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Foundation Engineering
Pile Foundations
Carsten H. Floess, PE
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Deep Foundations
Driven Piles
Drilled Piles
Drilled Piers (Drilled Shafts;
Caissons)
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Deep Foundations: Applications
1.To transmit loads through weak soils todeeper competent soils
2.To transmit foundation loads below scourlevel
3.To provide support in areas where shallow
foundations are impractical; e.g.,
waterfront structures
4.To provide uplift resistance and/or lateral
load capacity
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Pile Foundations: History
Used for more than 2000 years
Alexander the Great - City of Tyre, 330 BC
Romans used piles extensively
Chinese bridge builders - Han Dynasty,
200 BC to 200 AD
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Pile Drivers: Early Builders
from Chellis
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Pile Drivers: Middle Ages
from Chellis
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Pile Drivers: Wheel Power
from Chellis
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Pile Driving: Modern
Montgomery
County
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Pile Capacity
Structural Capacity
Soil Capacity
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Design Procedure: Soil Capacity
Static Analysis (Desk top)
Load Test in Field Driving Resistance
Static Load Test
Dynamic Load Test
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Static Analysis
Qfriction
Qtip
Qultimate
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Static Analysis
Qult= Qtip +Qfriction
Qult= (qult Atip)+(f Asurface)
where, qult= ultimate tip bearing
capacity
Atip
= area of pile tip
f = unit friction factor = Htan #
Asurface= surface area of pile
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Static Analysis
Methods:
1.DM-7
2.Corps of Engineers
3.Meyerhof (Granular Soil)
4.Nordlund (Granular Soil)
5.Tomlinson (Cohesive Soil) $method6.%method (Granular & Cohesive Soil)
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Static Analysis: Granular Soils
Pile Tip Resistance:
qult= & D Nq+ !& B N& (Bearing Capacity Equation)
B is small; therefore, ignore the B term
qult= & D Nq
Note: Nqvalues for driven piles are higher than Nqvalues for footings because the soil around the piletip is compacted by installing the piles.
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Static Analysis: Granular Soils
Side Friction (ultimate):
f = Htan # = K Vtan #
where, K = coefficient of lateral pressure
V= vertical effective stress
#= angle of interface friction
between pile and soil
Total Friction = 'K Vtan #Asurface
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Lateral Earth Pressures:
Lateral Earth Pressures are determined using aCoefficient of Lateral Earth Pressure, K:
K = H/V
or, H = K V
where, H = lateral effective earth pressure
V= vertical effective pressure
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Lateral Earth Pressures:
Three general conditions:
At-rest, Ko (no lateral movement)
Active, Ka (movement away from soil)
Passive, Kp (movement toward soil)
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At-Rest Conditions
Korefers to case where there is no lateralmovement or strain
Examples:
In the ground (level ground surface)
A stiff, unyielding wall
Ko= 1-sin( J. Jaky (1948)
Ka"0.5
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At-Rest Conditions: Example
What is lateral earth pressure 10 below ground surface?
V= 1200 psf
H = 480 psf
10
Sand
&= 120 pcf
(= 37
V= V= 10 120 pcf = 1200 psfKo= 1 sin 37 = 0.398
H= H= 0.398 1200 psf = 480 psf
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At-Rest Conditions: Example (cont)
What if water table is at ground surface?
(V)total= 10 120 pcf = 1200 psf
w = 10 62.4 pcf = 624 psf
V= 10 (120-62.4) pcf = 576 psf
or, V= 1200 psf 624 psf = 576 psf
H= Ko V= 0.398 x 576 = 229 psf
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At-Rest Conditions: Example (cont)
What is lateral earth pressure 10 below ground surface?
V= 576 psf
w= 624 psf
H= 229 psf
w= 624 psf
10Sand
&= 120 pcf
(= 37
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Active Conditions
Karefers to the case where a wall moves away fromthe retained earth. The soil will move downward
and outward. Lateral earth pressure will decrease to
a minimum value known as the active state.
Failure zone
Failure surface
(approximately a plane)wall
movement
45 + (/2
Ka"0.3
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Passive Conditions
KpRefers to the case where a wall moves toward theretained earth. The soil will move upward and
inward. Lateral earth pressure will increase to a
maximum value known as the passive state.
Failure zone
Failure surface
(generally not a plane)
wall
movement
45 - (/2
Kp"3
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Active & Passive Pressures
Active
Pressure
PassivePressure
Movement
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Static Analysis: Granular Soils
Values of K for pile analysis:
Installing piles displaces soil outward away
from the pile. This displacement
(squeezing) tends to increase lateralpressures against the pile. The
magnitude of the lateral pressure against
the pile is a function of the volume of soil
displaced by the pile.
Generally, K )1 to 2 (i.e., between at-rest
and passive conditions)
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Static Analysis: Granular Soils
General Procedure:
1.Compute Vas a function of depth. Assume
V
remains constant below a depth of )20 B.
(Actual depth varies, depending on analysis
method)
2.Determine K for the given pile type
3.Determine tan #for the given pile & soil type
4.Determine ultimate tip bearing capacity factor,
Nq
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Static Analysis: DM-7, Granular Soils
Earth Pressure
Coefficients
KHC & KHT:
Pile Type KHC KHT
Driven H-Pile 0.5 1.0 0.3-0.5
DrivenDisplacement Pile
1.0-1.5 0.6-1.0
Driven
DisplacementTapered Pile
1.5-2.0 1.0-1.3
Drilled Pile
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Static Analysis: DM-7, Granular Soils
Friction
Angle #:
Pile Type #
Steel 20
Concrete 3/4(
Timber 3/4(
Note: Limiting Depth for analysis = 20B
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Static Analysis: DM-7, Granular Soils
Bearing Capacity Factor Nq:
(
degrees
26 28 30 31 32 33 34 35 36 37 38 39 40
NqDisplacement
pile
10 15 21 24 29 35 42 50 62 77 86 120 145
NqDrilled pile 5 8 10 12 14 17 21 25 30 38 43 60 72
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Static Analysis: COE, Granular Soils
EarthPressure
Coefficients
KC & KT:
Soil Type KC KT
Sand 1.00 to 2.00 0.5 to 0.7
Silt 1.00 0.5 to 0.7
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Static Analysis: COE, Granular Soils
Friction
Angle #:
Pile Material #
Steel 0.67(to 0.83(
Concrete 0.90(to 1.0(
Timber 0.80(to 1.0(
Note: Limiting Depth for analysis:
Dc= 10B Loose sand
Dc= 15B Medium dense sand
Dc= 20B Dense sand
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Static Analysis: COE, Granular Soils
Bearing CapacityFactor Nqis
presented in chart
format as afunction of (
Static Analysis:
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Static Analysis:
Meyerhof, Granular Soils
Tip Resistance:
qtip= 0.4(N1)D/b < qlim
where: qtip= ultimate tip resistance, tsf
N1= blow count, corrected
D = pile embedment into bearing stratum, ft
b = pile diameter, ft
qlim= limiting point resistance = 4 N1(sand)= 3 N1(silt)
Note: N1represents the corrected blow count withinabout 3 pile diameters below the pile tip
Static Analysis:
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Static Analysis:
Meyerhof, Granular Soils
Unit Side Friction Resistance:
f= N1/50 < ql displacement pile
f= N1/100 < ql non-displacement pile (H-pile)
Where: f = ultimate skin friction, tsf
N1= blow count, corrected
ql= limiting skin friction = 1 tsf for driven pile
Note: N1represents soil along the pile shaft in thebearing zone. Subdivide into layers as needed.
Static Analysis:
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Static Analysis:
$Method, Cohesive Soils
Pile Tip Resistance:
qtip
= c Nc (net ultimate bearing capacity)
Adhesion along side of pile:
adhesion = $ cwhere, $= adhesion factor
c = undrained shear strength
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$values, COE
39
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40
$values,
Tomlinson
Static Analysis:
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Static Analysis:
$Method, Cohesive Soils
Ultimate Pile Capacity:
Qult= (c NcAtip) + #($ c Asurface)
Static Analysis:
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Static Analysis:
$Method, Cohesive Soils
General Procedure:
1. Delineate soil profile into layers and determine
the undrained shear strength for each layer
2. For each layer, compute the unit shaft resistance
= $ c
3. Sum the shaft resistances for each layer4. Add the tip resistance = c NcAtip
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Static Analysis:
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Static Analysis:
%Method, All Soils
Side frictionf = Htan # = K Vtan #
f = % V
where, %= K tan #V= average effective stress along pile shaft
#= interface friction between pile and soil
K = lateral earth pressure coefficientf = unit shaft resistance
Static Analysis:
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Static Analysis:
%Method, All Soils
Tip resistance
qt= Ntpt
where, Nt= toe bearing capacity coefficientpt = effective pressure at pile toe
qt= unit toe resistance
Note: Analysis based on effective stress
Static Analysis:
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Static Analysis:
%
Method, All Soils
Soil Type ( % Nt
Clay 25-30 0.23-0.40 3-30
Silt 28-34 0.27-0.50 20-40
Sand 32-40 0.30-060 30-150
Gravel 35-45 0.35-0.80 60-300
Approximate range of%
and Ntcoefficients (FHWA, Fellenius)
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Piles on Rock
Piles bearing on rock can normally carry high loads.
Piles on rock of fair to excellent quality (RQD > 50%)will support high loads, generally exceeding the
structural capacity of the piles.
Piles on soft, weathered rock, such as shale, or rockof very poor to poor quality (RQD
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Pile Design Procedure
1. Assume pile length2. Compute capacity (static analysis)
3. Compare with required capacity
4. Re-analyze again, if necessary. Plot data
Qult
length
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H-Piles: Which Surface Should be Used?
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H-Piles: Which Surface Should be Used?
Use block circumference for computing sidefriction.
Use block tip area in cohesive soils (assumes adense soil plug is formed at the tip)
Use steel tip area (or partial plug) in coarsecohesionless soil.
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Strain Compatibility
The equation for ultimate pile capacity assumes that
both the pile tip and the pile shaft have moved
sufficiently relative to the soil to simultaneously
develop shaft friction and toe resistance.
Generally, the displacement needed to mobilize shaft
resistance is smaller that that required to mobilize tip
resistance.
Nevertheless, this simple approach is commonly
used for all but very large diameter piles.