KS Design Manual Short

8/11/2019 KS Design Manual Short

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K e y s t o n e R e t ai n i n g W al l U n i t s

G e o s y n t h e t i c S oi l R ei nf o r c e m e n t

R e t ai n i n g W al l D es i g n T h e o r y

T h e D esi g n P r o c es s

K e y W al l O p e r a ti n g I n s t r u c ti o n s

A p p e n di c es

2011 KeystoneRetainingWallSystems, Inc

OriginalMarch2001

RevisedAugust2001

ReprintedJuly 2006

Revised&ReprintedFebruary, 2011

& K E Y W ALL O PERA T I N G G U ID E

Keystone

D ES I G N M A N U AL


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i

I N TR O D U C T I O N

TableofContents

PART ONE KeystoneRetainingWallUnits

KeystoneRetainingWallUnits.................................................................... ..................................................................... . 1.1

KeystoneMaterials................................................................... ...................................................................... ....................... 1.2

StandardUnit................................................................ ..................................................................... ................................... 1.2

CompacUnit.................................................................. ..................................................................... ................................... 1.3

KeystoneCenturyW

allU

nits............................................................ ..................................................................... ............ 1.3133EliteUnit.................................................................. ..................................................................... ................................... 1.4

UnitShearResistance.............................................................. ...................................................................... ....................... 1.4

BaseShearResistance.............................................................. ...................................................................... ....................... 1.4

Inter-UnitShearResistance................................................................ ..................................................................... ............ 1.5

ShearData&Analysis............................................................. ...................................................................... ....................... 1.5

TypicalShearResistanceBetweenUnits................................................................ ........................................................... 1.6

PART TWO GeosyntheticSoilReinforcement

GeosyntheticSoilReinforcement.................................................................. ..................................................................... . 2.1

DesignStrength............................................................. ..................................................................... ................................... 2.2

ReductionFactors.................................................................... ..................................................................... ........................ 2.3

ConnectionStrength .......................................................................................................................................................2.4-2.5

GeosyntheticSoilInteractionCoefficient ......................................................................................................................2.5-2.6

GeogridManufacturersData............................................................ ..................................................................... ............ 2.6

PART THREE RetainingWallDesignTheory

RetainingWallDesignTheory .......................................................... ..................................................................... ............ 3.1

ImportantTechnicalDefinitions.................................................................. ...................................................................... 3.2

LateralEarthPressureTheories................................................................... ..................................................................... . 3.3

CoulombEarthPressureTheory .................................................................. ..................................................................... . 3.4

CoulombEarthPressureEquation ..............................................................................................................................3.4-3.5

CoulombFailurePlaneLocation.................................................................. ..................................................................... . 3.6

RankineEarthPressureTheory ................................................................... ..................................................................... . 3.6

RankineEarthPressureEquations ..............................................................................................................................3.6-3.7

RankineFailurePlaneLocation................................................................... ..................................................................... . 3.7

TrialWedgeAnalysis.............................................................. ..................................................................... ........................ 3.8

BearingCapacity ........................................................... ..................................................................... ................................... 3.9AppliedBearingPressure.................................................................. ...................................................................... ............ 3.9

CalculatedBearingCapacity .............................................................. ..................................................................... .......... 3.10

BearingCapacity Factors ...........................................................................................................................................3.10-3.11

Settlement........................................................... ...................................................................... ............................................ 3.11

GlobalStability .............................................................. ..................................................................... ................................. 3.12

SeismicAnalysis............................................................ ..................................................................... ................................. 3.13

In troduc t ion......................................................................................................................................... iv

D esign Manual&KeyWall Operat ing Guide ..................................................................................... v

G e o t echn ica l R esp on s i b i l i ty............................................................................................................. vi

References.......................................................................................................................................... vii

TA B L E O F C O N T E N T S


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D ES I G N M A N U AL& K E Y W A L L O P E R A T I N G G U I D E

ii

TA B L E O F C O N T E N T S

PART FO UR TheDesignProcess

Introduction ..........................................................................................................................................................4.1

DesignMethodology ...............................................................................................................................................................4.2

UnitSelection...........................................................................................................................................................................4.2

WallBatter ...............................................................................................................................................................................4.3

HingeHeight...........................................................................................................................................................................4.3

WallGeometry ........................................................................................................................................................................4.4

WallEmbedment ....................................................................................................................................................................4.4

SlopingToe ..............................................................................................................................................................................4.5

SoilProperties................................................................... ..................................................................... ........................... 4.5-4.6

Surcharge..................................................................................................................................................................................4.6

ReinforcementType&Properties ........................................................................................................................................4.7

SoilReinforcementLength....................................................................................................................................................4.7

ExternalStability Analysis.....................................................................................................................................................4.8

BatteredWallDesignOptions ..............................................................................................................................................4.9

AASHTOLRFD............................................................. ...................................................................... ........................ 4.9-4.10

SlidingAnalysis .....................................................................................................................................................................4.11

Coulomb-NCMASliding ..................................................................................................................................................4.12

Rankine-AASHTOSliding ..............................................................................................................................................4.12

OverturningAnalysis ...........................................................................................................................................................4.13

Coulomb-NCMAOverturning ........................................................................................................................................4.14

Rankine-AASHTOOverturning.................................................................. ...................................................................4.14

Overturning ...........................................................................................................................................................................4.14

BearingCapacity ...................................................................................................................................................................4.14

UltimateBearingCapacity ..................................................................................................................................................4.15

InternalStability ....................................................................................................................................................................4.15TensileCapacity ....................................................................................................................................................................4.16

TensionLevelCalculation...................................................................................................................................................4.17

AASHTOInternalTension................................................................................................................................................4.18

ConnectionCapacity .............................................................................................................................................................4.18

PulloutCapacity ............................................................... ..................................................................... ....................... 4.19-4.20

Stability ofFacing .................................................................................................................................................................4.20

P A R T FI V E KeyWallOperatingInstructions

Installation&UsersGuide ....................................................................................................................................................5.1

Installation ...............................................................................................................................................................................5.2

Registration ..............................................................................................................................................................................5.3

KeywallInstallation&Setup .................................................................................................................................................5.4

LaunchingtheKeywallProgram .........................................................................................................................................5.5KeywallWindowsInterface..................................................................................................................................................5.6

KeywallInterfaceOptions......................................................... ...................................................................... ............... 5.7-5.8

KeywallRegistrationDetails.................................................................................................................................................5.8

GeneralInput.................................................................... ..................................................................... ......................... 5.9-5.10


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PA RT O N E

K E Y S T O N E R E T AI NI N G W A L L U N I T S

Fordland, Lakewood, Colorado;KeystoneCentury&HalfCenturyWall


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eystoneretainingwallunitsareazero-slumpconcretemasonry productdevelopedspecifically

foruseinearthretainingwallstructures. Keystonehasdevelopedawidevariety ofshapesand

designstoaccommodatemostarchitecturalandstructuralrequirements. Localproducersof

theKeystoneproductshaveavariety ofcolorsavailable, complementingmostlandscapingand

structuralretainingwallapplications.

Keystonestructuralproductscurrently availableinclude:

StandardI/StandardII, Figure1:1

CompacI/CompacII/CompacIII, Figure1:2

KeystoneCentury Wall, Figure1:3

133Elite

, Figure1:4

TheKeystoneunitslistedabovearedesignedforuseasstructuralretainingwalls, i.e., thoseexceeding6.5 feet

(2m)inheightand/orsupportingstructuresorhighway loading.

Inadditiontotheaboveunits,Keystonehasacompletelineofsmallerlandscapeproductsthataremarketedand

soldthroughretaildistributionandlandscapesupply outlets. Theseproductsaregenerally notconsideredfor

structuralapplicationsandarenotdiscussedfurtherinthismanual.

KEYSTONE RETAIN ING WALL UN ITS

K

Brentwood Gate, Burnaby, BritishColumbia;Keystone133Elite

1.1


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1.2

P ART O N E

RetainingWallUnits

P ART O N E

RetainingWallUnits

Keystoneunitsaretypicallymanufacturedof concretewithaminimumcompressivestrengthof3000 psi

(21MPa)at28 daysandamaximumabsorptionof8%. Alldimensionsareplusorminus18inch(3mm)

exceptfortheunitdepth, whichvariesduetothesplitrockfinish. Themanufacturingprocessisautomated,

sothemixing, compaction, andcuringareperformedundercontrolledconditionsandprovideconsistent

quality.Theunitshavevariousfacetexturesavailable, dependingonyourlocalmanufacturer. Someofour

mostpopulartexturesaremoldedorsplit-rockfinishinvariousnaturalcolors. Faceshapescanbetri-plane,

straight, Victorian, orSculpterramoldedfacesuchasHewnstone.

Standard, Compac,KeystoneCentury Wall&133Elitearevertically interconnectedusinghigh-strength

pultrudedfiberglasspins.TheKeystoneunitshavecoresthatarefilledwithcleancrushedstonetoprovide

additionalmechanicalinterlockandinternaldrainage.Thepinsassurearunningbondconfigurationof

theunitsandprovidesignificantlateralconnectionstrengthbetweenunits.Thepinsimprovetheconnection

betweentheunitsandthestructuralsoilreinforcementwhileassuringproperplacementofthe

reinforcementmaterials.

Theconnectionpinsareavailableinstraightandshouldereddesigns. Straightpinsare5inches(133mm)

longandinch(12.7mm)indiameter. TheStandardandCompacunitsusestraightpins. Shoulderedpins

are3inches(95mm)longandinch(12.7mm)indiameter.Theshoulderedlengthis78inch(22mm)

andtheshouldereddiameterisinch(20mm). TheCentury Walland133Eliteunitsuseshoulderedpins.

Theminimumpinstrengthis6,400 psi(44MPa)shortbeamshearstrengthand110,000 psi(750MPa)tensile

strength. Thepinsaremanufacturedofpultrudedfiberglassandwillnotcorrodeordeteriorate. Inaddition,

thefiberglasspindoesnotchangeproperties(softenorbecomebrittle)duetothetemperaturechanges

typicalinretainingwallapplications.

TheStandardunitvariesduetomanufacturingconsiderationsfrom18 to24 inches(457 to600mm)in

depth, withatypicalfacewidthof18 inches(457mm)andheightof8 inches(203mm).Thegeometry yields

exactly 1 squarefoot(0.09m)offaceareaperunit. Unitsweighfrom95 to125 pounds(43 to56kg)each,

varyingwithlocalmanufacturingandaggregates.Thecentroidoftheunitisslightly forwardofcenter

towardtheface, butfordesignpurposes, itistakenatthecenter. Fordesignpurposes, thein-placedensity

oftheaggregatefilledunitis120 pcf(18.85 kN /m ).

Standardunitsaremanufacturedwithadualpinholeconfiguration.Thefrontpinsettingallowstheunits

tobeplacedataminimumsetbackofapproximately 18-inch(3.2mm)per8 inch(203mm)unitheight

(1batter, fordesignpurposesuse0). Therearpinsettingallowsplacementoftheunitsataminimum

1-inch(31.7mm)setbackper8 inch(203mm)unitheight(8batter). Analternateplacementoffront/back

pinholeallowsasetbackof58-inch(15.9mm)per8 inch(203mm)unitheight(4batter).

Note:

Notallunitstypes,

facetreatmentsand

colorsareavailable

atallmanufacturing

locations. Please

check withyourlocal

manufactureror

Keystonesupplierfor

availability.

KEYSTONE MATERIALS

STAN D ARD UN IT

F igure 1:1 Standard/Standard IIU n it


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1.3

D ESI G N M A N U AL& K E Y W A L L O P E R A T I N G G U I D E

TheKeystoneCompacunitisa12 inch(305mm)deepunitwithatypicalfacewidthof18 inches(457mm)

by 8 inches(203mm)high. Thisgeometry yieldsexactly 1 squarefoot(0.09m)offaceareaperunit.Depth

may vary from11 to12.5 inches(280 to317mm)dependinguponlocalmanufacturingandsplitting

requirements.Unitsweighfrom70 to95 pounds(32 to43kg)each, varyingwithlocalmanufacturingand

aggregates. Fordesignpurposes, thein-placedensity oftheaggregatefilledunitis120 pcf(18.85 kN/m).

Thedualpinholeconfigurationallowsthesame1(0fordesignpurposes), 4, and8setbackasthe

Standardunit.

Century Wallisathreepiecesystemthatconsistsofasmall,medium, andlargeunit.Thewidthofthe

unitsisthevaryingdimensionthatdictatesthesize. Thesmallunitis7 inches(178mm)wide, themedium

unitis11 inches(279mm)wide, andthelargeunitis18 inches(457mm)wide.ThethreeCentury Wall

unitsare12 inches(305mm)deepand8 inches(203mm)high.Thesmallunitweighs45 pounds(20kg), the

mediumunitweighs58 pounds(26kg), andthelargeunitweighs90 pounds(41kg). Weightsmay vary with

localmanufacturingandaggregates.

SimilartotheCompacandStandardunits, adualpinholeconfigurationallows1(0fordesignpurposes),

4, and8setback.

CO MPAC UN IT

KEYSTONE CENTURY WALLUN ITS

F igure 1:3 Century Wa llU n its

F igure 1:2 Compac / CompacII / CompacIIIU n it


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1.4

P ART O N E

RetainingWallUnits

P ART O N E

RetainingWallUnits

133Eliteunitsare8 inches(203mm)highand24 inches(610mm)widetocreateafaceareaof1.33 square

feet(0.124m), hencethename133Elite.Thedepthoftheunitis11.5 inches(292mm).Dependingonface

treatment, theweightofthe133Eliteunitisapproximately 100 pounds(45kg).

133Eliteunitsaremanufacturedwithonepinpositionthatcreatesanearverticalsetbackequalto38

inch(9.6mm)per8 inches(203mm)ofunitheight(2.5batter).

Therearetwoareaswheretheshearresistanceisimportant:

Levelingpadshearresistance

Inter-unitshearresistance

Bothareimportanttothewallsability toresistlateralmovementsduringconstructionandtoholdtheretained

soilinplace.Theshearandmomentcapacity ofthewallfacingpreventsbulgingofthewallface.

Apreparedlevelingpadisrequiredtoprovideafirm, levelsurfaceonwhichtoplacethebasecourseunits

atthedesignelevationsandprovidelocalizedbearingcapacity fortheunits.

Levelingpadsmay beconstructedofwell-compactedgravel/crushedstoneorunreinforcedconcrete.Formost

walls, thegravel/crushedstonelevelingpadisadequate.Fortallerwalls(over15 feetor5m), contractorshave

foundthatconcretecanleadtofasterwallinstallationandiseasiertouseonthelargerprojects.Theconcrete

padrequiresmorecareinplacementandmoreexpensivematerials(concreteversusaggregate), butthespeedof

placingthefirstcoursegenerally offsetstheextracostofmaterials.

InKeystonewallswithnoearthreinforcement(gravity walls), thetotalresistanceofthewalltolateral

movement(sliding)isprovidedby thefrictionalongthebaseoftheunits. InsoilreinforcedKeystonewalls,

unitbasefrictionisalessercomponentoftheslidingcalculationasthereinforcedzoneprovidesmostof

theresistancealongthebase. Sincethelevelingpadmay beconstructedofvariousmaterials, thefrictional

resistancevarieswiththeroughnessandshearstrengthofthematerials.

133ELITEUN IT

UN ITSHEARRESISTANCE

BASE SHEARRESISTANCE

F igure 1:4 133E li te U n it


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1.5


Laboratory testinghasbeenperformedtodeterminetheinter-unitshearresistanceofthevariousKeystone

units. Theinter-unitshearresistanceistheinternalshearcapacity ofthewallfacing. Withoutadequate

shearresistancebetweenunits, awallcouldbulgebetweenlayersofreinforcing, shearduringconstruction,

orinthecaseofagravity wall, shearbetweenany unitabovethebase.

Forgravity walls, theinter-unitshearcapacity isobtainedbasedonthecalculatednormalforce. Whena

layerofgeogridreinforcingisincludedinthewallsystem, theshearingresistancebetweenunitsmay be

reducedbecausethereinforcingcanreducefrictionbetweenunits.Thegranularinterlockisdecreasedand

theunit-to-unitfrictionmay bereduced. Formany systems, reinforcingmay actually decreasethestability

ofthefacewhileprovidingstability totheoverallearthmass.

Inter-unitsheartestinghasbeenperformedonallKeystonestructuralunits. Testingwasinitially doneat

UtahStateUniversity ontheCompacandStandardunits. Theinter-unitsheartestingontheremainderof

theunitshasbeencompletedby Bathurst, ClarabutGeotechnicalTestingInc.Theresultsofthetestforthe

CompacandStandardunitsaregraphically depictedinFigures1:5 and1:6. Laboratory testingprovidesthe

followingderivedequationsforshearresistancebasedonatotalcalculatednormalforce, N , inlbs/lf.

N =

h Wuunitwhere:

h = DepthtoInterface

Wu = Widthofunitface

un it = Unitweightofunitface

AdditionaldirectsheartestswerecompletedatUtahStateUniversity toevaluatebaseshearusingthree

typesoflevelingpadmaterials.Theresultsofthattestingislistedbelow:

TodeterminemetricequivalentsinkN /m , dividethey-interceptby 68.5. Forexample, theStandardunit

tounitshearequationwouldbeF=35.43+Ntan17.4. SheartestreportsareavailablefromKeystone.

INTER-UN ITSHEARRESISTANCE

SHEARD ATA AND ANALYSIS

Un it to UnitUn it to Unit

w/geogrid

Standard F=2427+Ntan17.4 F =1550+ Ntan17.4

Standard II F =1375 +N tan35 F =1556 +N tan19

Compac F=769+Ntan26.9 F =769+ Ntan26.9

CompacII F =1475 +N tan29 F =1250 +N tan29

CompacIII F =1393+ Ntan34 F =900+ Ntan34

Century Wall F =900+ Ntan30 F =900+ Ntan30

133E lite F =1100 +N tan29 F =1100 +N tan29

Standard Compac

Crushed Stone Pad F=995+0.31N F=0.92 N

Concrete Pad F=205+0.30N F=0.90 N

Sand Pad F=290+0.29N F=0.49 N

INTER-UNIT SHE AR TABLE

BASE SHEAR TABLE


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1.6

P ART O N E

RetainingWallUnits

P ART O N E

RetainingWallUnits

Inter-Un it Shear Strength

0

1000

2000

3000

4000

5000

6000

0 2000 4000 6000 8000 10000

NormalF orce (p lf)

S

hear

Force

(

pl

f)

Standard

Standard

w/geog

rid

1000 3000 5000 7000 9000 11000

Un it to Unit

Un it to Unitw/Geogrid

TYP ICALSHEARRESISTAN CE BETWEEN UN ITS

Un it to Un it

Un it to Un itw/Geogrid

Inter-Unit Shear Strength

0

1000

2000

3000

4000

0 2000 4000 6000

NormalF orce (p lf)

Shear

Forc

e

(

pl

f)

1000 3000 5000

Comp

acII

Comp

acIIwi

thGe

ogrid

F igure 1:5 Standard Un it Inter-unit Shear Strength

F igure 1:6 CompacIIU n it Inter-unit Shear Strength


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PA RT T W O

G E O S Y N T H E T I C S O I L R E I N F O R C E M E N T

MartinStreetImprovements, Fredonia, Wisconsin;KeystoneCompacHewnstone


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eystoneretainingwallsmay performasgravity retainingwallsforheightsupto6 ft(1.8m)forStandardunits

and3.3ft(1m)forCompacunits, dependingongeometry, soiltype, andspecificloading. Whenawallexceeds

safegravity heights, soilreinforcementisrequiredtoprovidestability againstoverturningandsliding. The

majority ofKeystoneretainingwallsareconstructedusinggeosyntheticreinforcement, whichisthefocusofthis

manualandtheKeyWallprogram.DesignforinextensiblesteelreinforcementisdiscussedintheKeystoneKeySystem

designmanualandHITECEvaluation, August2000.

Soil-reinforcedwallstypically consistofgeosyntheticmaterials, primarily geogrids, whichareconnectedtothe

Keystoneunitsandplacedinhorizontallayersinthecompactedbackfill. Alimitequilibriumdesignprocedureisused

todeterminethenumber, strength, length, anddistributionofgeosyntheticreinforcementlayersrequiredtoforma

stablesoil-reinforcedmass.

Geosyntheticmaterialdesignparametersinthelimitequilibriumanalysisare:

Long-TermDesignStrength(LTDS)andallowablestrength, Tal

Geosynthetic-Keystoneunitconnectionstrength, Tcl, Tsc

Geosynthetic-soilpulloutinteractioncoefficient, C i

Geosynthetic-soildirectshearcoefficient, Cds

Terminology usedtodefinegeosyntheticsoilreinforcementtensilestrengthvariessomewhatbetweenauthors,

specifiers, andsuppliers. Theterminology usedwithinthissectionisconsistentwiththatofNCMAandAASHTO/

FHWA, unlessotherwisenoted.

GEOSYNTHETIC SO ILREINFORCEMENT

WallconstructionwithGeogrid Reinforcement.

K

2.1


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2.2

P ART T W O

GeosyntheticSoilReinforcement

Thepracticefordeterminingtheallowabletensilestrengthofgeosyntheticreinforcement, (Tal), isbased

upontheU.S. FederalHighway Administrationguidelines. ThismethodestablishestheLongTerm

DesignStrength(LTDS), bebasedonsustainedloadtesting, extrapolatedtoadesignlifeforthestructure.

TheLong-TermDesignStrength(LTDS), forgeogridreinforcementutilizedintheKeystoneRetaining

Walldesignis:

Equation(2a) LTDS =

Therearetwodesignphilosophiescurrently employedinretainingwalldesignandanalysis. AllowableStress

Design(ASD)istheconventionalworkingstressandfactorofsafetymethodofanalysisthathasbeenused

foryears. LimitStateDesign(LSD)orLoadandResistanceFactorDesign(LRFD)isthenewermethodthat

comparesfactoredloadstofactoredresistances.

Allowable StressD esign

Thelong-termdesignstrength(LTDS), isreducedby anoverallsafety factor(FS), inAllowableStress

designtoaccountforallfactorsonloads, uncertainties, etc.

Equation(2b) Tal =

Limit State Design (LRFD)

TheLongTermDesignStrength(LTDS)isreducedby aresistancefactor()inLimitStateDesign/

(LRFD)toaccountformaterialuncertainty.

Equation(2c) Tal = GE OLTDS

TheNCMA, Rankine, AASHTO96, andAASHTOSimplifieddesignmethodsinKeyWallemploy

AllowableStressdesignprocedures.TheAASHTOLRFD, CanadianLRFDandAustraliandesignmethods

inKeyWallemployLimitStatedesignprocedures.

Tensile Strength (Tult)

TultistheultimatestrengthofgeosyntheticreinforcingwhentestedinawidewidthtestperASTMD4595(geotextile)orD6637 (geogrid).ThisvalueisreportedastheMeanAverageRollValue(MARV),

asdeterminedby themanufacturersquality controlprocessandaccountingforstatisticalvariation.

DESIG N STREN GTH

LTDS

FS

Tult

RFcrx RF idx RFd


15/33

2.3


Note:

KeyWallpermitsthe

choiceofthreediffer

backfillmaterials

(fine-grained soils,

34-inch(20 mm)mi

and 2 inch(50 mm)

minus), whichestabl

adefaultvalueforth

installationdamage

factorsused inthe

LTDSdetermination

based onthe

manufacturers

recommendations

and testing.

REDUCTION FACTORS

RFcr

RFcristhereductionfactortoaccountforthelong-termcreepcharacteristicofpolymetricmaterials. The

long-termtension-strain-timebehaviorofpolymericreinforcementisdeterminedfromresultsofcontrolled

laboratory creeptestsconductedonfinished-productspecimensforperiodsupto10,000 hoursperASTM

D5262 andD6992.Thedataisthenextrapolatedtotheprojectdesignlifeofthestructure, 75 yearsor100

years. Creeprupturetestingissimilartotheproceduredescribedabove, however, theloadatwhichrupture

may occurattheendofthedesignlifeispredicted. Acombinationof10,000 hourtestingandcreeprupture

testingappearstobethecurrentstandardforevaluatinggeosyntheticmaterialcreep.TypicalrangeofRFcr

is1.4 to5.0.

RF id

RFidisthereductionfactorforinstallationdamage(i.e., cuts, nicks, tears, etc.)createdby fillplacementand

constructionequipmentoperationswithvariousbackfillmaterialthatcanpotentially reducereinforcing

strengthandperformance. Therecommendedreductionfactorforreinforcementinstallationdamageis

basedonresultsoffull-scaleconstructiondamagetests. Sitespecificvaluesmay bedeterminedby

performingconstructiondamagetestsfortheselectedgeosyntheticmaterialwithprojectspecific

backfillandequipment. TypicalrangeofRFidis1.05 to2.00.

RFd

RFdisthereductionfactortoaccountfortheeffectsofchemicalandbiologicalexposuretothe

reinforcementthataredependentonmaterialcomposition, includingresintype, resingrade, additives,

manufacturingprocess, andfinalproductphysicalstructure. FormostsoilsusedwiththeKeystoneSystem,

themanufacturershaveincludedrecommendedfactorstoaccountforpossiblechemicalandbiologicaldegradation. Insoilswherehighalkalinity orotheraggressivefactors(ph9)may bepresent, the

manufacturershouldbecontactedforspecificrecommendations.TypicalrangeofRFdis1.0 to2.0.

Forfurtherinformationonthechemicalandbiologicaldurability ofareinforcement, areviewofdurability

ispresentedinFHWA-NHI-09-087 Corrosion/DegradationofSoilReiforcementforMSEWallsand

ReinforcedSoilSlopes.

FS

FSistheoveralltensionsafety factorformaterial, geometric, andloadinguncertaintiesthatcannotbe

specifically accountedfor. FSissimilartootheroverallsafety factorsinAllowableStressdesign. A

minimumfactorofsafety of1.5 isrequiredformostpermanentapplications. Forunusualloading

conditions, variableorpoorly definedsoilconditions, thisfactormay beincreasedatthediscretionof

thedesigner.

GE OGE O isthegeosyntheticresistancefactorformaterialuncertainty usedinLimitStateDesign. TheUSAASHTOLRFDCodeuses0.90 forthetensileresistancefactor. Resistancefactorswillvary inLimit

StateDesignbasedontheload/resistancefactorsystemadoptedby specificdesigncodes.


16/33

2.4

P ART T W O


Theconnectionstrengthisthestrengthstatereinforcement-facingconnectionstrength. Thecapacity is

dependentupontheverticaldepthtothereinforcement, wallgeometry, typeofKeystoneunitutilized, and

thespecificgeosyntheticutilized.

Laboratory testingisrequiredtodefinetheconnectionstrengthforspecificunitsandgeosyntheticmaterials

atvaryingnormalpressures.Typicalgraphsforanindividualstress-straintestandcompleteseriesplotis

showninFigure2:1 andFigure2:2 perNCMATestMethodSRWU-1/ASTMD6638.

Note:

Reinforced soilwall

designsareuniqueto

thespecificKeystone

unitsandgeosynthetic

reinforcementused.

Connectiondatais

specifictoeach

combinationand

reinforcementlevel.

Substitutionofany

materialsinvalidatesa

givenwalldesign.

F igure 2:1 Load Test at one Norma lF orce

F igure 2:2 Connect ion Load P lotsa t D iff erent Norma lF orces

CONNECTIO N STREN GTH

Peak Load

Failure

3 / 4 " De formation

Load at 3/4"

Stress/strain p lot

D isp lacement

C

onnecti

on

Load

Norma lF orce

C

onne

cti

on

Load

Peak Load Plot

3/4" Load Plot

Tconn = y + N tan Max

y

Max

Max


17/33

2.5


InAllowableStressdesign, thecalculatedtensileloadateachreinforcementlevelinthegeosyntheticmust

belessthan1)theallowablegeosyntheticdesignstrength, Tal, and2)anultimateconnectionstrengthlimit,

Tcl, dividedby asafety factor(Tcl/FS).Therecommendedminimumfactorofsafety ontheultimate

connectionstrengthis1.5.

InLimitState/LRFDdesign, thefactoredtensileloadateachreinforcementlevelinthegeosyntheticmust

belessthan1)theallowablegeosyntheticdesignstsrength, Tal, and2)theultimateconnectionstrengthlimit,

Tcl, timestheappropriateresistancefactor().

Serviceability strength, Tsc, isdefinedastheconnectionstrengthatamaximum0.75-inch(20mm)

movement, asdeterminedwiththeNCMATestMethodSRWU-1/ASTMD6638. Serviceability criteriais

only consideredwhenaservicestateanalysisisbeingperformed. Thisisrarely consideredincurrentMSE

walldesignasservicestatedeformationisnotwellundersood.NCMAandAASHTOdesigncodes

generally ignoretheservicestateconditionandrequireastrengthanalysisonly.

Twotypesofsoil-reinforcementinteractioncoefficientsorinterfaceshearstrengthparametersareusedfor

designofsoilreinforcedstructures:pulloutinteractioncoefficient, C i,anddirectshearcoefficient, Cds.

Thepulloutinteractioncoefficientisusedinstability analysistocomputethefrictionalresistancealong

thereinforcement/soilinterfaceinthezonebeyondadefinedplaneoffailure.

Thecalculationyieldsthe

capacity toresistpulloutofthereinforcementfromthesoil.

Thedirectshearcoefficientisusedtodeterminethefactorofsafety againstoutwardslidingofthewallmass

alongthelayersofreinforcement.Thecoefficientsaredeterminedinthelaboratory andareafunctionof

soilandgeosyntheticmaterialtypes.

Designpulloutresistanceofthegeosyntheticreinforcementisdefinedastheultimatetensileload

requiredtogeneratemovementofthereinforcementthroughthesoilmassmeasuredatamaximum

inch(19mm)displacement. Therecommendedminimumfactorofsafety againstgeosyntheticpulloutis

1.5. Equivalentresistancefactorsareusedinlimitstatedesign. ASTMD6706may beusedtodetermine

pulloutcoefficientsforgeogrids.

Note:AASHTO requiresthat1000 hoursustained load testingbeperformed onallgeosyntheticconnection

schemesforMSEwallsperFHWAguidelines. Thistestingcanresultinanadditionalreductionfactorthat

reducesconnectioncapacityoverthelifeofthestructure. Thereisnoevidencethatconnectioncreep isalongterm

performanceconsiderationbased onthethousandsofwallsconstructed sincethe1980s. NCMAdoesnotrecognize

theconceptintheirDesignManualforSegmentalRetainingWallsand Keystonehasnotobserved thisin

practice. ThemostprobableexplanationisthattheDesignloadsnevermaterializeattheconnectionasextensible

reinforcementcanyieldtoreleaseanystressbuild up whilethesurroundingsoiland reinforcementpicksup the

load (ie, arching). CurrentUSpracticeistoanalyzetheconnectionat100%ofthetheoreticaldesignload inthe

reinforcementwhichoverstatestheload and probablyexplainsthelack ofcreep related connectionisuses.

CONNECTIO N STREN GTH

GEOSYNTHETIC-SO ILINTERACTION COEFFICIENT


18/33

2.6

P ART T W O


KeyWall2010 usesthefollowingdefaultvaluesforC iandC dsbaseduponthephiangleinputtedforthe

reinforcedfillmaterial.

* (USCistheUnifiedSoilClassificationSystemperASTMD-2487)

** (Consultgeogridmanufacturer)

GEOGRID MANUFACTURERS DATA

GeogridmanufacturerswerecontactedduringthedevelopmentofthisKeystoneDesignManualandasked

toprovidetherequiredinformationtoevaluatetheirdesignparameters.Thematerialprovidedandtesting

performedbyKeystoneisthebasisforthevaluesprogrammedintoKeyWall. Informationprovidedto

Keystoneby thegeogridmanufacturers/suppliersisavailabledirectly fromthegeogridmanufacturers.

Note:

Asnewmaterialsare

developed ornewdatais

provided, KeyWallsdata

fileswillbeupdated.

Keystoneprovidesthe

dataasaservicetoits

customersand toensure

datauniformityfor

Keystonedesign. This

informationisprovided

withnowrittenor

implied warrantyasto

theaccuracyofthedata

supplied. Datavalues

should beverified with

themanufacturers.

Note:

Thesecoefficientsdonot

applytoGeotextilesor

fatsoils.So ilType (USC*) A N GLE C i Cd

Crushed Stone ,Grave l(G W ,GM) > 32 0.9 0.9

Sand ,Grave l,Silty Sands(SW ,SM ,SP) > 28 0.8 0.8

Sandy Silt,Lean C lay (SC ,ML,CL) > 25 0.7 0.7

Other clay (CL/CH) < 25 0.6** 0.6**

GEOSYNTHETIC-SO ILINTERACTION COEFFICIENT


19/33

P A RT T H R E E

R E T AI NI N G W A L L D E S I G N T H E O R Y

ArborLakes, Beaverton, Oregon;KeystoneCenturyWall


20/33


3.1

arthretainingwallstructuresrequirethreeprimary areasofdesignanalysis:1)lateralearthpressures

2)foundationbearingcapacity, and3)globaloroverallstability.Theanalysisofeachisbasedonthe

followingengineeringpropertiesofthesoil(s):angleofinternalfriction(), soilcohesion(c), andthe

density ()ofthesoils.

Inthischapter, thebasicmechanismsoflateralearthpressuresandstability offoundationsarepresented.Global

stability andseismicanalysisarebeyondthescopeofthisdesignmanualbutabriefdescriptionisprovided.Once

thebasicconceptsandmechanismsofearthpressuresareunderstood, simplificationofthecalculationstodevelop

theCoulombandRankineearthpressuretheoriescanbeexamined.Therearefurthersimplificationsmadetothe

theorieswhenadaptedfordesignofmechanically stabilizedearth(MSE)structures. By startingwiththebasictheory,

itiseasiertounderstandthemechanismsofperformanceandfailureandadaptthedesigntospecialconditionsnot

directly addressedby thesimplifiedmethods.

Theusershouldrefertorecentgeotechnicaltextbooks, theNCMADesignManualforSegmentalRetaining

Walls,FHWADesignand ConstructionofMechanicallyStabilized EarthWallsand Slopes, andAASHTOStandard

SpecificationsforHighwayBridges,foradditionalmaterialandinformationonsoilsandMSEstructures. Thismanual

issolely intendedtoprovideinsightintotheKeyWalldesignsoftwareandthegeneralprinciplesofmodularwall

designwithoutbeinganexhaustivesummary ofsoilmechanics.

RETA IN ING WALL DESIG N THEORY

E

Thurgood MarshallMiddleSchool, SanDiego, California;KeystoneStandard &Compac


21/33

3.2

P ART T HR E E

RetainingWallDesignTheory

Effective StressD esign

Thesoilstrengthparametersarebasedondrainedconditionsthatareapplicabletogranularsoilsandfine

grainedsoilsforlongterm, drainedconditions. (Note:thesepropertiesarereferredtoasandcinmost

textbooks. Inthismanual, andcwillbeusedforsimplicity andrepresentseffectivestressanalysis.)

Angle of InternalFriction ()Thisvaluerepresentsthefrictionalshearstrengthofthesoilwhentestedundercompactedandconfined

conditions.Thisvalueshouldnotbeconfusedwithasoilangleofrepose,whichreflectstheanglethata

pileofloosesoilwillnaturally stand.

Peak Strength

Thepeakshearstrengthofasoilisthemaximumloadmeasuredduringatestatanominaldisplacement.

Thismanualwillutilizepeakshearstrengthvaluesineffectivestressanalysisunlessotherwisenoted.

Residualstrengthvaluesrequiregreatermovementofthesoilthanisintendedby thedesignofreinforced

soilstructuresbutmay beappropriateinsomecaseswithcohesivesoils.

Globa lStabili ty

Conventionalretainingwalldesignonly looksatsimplesliding, overturning, andbearingasfailuremodes.

Thismanualreferstoglobalstability asallothercombinationsofinternalandexternalstability, slope

stability, andcompoundfailureplanesthatmay compromisethewallstructure.

Fa ilure Plane

Soilfailureplanesaretypically non-linearandareoftenrepresentedby alog-spiralcurve. Internally, thefailureplane(locusofmaximumstresspoints)ismodeledasastraightlinefollowingtheappropriate

RankineorCoulombdefinitionoftheslopeangleforsimplification.

Bearing Capacity Factors

Thegeneralbearingcapacity formulaasproposedbyTerzaghiisused.However, differentbearingcapacity

factorshavebeenpublishedby Meyerhof,Hansen, andVesicovertheyears. Thismanualusesthefactors

proposedby Vesic(1975), whichisconsistentwiththeotherdocumentsdiscussingMSEwalls. (Note:All

factorsassumelevelgroundandmustbeadjustedforslopinggroundconditions.)

Soilmechanicstextbooksincludesectionsonpassiveandactiveearthpressures. They describethe

theoriesofCoulombandRankineandmethodsofsolutionviaformulas, graphicalmethods, and

computeranalysis. Thismanualwillbriefly discussthemethodsofactiveearthpressurecalculation

asitrelatestoreinforcedsoilstructuresandaccepteddesignprincipals. Passivepressuresaretypically

neglectedandnotcoveredinthismanual.

IMPORTA NTTECHNICALDEFIN ITIO NS


22/33

3.3


TheNCMADesignManualforSegmentalRetainingWalls, Second and Third editions, isbasedonCoulomb

earthpressuretheory.Thebasicassumptionsforthisactivewedgetheory weredevelopedby Coulomb

(1776).Theothermajormethodology isRankineearthpressuretheory (1857), whichisbasedonthestate

ofstressthatexistsintheretainedsoilmass. Boththeoriesessentiallymodeltheweightofthesoilmass

slidingalongatheoreticalplaneoffailure(Figure3.2 and3.3). Thelateralearthpressure, Pa, isthenetforce

requiredtoholdthewedgeofsoilinplaceandsatisfy equilibrium.

ThemajordifferencebetweenthetwotheoriesisthattheCoulombmodelandequationsaccount

forfrictionbetweenthebackofthewallandthesoilmassaswellaswallbatter. Rankineequations

moreconservatively assumenowallfrictionatthesoil-wallinterfaceandaverticalwallstructure

whichgreatly simplifiesthemathematicsoftheproblem. Thefrictionatthebackofthewallfaceand

atthebackofthereinforcedzoneforexternalstability computations, providesanadditionalforce

componentthathelpssupporttheunstablewedgeofsoil. Becauseoftheseadditionalresistingforces,

thelateralearthpressurecalculatedby Coulombisgenerally lessthantheearthpressurethatwouldbe

predictedby theRankineequations.

AASHTOdesignmethodologiesgenerally appliesRankineearthpressuretheory forearthreinforced

structures. AASHTOdesignmethodology isrequiredonmosttransportationrelatedprojectsoperating

underthismoreconservativedesigncriteria.

LATERALEARTH PRESSURE THEORIES

Note:

Whenthebackslope

equaltotheassumed

frictionattheback

ofthewall(=),

Coulomband Ranki

formulasprovide

identicalearthpressu

coefficientsand resul

forcesforverticalwa

Note:

Forthoseinterested i

comparingCoulomb

versusRankineversu

AASHTO, KeyWall

allowstheuserto

selecteachdesign

methodology. NCMA

istheCoulombanaly

pertheNCMAdesign

manual;Rankinean

AASHTOuseaRan

approach, butwill

accountforwallbatt

entered intheKeyWa

Geometryselection


23/33

3.4

P ART T HR E E


Thereadershouldnotethatforhorizontalsurfaces(levelsurcharge)orinfiniteslopingsurfaces(extending

beyondthetheoreticalCoulombfailureplane), aclosed-formequationsolutionisapplicableandeasily

derived. Forgeometrieswheretheslopechangeswithinthezoneoffailure(brokenbackslope), thesimple

equationsarenolongerapplicableandmay beunnecessarily conservative. Forexample, ifashortbroken

backslopeismodeledasaninfiniteslope, thedesignmay requiresignificantlymorereinforcementand

excavationthanifmodeledcorrectly. Fortheseconditions, thetrialwedgemethodisusedintheanalysis.

Thisisaniterativetrialwedgeprocesswheresuccessivefailuresurfacesaremodeleduntilamaximum

earthpressureforceiscalculatedforthegeometry andloadinggiven(SeeFigure3:4).

Theearthpressurebehindthewallfaceoratthebackofthereinforcedzoneisrepresentedby atriangular

pressuredistributionforactivesoilpressureandarectangulardistributionforuniformsurchargepressure

asisshowninFigure3:1.

TheappropriateCoulombearthpressureequationsforearthandsurchargepressureareasfollows:

Equation(3a) Pa =H ka

Equation(3b) Pq =q Hka

where:

ka = coefficientofactiveearthpressure

= moistunitweightofthesoil

H = totaldesignheightofthewall

q = uniformsurcharge

COULOMB EARTH PRESSURE THEORY

H/3H/2

H Pa

Pq

q

F igure 3:1 Earth Pressure D iagram

COULOMB EARTH PRESSURE EQUATIO N


24/33

3.5


COULOMB EARTH PRESSURE EQUATIO N

Theactiveearthpressurecoefficient, ka, isdeterminedfromanevaluationoftheCoulombwedgegeometry

showninFigure3:2 andresultsinthefollowingkacoefficient:

Equation(3c) ka =

where:

= angleofbatterfromhorizontal

= angleofinternalfrictionofsoil

=

slopeangleabovewall = angleoffrictionatbackofwall

Thisequationisfoundindifferingformsinothertextsduetothetrigonometricassumptionsmadeinthe

formuladerivation.ThederivationofthisCoulombformulacanbefoundingeotechnicaltextbookssuchas

FoundationAnalysisand Designby Bowles(1996).

H

R

W

Pa

F igure 3:2 Cou lomb Wedge D iagram

sin(+ )

sinsin () 1 +

sin(+ )sin()sin()sin(+ )


25/33

3.6

P ART T HR E E


TheCoulombfailureplanevariesasafunctionofthewallgeometry andfrictionanglesforboththesoils

andthesoil/wallinterface. Forlevelsurchargeandinfiniteslopeconditions, therelationshipforis:

Equation(3d) tan (- ) =

where:

= angleofinternalfriction

= batterofwallmeasuredfromvertical(-90)

= slopeangleabovethewall

= angleoffrictionatbackofwall

(orreinforcedmass)

TablesareavailableintheNCMADesignManualandelsewherethattabulatethesevaluesandassistin

determiningtheappropriateCoulombearthpressurecoefficientsandfailureplaneorientationbasedupon

thewallgeometry andsoilparameters. TheKeyWallprogramcalculatesthesevaluesforeachgeometry.

Forbrokenbackconditions, atrialwedgecalculationisusedinsteadoftheformulas.

Rankineearthpressureisastateofstressevaluationofthesoilbehindaretainingstructurethattraditionally

assumesaverticalwallandnofrictionbetweenthesoil/wallinterface.Theorientationoftheresultantearth

pressureisparalleltothebackslopesurface.

Theearthpressurebehindthewallfaceoratthebackofthereinforcedzoneisrepresentedby a

triangularpressuredistributionsimilartothatshowninFigure3:1.Theearthpressureequations

arethesameasCoulomb:

Equation(3e) Pa =H k a

Equation(3f) Pq =q Hka

where:

ka = coefficientofactiveearthpressure

= moistunitweightofthesoil

h = totaldesignheightofthewall

q = uniformliveloadsurcharge

RA NKINE EARTH PRESSURE EQUATIO NS

RA NKINE EARTH PRESSURE THEORY

COULOMB FA ILURE PLANE LOCATIO N

tan"#$tan"#%tan"# $ cot"+ #&%' $ tan"# cot"+ #&

1 + tan"#%tan "# $ cot "+ #&


26/33

3.7


kacandeterminedfromanevaluationoftheRankinewedgegeometry similartotheCoulombwedge

analysisasshowninFigure3:3.

Thisresultsinthefollowingequationsforka:

Verticalwall, levelbackslope

Equation(3g)

ka=

tan(45 -2)

Verticalwall, backslope

Equation(3g) ka = cos

where:

= angleofinternalfrictionofsoil = slopeangleabovewall

TheRankinefailureplanelocationistypically assumedtobeat:

Equation(3h) = 45+2

Whereisfixedandmeasuredfromhorizontalunderalldesignscenarios, whichisonly technically correct

forlevelsurchargeapplicationsandminimalwallbatter. Intheory, theRankinefailureplanevariesunder

backslopeconditions.However, itiscustomary tofixthefailureplaneat45+2inearthreinforcement

design, thusbestrepresentingthecurvedfailuresurfaceandlocusofmaximumstresspointsfora

reinforcedsoilmass.

Note:

TheCoulombearth

pressureequation

willprovideidentica

Rankineearthpressu

coefficientsbysetting

theinterfacefriction

angle, , equaltothe

backslope, , fora

specificdesigncase.

TheKeyWallprogra

actuallyusesthis

method tocalculate

Rankineearthpressu

coefficientsasit

permitswallbatter

tobeincluded in

thecalculation

whenrequired.

RA NKINE FAILURE PLANE LOCATIO N

H

= 45 + /2

W

R

Pa

F igure 3:3 Rank ine Wedge D iagram

RA NKINE EARTH PRESSURE EQUATIO NS

cos coscos

cos $coscos


27/33

3.8

P ART T HR E E


Thelimitationofclosedformsolutions, suchastheCoulombandRankineequations, isthatonly simple

levelandinfiniteslopingsurchargeswithuniformloadingscanbeanalyzed. Itisnecessary tolookata

trialwedgemethodorapproximationmethodwhenattemptingtoanalyzebrokenbackslopesorother

slope/loadcombinations.

AASHTOandNCMAsuggestanapproximationmethodforbroken-backslopeconditionsthatdefines

equivalentdesignslopesfortheexternalanalysis.However, theinternalanalysisisnotwelldefinedfor

unusualslopesandloadingconditionsandthedesignerisexpectedtouseengineeringjudgementwiththe

simplifiedmethods.

TheKeyWallprogramusesatrialwedgeanalysisfordeterminingtheinternalandexternalforces

inordertoprovidethecorrectresultsformorecomplicateddesigngeometries.Thetrialwedge

calculationisaniterativeprocessthatdeterminestheloadingatsuccessivefailureplaneorientationsuntil

amaximumloadingisdeterminedforthegeometry andsurchargeloading(SeeFigure3:4).

TheKeyWalltrialwedgeanalysisusedisconsistentwiththefundamentalassumptionsoftheapplicable

Coulomband Rankinetheoriesby setting=. Trialwedgeresultsmatchtheequationsolutionsfor

thelevelandinfiniteslopeconditions, butwilldeterminethecorrectinternalandexternalvaluesfor

brokenbackslopeconditionsandoffsetliveanddeadloads. Thismethodofanalysispermitsthedesigner

toproperlymodelmany typicaldesignconditionsandnotoverly simplify theanalysisduetolimitationsof

equationsolutionsandotherdesignsoftware.

F igure 3:4 Tr ia lWedge D iagram

H

R

W

WR

ForceD iagram

W1W2

W3 W4 W5 W6

Wedges

TRIALWEDGE ANALYSIS

Note:

TheAASHTO

Simplified, AASHTO

LRFD, and CAN

LRFDmethodsusethe

AASHTO Simplified

method forcalculatinginternalpressuresand

thetrialwedgefor

calculatingexternal

loadingconditions.


28/33

3.9


Bearingcapacity istheability ofthefoundationsoiltosupportadditionalloadingimposedonthesurface

fromthecompletedwallsystem. Bearingcapacity isanalyzedconsideringtwocriteria:

Shearcapacity ofthesoil

Totalanddifferentialsettlement

Shearcapacity ofthesoilisafunctionofthefoundationsoilstrength, thesoilmassequivalentfootingsize,

thedepthofembedment, andany groundwaterconditionsasdeterminedby thegeotechnicalinvestigation.

Figure3:5 showstheMeyerhofdistributionofappliedbearingpressureforflexiblefoundationsystemsthat

istypically utilizedwithearthreinforcementstructures.

Theequivalentfootingwidthandappliedbearingpressurearecalculatedasfollows:

Equation(3i) e = B 2- (Mr- M o)/Rv

Equation(3j) v = Rv(B - 2e)

where: e = eccentricity ofreaction

B = totallengthofbase

Mr = sumofresistingmoments

Mo = sumofoverturningmoments

Rv = sumofverticalreactions

BEARING CAPACITY

W

q

e

D

R

v

B-2e

B

Foundation So

il

- shear strengthc- coh esion- un it w e ight

F igure 3:5 App li ed Bearing Pressure D iagram

APPLIED BEARIN G PRESSURE


29/33

3.10

P ART T HR E E


Qultistheultimatebearingcapacity ofthefoundationsoilsbasedonthesoilandgeometry parameters.

Theultimatefoundationbearingcapacity canbecalculatedfromthefollowingMeyerhofequation:

Q ult = cNcscdc+ D N qsqdq+ 0.5BNsd

Forwhichinfinitely longstripfootingwithshapeanddepthfactors=1.0, andeffectivebasewidthof

B - 2e, simplifiesto:

Equation(3k) Q ult = cN c+D N q+ 0.5(B - 2e)N

where:

c = cohesionoffoundationsoil

= unitweightoffoundationsoil

D = depthofembedmentbelowgrade

B-2e = effectivefootingwidth

Nc = bearingcapacity factorforcohesion

N q = bearingcapacity factorforembedment

N = bearingcapacity forfootingwidth

Bearingcapacity factorsforthebearingcapacity equation(throughVesic1975)areasfollows:

Nc = (N q- 1)cot N q = e

tantan(45+2) N = 2(N q +1)t an

Thefactorofsafety forbearingcapacity istheratioofultimatebearingcapacity tothecalculated

appliedbearingpressure.

Equation(3l) FSbearing = Q ult/ v

Aminimumsafety factorof2.0 (NCMA)and2.5 (AASHTOASD)againstbearingcapacity failureis

consideredacceptableforflexibleearthreinforcedstructures.

AASHTOLRFDcomputesacapacity demandratio(CDR)forbearingcapacity usingtheequationbelow:

Equation(3m) C DRbearing = Q ultRFb/v(factored loa ds)

TheMSEwallbearingresistancefactorisRFb=0.65. Asisalwaysthecase, thebearingcapacity

demandratiois(1.0.

CALCULATED BEARING CAPACITY

BEARING CAPACITY FACTORS

Note:

Insomecases, bearing

capacityisdetermined

withoutconsideringthe

wallembedment

portionoftheequation,

D N q. SeeKeyWall

DesignPreferences

forthisoption.


30/33

3.11


Note:

Thedesigneris

cautioned about

spanningboxculvert

constructingwallsov

uncompacted utility

trenches,goingdirect

fromarock foundati

toasoilfoundation,

transitioningfroman

elasticbearingsurfac

toarigid foundation

surface. Differential

settlementsinsuch

ashortdistancewill

bedetrimentaltoany

structure, and vertica

constructionslipjoin

arerecommended.

BEARING CAPACITY FACTORS

Bearingcapacity inKeyWallisbasedsolely onthesoilparametersinputforthefoundationsoilandthe

embedmentdepthassumingthatthegroundislevelinfrontofthewall.Thedesignermay checkforatotal

stressconditionby insertingtotalstressparametersforthefoundationsoil.

Settlementcriteriamay limitdesignbearingpressuresforstructureshavinglargefootingareas, suchas

mat-typefoundationsandbearingareasunderMSEwallsystems. By reviewingequation(3k), itiseasy

toseethatwithalargeB, theshearcapacity ofthefoundationisusually sufficient.However, with

largerfootingwidths, theareaofinfluencebelowtheloadedareabecomesquitelarge, typically 2B, and

theadditionofthisverticalstressoveralargeareacaninducesignificantsettlement. Itisimportantthat

thedesignerdistinguishesbetweenallowablebearingcapacity forshearfailure(acatastrophicfailure

mechanism)andasettlementcriterion(anon-catastrophicevent).

Totalsettlementislimitedby thedesignersperformancecriteriaandimpactonadjacentstructuresor

tolerancesonverticalmovements. Aslongasthestructuresettlesuniformly, thereisnosignificant

structuraleffectonthewallsystem.Differentialsettlement, however, willcauseaflexuralmovementin

thewallfaceandmay leadtounitrealignmentandcrackstorelievetensilestressesintheconcrete.

Differentialsettlementstypically shouldbelimitedto1%(i.e., 1 footin100 feet)(NCMA)or%

(i.e., 1 footin200 feet)(AASHTO).

Settlementanalysisisbeyondthescopeofthisdocument, andisnotincludedintheKeyWallanalysis.

Duetothevariability offoundationconditions, potentialinfluencesofgroundwater, andother

subsurfaceconditions, itisrecommendedtoconsultaqualifiedgeotechnicalengineerforproper

analysisandspecifications.

SETTLEMENT

Bearing Capac ity Facto rs (Vesic1975)

N c Nq N

0 5.14 1.00 0.00

5 6.49 1.57 0.45

10 8.34 2.47 1.22

15 10.98 3.94 2.65

20 14.83 6.40 5.39

25 20.72 10.66 10.88

30 30.14 18.40 22.40

35 46.12 33.30 48.03

40 75.31 64.20 109.41

Note:

LimitStateorLRFD

analysisrequiresa

servicestateanalysis

inadditiontothe

strengthanalysis.

KeyWallprovidesth

bearingpressurein

additiontothefactor

bearingpressurefor

comparison.


31/33

3.12

P ART T HR E E


Globalstability analysisisbeyondthescopeofthisdocumentandisnotincludedintheKeyWallprogram.

However, itcanbeanecessary partofacomprehensivedesignanalysisonlargerprojectsandisbest

performedby thesitegeotechnicalengineer.

Globalstability shouldbeinvestigatedany timethefollowingsituationsoccur:

Steepslopesaway fromthetoeofwall

Steepslopesabovethetopofwall

T

ieredwallconstruction Poorfoundationsoils

Slopestability isacomplicatedanalysisthatdependsonsitegeometry, constructionmethods, testedsoil

parametersandpotentialinfluenceofgroundwater. Itisrecommendedthataqualifiedgeotechnical

engineerbeconsultedforproperanalysisandrecommendations.

AminimumFactorofSafety of1.3 isrequiredbyNCMAandAASHTO. Ahigherfactor(FS = 1.5)may

berequiredforcriticalstructuressuchasbridgeabutments. AASHTOLRFDrequiressimilarratios.

NCMA'sDesignManual, 3rdEdition, introducedtheconceptofinternalcompoundstability (ICS)whichisalimitedformofglobalstability analysisthatcheckscircularfailureplanesthroughthereinforced

zoneforalimitedsetofconditions.Keystonebelievesthatglobalstability analysisshouldbedoneona

comprehensivebasis, whenrequired, toavoidthelimitationsoftheICSanalysisanddoesnotcurrently

includethatfunctioninKeyWall.

ExternalG lob alStabili ty S lid ing Surface

InternalG lob a l

Stabili ty S lid ing Surface

2nd Tier Load ing

Method ofSlices

W

E1E2

T1

T2

N

L

FS = 1.30 m i

F igure 3:7 G loba lStab il i ty Section

GLOBALSTABILITY

INTERN ALCOMPOUND STABILITY ANALYSIS


32/33

3.13


Keystoneretainingwallstructureshaveproventobeearthquakeresistantduetothesystemsinherent

flexibility thatpermitsminoryieldingduringamajorseismicevent.

ThemostrecentseismicdesignstandardsarecontainedintheAASHTOStandardSpecificationsforHighway

Bridges(Chapter11)andinthe3rdeditionoftheNCMADesignManual, whichdescribeapseudo-static

methodofanalysisbasedontheMononobe-Okabeapplicationofconventionalearthpressuretheory.

Aschematicofpseudo-staticanalysisconsiderationsisshowninFigure3:8 belowasitpertainsto

reinforcedsoilstructures.

Thedetailsofseismicanalysisarebeyondthescopeofthismanualandotherdocumentsshouldbe

consulted.Therearemany waystoevaluateseismicforces, whicharequitecomplicated.TheKeyWall

programusesthreedifferentmethodsthatparallelthethreedifferentdesignmethodologiesofCoulomb,

Rankine, andAASHTO.

SEISM IC ANALYSIS

Peak Ground Acce leration ,A

LateralInert ialForce

StaticEarthPressure

DynamicEarthPressure

EXTERN AL STABILITY

Structure Acce leration ,Am

Act iveZone

Active WedgeInert ialF orce

FacingInert ialF orce

INTERNAL STABILITY

F igure 3:8 Externa lStab il ity

F igure 3:9 InternalStab il ity


33/33

KS Design Manual Short

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