Pile & Pier Foundation Analysis & Design - Peter Bosscher.pdf

8/21/2019 Pile & Pier Foundation Analysis & Design - Peter Bosscher.pdf

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Pile & Pier FoundationAnalysis & Design

by

Peter J. Bosscher

University of Wisconsin-Madison


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5

Topic Outline

Overview

Axial Load Capacity

Group Effects

Settlement


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6

Overview

Shallow vs Deep

Foundations

– A deep foundation is one

where the depth of

embedment is larger than

2X the foundation width.


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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


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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


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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)


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FoundationDesign

Process(FHWA)


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FoundationDesign

ProcessContinued

(FHWA)


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Foundation

Classification


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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


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Steel H-Piles


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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


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Composite Piles


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Drilled Shafts


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Augered, Pressure Injected

Concrete Piles


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Micropiles


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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 iscritical.


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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 usuallydominate 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 exceededduring 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

Normal

Somewhat

variable Good None Average

Poor Erratic Good None Variable

Very 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

Failure

Good

Control

Normal

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 3.4

Temporary 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


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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 inducedloads. 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 anddriving stress limits, may apply to prevent buckling of pileswhen a portion of the pile is in air, water, or soil not capable ofadequate lateral support. In these cases, the structural designof the pile should also be in accordance with the requirements

of Sections 8, 9, 10, and 13 of AASHTO code (1994) forcompression members.

• See excerpt from FHWA’s Design and Construction of DrivenPile Foundations

27


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Axial Pile Capacity

In general:

Three general cases shown (from Das)

30

F

A f Aq

F

P P P

s see sea ∑+′=+′=


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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 controlledstrain, 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 of Piling (ft)

Length of Piling (m)

Number of Load Tests

0-6000 0-1800 06000-10000 1800-3000 1

10000-20000 3000-6000 2

20000-30000 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 increaseslinearly with depth.

( )q N BN e D q' .= ′ − +σ γ γ 1 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 andcompressibility of the soil

– friction angle varies witheffective stress

– overconsolidation causeschanges in bearing capacity


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Vesic/Kulhawy Method Based on Vesic’s work, Kulhawy gives the

two bearing capacity factors:

38

( ) φ σ ν tan12 D sr

E I

′+=

( ) φ σ ν tan12 D sr

E I

′+=


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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 φ tan′ ==

σ

φ

h horizontal effective stress

tan coef. of friction between soil and piles

′ = ′σ σ h v K


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General Method (Kulhawy) rewrite equation:

Suggest using:

f K K

K s v s= ′

σ φ

φ

φ 0 0tan

Pile & Soil Types φs/φSand/Rough concrete 1.0

Sand/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/K 0

Jetted pile ½ -2/3

Drilled shaft 2/3 - 1Pile-small displacemnt ¾-1¼

Pile-large displacement 1 – 1.2

( ) φ φ ′′−= sin0 sin1 OCR K


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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

D

r

r where is the relative density in decimal form


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Coyle & Castello’s Method empirical correlation

of f s to φ and z/B. z is depth to midpoint

of strata.

CAUTION: No effect of

pile material, installation

effects, and initial insitu

stresses


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Cohesive Soil excess pore pressures produced by soil

displacement during driving takes time to

dissipate. This means capacity increases withtime. 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 K 0 therefore use βmethod.

′ =q s

s

e u

u

9

where = undrained shear strength

Skin Friction


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β Method for Clay use Randolph

and Wroth(1982):

upper limit:

β φ

≤ +

tan

2 452


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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 as3 times the actual capacity.


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In-Situ Soil Test Methods can determine φ or su and then use previous

methods or can use direct correlationmethods.

direct in-situ methods especially important

for sand as sampling and testing is difficult.

In-situ tests:

– SPT & CPT


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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


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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 3 0

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


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Briaud Method based on regression analyses:

( )

( )

′ =

=

q N

f N

e r

s r

19 7

0224

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 ofcapacity.

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 - thestability 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 0.5inches 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 sufficientlyaccurate to match column location

To build in redundancy

Increased efficiency gained by multiple

piles driven in close proximity


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Group characteristics Common C-C spacing: 2.5 to 3.0 diameters

Efficiency:( )

η = =′+

Group CapacitySum of Individual Piles

P F N P P

ag

e swhere:

group efficiency factor

net allowable capacity of pile group

factor of safety

number of piles in groupnet end bearing capacity of single pile

skin friction capacity of single pile

η ==

=

=′ ==

P

F

N P

P

ag

e

s

56


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Individual vs Block Failure Modes

Individual Failure Mode Block Failure Mode


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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/B2, eventual η ≅

1.0 but early values range from 0.4 to 0.8.

In cohesionless soils, design for η between 1.0and 1.25 if driven piling w/o predrilling. If

predrilling 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 tothe structural load when determining stresses inthe pile. Negative skin friction generally

increases pile settlement but does not change pilecapacity.


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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


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Laterally Loaded Deep Fnds Deep foundations must also commonly

support lateral loads in addition to axialloads.

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


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From Karl Terzaghi, 1943“The problems of soil mechanics may be

divided into two principal groups - thestability problems and the elasticity

problems.”

Ultimate lateral load capacity is a stability

problem, load-deformation analysis is

similar to an elasticity problem.


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Ultimate Lateral Load Dependent on the diameter and length of the

shaft, the strength of the soil, and otherfactors.

Use Broms method (1964, 1965)

Divide world into:

– cohesive & cohesionless

– free & fixed head – 0, 1, or 2 plastic hinges

Cohesive Soil Diagrams


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Cohesive Soil Diagrams

LateralResistance

Free-Head

Distributions Fixed-Head

Distributions

Cohesionless Soil


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Diagrams

Free-Head Distributions

Fixed-Head Distributions

Summary Instructions

for

Laterally Loaded Piles

by

B. Broms

Cohesive Soil:


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Cohesive Soil:

Cohesionless Soil:

(a)

Short-Free:( )

H dg ce d f

uu=

+ +

2 2515 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≤ 2 252. 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> 2 252. 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 uu yield

=

+

+

2 25

15 05

2.

. .

Long-Fixed: H M

d f u yield

=+

2

15 0 5. . or Fig (b)

Short-free: H dK L

e Lu

p=

+

0 5 3. γ or Fig (a)

Long-free: H M

e f u yield

=+ 0 67.

or Fig (b)

where f

H

dK

u

p= 0 82. γ

Check if M dK L yield p> γ 3. If so, pile is short,

l il i i di l

Load-Deformation Method


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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


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p y et od

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


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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 programsolves 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


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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


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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 shareof load.

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