DRILLED DEEP FOUNDATIONS

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1 GROUPS: ADVANTAGES Figure Group vs. Single Shaft (FHWA NHI ). Large overturning moments are most effectively resisted using groups of shafts. Higher axial capacities. May be cost effective (larger shafts = larger, heavier equipment).

2 GROUPS:DISADVANTAGES Construction Time. Construction of Pile Cap (especially over water). Limited Foundation Footprint Required. Arthur Ravenel Jr. West Pier Figure courtesy of Marvin Tallent (PBC). Scour (may be less for single shaft).

3 DRILLED DEEP FOUNDATIONS Figure Overlapping Zones of Influence (FHWA NHI after Bowles, 1998). GROUP ANALYSIS: EFFECT ON AXIAL RESISTANCE (STRENGTH LIMIT STATE) Construction Effects: DS & CFA generally decrease effective stress. DD generally increase effective stress. More pronounced in cohesionless soils

4 GROUP ANALYSIS: COHESIVE SOILS R Block = Nominal resistance of Block Failure = 1, Figure Block Type Failure (FHWA NHI after Tomlinson, 1994). *Must take into account 2-3Z below block. = Pile Group Efficiency R n = Nominal resistance of a single shaft

5 GROUP ANALYSIS: MICROPILES IN COHESIVE SOILS Table 5-4. Efficiency Factors in Cohesive Soils (FHWA NHI ). AASHTO : Minimum Micropile Spacing should not be less than 30 inches or 3D. Figure 5-3. Block Failure Model for Micropiles (FHWA NHI ).

6 GROUP ANALYSIS: GROUP EFFICIENCY (COHESIONLESS) = Pile Group Efficiency R ng = Nominal resistance of the shaft group = R n = Nominal resistance of a single shaft

7 GROUP ANALYSIS: GROUP EFFICIENCY (COHESIONLESS) Drilled Shafts (AASHTO ) = 0.65 for center to center spacing of 2.5D = 1.0 for center to center spacing 4.0D Use Linear Interpolation between 2.5 < D < 4.0 Micropiles (AASHTO refers to )* = 0.65 for center to center spacing of 2.5D = 1.0 for center to center spacing 6.0D Use Linear Interpolation between 2.5 < D < 6.0 *FHWA GEC8 recommends same for CFA Piles.

8 GROUP ANALYSIS: GROUP EFFICIENCY (COHESIONLESS)

9 GROUP ANALYSIS: SETTLEMENT (SERVICEABILITY LIMIT) Figure Deeper Zone of Influence for End Bearing Shaft Group (FHWA NHI after Tomlinson, 1994). Simplified Methods. Formulated for use with driven pile groups. Considered to be generally representative of drilled shaft group settlements. The deeper zone of influence unlikely to be significantly affected by deep foundation type.

10 GROUP ANALYSIS: SETTLEMENT (SERVICE LIMIT STATE) Bridge Abutment on Piles - 30 inches of Settlement over 10 years Photograph courtesy of FHWA-NHI Soils and Foundations Workshop Participants Workbook

11 GROUP ANALYSIS: SETTLEMENT Foundation is Rigid if 0.01 Elastic Compression If Rigid, the elastic shortening of pile is very small compared to settlements related to soil. If not Rigid, the elastic shortening of pile should be estimated and included in settlement calculations.

12 GROUP ANALYSIS: SETTLEMENT For Uniform Unit Skin Friction Upper Bound Elastic Compression (No Downdrag)

13 GROUP ANALYSIS: SETTLEMENT (COHESIONLESS -CFA) From FHWA GEC8 after Meyerhof (1976).

14 GROUP ANALYSIS: SETTLEMENT (COHESIONLESS DRILLED SHAFTS) From FHWA NHI after Meyerhof (1976).

15 GROUP ANALYSIS: SETTLEMENT (COHESIVE) Figure Equivalent Footing Concept for Pile Groups (FHWA NHI after Terzaghi and Peck, 1967).

16 GROUP ANALYSIS: SETTLEMENT (COHESIVE) Figure Pressure Distribution Below Equivalent Footing for Pile Group (FHWA NHI after Cheney and Chassie, 2002).

17 GROUP ANALYSIS: SETTLEMENT Same Analysis as 1D Consolidation Settlement. Figure Typical e vs. Log P Curve from Laboratory Consolidation Testing (FHWA NHI ).

18 GROUP ANALYSIS: SETTLEMENT (COHESIONLESS MICROPILES) H=H o Figure 5-9. Bearing Capacity Index vs. N1 60 (FHWA NHI modifed after Hough, 1959). Similar Analysis as 1D Consolidation Settlement.

19 GROUP ANALYSIS: SETTLEMENT Same Analysis as 1D Consolidation Settlement. Figure Typical e vs. Log P Curve from Laboratory Consolidation Testing (FHWA NHI ).

20 GROUP ANALYSIS: UPLIFT (STRENGTH LIMIT STATE) Figure Typical Loading Combination Resulting in Uplift (FHWA NHI ). Figure Forces and Idealized Geomaterial Layering for Computation of Uplift (FHWA NHI ).

21 GROUP ANALYSIS: UPLIFT (DRILLED SHAFTS) Do not use suction for drilled shafts. Same ultimate f s for uplift and compression (FHWA). FHWA IF : Reduction Factors for uplift. FHWA NHI : Account for reduction in Resistance Factor (Read Manual!) AASHTO: Use lesser of the following: Sum of individual shaft uplift resistance. Uplift resistance of pile group as block.

22 GROUP ANALYSIS: UPLIFT (CFA) CFA Piles behave essentially like drilled shafts in response to uplift (FHWA GEC 8). CFA Piles need corresponding reinforcement. Same Ultimate f s for uplift and compression (FHWA). Reduction Factor of 0.8 recommended in cohesionless soils. Don t use reduction factor when uplift is due to soil load (e.g. swelling). Still need safety factors. AASHTO (1996): Use lesser of the following: Sum of individual shaft uplift resistance. 2/3 Effective Weight of Group and Soil in Block. ½ Effective Weight of Group and Soil in Block + ½ Total Shear Resistance of Block.

23 GROUP ANALYSIS: UPLIFT (MICROPILES -COHESIVE) Undrained shear strength of the block of soil enclosed by the group plus the effective weight of the pile cap and pile-soil block Effective weight of a block of soil Figure 5-5. Uplift Cohesive Soils (FHWA NHI after Tomlinson, 1994) *See also FHWA NHI Figure 5-6. Uplift Cohesionless Soils (FHWA NHI after Tomlinson, 1994)

24 LATERAL ANALYSIS (SERVICE LIMIT) Deep Foundations can be subjected to Lateral Loads due to: Vehicle Acceleration/Braking. Wind and/or Wave action. Pile supported earth retaining structures. Debris or Ice Loading. Vessel Impacts. Construction Procedures. Thermal Changes. Slope Movements. Seismic Events. Figure Elevation View of an Overhead Sign Structure (FHWA NHI from FHWA IP-84-11).

25 LATERAL ANALYSIS (SERVICE LIMIT) Figure Soil Resistance to a Lateral Pile Load (FHWA NHI adapted from Smith, 1989).

26 DRILLED DEEP FOUNDATIONS LATERAL ANALYSIS Available Analysis Methods: Brooms Method equilibrium, simplified and practical (FHWA NHI ). Non-Dimensional Solutions. Computer Codes based on Numerical Solutions. FEM FB Pier (UF). FD p-y curves like L-PILE or SWM (Strain Wedge Method). FB-Pier Figure courtesy of Bridge Software Institute (

27 LATERAL ANALYSIS LATERAL DEFLECTION Coefficient of Subgrade Reaction (K h ) K h = P = Load per Unit Area Y = Lateral Deflection P = K h y The soil can be replaced by a series of linearly elastic springs and the coefficient of subgrade reaction corresponding to the stiffness of the springs.

28 DRILLED DEEP FOUNDATIONS LATERAL ANALYSIS COEFFICIENT OF SUBGRADE REACTION (K h ) K h is not a material constant since it varies with the size of the loaded area and thus with the diameter of the pile. K h f z D P y f = Coefficient of variation (tons/ft 3 ) (see NAVFAC 7.02) z = Depth (ft) D = Width or diameter of loaded area (ft) K h = Coefficient of Subgrade Reaction (tons/ft 3 ) Figure 9. Coefficient of Subgrade Reaction (NAVFAC DM7.02).

29 LATERAL ANALYSIS K h COHESIVE SOILS For short term loading E >>, K h >> so that an increase in Su will bring an increase in K h. Heavily over consolidated cohesive soils - assume K h constant with depth K h ~(35 to 70)S u Normally consolidated clays - since S u for NC will increase with depth K h will increase with depth Based on theory of elasticity K h (short term) for a pile with L>5D K h Cu 120 D Due to creep and consolidation, the lateral deflection will increase with time. For inorganic clays, the lateral deflection will normally be 2 to 6 times (25% to 50% for stiff, 20% to 30% for soft) the short term loading deflection. Hence for the long term, Kh = 20 Cu/D can be used.

30 LATERAL ANALYSIS COEFFICIENT OF SUBGRADE REACTION (K h ) Cohesionless Soils Other Considerations E is a function of density and overburden pressure and hence will increase approximately linearly with depth. K h decreases under cyclic loading and the largest reduction is for lowest density. D r <35%: K h =0.25K h of 1 st loading 35%<D r <65%: K h =0.33K h of 1 st loading D r >65% : K h =0.50K h of first loading *For clays correlate soft with loose and stiff with dense. K h decreases with an increase in the size of the pile or pile group. Group action should be considered when the pile spacing in the direction of loading is less than 6 to 8 D. Group action can be evaluated by reducing the effective K h in the direction of loading by the following reduction factors (multiply K h by): Spacing 8D 6D 4D 3D Reduction Factor

31 LATERAL ANALYSIS (p-y METHOD a.k.a. REESE S LPILE METHOD) Pile is modeled as a beam-column with a distributed load along the length of the beam produced by the elastic (spring) foundation. The governing differential equations derived by Hetenyi (1946). Figure LPILE Pile-Soil Model (FHWA NHI ).

32 LATERAL ANALYSIS (p-y METHOD a.k.a. REESE S LPILE METHOD) E s = - E s = Soil Modulus (secant modulus) p = Soil Resistance per Unit Pile Length y = Lateral Soil or Pile Deflection Figure LPILE Pile-Soil Model (FHWA NHI ).

33 LATERAL ANALYSIS (p-y METHOD a.k.a. REESE S LPILE METHOD) p-y Curves influenced by: Soil Properties (most) Depth Pile Width Water Table Loading (Static or Cyclic) Figure Typical p-y Curves for Ductile and Brittle Soil (FHWA NHI after Coduto, 1994). Ductile Soils: Soft Clays, Sands Brittle Soils: Stiff Clays under Dynamic Loading

34 LATERAL ANALYSIS (p-y METHOD a.k.a. REESE S LPILE METHOD) integrate differentiate Figure Graphical Presentation of LPILE Results (FHWA NHI after Reese et al. 2000).

35 LATERAL ANALYSIS FAILURE MECHANISMS OF LATERALLY LOADED PILES FREE HEAD FIXED HEAD (RESTRAINED) Plastic Hinge From Gunaratne (2006) after Broms (1964)

36 LATERAL ANALYSIS LATERAL LOAD CAPACITY BASED ON STRENGTH (BROMS 1964a,b) Simplified solutions for the ultimate lateral load capacity based on strength of the pile material, pile dimension, and the soil shear strength. At a lateral displacement of about 0.2D the passive resistance will be mobilized as shown with soil located in front of the pile moving upwards towards the ground surface. See Gunaratne (2006) for Step by Step Details. Figure 8.5. Failure mechanisms for laterally loaded restrained piles in cohesive soils (c) long piles. (From Broms, B., 1964a, J. Soil Mech. Found. Div., ASCE, 90(SM3):27 56.) * See also FHWI NHI & FHWA NHI

37 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE (AFTER REESE AND MATLOCK, 1956) Three (3) Principal Loading Conditions (i.e. Cases) Figure 10. Design Procedure for Laterally Loaded Piles. (NAVFAC 7.02)

38 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE: CASE I Figure 10. Design Procedure for Laterally Loaded Piles (NAVFAC 7.02) Pile with Flexible Cap or Hinged End Condition. Thrust and moment are applied at the top, which is free to rotate. Obtain total deflection, moment and shear in the pile by algebraic sum of the effects of thrust and moment, given in Figure 11.

39 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE: CASE I Figure 11. Case I Influence Values (NAVFAC 7.02)

40 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE: CASE II Figure 10. Design Procedure for Laterally Loaded Piles (NAVFAC 7.02) Pile with Rigid Cap Fixed against Rotation at Ground Surface. Thrust is applied at the top, which must maintain a vertical tangent. Obtain deflection and moment from influence values of Figure 12.

41 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE: CASE II Figure 12. Case II Influence Values (NAVFAC 7.02)

42 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE: CASE III Figure 10. Design Procedure for Laterally Loaded Piles (NAVFAC 7.02) Pile with Rigid Cap above Ground Surface. Rotation of pile top depends on combined effect of superstructure and resistance below ground. Express rotation as a function of the influence values of Figure 13 and determine moment at pile top. Knowing thrust and moment applied at pile top, obtain total deflection, moment and shear in the pile by algebraic sum of the separate effects from Figure 11.

43 LATERAL ANALYSIS NAVFAC DESIGN PROCEDURE: CASE III Figure 12. Case III Influence Values (NAVFAC 7.02)

44 LATERAL ANALYSIS LATERAL PILE GROUPS Figure 1.1 Illustration of shadow and edge effects on a laterally loaded pile group (Walsh, 2005).

45 LATERAL ANALYSIS LATERAL PILE GROUPS (P-MULTIPLIER METHOD, BROWN ET AL. 1988) Figure The p-mulitplier (P m ) (FHWA GEC 8 from FHWA NHI ).

46 LATERAL ANALYSIS LATERAL PILE GROUPS (USING LPILE, FHWA NHI ) STEP 1: Obtain Lateral Loads. STEP 2: Develop p-y curves for single pile. STEP 3: Perform LPILE analyses. STEP 4: Estimate group deflection under lateral load (see Figure 9.69). Figure Typical Plots of Load versus Deflection and Bending Moment versus Deflection for Pile Group Analysis (FHWA NHI adapted from Brown and Bollman, 1993).

47 LATERAL ANALYSIS FHWA DRILLED SHAFT LATERAL LOADING DESIGN PROCESS Figure Drilled Shaft Design Process for Lateral Loads (FHWA NHI )