6.1 pile foundations 2013

8/11/2019 6.1 Pile Foundations 2013

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Foundation ngineering

1

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



Pile Driving: Modern

Montgomery

County


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Pile Capacity

Structural Capacity

Soil Capacity



Design Procedure: Soil Capacity

Static Analysis (Desk top)

Load Test in Field Driving Resistance

Static Load Test

Dynamic Load Test



Static Analysis

Qfriction

Qtip

Qultimate



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



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)



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.




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



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



Lateral Earth Pressures:

Three general conditions:

At-rest, Ko (no lateral movement)

Active, Ka (movement away from soil)

Passive, Kp (movement toward soil)



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



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



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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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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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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Friction

Angle #:

Pile Type #

Steel 20

Concrete 3/4(

Timber 3/4(

Note: Limiting Depth for analysis = 20B


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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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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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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:


Ultimate Pile Capacity:

Qult= (c NcAtip) + #($ c Asurface)

Static Analysis:


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Static Analysis:


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.

6.1 pile foundations 2013

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