1. AASHTO LRFD Section 11 Abutments, Piers, and Walls

2. AASHTO Section 11 Design specifications for:
Conventional gravity/semigravity walls
Non-gravity cantilevered walls
Anchored walls
Mechanically Stabilized Earth (MSE) walls
Prefabricated modular walls

3. Common Load Groups for Walls
gDC
gEV
gEH
(Active)
gES
gLS
Strength Ia
0.90
1.00
1.50
1.75
Strength Ib
1.25
1.35
Service I

4. 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)

5. Surcharge Loads Earth surcharge AASHTO Section 3.11.6.1 and 3.11.6.2
Live load surcharge AASHTO

6. Conventional Retaining Walls
Strength Limit States
Sliding
Bearing resistance
Eccentricity
Service Limit States
Vertical settlement
Lateral wall movement
Overall stability

7. External Failure Mechanisms
Sliding Failure
Overturning Failure
Deep-Seated
Sliding Failure
Bearing Failure

8. Load Factors for Conventional Wallsb b
1.50 EHsin(b+d)
1.50 EHsin(b+d)
1.25 DC
1.50 EH
0.90 DC
1.50 EH
1.35 EV
1.00 EV
b+d
b+d
1.50
EHcos(b+d)
1.50
EHcos(b+d)
1.00 WAV
1.00 WAV
1.00 WAH
1.00 WAH
Load Factors for Bearing Resistance
Load Factors for Sliding and Eccentricity

9. 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 < for rock (compare to ASD 0.25)

10. 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

11. 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

12. Pressure Diagrams – Discrete Elements
ASD
LRFD

13. 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)

14. 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)

15. 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)

16. Example Geometry

17. Example Results
Method
Mmax
(k-ft)
Embedment
(ft)
Section Modulus
(in3/ft)
ASD
15.4
12.2
9.23 (S)
(elastic)
LRFD
29.2
10.83 (Z)
(plastic)
Since Z is about 1.15 to 1.20 times S, similar section would be acceptable

18. Anchored Walls Strength Limit States Service 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

19. 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

20. Recommended AEP for Sands
2/3 H1
2/3 H1 H1 H1
Th1 p p
Th1
1/3 H H2 H
Th2 Hn
Thn
2/3 (H-H1)
Hn+1
2/3 Hn+1 R R
(a) Walls with one level of ground anchors
(b) Walls with multiple levels of ground anchors

21. LRFD Check on Tensile Breakage
Guaranteed Ultimate Tensile Strength (GUTS)
Select tendon with:

22. Resistance Factors for Ground Anchors – Tensile Rupture
Mild Steel
0.90
High Strength Steel
0.80
Resistance factors are applied to maximum proof test load
For high strength steel, apply resistance factor to GUTS

23. Comparison to ASD – Tensile Rupture
0.8 GUTS > 1.33 Design Load (DL = EH + LS)
0.8 GUTS > 1.33 EH LS
LRFD
f GUTS > gp EH LS
0.8 GUTS > 1.5 EH LS
Maximum proof test load must be at least equal to the factored load

24. Anchor Bond Length Lb = anchor bond length Tn = factored anchor load
Qa = nominal anchor pullout resistance

25. Nominal Anchor Pullout Resistance
Qa = nominal anchor pullout capacity
d = anchor hole diameter
ta = nominal anchor bond stress
Lb = anchor bond length

26. Preliminary Evaluation Only
Bond stress values in AASHTO should be used for FEASIBILITY evaluation
AASHTO values for cohesionless and cohesive soil and rock

27. 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
Pressure Grouted Anchors
(50 to 400 psi)
Fine to Medium Sand
Medium to Coarse Sand w/Gravel
Silty Sands
Sandy Gravel
Glacial Till
Medium Dense 11-30
Dense to Very Dense 30-50
-----
Medium Dense to Dense 11-40
Dense to Very Dense
Dense 31-50
1.7 to 7.9
2.3 to 14
5.2 to 20
3.5 to 8.5
4.4 to 29
5.8 to 29
6.3 to 11

28. Resistance Factors – Anchor Pullout
Cohesionless (Granular) Soils
0.65(1)
Cohesive Soils
0.70(1)
Rock
0.50(1)
Where Proof Tests Preformed
1.00(2)
Using presumptive values for preliminary design only
Where proof tests conducted to at least 1.0 times the factored anchor load

29. Comparison to ASD – Anchor Pullout
1.1
1.05
1.0
Rock (FS = 3.0, f = 0.50)
LRFD/ASD
0.95
Sand (FS = 2.5, f = 0.65)
0.9
0.85
Clay (FS = 2.5, f = 0.70)
0.8 5 10 15 20
Dead Load / Live Load

30. 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

31. 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

32. MSE Walls Strength Limit States Service Limits 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

33. MSE Walls – External Stability

34. MSE Walls – Internal Stability
Check pullout and tensile resistance at each reinforcement level and compare to maximum factored load, Tmax

35. Maximum Factored Load Apply factored load to the reinforcements
sH = factored horizontal soil stress at reinforcement (ksf)
Sv = vertical spacing of reinforcement
AASHTO

36. Factored Horizontal Stresses
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

37. Reinforcement Tensile Resistance
Tal = Nominal long-term reinforcement design strength
f = Resistance factor for tensile resistance
AASHTO

38. Resistance Factors for Tensile Resistance
Metallic Reinforcement
Strip Reinforcement
Static loading
Combined static/earthquake loading
Grid Reinforcement
0.75
1.00
0.65
0.85
Geosynthetic Reinforcement
0.90
1.20

39. ASD/LRFD Tensile Breakage
Example of Steel Strip Reinforcement

40. Other Developments LRFD for Soil Nails – NCHRP 24-21
Draft LRFD Design and Construction Specification for Micropiles

41. The
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