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Pile & Pier Foundation Analysis & Design
byPeter J. Bosscher
University of Wisconsin-Madison
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Topic Outline
● Overview● Axial Load Capacity● Group Effects● Settlement
6
Overview
● Shallow vs Deep Foundations– A deep foundation is one
where the depth of embedment is larger than 2X the foundation width.
7
Historic Perspective
• one of the oldest methods of overcoming the difficulties of founding on soft soils• Alexander the Great, 332BC in Tyre
• “Amsterdam, die oude Stadt, is gebouwed op palen, Als die stad eens emmevelt, wie zal dat betalen?” an old Dutch nursery rhyme
• “If in doubt about the foundation, drive piles.” 1930-1940 practice methodology
8
Contrast in Performance
● Example– deep clay
» cu = 500 psf
– Load = 340 kips– Factor of Safety = 2
Settlements at working load Pad Single Pile Pile & Pad 4-Pile Grp.Immediate 4.1 0.9 2.3 0.8Consolidation 1.2 0.1 0.4 0.2Total 5.3 1.0 2.7 1.0
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Modern Uses● weak upper soils
– shallow (a)– deep (b)
● large lateral loads (c)● expansive &
collapsible soils (d)● uplift forces (e)● bridge abutments &
piers (f)
Foundation Design Process(FHWA)
Foundation Design Process
Continued(FHWA)
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FoundationClassification
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Pile Types
• Timber Piles• Steel H-Piles• Steel Pipe Piles• Precast Concrete
Piles• Mandrel-Driven Piles• Cast-in-Place
Concrete Piles
• Composite Piles• Drilled Shafts• Augered, Pressure
Injected Concrete Piles
• Micropiles• Pressure Injected
Footings
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Timber Piles
13
Steel H-Piles
14
Steel Pipe Piles
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Precast Concrete Piles
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Mandrel-Driven Piles
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Cast-in-place Concrete Piles
Composite Piles
Drilled Shafts
Augered, Pressure Injected Concrete Piles
Micropiles
Pressure Injected Footings
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Evaluation of Pile Types• Load Capacity & Pile Spacing• Constructability
• soil stratigraphy• need for splicing or cutting• driving vibrations• driving speed (see next slide)
• Performance• environmental suitability (corrosion)
• Availability• Cost
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Soil Properties for Static Pile Capacity
● Proper subsurface investigations yield critical information regarding stratigraphy and also provide quality soil samples.
● Boring depths minimally should extend 20 feet beyond the longest pile. Looking for critical information such as soft, settlement prone layers, or other problem soils such as cobbles. Want additional information from in-situ field tests (SPT and CPT). Location of groundwater table is critical.
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Soil Properties for Static Pile Capacity, cont.
● From soil samples, determine shear strength and consolidation properties. For clays, both quick and long term strengths (from UU and CU/CD) should be determined. For sands, only CD tests are used.
● For clays, the pile capacities in the short and long terms should be compared and the lower of the two cases selected for use. If the design is verified by pile load tests, these results will usually dominate the final design.
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Factor of Safety
● Depends on many factors, including:– type and importance of the structure– spatial variability of the soil– thoroughness of the subsurface investigation– type and number of soil tests– availability of on-site or nearby full-scale load
tests– anticipated level of construction monitoring– probability of design loads being exceeded
during life of structure
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Classification of Structure & Level of Control
● Structure:– monumental: design life > 100 years– permanent: design life >25 yrs and < 100 yrs– temporary: design life < 25 yrs
● Control:Control
Subsurface Conditions
Subsurface Exploration
Load Tests
Construction Monitoring
Good Uniform Thorough Available Good
NormalSomewhat
variable Good None AveragePoor Erratic Good None VariableVery Poor V. Erratic Limited None Limited
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Factors of Safety for Deep Foundations for Downward Loads
Design Factor of Safety, F
Classification of Structure
Acceptable Probability of
FailureGood
ControlNormal Control
Poor Control
Very Poor Control
Monumental 1E-05 2.3 3.0 3.5 4.0Permanent 1E-04 2.0 2.5 2.8 3.4Temporary 1E-03 1.4 2.0 2.3 2.8
Expanded from Reese and O’Neill, 1989.
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Methods for Computing Static Pile Capacity
● Allowable Stresses in Structural Members● Pile Capacity
– Many different methods (α, β, λ, Meyerhof, Vesic, Coyle & Castello, etc).
– Soil Type (Cohesionless, Cohesive, Silt, Layered Soils)– Point Bearing– Skin Resistance
» Normal (Positive) Skin Friction» Negative Skin Friction
● Settlement of Piles
Allowable Stresses in Structural Members
• Any driven pile has to remain structurally intact and not be stressed to its structural limit during its service life under static loading conditions as well as under dynamic driving induced loads. Therefore, material stress limits are placed on:
• The maximum allowable design stress during the service life.• The maximum allowable driving stresses.
• Additional material stress limits, beyond the design and driving stress limits, may apply to prevent buckling of piles when a portion of the pile is in air, water, or soil not capable of adequate lateral support. In these cases, the structural design of the pile should also be in accordance with the requirements of Sections 8, 9, 10, and 13 of AASHTO code (1994) for compression members.
• See excerpt from FHWA’s Design and Construction of Driven Pile Foundations
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Axial Pile Capacity
● In general:
● Three general cases shown (from Das)
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FAfAq
FPPP sseese
a∑+′
=+′=
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Methods of Evaluating Axial Load Capacity of Piles
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Full-Scale Load Tests
● Most precise way to determine axial load capacity. All other methods are indirect.
● Quite expensive thus use judiciously.● Two types: controlled stress or controlled
strain, also quick and slow versions.● Results are open to interpretation:
– 9 methods to analyze results
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When to use Full-scale Load Tests● many piles to drive● erratic or unusual soil conditions● friction piles in soft/medium clay● settlement is critical● engineer is inexperienced● uplift loads on piles
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How many load tests?
● From Engel (1988):Length ofPiling (ft)
Length ofPiling (m)
Number ofLoad Tests
0-6000 0-1800 06000-10000 1800-3000 1
10000-20000 3000-6000 220000-30000 6000-9000 330000-40000 9000-12000 4
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Static Methods(Based on Soil Tests or In-situ Tests)
● More difficult to interpret than load tests:– pile driving changes soil properties– soil-structure interaction is complex
● Less expensive than load tests● Used for:
– preliminary analysis to plan pile load testing– extend results of pile load testing– design purposes on small projects
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Cohesionless Soil● no excess pore pressure● End Bearing:
– many use shallow bearing capacity formulas
– use – but real piles do not behave
like shallow foundations where capacity increases linearly with depth.
( )q N BNe D q' .= ′ − +σ γ γ1 0 5
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Max Limit on End Bearing?● Some suggest a limit on end
bearing to match experience.● Problems with that approach:
– more complex than that; need to consider both strength and compressibility of the soil
– friction angle varies with effective stress
– overconsolidation causes changes in bearing capacity
Vesic/Kulhawy Method● Based on Vesic’s work, Kulhawy gives the
two bearing capacity factors:
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( ) φσν tan12 Dsr
EI′+
=
( ) φσν tan12 Dsr
EI′+
=
39
Coyle & Castello’s Method
● Based on 16 pile load tests
● Based on φ and D/B.
● CAUTION: No effect of pile material, installation effects, and initial insitu stresses
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Cohesionless Soil
● Skin (Side) Friction– use a simple sliding model:
» where
» often rewrite using» K varies with:
● amount of soil displacement● soil consistency● construction techniques
f s h s= ′σ φtan′ =
=σ
φh horizontal effective stress
tan coef. of friction between soil and piles
′ = ′σ σh vK
41
General Method (Kulhawy)
● rewrite equation:
● Suggest using:
f KKKs v
s= ′
σ φ
φφ0
0tan
Pile & Soil Types φs/φSand/Rough concrete 1.0Sand/Smooth concrete 0.8-1.0
Sand/Rough steel 0.7-0.9Sand/Smooth steel 0.5-0.7
Sand/timber 0.8-0.9
Foundation Type &Construction Method
K/K0
Jetted pile ½ -2/3Drilled shaft 2/3 - 1
Pile-small displacemnt ¾-1¼Pile-large displacement 1 – 1.2
( ) φφ ′′−= sin0 sin1 OCRK
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Simplistic β Method
● lumps K and tanφ into one term: β=Ktanφs
● can develop site-specific β or use empirical formulas in literature.
● Eg: for large displacement piles in sand,Bhushan (1982)suggests: β = +018 0 65. . D
Dr
rwhere is the relative density in decimal form
43
Coyle & Castello’s Method
● empirical correlation of fs to φand z/B.
● z is depth to midpoint of strata.
● CAUTION: No effect of pile material, installation effects, and initial insitustresses
44
Cohesive Soil● excess pore pressures produced by soil
displacement during driving takes time to dissipate. This means capacity increases with time. Usually assume full capacity is achieved by the time the full dead load is applied.
● but usually need to consider live load too.– end bearing affected by live load (soil compression)
» use undrained strength if significant live load– side friction not affected
» use drained strength always
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End Bearing
● most engineers use:
● not adhesion but rather frictional behavior● could use cohesionless equation but
problems again with K0 therefore use βmethod.
′ =q ss
e u
u
9where = undrained shear strength
Skin Friction
46
β Method for Clay
● use Randolph and Wroth (1982):
● upper limit:
βφ
≤ +
tan2 45
2
47
Traditional Methods
● a large number of engineers still use “adhesion” concepts.
● The α and λ methods are based onundrained strength. See Sladen (1992) for an analysis of these methods.
● These methods have wide scatter, sometimes being as low as 1/3 or as high as 3 times the actual capacity.
48
In-Situ Soil Test Methods
● can determine φor su and then use previous methods or can use direct correlation methods.
● direct in-situ methods especially important for sand as sampling and testing is difficult.
● In-situ tests:– SPT & CPT
49
Standard Penetration Test
● SPT is inconsistent thus correlation is less reliable than CPT.
● Two methods (for sand only): Meyerhof &Briaud
● SPT does not seem reliable for clays
Meyerhof Method
● End Bearing:For sands and gravels:
For nonplastic silts:
′ = ′ ≤ ′
′ = ′ ≤ ′
q NDB
N
q NDB
N
e r r
e r r
0 40 4 0
0 40 30
60 60
60 60
. .
. .
σ σ
σ σ
For large displacement piles:
For small displacement piles:
f N
f N
sr
sr
=
=
σ
σ
50
100
60
60
● Skin Friction:
NOTE NN
r:σ =′
1 60
60
tsf; = SPT N corrected for field procedures;= SPT N corrected for field procedures and overburden stress
50
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Briaud Method
● based on regression analyses:
( )( )
′ =
=
q N
f N
e r
s r
19 7
0 224
60
0 36
60
0 29
.
.
.
.
σ
σ
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CPT Correlations
● the CPT is very similar to driving piles therefore this test is a good predictor of capacity.
● unfortunately, the test is rarely run in the U.S. because of the inertia of the engineering community.
● for correlations based on CPT see Coduto(1994)
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From Karl Terzaghi, 1943
“The problems of soil mechanics may be divided into two principal groups - the stability problems and the elasticity problems.”
● Bearing capacity is a stability problem, settlement is an elastic problem.
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Pile Settlement
● Isolated piles designed using the previously mentioned methods usually settle less than 0.5 inches at their working loads. Pile groups may settle somewhat more but generally within acceptable limits. Most engineers do not conduct a settlement analysis unless:
– the structure is especially sensitive to settlement,– highly compressible strata are present,– sophisticated structural analyses are also being used.
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Why put piles in groups?
● Single pile capacity is insufficient● Single pile location may not be sufficiently
accurate to match column location● To build in redundancy● Increased efficiency gained by multiple
piles driven in close proximity
Group characteristics
● Common C-C spacing: 2.5 to 3.0 diameters● Efficiency:
( )η = =′ +
Group CapacitySum of Individual Piles
P FN P P
ag
e swhere:group efficiency factor
net allowable capacity of pile groupfactor of safetynumber of piles in groupnet end bearing capacity of single pileskin friction capacity of single pile
η ==
==
′ ==
PFNPP
ag
e
s
56
57
Individual vs Block Failure Mode
s
Individual Failure Mode Block Failure Mode
Group characteristics● Do not use Converse-Labarre formula for
group efficiency (not accurate)● From O’Neill (1983):
– in loose cohesionless soils, η > 1 and is highest at s/B = 2. Increases with N.
– in dense cohesionless soils at normal spacings(2 < s/B < 4), η is slightly greater than 1 if the pile is driven.
– in cohesive soils, η < 1. Cap in contact w/ ground increases efficiency but large settlement is required. 58
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Design Guidelines
● Use engineering judgment - no good recipes● Block failure not likely unless s/B<2● In most cohesive soil, if s/B>2, eventual η ≅
1.0 but early values range from 0.4 to 0.8.● In cohesionless soils, design for η between 1.0
and 1.25 if driven piling w/o predrilling. Ifpredrilling or jetting used, efficiency may drop below 1.0.
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Negative skin friction
● Occurs when upper soils consolidate, perhaps due to weight of fill.
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Negative skin friction● The downward drag due to negative skin friction
may occur in the following situations:– consolidation of surrounding soil– placement of a fill over compressible soil– lowering of the groundwater table– underconsolidated soils– compaction of soils
● This load can be quite large and must be added to the structural load when determining stresses in the pile. Negative skin friction generally increases pile settlement but does not change pile capacity.
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Methods to reduce downdrag
● Coat piles w/ bitumen, reducing φs
● Use a large diameter predrill hole, reducing lateral earth pressure (K)
● Use a pile tip larger than diameter of pile, reducing K
● Preload site with fill prior to driving piling
Laterally Loaded Deep Fnds
● Deep foundations must also commonly support lateral loads in addition to axial loads.
● Sources include:– Wind loads– Impacts of waves & ships on marine structures– Lateral pressure of earth or water on walls– Cable forces on electrical transmission towers
From Karl Terzaghi, 1943
“The problems of soil mechanics may be divided into two principal groups - the stability problems and the elasticity problems.”
Ultimate lateral load capacity is a stability problem, load-deformation analysis is similar to an elasticity problem.
Ultimate Lateral Load
● Dependent on the diameter and length of the shaft, the strength of the soil, and other factors.
● Use Broms method (1964, 1965)● Divide world into:
– cohesive & cohesionless– free & fixed head– 0, 1, or 2 plastic hinges
Cohesive Soil Diagrams
LateralResistance
Free-HeadDistributions Fixed-Head
Distributions
Cohesionless Soil Diagrams
Free-Head DistributionsFixed-Head Distributions
Summary Instructionsfor
Laterally Loaded Pilesby
B. BromsCohesive Soil:
Cohesionless Soil:
(a)
(b)
Short-Free: ( )Hdg c
e d fuu=
+ +2 25
15 05
2.
. . or Fig (a)
where fH
c du
u
=9
and L d f g= + +15.
If M dg cyield u≤ 2 25 2. then pile has one plastic
hinge and is “long”.
Long-Free: ( )HM
e d fu
yield=+ +15 05. .
or Fig (b)
Check if ( )M H L dyield u> +0 5 0 75. . . If so, pile is
short, else pile is intermediate or long.
Then if M c dgyield u> 2 25 2. then pile is
intermediate, else pile is long.
Short-Fixed: ( )H c d L du u= −9 15. or Fig (a)
Intermediate-Fixed: Hc dg M
d fu
u yield=+
+2 25
15 05
2.
. .
Long-Fixed: HM
d fu
yield=+
2
15 0 5. . or Fig (b)
Short-free: HdK L
e Lu
p=+
0 5 3. γ or Fig (a)
Long-free: HM
e fu
yield=+ 0 67.
or Fig (b)
where fH
dKu
p
= 0 82. γ
Check if M dK Lyield p> γ 3 . If so, pile is short,
else pile is intermediate or long.Then if M yield > the moment at depth f, then
pile is intermediate, else pile is long.
Short-fixed: H L dKu p= 15 2. γ or Fig (a)
Interm.-fixed: H L dKM
Lu p
yield= +05 2. γ
Long-fixed: HM
e fu
yield=+2
0 67. or Fig (b)
Load-Deformation Method
● Due to the large lateral deflection required to mobilize full lateral capacity, typical design requires a load-deformation analysis to determine the lateral load that corresponds to a certain allowable deflection.
● Considers both the flexural stiffness of the foundation and the lateral resistance from the soil.
● Main difficulty is accurate modeling of soil resistance.
p-y Method● Can handle:
– any nonlinear load-deflection curve– variations of the load-deflection curve w/ depth– variations of the foundation stiffness (EI) w/ depth– elastic-plastic flexural behavior of the foundation– any defined head constraint
● Calibrated from full-scale load tests● Reese (1984, 1986) are good references.● Requires computer program
COM624P● COM624P -- Laterally Loaded Pile Analysis Program for
the Microcomputer, Version 2.0. Publication No. FHWA-SA-91-048.
● Computer program C0M624P has been developed for analyzing stresses and deflection of piles or drilled shafts under lateral loads. The technology on which the program is based is the widely used p-y curve method. The program solves the equations giving pile deflection, rotation, bending moment, and shear by using iterative procedures because of the nonlinear response of the soil.
p-y Method: Chart solutions
● Evans & Duncan (1982) developed chart solutions from p-y computer runs.
● Advantages:– no computer required– can be used to check computer output– can get load vs max moment and deflection
directly
Group Effects
● Complexities arise:– load distribution amongst piles in group– differences between group effect and single pile
● O’Neill (1983) has identified an important characteristic: pile-soil-pile interaction (PSPI). Larger interaction in closely spaced piles.
● Lateral deflection of pile group is greater than single isolated pile subjected to proportional share of load.
