AASHTO LRFD Section 11
Abutments, Piers, and Walls
AASHTO 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 Walls
gDC
gEV
gEH
(Active)
gES
gLS
Strength Ia
0.90
1.00
1.50
1.50
1.75
Strength Ib
1.25
1.35
1.50
1.50
1.75
Service I
1.00
1.00
1.00
1.00
1.00
Group
Load Definitions





DC – dead load of structural components
and attachments
EV – vertical pressure from dead load of
earth fill
EH – horizontal earth pressure load
ES – earth surcharge load
LS – live load surcharge (transient load)
Surcharge Loads


Earth surcharge AASHTO Section
3.11.6.1 and 3.11.6.2
Live load surcharge AASHTO
3.11.6.4
Conventional Retaining Walls

Strength Limit States




Sliding
Bearing resistance
Eccentricity
Service Limit States



Vertical settlement
Lateral wall movement
Overall stability
External Failure Mechanisms
Sliding Failure
Overturning Failure
Bearing Failure
Deep-Seated
Sliding Failure
b
1.50 EHsin(b+d)
1.50 EHsin(b+d)
1.50
EHcos(b+d)
1.00 WAH
Load Factors for
Bearing Resistance
1.00 EV
b+d
1.00 WAV
1.00 WAV
1.50 EH
0.90 DC
b
1.35 EV
1.25 DC
Load Factors for Conventional Walls
1.50 EH
b+d
1.50
EHcos(b+d)
1.00 WAH
Load Factors for
Sliding and Eccentricity
Conventional Walls - Summary

Use resistance factors for spread footings
or deep foundations, as appropriate
(Section 10.5)

Eccentricity limited to:


e/B < 0.25 for soil (compare to ASD 0.167)
e/B < 0.375 for rock (compare to ASD 0.25)
Non-gravity Cantilevered Walls

Strength Limit States




Bearing resistance of embedded portion of wall
Passive resistance of embedded portion of wall
Flexural resistance of wall/facing elements
Service Limit States



Vertical wall movement
Lateral wall movement
Overall stability
Resistance Factors
Bearing Resistance
Passive Resistance
Flexural Resistance

Section 10.5
1.00
0.90
Code allows increase in Resistance Factors
for temporary walls but specific guidance is
not provided
Pressure Diagrams – Discrete Elements
ASD
LRFD
Non-gravity Cantilevered Walls

Below excavation line, multiply by 3b
on passive side of wall and 1b on active
side of wall for discrete elements

Look at forces separately below
excavation line on passive side and
active side (because different load
factors)
Non-gravity Cantilevered Walls

Factor embedment by 1.2 for
continuous wall elements

Do not factor embedment for discrete
wall elements (conservatism of 3b
assumption)
Example




Cantilevered sheet pile wall retaining a
10-ft deep cut in granular soils
Assume 36 ksi yield stress for sheet
pile
Compare required embedment depth
and structural section for ASD and
LRFD
Load Factor of 1.5 used for EH (active)
Example Geometry
g=
Ka =
gp =
10'
125 pcf
0.33
1.5
Pa
Pp
L
La
Lp
Kp =
jp =
3
1
A
Factored Pa = gp * 0.5 * (L+10)2 * Ka * g
Factored Pp = jp * 0.5 * L2 * Kp * g
Example Results
Method
Mmax
Embedment
ASD
(k-ft)
15.4
(ft)
12.2
LRFD
29.2
12.2
Section
Modulus
(in3/ft)
9.23 (S)
(elastic)
10.83 (Z)
(plastic)
Since Z is about 1.15 to 1.20 times S,
similar section would be acceptable
Anchored Walls

Strength Limit States






Bearing resistance of embedded portion of wall
Passive resistance of embedded portion of wall
Flexural resistance of wall/facing elements
Ground anchor pullout
Tensile resistance of anchor tendon
Service Limit States

Same as non-gravity cantilevered wall
Apparent Earth Pressure
Diagrams




Based on FHWA-sponsored research
Builds upon well-known Terzaghi-Peck
envelopes
Appropriate for walls built in competent
ground where maximum wall height is
critical design case
Same diagram shape for single or
multi-leveled anchored walls
H1
2/
Hn
Th2
p
Thn
2/
3
Hn+1
Hn+1
3
2/
3
H1
H2
H
Th1
3
1/
p
(H-H1)
H
Th1
2/
H1
3
H1
Recommended AEP for Sands
R
p=
R
TOTALLOAD
 K A γH
2 H
3
(a) Walls with one level
of ground anchors
p=
TOTALLOAD
H - 1 3 H1  1 3 Hn+1
(b) Walls with multiple
levels of ground anchors
LRFD Check on Tensile Breakage

Guaranteed Ultimate Tensile
Strength (GUTS)
Tn
GUTS 


Select tendon with:
Σγ iQi
GUTS 

Resistance Factors for Ground
Anchors – Tensile Rupture
Mild Steel
High Strength Steel


0.90
0.80
Resistance factors are applied to
maximum proof test load
For high strength steel, apply
resistance factor to GUTS
Comparison to ASD –
Tensile Rupture

ASD



0.8 GUTS > 1.33 Design Load
(DL = EH + LS)
0.8 GUTS > 1.33 EH + 1.33 LS
LRFD

 GUTS > gp EH + 1.75 LS

0.8 GUTS > 1.5 EH + 1.75 LS
Maximum proof test load must be at least
equal to the factored load
Anchor Bond Length
Tn
L b(min) =
  Qa



Lb = anchor bond length
Tn = factored anchor load
Qa = nominal anchor pullout resistance
Nominal Anchor Pullout
Resistance
Qa =   d  a  L b




Qa = nominal anchor pullout capacity
d = anchor hole diameter
a = nominal anchor bond stress
Lb = anchor bond length
Preliminary Evaluation Only

Bond stress values in AASHTO
should be used for FEASIBILITY
evaluation

AASHTO values for cohesionless
and cohesive soil and rock
Presumptive Nominal Bond Stress
in Cohesionless Soils
Anchor/Soil Type
(Grout Pressure)
Soil Compactness or SPT
Resistance
Presumptive
Ultimate Bond
Stress, n (ksf)
Gravity Grouted Anchors
(<50 psi)
Sand or Sand-Gravel Mixtures
Medium Dense to Dense 11-50
1.5 to 2.9
Medium Dense to Dense 11-50
Medium Dense 11-30
Dense to Very Dense 30-50
1.7 to 7.9
2.3 to 14
5.2 to 20
Silty Sands
-----
3.5 to 8.5
Sandy Gravel
Medium Dense to Dense 11-40
Dense to Very Dense 40-50+
Dense 31-50
4.4 to 29
5.8 to 29
6.3 to 11
Pressure Grouted Anchors
(50 to 400 psi)
Fine to Medium Sand
Medium to Coarse Sand w/Gravel
Glacial Till
Resistance Factors –
Anchor Pullout
Cohesionless (Granular)
Soils
Cohesive Soils
Rock
Where Proof Tests Preformed
1)
2)
0.65(1)
0.70(1)
0.50(1)
1.00(2)
Using presumptive values for preliminary
design only
Where proof tests conducted to at least 1.0
times the factored anchor load
Comparison to ASD –
Anchor Pullout
1.1
LRFD/ASD
1.05
1.0
Rock (FS = 3.0,  = 0.50)
0.95
Sand (FS = 2.5,  = 0.65)
0.9
0.85
0.8
Clay (FS = 2.5,  = 0.70)
0
5
10
15
20
Dead Load / Live Load
 EH

L b(min)(ASD) = 
+ 1 FS
 LS

EH


+ 1.75 
 1.5
LS


L b(min)(LRFD) =

Final Anchor Design

Section 11.9.4.2 Anchor Pullout
Capacity
“For final design, the contract documents
shall require that verification tests or
pullout tests on sacrificial anchors in each
soil unit be conducted …”
 Different than current ASD practice, but
intent is not to require, in general, pullout
testing

Bearing Resistance of Wall
Element




Assume all vertical loads carried by portion
of wall below excavation level
Code refers designer to section on spread or
deep foundations for analysis methods
Resistance factors used are for static
capacity evaluation of piles or shafts (i.e.,  =
0.3 to 0.5  FS ~ 3.0 to 4.5)
Resistance factors should be modified to
correlate to FS = 2.0 to 2.5 for bearing
resistance evaluation
MSE Walls

Strength Limit States





Same external stability checks as for
conventional gravity walls
Tensile resistance of reinforcement
Pullout resistance of reinforcement
Structural resistance of face elements and face
element connection
Service Limits States

Same as for conventional gravity walls
MSE Walls – External Stability
MSE Walls – Internal Stability

Check pullout and tensile
resistance at each reinforcement
level and compare to maximum
factored load, Tmax
Maximum Factored Load

Apply factored load to the
reinforcements
Tmax = σ HS v


sH = factored horizontal soil stress
at reinforcement (ksf)
Sv = vertical spacing of
reinforcement
AASHTO 11.10.6.2.1-2
Factored Horizontal Stresses

Factored Horizontal Stress
σH = gP σ Vk r + ΔσH 




gP = load factor (=1.35 for EV)
kr = pressure coefficient
sV = pressure due to resultant of
gravity forces from soil self weight
DsH = horizontal stress
AASHTO 11.10.6.2.1-1
Reinforcement Tensile
Resistance
Tmax   Tal R c


Tal = Nominal long-term
reinforcement design strength
 = Resistance factor for tensile
resistance
AASHTO 11.10.6.4.1-1
Resistance Factors for Tensile
Resistance
Metallic
Reinforcement
Geosynthetic
Reinforcement
Strip Reinforcement
• Static loading
• Combined static/earthquake loading
Grid Reinforcement
• Static loading
• Combined static/earthquake loading
•
•
Static loading
Combined static/earthquake
loading
0.75
1.00
0.65
0.85
0.90
1.20
ASD/LRFD Tensile Breakage
Example of Steel Strip Reinforcement
ASD
LRFD
Tmax = shSv
Tmax = gpshSv
Tmax = (svkr + Dsh) Sv
Tmax = 1.35 (svkr + Dsh) Sv
Tal = (0.55 Fy Ac) / b
Tal = ( Fy Ac) / b
Tal / Tmax = 0.55 / 1 = 0.55
Tal / Tmax = 0.75 / 1.35 = 0.55
with  = 0.75
Other Developments

LRFD for Soil Nails – NCHRP 24-21

Draft LRFD Design and Construction
Specification for Micropiles
?
The
End