14.528 DRILLED DEEP FOUNDATIONS Group, Uplift,...
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Revised 10/2012 14.528 DRILLED DEEP FOUNDATIONS Group, Uplift, and Lateral Analyses GROUPS: ADVANTAGES Figure 14-2. Group vs. Single Shaft (FHWA NHI-10-016). • Large overturning moments are most effectively resisted using groups of shafts. • Higher axial capacities. • May be cost effective (larger shafts = larger, heavier equipment).
Transcript of 14.528 DRILLED DEEP FOUNDATIONS Group, Uplift,...
Revised 10/2012
14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUPS: ADVANTAGES
Figure 14-2. Group vs. Single Shaft(FHWA NHI-10-016).
• Large overturning moments are most effectively resisted using groups of shafts.
• Higher axial capacities.
• May be cost effective (larger shafts = larger, heavier equipment).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUPS:DISADVANTAGES
Arthur Ravenel Jr. West PierFigure courtesy of Marvin Tallent
(PBC).
• Construction Time.
• Construction of Pile Cap (especially over water).
• Limited Foundation Footprint Required.
• Scour (may be less for single shaft).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
Figure 14-3. Overlapping Zones of Influence (FHWA NHI-10-016 after Bowles, 1998).
Construction Effects:• DS & CFA generally
decrease effective stress.• DD generally increase
effective stress.• More pronounced in
cohesionless soils
GROUP ANALYSIS:EFFECT ON AXIAL
RESISTANCE(STRENGTH LIMIT STATE)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
Figure 14-3. Block Type Failure(FHWA NHI-10-016 after Tomlinson, 1994).
GROUP ANALYSIS:COHESIVE SOILS
,
=
RBlock = Nominal resistance of Block Failure
Rn = Nominal resistance of a single shaft
= Pile Group
Efficiency
≤ 1
*Must take into account 2-3Z below block.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS:MICROPILES IN COHESIVE SOILS
Table 5-4. Efficiency Factors in Cohesive Soils(FHWA NHI-05-039).
Figure 5-3. Block Failure Model for Micropiles(FHWA NHI-05-039).
AASHTO 10.0.1.2: Minimum Micropile Spacing should not be less than 30 inches or 3D.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS:GROUP EFFICIENCY (COHESIONLESS)
=
Rng = Nominal resistance of the shaft group
Rn = Nominal resistance of a single shaft
= Pile Group
Efficiency
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS:GROUP EFFICIENCY (COHESIONLESS)
Drilled Shafts (AASHTO 10.8.3.6) = 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 10.9.3.6 refers to 10.7.3.9)* = 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUPANALYSIS:
GROUPEFFICIENCY
(COHESIONLESS)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENT(SERVICEABILITY LIMIT)
Figure 14-8. Deeper Zone of Influence for End Bearing Shaft Group (FHWA NHI-10-016 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
Bridge Abutment on Piles - 30 inches of Settlement over 10 years
Photograph courtesy of FHWA-NHI-132012 Soils and Foundations Workshop Participants Workbook
GROUP ANALYSIS: SETTLEMENT(SERVICE LIMIT STATE)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENT• If “Rigid”, the elastic
shortening of pile is very small compared to settlements related to soil.
≤ 0.01
Foundation is “Rigid” if
Elastic Compression • If not “Rigid”, the elastic shortening of pile should be estimated and included in settlement calculations.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENTFor Uniform Unit Skin Friction
Upper Bound Elastic Compression(No Downdrag)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENT(COHESIONLESS - CFA)
From FHWA GEC8 after Meyerhof (1976).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENT(COHESIONLESS – DRILLED SHAFTS)
From FHWA NHI-10-016 after Meyerhof (1976).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUPANALYSIS:
SETTLEMENT(COHESIVE)
Figure 14-9. Equivalent Footing Concept for Pile Groups (FHWA NHI-10-016 after Terzaghi and Peck, 1967).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUPANALYSIS:
SETTLEMENT(COHESIVE)
Figure 14-10. Pressure Distribution Below Equivalent Footing for Pile Group (FHWA
NHI-10-016 after Cheney and Chassie, 2002).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENT
Figure 14-11. Typical e vs. Log P Curve from Laboratory Consolidation Testing (FHWA NHI-10-016).
• Same Analysis as 1D Consolidation Settlement.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUPANALYSIS:
SETTLEMENT(COHESIONLESS –
MICROPILES)
Figure 5-9. Bearing Capacity Index vs. N160(FHWA NHI-05-039 modifed after Hough, 1959).
H=Ho
Similar Analysis as 1D Consolidation Settlement.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS: SETTLEMENT
Figure 14-11. Typical e vs. Log P Curve from Laboratory Consolidation Testing (FHWA NHI-10-016).
• Same Analysis as 1D Consolidation Settlement.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS:UPLIFT (STRENGTH LIMIT STATE)
Figure 13-14. Typical Loading Combination Resulting in Uplift (FHWA NHI-10-016).
Figure 13-15. Forces and Idealized Geomaterial Layering for Computation of
Uplift (FHWA NHI-10-016).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS:UPLIFT (DRILLED SHAFTS)
• Do not use suction for drilled shafts.• Same “ultimate” fs for uplift and
compression (FHWA).• FHWA IF-99-025: Reduction Factors for uplift.• FHWA NHI-10-016: 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
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” fs 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
GROUP ANALYSIS:UPLIFT (MICROPILES - COHESIVE)
Figure 5-5. Uplift Cohesive Soils(FHWA NHI-05-039 after Tomlinson, 1994)
Figure 5-6. Uplift Cohesionless Soils(FHWA NHI-05-039 after Tomlinson, 1994)
Effective weight of a block of soil
Undrained shear strength of the
block of soil enclosed by the group plus the
effective weight of the pile cap and pile-soil block
*See also FHWA NHI-05-042
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
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 12-5. Elevation View of an Overhead Sign Structure (FHWA NHI-10-016 from
FHWA IP-84-11).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSIS (SERVICE LIMIT)
Figure 9.36. Soil Resistance to a Lateral Pile Load (FHWA NHI-05-042 adapted from Smith,
1989).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISAvailable Analysis Methods:• Brooms Method – equilibrium,
simplified and practical (FHWA NHI—05-042).
• 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 (https://bsi-web.ce.ufl.edu)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISLATERAL DEFLECTION
Kh =
Coefficient ofSubgrade Reaction
(Kh)
P = Load per Unit AreaY = Lateral Deflection
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.
P = Khy
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISCOEFFICIENT OF SUBGRADE REACTION (Kh) Kh is not a material constant
since it varies with the size of the loaded area and thus with
the diameter of the pile.
yP
DzfKh
f = Coefficient of variation (tons/ft3)(see NAVFAC 7.02)
z = Depth (ft)D = Width or diameter of loaded
area (ft)Kh = Coefficient of Subgrade
Reaction (tons/ft3) Figure 9. Coefficient of Subgrade Reaction (NAVFAC DM7.02).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISKh – COHESIVE SOILS
• For short term loading E >>, Kh >> so that an increase in Su will bring an increase in Kh.
• Heavily over consolidated cohesive soils - assume Khconstant with depth Kh~(35 to 70)Su
• Normally consolidated clays -since Su for NC will increase with depth Kh will increase with depth
• Based on theory of elasticityKh (short term) for a pile with L>5D
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.
K CDh
u 120
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISCOEFFICIENT OF SUBGRADE REACTION (Kh)
• E is a function of density and overburden pressure and hence will increase approximately linearly with depth.
• Kh decreases under cyclic loading and the largest reduction is for lowest density.
Dr<35%: Kh=0.25Kh of 1st loading35%<Dr<65%: Kh=0.33Kh of 1st loading
Dr>65% : Kh=0.50Kh of first loading*For clays correlate soft with loose and
stiff with dense.
• Kh 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 Kh in the direction of loading by the following reduction factors (multiply Kh by):
Cohesionless Soils Other Considerations
Spacing 8D 6D 4D 3DReduction Factor 1.00 0.70 0.40 0.25
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSIS(p-y METHOD a.k.a.
REESE’S LPILE METHOD)
Figure 9-44. LPILE Pile-Soil Model (FHWA NHI-05-042).
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).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSIS(p-y METHOD a.k.a.
REESE’S LPILE METHOD)
Figure 9-44. LPILE Pile-Soil Model (FHWA NHI-05-042).
Es = -Es = Soil Modulus (secant modulus)p = Soil Resistance per Unit Pile
Lengthy = Lateral Soil or Pile
Deflection
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSIS(p-y METHOD a.k.a. REESE’S LPILE METHOD)
Figure 9-45. Typical p-y Curves for Ductile and Brittle Soil
(FHWA NHI-05-042 after Coduto, 1994).
p-y Curves influenced by:• Soil Properties (most)• Depth• Pile Width• Water Table• Loading (Static or Cyclic)
Ductile Soils:Soft Clays, Sands
Brittle Soils:Stiff Clays under Dynamic Loading
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSIS(p-y METHOD a.k.a. REESE’S LPILE METHOD)
Figure 9-46. Graphical Presentation of LPILE Results(FHWA NHI-05-042 after Reese et al. 2000).
differentiate integrate
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISFAILURE MECHANISMS OF LATERALLY LOADED PILES
From Gunaratne (2006) after Broms (1964)
FREE HEAD FIXED HEAD (RESTRAINED)
PlasticHinge
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISLATERAL LOAD CAPACITY
BASED ON STRENGTH(BROMS 1964a,b)
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.)
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 also FHWI NHI-05-042 & FHWA NHI-10-016
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERALANALYSIS
NAVFAC DESIGNPROCEDURE
(AFTER REESE ANDMATLOCK, 1956)
Three (3) Principal Loading Conditions (i.e. Cases)
Figure 10. Design Procedure for Laterally Loaded Piles.
(NAVFAC 7.02)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISNAVFAC 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISNAVFAC DESIGN PROCEDURE: CASE I
Figure 11. Case I Influence Values (NAVFAC 7.02)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISNAVFAC 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERALANALYSIS
NAVFAC DESIGNPROCEDURE: CASE II
Figure 12. Case II Influence Values (NAVFAC 7.02)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISNAVFAC 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.
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERALANALYSIS
NAVFAC DESIGNPROCEDURE: CASE III
Figure 12. Case III Influence Values (NAVFAC 7.02)
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISLATERAL PILE GROUPS
Figure 1.1 Illustration of shadow and edge effects on a laterally loaded pile group(Walsh, 2005).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERALANALYSIS
LATERAL PILE GROUPS(P-MULTIPLIER METHOD,
BROWN ET AL. 1988)
Figure 5.27. The p-Mulitplier (Pm)(FHWA GEC 8 from FHWA NHI-05-042).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISLATERAL PILE GROUPS
(USING LPILE,FHWA NHI-05-041)
Figure 5.27. Typical Plots of Load versus Deflection and Bending Moment versus Deflection for Pile
Group Analysis (FHWA NHI-05-042 adapted from Brown and Bollman, 1993).
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).
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14.528 DRILLED DEEP FOUNDATIONSGroup, Uplift, and Lateral Analyses
LATERAL ANALYSISFHWA DRILLED SHAFT LATERAL LOADING
DESIGN PROCESS
Figure 12-13. Drilled Shaft Design Process for Lateral Loads
(FHWA NHI-10-016)
