AASHTO LRFD Section 11 AASHTO LRFD Section 11 Abutments, Piers, and WallsAbutments, Piers, and Walls

AASHTO Section 11AASHTO Section 11

Design specifications for: Conventional gravity/semigravity walls Non-gravity cantilevered walls Anchored walls Mechanically Stabilized Earth (MSE)

walls Prefabricated modular walls

Common Load Groups for WallsCommon Load Groups for Walls

GroupGroupDCDC EVEV EHEH

(Active)(Active)

ESES LSLS

Strength IaStrength Ia 0.900.90 1.001.00 1.501.50 1.501.50 1.751.75

Strength IbStrength Ib 1.251.25 1.351.35 1.501.50 1.501.50 1.751.75

Service IService I 1.001.00 1.001.00 1.001.00 1.001.00 1.001.00

Load DefinitionsLoad Definitions

DC – dead load of structural components DC – dead load of structural components and attachmentsand attachments

EV – vertical pressure from dead load of EV – vertical pressure from dead load of earth fillearth fill

EH – horizontal earth pressure loadEH – horizontal earth pressure load ES – earth surcharge loadES – earth surcharge load LS – live load surcharge (transient load)LS – live load surcharge (transient load)

Surcharge LoadsSurcharge Loads

Earth surcharge Earth surcharge AASHTOAASHTO Section Section 3.11.6.1 and 3.11.6.23.11.6.1 and 3.11.6.2

Live load surcharge Live load surcharge AASHTOAASHTO 3.11.6.43.11.6.4

Conventional Retaining WallsConventional Retaining Walls

Strength Limit StatesStrength Limit States SlidingSliding Bearing resistanceBearing resistance EccentricityEccentricity

Service Limit StatesService Limit States Vertical settlementVertical settlement Lateral wall movementLateral wall movement Overall stabilityOverall stability

External Failure MechanismsExternal Failure Mechanisms

Sliding FailureSliding Failure Overturning FailureOverturning Failure

Bearing FailureBearing FailureDeep-Seated Deep-Seated Sliding FailureSliding Failure

1.2

5 D

C1

.25

DC

0.9

0 D

C0

.90

DC

1.0

0 W

A1

.00

WA

VV

1.0

0 W

A1

.00

WA

VV

1.50 1.50

EHcos(EHcos())1.50 1.50

EHcos(EHcos())

1.50 EH1.50 EH 1.50 EH1.50 EH1

.35

EV

1.3

5 E

V

1.0

0 E

V1

.00

EV

1.50 EHsin(1.50 EHsin()) 1.50 EHsin(1.50 EHsin())

1.00 WA1.00 WAHH 1.00 WA1.00 WAHH

Load Factors for Load Factors for Bearing Bearing

ResistanceResistance

Load Factors for Load Factors for Sliding and Sliding and EccentricityEccentricity

Load Factors for Conventional WallsLoad Factors for Conventional Walls

Conventional Walls - SummaryConventional Walls - Summary

Use resistance factors for spread footings Use resistance factors for spread footings or deep foundations, as appropriate or deep foundations, as appropriate (Section 10.5)(Section 10.5)

Eccentricity limited to:Eccentricity limited to: e/B < 0.25 for soil (compare to ASD 0.167)e/B < 0.25 for soil (compare to ASD 0.167) e/B < 0.375 for rock (compare to ASD 0.25)e/B < 0.375 for rock (compare to ASD 0.25)

Non-gravity Cantilevered WallsNon-gravity Cantilevered Walls

Strength Limit StatesStrength Limit States Bearing resistance of embedded portion of wallBearing resistance of embedded portion of wall Passive resistance of embedded portion of wallPassive resistance of embedded portion of wall Flexural resistance of wall/facing elementsFlexural resistance of wall/facing elements

Service Limit StatesService Limit States Vertical wall movementVertical wall movement Lateral wall movementLateral wall movement Overall stabilityOverall stability

Resistance FactorsResistance Factors

Bearing ResistanceBearing Resistance

Passive ResistancePassive Resistance

Flexural ResistanceFlexural Resistance

Section 10.5Section 10.5

1.001.00

0.900.90

Code allows increase in Resistance Factors Code allows increase in Resistance Factors for temporary walls but specific guidance is for temporary walls but specific guidance is not provided not provided

Pressure Diagrams – Discrete ElementsPressure Diagrams – Discrete Elements

ASDASD

LRFDLRFD

Non-gravity Cantilevered WallsNon-gravity Cantilevered Walls

Below excavation line, multiply by 3b Below excavation line, multiply by 3b on passive side of wall and 1b on active on passive side of wall and 1b on active side of wall for discrete elementsside of wall for discrete elements

Look at forces separately below Look at forces separately below excavation line on passive side and excavation line on passive side and active side (because different load active side (because different load factors)factors)

Factor embedment by 1.2 for Factor embedment by 1.2 for continuous wall elementscontinuous wall elements

Do not factor embedment for discrete Do not factor embedment for discrete wall elements (conservatism of 3b wall elements (conservatism of 3b assumption)assumption)

Non-gravity Cantilevered WallsNon-gravity Cantilevered Walls

ExampleExample

Cantilevered sheet pile wall retaining a Cantilevered sheet pile wall retaining a 10-ft deep cut in granular soils10-ft deep cut in granular soils

Assume 36 ksi yield stress for sheet Assume 36 ksi yield stress for sheet pilepile

Compare required embedment depth Compare required embedment depth and structural section for ASD and and structural section for ASD and LRFDLRFD

Load Factor of 1.5 used for EH (active)Load Factor of 1.5 used for EH (active)

= 125 pcfKa = 0.33

p = 1.5

Kp = 3j

p = 1

Factored Pa = p * 0.5 * (L+10)2 * Ka *

Factored Pp = jp * 0.5 * L2 * Kp *

Pa

Lp

La

L

A

10'

Pp

Example GeometryExample Geometry

Example ResultsExample Results

MethodMethod MMmaxmax

(k-ft)(k-ft)

EmbedmentEmbedment

(ft)(ft)

Section Section ModulusModulus

(in(in33/ft)/ft)

ASDASD 15.415.4 12.212.2 9.23 (S)9.23 (S)

(elastic)(elastic)

LRFDLRFD 29.229.2 12.212.2 10.83 (Z)10.83 (Z)

(plastic)(plastic)

Since Z is about 1.15 to 1.20 times S, similar section would be acceptable

Anchored WallsAnchored Walls

Strength Limit StatesStrength Limit States Bearing resistance of embedded portion of wallBearing resistance of embedded portion of wall Passive resistance of embedded portion of wallPassive resistance of embedded portion of wall Flexural resistance of wall/facing elementsFlexural resistance of wall/facing elements Ground anchor pulloutGround anchor pullout Tensile resistance of anchor tendonTensile resistance of anchor tendon

Service Limit StatesService Limit States Same as non-gravity cantilevered wallSame as non-gravity cantilevered wall

Apparent Earth Pressure Apparent Earth Pressure DiagramsDiagrams

Based on FHWA-sponsored researchBased on FHWA-sponsored research Builds upon well-known Terzaghi-Peck Builds upon well-known Terzaghi-Peck

envelopesenvelopes Appropriate for walls built in competent Appropriate for walls built in competent

ground where maximum wall height is ground where maximum wall height is critical design casecritical design case

Same diagram shape for single or Same diagram shape for single or multi-leveled anchored walls multi-leveled anchored walls

Recommended AEP for SandsRecommended AEP for SandsHH

HH11 HH11

HHn+

1n+

1

pp pp

22// 33

H H11

22// 33

H H11

22// 33

H Hn+

1n+

1

22// 33

(H

-H (

H-H

11))

11// 33

H HTTh1h1

TTh1h1

TTh2h2

TThnhn

HH22

HHnn

RR RR

(a) Walls with one level(a) Walls with one levelof ground anchorsof ground anchors

(b) Walls with multiple(b) Walls with multiplelevels of ground anchorslevels of ground anchors

HKHLOADTOTAL

p A3

2γ=

1n31

131 HH-H

LOADTOTALp

=

Guaranteed Ultimate Tensile Guaranteed Ultimate Tensile Strength (GUTS) Strength (GUTS)

Select tendon with:Select tendon with:

nTGUTS

iiQΣ

GUTSγ

LRFD Check on Tensile BreakageLRFD Check on Tensile Breakage

Resistance Factors for Ground Resistance Factors for Ground Anchors – Tensile RuptureAnchors – Tensile Rupture

Resistance factors are applied to Resistance factors are applied to maximum proof test loadmaximum proof test load

For high strength steel, apply For high strength steel, apply resistance factor to GUTSresistance factor to GUTS

Mild SteelMild Steel 0.900.90

High Strength SteelHigh Strength Steel 0.800.80

Comparison to ASD – Comparison to ASD – Tensile RuptureTensile Rupture

ASDASD 0.8 GUTS > 1.33 Design Load 0.8 GUTS > 1.33 Design Load

(DL = EH + LS)(DL = EH + LS) 0.8 GUTS > 1.33 EH + 1.33 LS0.8 GUTS > 1.33 EH + 1.33 LS

LRFDLRFD GUTS > GUTS > pp EH + 1.75 LS EH + 1.75 LS 0.8 GUTS > 1.5 EH + 1.75 LS0.8 GUTS > 1.5 EH + 1.75 LS

Maximum proof test load must be at Maximum proof test load must be at least equal to the factored loadleast equal to the factored load

Anchor Bond LengthAnchor Bond Length

a

nb(min) Q

TL

=

LLbb = anchor bond length = anchor bond length

TTnn = factored anchor load = factored anchor load

QQaa = nominal anchor pullout resistance = nominal anchor pullout resistance

Nominal Anchor Pullout Nominal Anchor Pullout ResistanceResistance

baa LdQ =

QQaa = nominal anchor pullout capacity = nominal anchor pullout capacity d = anchor hole diameterd = anchor hole diameter aa = nominal anchor bond stress = nominal anchor bond stress

LLbb = anchor bond length = anchor bond length

Preliminary Evaluation Only Preliminary Evaluation Only

Bond stress values in AASHTO Bond stress values in AASHTO should be used for FEASIBILITY should be used for FEASIBILITY evaluationevaluation

AASHTO values for cohesionless AASHTO values for cohesionless and cohesive soil and rockand cohesive soil and rock

Anchor/Soil TypeAnchor/Soil Type(Grout Pressure)(Grout Pressure)

Soil Compactness or SPT Soil Compactness or SPT ResistanceResistance

Presumptive Presumptive Ultimate Bond Ultimate Bond Stress, Stress, nn (ksf) (ksf)

Gravity Grouted AnchorsGravity Grouted Anchors(<50 psi)(<50 psi)Sand or Sand-Gravel MixturesSand or Sand-Gravel Mixtures Medium Dense to Dense 11-50Medium Dense to Dense 11-50 1.5 to 2.91.5 to 2.9

Pressure Grouted AnchorsPressure Grouted Anchors(50 to 400 psi)(50 to 400 psi)Fine to Medium SandFine to Medium SandMedium to Coarse Sand w/GravelMedium to Coarse Sand w/Gravel

Silty SandsSilty Sands

Sandy GravelSandy Gravel

Glacial TillGlacial Till

Medium Dense to Dense 11-50Medium Dense to Dense 11-50Medium Dense 11-30Medium Dense 11-30Dense to Very Dense 30-50Dense to Very Dense 30-50

----------

Medium Dense to Dense 11-40Medium Dense to Dense 11-40Dense to Very Dense 40-50+Dense to Very Dense 40-50+Dense 31-50Dense 31-50

1.7 to 7.91.7 to 7.92.3 to 142.3 to 145.2 to 205.2 to 20

3.5 to 8.53.5 to 8.5

4.4 to 29 4.4 to 29 5.8 to 295.8 to 296.3 to 116.3 to 11

Presumptive Nominal Bond Stress Presumptive Nominal Bond Stress in Cohesionless Soilsin Cohesionless Soils

Resistance Factors – Resistance Factors – Anchor PulloutAnchor Pullout

1)1) Using presumptive values for preliminary Using presumptive values for preliminary design onlydesign only

2)2) Where proof tests conducted to at least 1.0 Where proof tests conducted to at least 1.0 times the factored anchor loadtimes the factored anchor load

Cohesionless (Granular) Cohesionless (Granular) SoilsSoils 0.650.65(1)(1)

Cohesive SoilsCohesive Soils 0.700.70(1)(1)

RockRock 0.500.50(1)(1)

Where Proof Tests Where Proof Tests PreformedPreformed 1.001.00(2)(2)

FS1LSEH

(ASD)Lb(min)

=

=1.75

LSEH

1.5(LRFD)Lb(min)

LRFD

/AS

DLR

FD/A

SD

1.051.05

1.11.1

1.01.0

0.950.95

0.90.9

0.850.85

0.80.800 55 1010 1515 2020

Dead Load / Live LoadDead Load / Live Load

Rock (FS = 3.0, Rock (FS = 3.0, = 0.50) = 0.50)

Sand (FS = 2.5, Sand (FS = 2.5, = 0.65) = 0.65)

Clay (FS = 2.5, Clay (FS = 2.5, = 0.70) = 0.70)

Comparison to ASD – Comparison to ASD – Anchor PulloutAnchor Pullout

Final Anchor DesignFinal Anchor Design

Section 11.9.4.2 Anchor Pullout Section 11.9.4.2 Anchor Pullout CapacityCapacity ““For final design, the contract documents For final design, the contract documents

shall require that verification tests or shall require that verification tests or pullout tests on sacrificial anchors in each pullout tests on sacrificial anchors in each soil unit be conducted …”soil unit be conducted …”

Different than current ASD practice, but Different than current ASD practice, but intent is not to require, in general, pullout intent is not to require, in general, pullout testingtesting

Bearing Resistance of Wall Bearing Resistance of Wall ElementElement

Assume all vertical loads carried by portion Assume all vertical loads carried by portion of wall below excavation levelof wall below excavation level

Code refers designer to section on spread or Code refers designer to section on spread or deep foundations for analysis methodsdeep foundations for analysis methods

Resistance factors used are for static Resistance factors used are for static capacity evaluation of piles or shafts (i.e., capacity evaluation of piles or shafts (i.e., = = 0.3 to 0.5 0.3 to 0.5 FS ~ 3.0 to 4.5) FS ~ 3.0 to 4.5)

Resistance factors should be modified to Resistance factors should be modified to correlate to FS = 2.0 to 2.5 for bearing correlate to FS = 2.0 to 2.5 for bearing resistance evaluation resistance evaluation

MSE WallsMSE Walls

Strength Limit StatesStrength Limit States Same external stability checks as for Same external stability checks as for

conventional gravity wallsconventional gravity walls Tensile resistance of reinforcementTensile resistance of reinforcement Pullout resistance of reinforcementPullout resistance of reinforcement Structural resistance of face elements and face Structural resistance of face elements and face

element connectionelement connection

Service Limits StatesService Limits States Same as for conventional gravity wallsSame as for conventional gravity walls

MSE Walls – External StabilityMSE Walls – External Stability

MSE Walls – Internal StabilityMSE Walls – Internal Stability

Check pullout and tensile Check pullout and tensile resistance at each reinforcement resistance at each reinforcement level and compare to maximum level and compare to maximum factored load, Tfactored load, Tmaxmax

Apply factored load to the reinforcements Apply factored load to the reinforcements

HH = factored horizontal soil stress at = factored horizontal soil stress at

reinforcement (ksf)reinforcement (ksf) SSvv = vertical spacing of reinforcement = vertical spacing of reinforcement

vHmax SσT =

AASHTO 11.10.6.2.1-2

Maximum Factored LoadMaximum Factored Load

Factored Horizontal StressesFactored Horizontal Stresses Factored Horizontal StressFactored Horizontal Stress

PP = load factor (=1.35 for EV) = load factor (=1.35 for EV)

kkrr = pressure coefficient = pressure coefficient

VV = pressure due to resultant of gravity forces from soil = pressure due to resultant of gravity forces from soil

self weight self weight HH = horizontal stress = horizontal stress

HrVPH Δσkσσ =

AASHTO 11.10.6.2.1-1

TTalal = Nominal long-term = Nominal long-term

reinforcement design strengthreinforcement design strength = Resistance factor for tensile = Resistance factor for tensile

resistanceresistance

calmax RTT

AASHTO 11.10.6.4.1-1

Reinforcement Tensile Reinforcement Tensile ResistanceResistance

Resistance Factors for Tensile Resistance Factors for Tensile ResistanceResistance

Metallic Metallic ReinforcementReinforcement

Strip ReinforcementStrip Reinforcement• Static loadingStatic loading• Combined static/earthquake loadingCombined static/earthquake loadingGrid ReinforcementGrid Reinforcement• Static loadingStatic loading• Combined static/earthquake loadingCombined static/earthquake loading

0.750.751.001.00

0.650.650.850.85

Geosynthetic Geosynthetic ReinforcementReinforcement

• Static loadingStatic loading• Combined static/earthquake loadingCombined static/earthquake loading

0.900.901.201.20

ASD/LRFD Tensile BreakageASD/LRFD Tensile Breakage

Example of Steel Strip ReinforcementExample of Steel Strip Reinforcement

ASD LRFD Tmax = hSv

Tmax = (vkr + h) Sv

Tal = (0.55 Fy Ac) / b Tal / Tmax = 0.55 / 1 = 0.55

Tmax = phSv

Tmax = 1.35 (vkr + h) Sv

Tal = ( Fy Ac) / b with = 0.75 Tal / Tmax = 0.75 / 1.35 = 0.55

Other DevelopmentsOther Developments

LRFD for Soil Nails – NCHRP 24-21LRFD for Soil Nails – NCHRP 24-21

Draft LRFD Design and Construction Draft LRFD Design and Construction Specification for MicropilesSpecification for Micropiles

? TheEnd