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 1.75 Strength Ib 1.25 1.35 Service I

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 Deep-Seated Sliding Failure Bearing Failure

Load Factors for Conventional Walls b 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

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

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

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

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

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

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

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

Comparison to ASD – Tensile Rupture 0.8 GUTS > 1.33 Design Load (DL = EH + LS) 0.8 GUTS > 1.33 EH + 1.33 LS LRFD f 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 Lb = anchor bond length Tn = factored anchor load Qa = nominal anchor pullout resistance

Nominal Anchor Pullout Resistance Qa = nominal anchor pullout capacity d = anchor hole diameter ta = 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 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 40-50+ 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

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

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

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 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

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 sH = factored horizontal soil stress at reinforcement (ksf) Sv = vertical spacing of reinforcement AASHTO 11.10.6.2.1-2

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 11.10.6.2.1-1

Reinforcement Tensile Resistance Tal = Nominal long-term reinforcement design strength f = Resistance factor for tensile resistance AASHTO 11.10.6.4.1-1

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

ASD/LRFD Tensile Breakage Example of Steel Strip Reinforcement

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

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