Pile & Pier Foundation Analysis & Design by Peter J. Bosscher University of Wisconsin-Madison
Topic Outline q
Overview
q
Axial Load Capacity
q
Group Effects
q
Settlement
5
Overview q
Shallow vs Deep Foundations –
A deep foundation is one where the depth of embedment is larger than 2X the foundation width.
6
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
7
Contrast in Performance q
Example –
deep clay »
cu = 500 psf
–
Load = 340 kips
–
Factor of Safety = 2
Settlements at working load Immediate Consolidation Total
Pad 4.1 1.2 5.3
Single Pile 0.9 0.1 1.0
Pile & Pad 4-Pile Grp. 2.3 0.8 0.4 0.2 2.7 1.0 8
Modern Uses q
q q
q q
weak upper soils – shallow (a) – deep (b) large lateral loads (c) expansive & collapsible soils (d) uplift forces (e) bridge abutments & piers (f)
9
Foundation Design Process (FHWA)
Foundation Design Process Continued (FHWA)
Foundation Classification
10
Pile Types •
Timber Piles
•
Composite Piles
•
Steel H-Piles
•
Drilled Shafts
•
Steel Pipe Piles
•
•
Precast Concrete Piles
Augered, Pressure Injected Concrete Piles
•
Mandrel-Driven Piles
•
Micropiles
•
Cast-in-Place Concrete Piles
•
Pressure Injected Footings
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Timber Piles
12
Steel H-Piles
13
Steel Pipe Piles
14
Precast Concrete Piles
15
Mandrel-Driven Piles
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Cast-in-place Concrete Piles
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Composite Piles
Drilled Shafts
Augered, Pressure Injected Concrete Piles
Micropiles
Pressure Injected Footings
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
18
Soil Properties for Static Pile Capacity q
Proper subsurface investigations yield critical information regarding stratigraphy and also provide quality soil samples.
q
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. 21
Soil Properties for Static Pile Capacity, cont. q
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.
q
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. 22
Factor of Safety q
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 23
Classification of Structure & Level of Control q
q
Structure: –
monumental: design life > 100 years
–
permanent: design life >25 yrs and < 100 yrs
–
temporary: design life < 25 yrs
Control: Subsurface Subsurface Load Construction Control Conditions Exploration Tests Monitoring G oo d Uniform Thorough Available G oo d Somewhat Normal variable G o od None Average P oo r Erratic G o od None Variable Very Poor V. Erratic Limited None Limited 24
Factors of Safety for Deep Foundations for Downward Loads Design Factor of Safety, F Acceptable Classification Probability of Good Normal Poor Very Poor Control Control Control Control of Structure Failure Monumental 1E - 0 5 2. 3 3.0 3. 5 4. 0 Permanent 1E-04 2.0 2.5 2. 8 3.4 Temporary 1E - 0 3 1. 4 2.0 2.3 2.8
Expand Expanded ed from from Reese Reese and O’Neil O’Neill, l, 1989. 1989.
25
Methods for Computing Static Pile Capacity q
Allowable Stresses in Structural Members
q
Pile Capacity –
q
Many different methods (α, β, λ, Meye Meyerh rhof of,, Vesi Vesic, c, Coyle Coyle & Castel Castello, lo, etc). etc).
–
Soil Type (Cohesionless, Cohesive, Silt, Layered Soils)
–
Point Bearing
–
Skin Resistance »
Normal (Positive) Skin Friction
»
Negative Skin Friction
Settlement of Piles 26
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 27
Axial Pile Capacity q
In general: Pa
q
=
Pe′ + Ps F
=
qe′ Ae
+ ∑ f s As F
Three general cases shown (from Das)
30
Methods of Evaluating Axial Load Capacity of Piles
31
Full-Scale Load Tests q
Most precise way to determine axial load capacity. All other methods are indirect.
q
Quite expensive thus use judiciously.
q
Two types: controlled stress or controlled strain, also quick and slow versions.
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Results are open to interpretation: –
9 methods to analyze results
32
When to use Full-scale Load Tests q
many piles to drive
q
erratic or unusual soil conditions
q
friction piles in soft/medium clay
q
settlement is critical
q
engineer is inexperienced
q
uplift loads on piles
33
How many load tests? q
From Engel (1988): Length of Piling (ft) 0-6000 6000-10000 10000-20000 20000-30000 30000-40000
Length of Number of Piling (m) Load Tests 0-1800 1800-3000 3000-6000 6000-9000 9000-12000
0 1 2 3 4 34
Static Methods (Based on Soil Tests or In-situ Tests) q
More difficult to interpret than load tests: –
pile driving changes soil properties
–
soil-structure interaction is complex
q
Less expensive than load tests
q
Used for: –
preliminary analysis to plan pile load testing
–
extend results of pile load testing
–
design purposes on small projects 35
Cohesionless Soil q
no excess pore pressure
q
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 ' e = σ D′ N q
− 1) + 0.5γ BN γ
36
Max Limit on End Bearing? Some suggest a limit on end bearing to match experience. q Problems with that approach: q
–
– –
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
37
Vesic/Kulhawy Method q
Based on Vesic’s work, Kulhawy gives the two bearing capacity factors: I r =
I r =
E
2(1 + ν s )σ D′ tan φ
E
2(1 + ν s )σ D′ tan φ
38
Coyle & Castello’s Method q
q
q
Based on 16 pile load tests Based on φ and D/B. CAUTION:
No effect of pile material, installation effects, and initial insitu stresses
39
Cohesionless Soil q
Skin (Side) Friction –
use a simple sliding model: f s »
= σ h′ tan φ s
where σ h′ = horizontal effective stress tanφ s = coef. of friction between soil and pile
′ = K σ v′
»
often rewrite using σ h
»
K varies with: q q q
amount of soil displacement soil consistency construction techniques
40
General Method (Kulhawy) K φ s tanφ q rewrite equation: f s = σ v′K 0 K 0 φ Pile & Soil Types
φs / φ
Sand/Rough concrete Sand/Smooth concrete Sand/Rough steel Sand/Smooth steel Sand/timber
1.0 0.8-1.0 0.7-0.9 0.5-0.7 0.8-0.9
q
Foundation Type & Construction Method Jetted pile Drilled shaft Pile-small displacemnt Pile-large displacement
K/K 0 ½ -2/3 2/3 - 1 ¾-1¼ 1 – 1.2
Suggest using: K 0 = (1 − sin φ ′)OCR sin φ ′ 41
Simplistic β Method q
lumps K and tanφ into one term: β=Ktanφs
β or use empirical
q
can develop site-specific formulas in literature.
q
Eg: for large displacement piles in sand, Bhushan (1982)suggests: β = 0.18 + 0.65 Dr where Dr is the relative density in decimal form
42
Coyle & Castello’s Method q
empirical correlation of f s to φ and z/B.
q
z is depth to midpoint of strata.
q
CAUTION:
No effect of pile material, installation effects, and initial insitu stresses
43
Cohesive Soil q
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.
q
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
44
End Bearing q
most engineers use: qe′ = 9 su where su = undrained shear strength
Skin Friction q
not adhesion but rather frictional behavior
q
could use cohesionless equation but problems again with K0 therefore use β method. 45
β Method for Clay q
use Randolph and Wroth (1982):
q
upper limit:
β ≤ tan
2
φ 45 + 2
46
Traditional Methods q
q
q
a large number of engineers still use “adhesion” concepts. The α and λ methods are based on undrained 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. 47
In-Situ Soil Test Methods q
can determine φ or su and then use previous methods or can use direct correlation methods.
q
direct in-situ methods especially important for sand as sampling and testing is difficult.
q
In-situ tests: –
SPT & CPT
48
Standard Penetration Test q
SPT is inconsistent thus correlation is less reliable than CPT.
q
Two methods (for sand only): Meyerhof & Briaud
q
SPT does not seem reliable for clays
49
Meyerhof Method q
End Bearing:
For sands and gravels: qe′
= 0.40 N 60′
D
σr
≤ 4.0 N 60′ σ r
B For nonplastic silts:
qe′
= 0.40 N 60′
D B
σr
:σ r = 1 tsf; NOTE
≤ 3.0 N 60′ σ r 60
q
Skin Friction:
For large displacement piles: f s
=
σ r
N 60
50 For small displacement piles:
f s
=
σ r 100
N 60
= N corrected for field procedures; N SPT
N 60 ′ = SPT N corrected for field procedures and overburden stress 50
Briaud Method q
based on regression analyses:
qe′ f s
0.36
= 19.7σ r ( N 60 )
0.29
σ r ( N 60 ) = 0224 .
51
CPT Correlations q
the CPT is very similar to driving piles therefore this test is a good predictor of capacity.
q
unfortunately, the test is rarely run in the U.S. because of the inertia of the engineering community.
q
for correlations based on CPT see Coduto (1994) 52
From Karl Terzaghi, 1943 “The problems of soil mechanics may be divided into two principal groups - the stability problems and the elasticity problems.” q
Bearing capacity is a stability problem, settlement is an elastic problem.
53
Pile Settlement q
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? q
Single pile capacity is insufficient
q
Single pile location may not be sufficiently accurate to match column location
q
To build in redundancy
q
Increased efficiency gained by multiple piles driven in close proximity
55
Group characteristics q
Common C-C spacing: 2.5 to 3.0 diameters
q
Efficiency: η = where:
Group Capacity Sum of Individual Piles
=
Pag F N( Pe′+ Ps )
η = group efficiency factor
= net allowable capacity of pile group F = factor of safety N = number of piles in group Pe′ = net end bearing capacity of single pile Ps = skin friction capacity of single pile Pag
56
Individual vs Block Failure Mode s
Individual Failure Mode
Block Failure Mode
57
Group characteristics q
Do not use Converse-Labarre formula for group efficiency (not accurate)
q
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
Design Guidelines q
Use engineering judgment - no good recipes
q
Block failure not likely unless s/B<2
q
q
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. If predrilling or jetting used, efficiency may drop below 1.0. 59
Negative skin friction q
Occurs when upper soils consolidate, perhaps due to weight of fill.
60
Negative skin friction q
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
q
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.
61
Methods to reduce downdrag φs
q
Coat piles w/ bitumen, reducing
q
Use a large diameter predrill hole, reducing lateral earth pressure (K)
q
Use a pile tip larger than diameter of pile, reducing K
q
Preload site with fill prior to driving piling
62
Laterally Loaded Deep Fnds q
Deep foundations must also commonly support lateral loads in addition to axial loads.
q
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 q
Dependent on the diameter and length of the shaft, the strength of the soil, and other factors.
q
Use Broms method (1964, 1965)
q
Divide world into: – cohesive & cohesionless – free & fixed head – 0, 1, or 2 plastic hinges
Cohesive Soil Diagrams
Lateral Resistance Free-Head Distributions
Fixed-Head Distributions
Cohesionless Soil Diagrams
Free-Head Distributions Fixed-Head Distributions
Summary Instructions for Laterally Loaded Piles by B. Broms
Cohesive Soil: Short-Free: H u
=
where f
=
If M
2.25dg 2 cu
or Fig (a)
(e + 15 . d + 0.5 f ) H u
. d+ f and L = 15
9cu d
+
g
dg2 c thenu pile has one plastic
≤yield 2.25
hinge and is “long”. Long-Free: H u
Check if M
=
M yield
or Fig (b)
(e + 15 . d + 0.5 f )
) H (0 u .5 L+ 0.75 d . If so, pile is
> yield
short, else pile is intermediate or long. Then if M
2 2.25 c dg u then pile is
> yield
intermediate, else pile is long. Short-Fixed: Hu
=
9 cu d( L − 15 . d) or Fig (a)
Intermediate-Fixed: H u Long-Fixed:
H u
=
=
2.25cu dg 2 15 . d
2 M yield 15 . d
Cohesionless Soil:
+ 0.5 f
+
+
M yield
0.5 f
or Fig (b)
Load-Deformation Method q
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.
q
Considers both the flexural stiffness of the foundation and the lateral resistance from the soil.
q
Main difficulty is accurate modeling of soil resistance.
p-y Method q
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
q
Calibrated from full-scale load tests
q
Reese (1984, 1986) are good references.
q
Requires computer program
COM624P q
COM624P -- Laterally Loaded Pile Analysis Program for the Microcomputer, Version 2.0. Publication No. FHWASA-91-048.
q
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 q
Evans & Duncan (1982) developed chart solutions from p-y computer runs.
q
Advantages: – no computer required – can be used to check computer output – can get load vs max moment and deflection directly
