3. 6
Overview
● Shallow vs Deep
Foundations
– A deep foundation is one
where the depth of
embedment is larger than
2X the foundation width.
4. 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
5. 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 0.8
Consolidation 1.2 0.1 0.4 0.2
Total 5.3 1.0 2.7 1.0
22. 18
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
23. 21
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.
24. 22
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.
25. 23
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
26. 24
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
Normal
Somewhat
variable Good None Average
Poor Erratic Good None Variable
Very Poor V. Erratic Limited None Limited
27. 25
Factors of Safety for Deep
Foundations for Downward Loads
Design Factor of Safety, F
Classification
of Structure
Acceptable
Probability of
Failure
Good
Control
Normal
Control
Poor
Control
Very Poor
Control
Monumental 1E-05 2.3 3.0 3.5 4.0
Permanent 1E-04 2.0 2.5 2.8 3.4
Temporary 1E-03 1.4 2.0 2.3 2.8
Expanded from Reese and O’Neill, 1989.
28. 26
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
29. 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
30. Axial Pile Capacity
● In general:
● Three general cases shown (from Das)
30
F
A
f
A
q
F
P
P
P s
s
e
e
s
e
a
∑
+
′
=
+
′
=
32. 32
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
33. 33
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
34. 34
How many load tests?
● From Engel (1988):
Length of
Piling (ft)
Length of
Piling (m)
Number of
Load Tests
0-6000 0-1800 0
6000-10000 1800-3000 1
10000-20000 3000-6000 2
20000-30000 6000-9000 3
30000-40000 9000-12000 4
35. 35
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
36. 36
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 BN
e D q
' .
= ′ − +
σ γ γ
1 0 5
37. 37
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
38. Vesic/Kulhawy Method
● Based on Vesic’s work, Kulhawy gives the
two bearing capacity factors:
38
( ) φ
σ
ν tan
1
2 D
s
r
E
I
′
+
=
( ) φ
σ
ν tan
1
2 D
s
r
E
I
′
+
=
39. 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
40. 40
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
fs h s
= ′
σ φ
tan
′ =
=
σ
φ
h horizontal effective stress
tan coef. of friction between soil and pile
s
′ = ′
σ σ
h v
K
41. 41
General Method (Kulhawy)
● rewrite equation:
● Suggest using:
f K
K
K
s v
s
= ′
σ φ
φ
φ
0
0
tan
Pile & Soil Types φs/φ
Sand/Rough concrete 1.0
Sand/Smooth concrete 0.8-1.0
Sand/Rough steel 0.7-0.9
Sand/Smooth steel 0.5-0.7
Sand/timber 0.8-0.9
Foundation Type &
Construction Method
K/K0
Jetted pile ½ -2/3
Drilled shaft 2/3 - 1
Pile-small displacemnt ¾-1¼
Pile-large displacement 1 – 1.2
( ) φ
φ ′
′
−
= sin
0 sin
1 OCR
K
42. 42
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
D
r
r
where is the relative density in decimal form
43. 43
Coyle & Castello’s Method
● empirical correlation
of fs to φ and z/B.
● z is depth to midpoint
of strata.
● CAUTION: No effect of
pile material, installation
effects, and initial insitu
stresses
44. 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
45. 45
End Bearing
● most engineers use:
● not adhesion but rather frictional behavior
● could use cohesionless equation but
problems again with K0 therefore use β
method.
′ =
q s
s
e u
u
9
where = undrained shear strength
Skin Friction
46. 46
β Method for Clay
● use Randolph
and Wroth
(1982):
● upper limit:
β
φ
≤ +
tan2
45
2
47. 47
Traditional Methods
● 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.
48. 48
In-Situ Soil Test Methods
● can determine φ or 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. 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
50. Meyerhof Method
● End Bearing:
For sands and gravels:
For nonplastic silts:
′ = ′ ≤ ′
′ = ′ ≤ ′
q N
D
B
N
q N
D
B
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
s
r
s
r
=
=
σ
σ
50
100
60
60
● Skin Friction:
NOTE N
N
r
:σ =
′
1 60
60
tsf; = SPT N corrected for field procedures;
= SPT N corrected for field procedures and overburden stress
50
51. 51
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
52. 52
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)
53. 53
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.
54. 54
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.
55. 55
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
56. Group characteristics
● Common C-C spacing: 2.5 to 3.0 diameters
● Efficiency:
( )
η = =
′+
Group Capacity
Sum of Individual Piles
P F
N P P
ag
e s
where:
group efficiency factor
net allowable capacity of pile group
factor of safety
number of piles in group
net end bearing capacity of single pile
skin friction capacity of single pile
η =
=
=
=
′ =
=
P
F
N
P
P
ag
e
s
56
58. 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
59. 59
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. If
predrilling or jetting used, efficiency may drop
below 1.0.
61. 61
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.
62. 62
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
63. 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
64. 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.
65. 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
68. Summary Instructions
for
Laterally Loaded Piles
by
B. Broms
Cohesive Soil:
Cohesionless Soil:
(a)
(b)
Short-Free:
( )
H
dg c
e d f
u
u
=
+ +
2 25
15 05
2
.
. .
or Fig (a)
where f
H
c d
u
u
=
9
and L d f g
= + +
15
.
If M dg c
yield u
≤ 2 25 2
. then pile has one plastic
hinge and is “long”.
Long-Free:
( )
H
M
e d f
u
yield
=
+ +
15 05
. .
or Fig (b)
Check if ( )
M H L d
yield u
> +
05 0 75
. . . If so, pile is
short, else pile is intermediate or long.
Then if M c dg
yield u
> 2 25 2
. then pile is
intermediate, else pile is long.
Short-Fixed: ( )
H c d L d
u u
= −
9 15
. or Fig (a)
Intermediate-Fixed: H
c dg M
d f
u
u yield
=
+
+
2 25
15 05
2
.
. .
Long-Fixed: H
M
d f
u
yield
=
+
2
15 05
. .
or Fig (b)
Short-free: H
dK L
e L
u
p
=
+
05 3
. γ
or Fig (a)
Long-free: H
M
e f
u
yield
=
+ 0 67
.
or Fig (b)
where f
H
dK
u
p
= 082
.
γ
Check if M dK L
yield p
> γ 3
. If so, pile is short,
else pile is intermediate or long.
Then if Myield > the moment at depth f, then
pile is intermediate, else pile is long.
Short-fixed: H L dK
u p
= 15 2
. γ or Fig (a)
Interm.-fixed: H L dK
M
L
u p
yield
= +
05 2
. γ
Long-fixed: H
M
e f
u
yield
=
+
2
067
.
or Fig (b)
69. 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.
70. 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
71. 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.
72. 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
73. 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.
