!...60

a.2

120lb

Figure 4-58. Values ofa) a2 applicable for very long pilesTheunit and bearing capacity may be computedasfollows: Qult=Qs + Qt

q - (J~Nq d. Tension Capacily. The tension capacity iscomput~d by applying the appropriate value of Ktfrom Table 4-7 to the unit skin friction equationabove.(J ~,. yD for D < De(J ~ ,. yDe for D ;;?;De Quh - Q'-onQt=Aq e. It is recommended that when designingpile foundations in silty soils, considerations be givento selecting a very conservative shear strength fromclassical R shear tests. It is further recommended that

test piles be considered as a virtual necessity, andthepossibility that pile length may have to be increasedioJhe field.should be considered~ . . ~-'-

4. Piles 'in ,Lciyered-'-'$oiis.':Pils're',rn~t" .,':frequently driven into' a layered soil 'stratigraphy.' Forthis condition, the preceding methods of computationmay be used on a layer by layer basis. The end bear-ing capacity of the pile should be determined fromtheproperties of the layer of soil where the tip is founded.

whereNq - Terzaghi bearing capacity factor, Figure.4-.4 .(J~- vertical earth pressure at the tip with limits. .At- -ar_e~f:)fth~_plle IIP, asdelerm1ectfor~C:Zt:!~s_. ' .-- .

c. Compression Capacily. Bycombining thetwo incremental contributors, skin friction and endbearing the ultimate capacity of the soil/pile may becomputed as follows:

Table 4-9. 5 case she. strengthSoilType Consisten,cy Angle of Internal Frictionfl1Fat clay (CH) Very soft 13. to 1l"Fat clay (CH) 50ft 1l" to 20.Fat clay (CH) Medium 20. to 21.Fat clay (CH) 5tiff 21 . to 23.

- ..:' ---'---5i1t""---'---(Ml)' -- no -- 25. to 28.Note: The designer should perform testing and select shear strengths.These general data ranges are from test on specific soils in site spe-cinc environments and may not represent the soil in question.

"". - ""---'--'U'


14/14

- 5. Point BearingPiles. Insomecases Ihepile will be driven to refusal upon firmgood quality >1'rock. In such cases the capacily of the pile is gov-erned by the structural capacity oFthe pile or the rackcapacity.6. Negative Skin Friction.a. Negative skin Friction is a downwardsheer drag acting on piles due to downward mova-ment oF surrounding soil strata relative to the piles.For such movement oFthe soils to occur, a segment ofthe pile must penetrate a compressible soil stratumthat consolidates. The dpwnward drag may becaused by the placement of fill on compressible soils,lowering of the groundwater table, or underconsoli-dated natural or compacted soils. The effect of theseoccurrences is to cause the compressible soils sur-rounding the piles to consolidate. Ifthe pile tip is in arelatively stiffsoil, the upper compressible stratum willmove down relative to the pile, inducing a drag loa~ .JThisload can be quite large and mustbe added Ivthe structuralload for purposes of assessing stresses inthe pile. Vesic (Item 60) stated that a relative down-ward movement of as little as 0.6 inch of the soil withrespect to the pile may be sufficient to mobilize fuI!negative skin friction. The geotechnical capacily ofthe pile is unaffected by downdrag, however down-drag does serve to increase settlement and increasethe stresses in the pile and pile cap.b. For a pile group, it can be assumed thatthere is no relative movement between the piles andthe soil between the piles. Therefore, the total forceacting down is equal to the weight of the block of solq ... cr~N held between the piles, plus the shear along the pileq - group perimeter due to negative skinfriction. Thecr~... y D for D < De average downwardload transferred to I:Jpile in a pilecr-~_.-for-f)->--9---~ grouP.Qnfcan-be estimated-by '--c .._~- -.----- - "- - .r.~~e e - -- .. - - - . - .,-

" --'_.'~ 1 - - -- -- -- ,.",'Qt,=;\q - - Qnf == [A'}'l.+ slP] (4- i .. -

22

However, when weak or dissimilar layers of soiJ existwithin approximately 5 feet or 8 pile tip diameters,whichever is the larger, of the tip founding elevationthe end beering capacity will be affected. It is neces-sary to compute this affect and account for it whenassigning end bearing capacity. Incomputing the skinresistance, the contribution of each layer is computedseparately, considering the layers above as a sur-charge and applying the appropriate reduction fac-tors for the soil type within that increment of pile shaft.a. Skin Friction.The skin friction contributedby different soil types may be computed incrementallyand summed to find the ultimate capacity. Considera-tion should be given to compatibility of strain betweenlayers when computing the unit skin resistance.NQs'" ~ fs. As.k,. I I.- i -1 -wherefs;'" unit skin resistance in layer iAs- surface area of pile in contact with layer iN - total number of layers

b. End Bearing. The pile tip bearing shouldbe computed based upon the sol type within whichthe tip is founded, with limits near layer boundariesmentioned above. Using the overlying soil layers assurcharge the following equations may be used.Sand or Silt:

m ----

Clay: q == 9cQt==Aq

c. Compression Capacity. Bycombining theskin resistance and end bearing, the ultimate capacityof the soil/pile may be computed as follows:Qult ... Qs + Qt

d. Tension Capacity. TI.e tension capacilyay be computed by applying the appropriate valuesf Ktfrom Table 4-7 as appropriate for granular soilso the incremental computation for eachlayer anden combining to yield:

Qult ... QSt.n.ion

DESIGN Of Pllf FOUNDAOONS

r

.. where- A == horizontalarea bounded by thepilegroup(cross-sectional area of piles and endosed soil)N == numberof piles in pile groupr == unit weight of fillor compressible soillayersl == length of embedment above the bottom of thecompressible soillayerss == sheer resistanceof the soil~.p; == perimeterof the area A

c. For a single pile, the downward loadtransferred to the pile is aqual to the shearing resis-tance along the pile as shown in Equation 4-2.

Qnf - slP' (4-2)

6821982-pilefounddeschap041

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