Pile Foundation Design -Smith

8/10/2019 Pile Foundation Design -Smith

1/38

Pile Foundation Design: A StudentGuide

Ascalew Abebe & Dr Ian GN Smith

School of the Built Environment, Napier University, Edinburgh

See Elements of Soil Mechanics(8th Edition)to learn how to design piles

(and other geotechnical structures) to Eurocode 7. Full details are here....

(Note:This Student Guide is intended as just that

- a guide for students of civil engineering. Use it

as you see fit, but please note that there is no

technical support available to answer anyquestions about the guide!)

PURPOSE OF THE GUIDE

There are many texts on pilefoundations. Generally, experienceshows us that undergraduates ndmost of these texts complicatedand dicult to understand.

This guide has extracted the mainpoints and puts together the wholeprocess of pile foundation design ina student friendly manner.

The guide is presented in twoversions: text-version (compendiumfrom and this we!-version that can!e accessed via internet or intranetand can !e used as a

supplementary self-assistingstudents guide.

STRUCTURE OF THE GUIDE

Intro!ction to "ile #o!nations

Pile #o!nation esi$n

%oa on "iles

Sin$le "ile esi$n

8/10/2019 Pile Foundation Design -Smith

2/38

Pile $ro!" esi$n

Installationtestan #actor o# sa#et'

Pile installation methos

Test "iles

Factors o# sa#et'

Pile Foundation Design: A StudentGuide

Chapter 1 Introduction to pile foundations

1.1 ile foundations

1. "istorical

1.# $unction of piles

1.% &lassification of piles

1.4.1 Classification of pile with respect to load transmission and functional behaviour

1.4.2 End bearing piles

1.4.3 Friction or cohesion piles

1.4.4 Cohesion piles

1.4. Friction piles

1.4.! Combination of friction piles and cohesion piles

1.4." .Classification of pile with respect to t#pe of material

1.4.$ %imber piles

1.4.& Concrete pile

1.4.1' (riven and cast in place Concrete piles

1.4.11 Steel piles

8/10/2019 Pile Foundation Design -Smith

3/38

1.4.12 Composite piles

1.4.13 Classification of pile with respect to effect on the soil

1.4.14 (riven piles

1.4.1 )ored piles

1.' (ide to classification of piles

1. (dvantages and disadvantages of different pile *aterial

1.+ &lassification of piles - eview

Chapter 2 Load on piles

.1 ntroduction

. ile arrange*ent

Chapter 3 Load Distribution

#.1 ile foundations vertical piles only

#. ile foundations vertical and ra/ing piles

#.# 0y**etrically arranged vertical and ra/ing piles

3.3.1 E*ample on installation error

Chapter 4 Load on Single Pile

%.1 ntroduction

%. The behaviour of piles under load

%.# eotechnical design *ethods

4.3.1 %he undrained load capacit# +total stress approach,

4.3.2 (rained load capacit# +effective stress approach,

4.3.3 -ile in sand

%.% 2yna*ic approach

Chapter Single Pile Design

'.1 3nd bearing piles

'. $riction piles

8/10/2019 Pile Foundation Design -Smith

4/38

8/10/2019 Pile Foundation Design -Smith

5/38

&.1.2 %5 the maintained increment load test

Chapter 1* Li%it State Design

18.1 eotechnical category & 1

18. eotechnical category &

18.# eotechnical category & #

1'.3.1 Conditions classified as in Eurocode "

18.% The partial factors *, n, d

Pile Foundation Design: A Student Guide

8/10/2019 Pile Foundation Design -Smith

6/38

Introduction to pile foundations

Objectives:Texts dealing with geotechnical and ground engineering techniques classify piles in a number of ways. The objective of thisunit is that in order to help theundergraduate student understand these classifications using materials extracted from several sources,this chapter gives an introduction to pile foundations.

1.1 Pile foundations

Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at somedepth below ground surface. The main components of the foundation are the pile cap and the piles. Piles are long and slender memberswhich transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity The main types ofmaterials used for piles are Wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground andconnected to pile caps. epending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly. !nthe following chapter we learnabout, classifications, functions and pros and cons of piles.

1.2 Historical

Pile foundations have been used as load carrying and load transferring systems for many years.

!n the early days of civilisation"#$, from the communication, defence or strategic point of view villages and towns were situated near torivers and lakes. !t was therefore important to strengthen the bearing ground with some form of piling.

Timber piles were driven in tothe ground by hand or holes were dug and filled with sand and stones.

!n %&'( )hristoffoer Polhem invented pile driving equipment which resembled to days pile driving mechanism. *teel piles have been usedsince %+(( and concrete piles since about %((.

The industrial revolution brought about important changes to pile driving system through the invention of steam and diesel drivenmachines.

-ore recently, the growing need for housing and construction has forced authorities and development agencies to exploit lands with poorsoil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advancedtechniques of pile installation.

1.3 Function of piles

s with other types of foundations, the purpose of a pile foundations is/

to transmit a foundation load to a solid ground

to resist vertical, lateral and uplift load

structure can be founded on piles if the soil immediately beneath its base does not have adequate bearing capacity. !f the results of siteinvestigation show that the shallow soil is unstable and weak or if the magnitude of the estimated settlement is not acceptable a pilefoundation may become considered. 0urther, a cost estimate may indicate that a pile foundation may be cheaper than any othercompared ground improvement costs.

!n the cases of heavy constructions, it is likely that the bearing capacity of the shallow soil wil l not be satisfactory, and the construction

should be built on

pile foundations. Piles can also be used in normal ground conditions to resist hori1ontal loads. Piles are a convenient method offoundation for works over water, such as jetties or bridge piers.

1.4 Classification of piles

1.4.1 Classification of pile with respect to load transmission and functional behaviour

2nd bearing piles 3point bearing piles4

0riction piles 3cohesion piles 4

)ombination of friction and cohesion piles

1.4.2 End bearing piles

8/10/2019 Pile Foundation Design -Smith

7/38

Pile Foundation Design: A StudentGuide

LO! O" PIL#$

2.1 Introduction

This section of the guide is divided into two parts. The first part gives brief summary on basic pile arrangementswhile part two deals with load distribution on individual piles.

Piles can be arranged in a number of ways so that they can support load imposed on them. 5ertical piles can bedesigned to carry vertical loads as well as lateral loads. !f required, vertical piles can be combined with raking pilesto support hori1ontal and vertical forces.

often, if a pile group is subjected to vertical force, then the calculation of load distribution on single pile that ismember of the group is assumed to be the total load divided by the number of piles in the group. 6owever if agroup of piles is subjected to lateral load or eccentric vertical load or combination of vertical and lateral load whichcan cause moment force on the group which should be taken into account during calculation of load distribution.

!n the second part of this section, piles are considered to be part of the structure and force distribution on

individual piles is calculated accordingly.

Objective/ !n the first part of this section, considering group of piles with limited number of piles subjected to

vertical and lateral forces, forces acting centrally or eccentrically, we learn how these forces are distributed onindividual piles.

The worked examples are intended to give easy follow through exercise that can help quick understanding of piledesign both single and group of piles. !n the second part, the comparison made between different methods usedin pile design will enable students to appreciate the theoretical background of the methods while exercising piledesigning.

Learnin% outco&e

When students complete this section, they will be able to/

)alculate load distribution on group of piles consist of vertical piles subjected to eccentric vertical

load.

)alculate load distribution on vertically arranged piles subjected to lateral and vertical forces.

)alculate load distribution on vertical and raking piles subjected to hori1ontal and eccentric

vertical loads.

)alculate load distribution on symmetrically arranged vertical and raking piles subjected to vertical

and lateral forces

8/10/2019 Pile Foundation Design -Smith

8/38

2.2 Pile arran%e&ent

7ormally, pile foundations consist of pile cap and a group of piles. The pile cap distributes the applied load to theindividual piles which, in turn,. transfer the load to the bearing ground. The individual piles are spaced andconnected to the pile cap or tie beams and trimmed in order to connect the pile to the structure at cut-off level,and depending on the type of structure and eccentricity of the load, they can be arranged in different patterns.0igure #.% bellow illustrates the three basic formation of pile groups.

a4 P!82 9:;

8/10/2019 Pile Foundation Design -Smith

9/38

LO! !I$'(I)*'IO"

To a great extent the design and calculation 3load analysis4 of pile foundations is carried out usingcomputer software. 0or some special cases, calculations can be carried out using the followingmethodsA...0or a simple understanding of the method, let us assume that the following conditionsare satisfied/

The pile is rigid

The pile is pinned at the top and at the bottom

2ach pile receives the load only vertically 3i.e. axially applied 4B

The force Pacting on the pile is proportional to the displacement < due to compression

- 6 7.0 C.%

Since - 6 E./

E./ 6 7.0 C.#

C.C

where:

P= vertical load component

k = material constant

U = displacement

E = elastic module of pile material

A = cross-sectional area of pile

8/10/2019 Pile Foundation Design -Smith

10/38

Pile Foundation Design: A StudentGuide

LO! O" $I"+L# PIL#

4.1 Introduction

!n this section, considering pileDsoil interaction, we learn to calculate the bearing capacity of single pilessubjected to compressive axial load. uring pile design, the following factors should be taken intoconsideration/

pile *aterial co*pression and tension capacity

defor*ation area of pile, bending *o*ent capacity

condition of the pile at the top and the end of the pile

eccentricity of the load applied on the pile

soil characteristics

ground water level ..etc.

7evertheless, calculation method that can satisfy all of these conditions will be complicated and difficult tocarry out manually, instead two widely used simplified methods are presented. These two methods arerefereed as geotechnical and dynamic methods. This section too has worked examples showing theapplication of the formulae used in predicting the bearing capacity of piles made of different types of materials.

Learnin% outco&e

When students complete this section, they will be able to

understand the theoretical back ground of the formulae used in pile design

carry out calculation and be able to predict design bearing capacity of single piles

appreciate results calculated by means of different formulae

4.2 ',e be,aviour of piles under load

Piles are designed that calculations and prediction of carrying capacity is based on the application of ultimateaxial load in the particular soil conditions at the site at relatively short time after installation.

8/10/2019 Pile Foundation Design -Smith

11/38

This ultimate load capacity can be determined by either/

the use of e*pirical for*ula to predict capacity fro* soil properties deter*ined by testing, or

load test on piles at the site

0ig.'@%, When pile is subjected to gradually increasing compressive load in maintained load stages, initiallythe pile@soil system behaves in a linear@elastic manner up to point on the settlement@load diagram and if theload is realised at any stage up to this point the pile head rebound to its original level. When the load is

increase beyond point ?there is yielding at, or close to, the pile@soil interface and slippage occurs until point

>?is reached, when the maximum skin friction on the pile shaft will have been mobilised. !f the load is realised

at this stage the pile head will rebound to point ) ?, the amount of permanent settlement being the distance

;). When the stage of full mobilisation of the base resistance is reached 3 point 4, the pile plungesdownwards with out any farther increase of load, or small increases in load producing large settlements.

?

8/10/2019 Pile Foundation Design -Smith

12/38

0igure @% axial compression of pile

4.3 +eotec,nical desi%n &et,ods

!n order to separate their behavioural responses to applied pile load, soils are classified aseither granularDnoncohesive or claysDcohesive. The generic formulae used to predict soilresistance to pile load include empirical modifying factors which can be adjusted accordingto previous engineering experience of the influence on the accuracy of predictions ofchanges in soil type and other factors such as the time delay before load testing.

30ig '@14 the load settlement response is composed of two separate components, the

linear elastic shaft friction :sand non@linear base resistance :b. The concept of theseparate evaluation of shaft friction and base resistance forms the bases of Estatic or soilmechanicsE calculation of pile carrying capacity. The basic equations to be used for this arewritten as/

F G FbH Fs@ Wpor

:cG :bH :s@ Wp

:t G :sH Wp

8/10/2019 Pile Foundation Design -Smith

13/38

8here9 FG :c G the ultimate compression resistance of the pile

Fb G :bG base resistance

FsG :sG shaft resistance

WpG weight of the pile

:tG tensile resistance of pile

!n terms of soil mechanics theory, the ultimate skin friction on the pile shaft is related to thehori1ontal effective stress acting on the shaft and the effective remoulded angle of frictionbetween the pile and the clay and the ultimate shaft resistance :scan be evaluated by

integration of the pile@soil shear strength aover the surface area of the shaft/

a6 Ca: n ?/>tana

8here9 n6 ;s ?/>v+refer geotechnical notes,

a6 Ca: ;S?/>v ?/>tana

and

where9 p 6 pile perimeter

6 pile length

6 angle of friction between pile and soil

;s6 coefficient of lateral pressure

the ultimate bearing capacit#5 b5of the base is evaluated from the bearing capacit# theor#9

/b6 area of pile base

C 6 undrained strength of soil at base of pile

>>>>>>>>>>>>>>>>%.1

8/10/2019 Pile Foundation Design -Smith

14/38

Nevertheless, in practise, for a given pile at a given site, the undrained shear strength Cavaries consideral!with man! factors, including, pile t!pe, soil t!pe, and methods of installations"

#deall!, Cashould e determined from a pile-load test, ut since this is not alwa!s possile, Cais correlated

with the undrained cohesion Cu! empirical adhesion factor so that the general e$pression in e"%" &'-()

could e simplified to the following e$pression:

IIIIIIIIIIIIIIIII'.#

*here: *s= weight of soil replaced ! the pile

=average value of shear strength over the whole shaft length

4.3.1 The undrained load capacity total stress approach!

+or piles in cla!, the undrained load capacit! is generall! taken to e the critical value unless the cla! is highl!over consolidated" #f the undrained or short-term ultimate load capacit! is to e computed, the soil parameters

C, ,, should e appropriate to undrained conditions and v and vshould e the total stresses" #f the

cla! is saturated?, the undrained angle of friction uis ero, and a&angle of friction etween pile and soil)

ma! also e taken as ero" #n addition, N%= (, N= (, so that the e% in&'-() reduces to:

>>>>>>>>>>>>>>>>>%.#

*here: Nc, N%, N,= earing capacit! factors and are functions of the internal angle of friction of the soil, the

relative compressiilit! of the soil and the pile geometr!"

4.3.2 "rained load capacity effective stress approach!

+or piles installed in stiff, over consolidated cla!s, the drained load capacit! is taken as design criterion" #f thesimplified assumption is made that the drained pile-soil adhesion C ais ero and that the term in e% &'-().

involving Nc, Nignoring the drained ultimate earing capacit! of the pile ma! e e$pressed as :

IIIIIIIIIIIIIIIII'.'

*here: s v,and s v = effective vertical stress at depth respective at pile ase

f a,= effective angle of friction etween pile/soil and implied can e taken as f ,

N% which is dependant up on the values of f ma! e taken to e the same as for piles in sand, and can e

8/10/2019 Pile Foundation Design -Smith

15/38

decided using tale (0-1 2 (0-3

4.3.3 #ile in sand

#f the pile soil adhesion Caand term Ncare taken as ero in e"% &'-(). and the terms 0"1d Nis neglectedas eing small in relation to the term involving N, the ultimate load capacit! of a single pile in sand ma! e

e$pressed as follows:

IIIIIIIIIIIIIIIII'.J

*here: s v,and s v = effective vertical stress at depth respective at pile ase

+w= correction factor for tapered pile & = ( for uniform diameter)

4.4 "ynamic approach

4ost fre%uentl! used method of estimating the load capacit! of driven piles is to use a driving formula ord!namic formula" All such formulae relate ultimate load capacit! to pile set &the vertical movement per low ofthe driving hammer) and assume that the driving resistance is e%ual to the load capacit! to the pile understatic loading the! are ased on an idealised representation of the action of the hammer on the pile in the laststage of its emedment"

Usuall!, pile-driving formulae are used either to estalish a safe working loador to determine the driving

re%uirements for a re%uired working load"

5he working load is usuall! determined ! appl!ing a suitale safet! factor to the ultimate load calculated !the formula" 6owever, the use of d!namic formula is highl! criticised in some pile-design literatures" 7!namicmethods do not take into account the ph!sical characteristics of the soil" 5his can lead to dangerous miss-interpretation of the results of d!namic formula calculation since the! represent conditions at the time ofdriving" 5he! do not take in to account the soil conditions which affect the long- term carr!ing capacit!,reconsolidation, negative skin friction and group effects"

specified load acting on the head of the pile

Pile Foundation Design: A StudentGuide

8/10/2019 Pile Foundation Design -Smith

16/38

8/10/2019 Pile Foundation Design -Smith

17/38

$I"+L# PIL# !#$I+"

-.1 #nd bearin% piles

!f a pile is installed in a soil with low bearing capacity but resting on soil beneath with high bearingcapacity, most of the load is carried by the end bearing.

!n some cases where piles are driven in to the ground using hammer, pile capacity can be estimatedby calculating the transfer of potential energy into dynamic energy . When the hammer is lifted andthrown down, with some energy lose while driving the pile, potential energy is transferred intodynamic energy. !n the final stage of the pileKs embedment,;n the bases of rate of settlement, it isable to calculate the design capacity of the pile.

0or standard pile driving hammers and some standard piles with load capacity &+8sp,), the workingload for the pile can be determined using the relationship between bearing capacity of the pile, the

design load capacity of the pile described by/ +8sp?n ?/>+9dand table J@%

where: +9dG design load for end baring.

The data is valid only if at the final stage, rate of settlement is %( mm per ten blow. nd pile lengthnot more than #( m and geo@category # . for piles with length #( @ C( m respective C( @ J( m thebearing capacity should be reduced by %( res. #JL.

'able -1 )arin% capacit/ of piles installed b/ ,a&&erin%

hammer DROP HA((ER )release b' tri$$er*ro" hammer )acti+ate b' ro"e an

#riction winch

crosssectional area o# "ile crosssectional area o# "ile

#all hei$ht "."##m$ "."%&m$ #all hei$ht "."##m$ "."%&m$

& T'

".&

".)

".#

)$",N

)*"

#+"

)#",N

#$"

#*"

".)

".#

".+

&*",N

)#"

#$"

)$",N

)"

#)"

) T'

".&

".)

".#

)%"

#)"

+"

#"

#*"

+"

".)

".#

".+

))"

#"

##"

)"

##"

+"

# T'

".&

".)

".#

#"

+%"

%+"

+)"

%)"

)"

".)

".#

".+

##"

+"

+%"

+""

++"

%&"

#0a&ple -.1

concrete pile with length #M m and cross@sectional area 3#CJ4?/>3#CJ4 is subjected to a

vertical loading of C( k7 3ultimate4 load. etermine appropriate condition to halt hammering. Type

8/10/2019 Pile Foundation Design -Smith

18/38

Pile Foundation Design: A StudentGuide

!#$I+" OF PIL# +(O*P

Introduction

+roup action in piled foundation/ -ost of pile foundations consists not of a single pile, but of agroup of piles, which act in the double role of reinforcing the soil, and also of carrying the appliedload down to deeper, stronger soil strata. 0ailure of the group may occur either by failure of theindividual piles or as failure of the overall block of soil. The supporting capacity of a group ofvertically loaded piles can, in many cases, be considerably less than the sum of the capacities theindividual piles comprising the group. 9rope action in piled foundation could result in failure orexcessive settlement, even though loading tests made on a single pile have indicated satisfactorycapacity. !n all cases the elastic and consolidation settlements of the group are greater than those ofsingle pile carrying the same working load as that on each pile within the group. This is because the1one of soil or rock which is stressed by the entire group extends to a much greater width and depththan the 1one beneath the single pile&fig"3-()

0igure M@% )omparison of stressed 1one beneath singlepile and pile group

Learnin% out co&e

When students complete this section, they will be able/

o to calculate and predict design bearing capacity of pile group in different soil types

8/10/2019 Pile Foundation Design -Smith

19/38

o to appreciate the governing factors in design of group of piles

o to design pile groups with appropriate pile spacing

.1 )earin% capacit/ of pile %roups

Pile groups driven into sand may provide reinforcement to the soil. !n some cases, the shaft capacityof the pile driven into sand could increase by factor of # or more.

>ut in the case of piles driven into sensitive clays, the effective stress increase in the surroundingsoil may be less for piles in a group than for individual piles. this will result in lower shaft capacities.

0igure M@# lock failure

!n general ,the bearing capacity of pile group may be calculated in consideration to block failure in asimilar way to that of single pile, by means of equation '@%,but hear sas the block surface area and

bas the base area of the block or by rewriting the general equation we get/

................................3M.%4

where:

s, surface area of !loc/

8/10/2019 Pile Foundation Design -Smith

20/38

!0 !ase area of !loc/ (see g.+-&

Cb, Cs0 average cohesion of clay around the group and !eneath thegroup.

c0 !earing capacity factor. 1or depths relevant for piles, theappropriate value of c is *

2pand 2s0 weight of pile respective weight of soil

!n examining the behaviour of pile groups it is necessary to consider the followingelements/

a free@standing group, in which the pile cap is not in contact with the underlying soil. a Epiled foundation,E in which the pile cap is in contact with the underlying soil. pile spacing independent calculations, showing bearing capacity of the block and bearing capacity of

individual piles in the group should be made. relate the ultimate load capacity of the block to the sum of load capacity of individual piles in

the group 3 the ratio of block capacity to the sum of individual piles capacity4 the higher thebetter.

!n the case of where the pile spacing in one direction is much greater than that in

perpendicular direction, the capacity of the group failing as shown in 0igure M@# b4 should beassessed.

$.1.1 #ile groups in cohesive soil

0or pile groups in cohesive soil, the group bearing capacity as a block may be calculated by mans ofe.q. '@J with appropriate 7cvalue.

$.1.2 #ile groups in non-cohesive soil

0or pile groups in non@cohesive soil, the group bearing capacity as a block may be calculated bymeans of e.q. '@&

$.1.3 #ile groups in sand

!n the case of most pile groups installed in sand, the estimated capacity of the block will be well inexcess of the sum of the individual pile capacities. s a conservative approach in design, the axialcapacity of a pile group in sand is usually taken as the sum of individual pile capacities calculatedusing formulae in '@+.

ored #0a&ple 1

)alculate the bearing capacity and group efficiency of pile foundation installed in uniform clay of bulk

unit weight, of #(k7DmCand undrained shear strength of )uof J(k7Dm#. The foundation consists of

8/10/2019 Pile Foundation Design -Smith

21/38

#J piles each %+m long ,(.'m in diameter and weight M(k7. The weight of the pile cap is M((k7 and

founded %m below the ground level. The adhesion factor for the soilDpile interface has a value of

(.+

0igure M@C Worked 2xample M@%

%&'(T)&*

)alculate single pile bearing capacity/

:sG ?/>)s?/>sG (.+ ?/>J( ?/>%+?/>?/>3(.'4 G ('k7?

:bG 7c ?/>)b?/>bG ?/>J(?/>?/>3(.#4#

G JM.Mk7

8/10/2019 Pile Foundation Design -Smith

22/38

:ciG :si H :bi G (' H JM.M G M(

3WpHWcap4 @ WsG 3M(?#JH3M((@#(?J.(?J.(?%.(44 @ 3#(?%+ ?/>?/>3(.#4#?/>#J G

'Mk7 ??

total load capacity of #J piles G :uc#JG 3:ciG :si H :bi4 ?/>#J @ N3WpHWcap4 @ WsO G M(?/>

#J @ 'M G #CJC%k7

calculate block load capacity /

G ' ?3%+?'.'?J(?/>(.+4H J(?/>'.' ?'.' ?/> G

#JMJ(k7

?surface area of pile group

??weight of soil replaced !y pile cap

Pile Foundation Design: A StudentGuide

Pile spacin% and pile arran%e&ent

!n certain types of soil, specially in sensitive clays, the capacity of individual piles within the a closelyspaced group may be lower than for equivalent isolated pile. 6owever, because of its insignificanteffect, this may be ignored in design. !nstead the main worry has been that the block capacity of thegroup may be less than the sum of the individual piles capacities. s a thumb rule, if spacing is morethan # @ C pile diameter, then block failure is most unlikely.

!t is vital importance that pile group in friction and cohesive soil arranged that even distribution ofload in greater area is achieved.

8arge concentration of piles under the centre of the pile cap should be avoided. This could lead toload concentration resulting in local settlement and failure in the pile cap. 5arying length of piles in

8/10/2019 Pile Foundation Design -Smith

23/38

the same pile group may have similar effect.

0or pile load up to C((k7, the minimum distance to the pile cap should be %(( mm

for load higher than C((k7, this distance should be more than %J( mm.

!n general, the following formula may be used in pile spacing/

3nd-!earing and friction piles: 4 0 $.#

?/> () + 0.02 . ...............&.%

5ohesion piles: 4 0 &.#?/> () + 0.02 ?/> ...............&.#

where:

d = assumed pile diameter

= assumed pile length

S = pile centre to centre distance !spacing"

E+ample ,-1

retaining wall imposing a weight of %#(k7Dm including self@weight of the pile cap

is to be constructed on pile foundation in clay. Timber piles of #J(mm in diameterand each %'m long with bearing capacity of (k7Dst has been proposed. ssessuitable pile spacing and pile arrangement.

%olution

%. recommended minimum pile spacing/

* G C.J?/>3d4 H (.(# ?/>8 G C.J ?/>3(.#J4 H (.(# ?/>%' G %.%M

m?

8/10/2019 Pile Foundation Design -Smith

24/38

#. try arranging the piles into tworows/

vertical load G %#(k7D-

single pile load capacity G(k7Dst

G .&&m

spacing in the two rows ?/>

minimum distance to the edge of the pile G (.%m ?/>)G #?/>(.% H (.#J H %.%( G %.JJm

?here because of the descending nature of the pile diameter a lesser value can be taken , say %.%(m

Pile Foundation Design: A StudentGuide

8/10/2019 Pile Foundation Design -Smith

25/38

PIL# I"$'L'IO" #'HO!$

5.1 Introduction

The installation process and method of installations are equally important factors as of the designprocess of pile foundations. !n this section we will discuss the two main types of pile installationmethodsB installation by pile hammer and boring by mechanical auger.

!n order to avoid damages to the piles, during design, installation -ethods and installationequipment should be carefully selected.

!f installation is to be carried out using pile@hammer, then the following factors should be taken in toconsideration/

the si1e and the weight of the pile

the driving resistance which has to be overcome to achieve the design penetration

the available space and head room on the site

the availability of cranes and

the noise restrictions which may be in force in the locality.

5.2 Pile drivin% &et,ods 6displace&ent piles7

-ethods of pile driving can be categorised as follows/

%. ropping weight#. 2xplosion

C. 5ibration

'. acking 3restricted to micro@pilling4

J. etting

.2.1 "rop hammers

hammer with approximately the weight of the pile is raised a suitable height in a guide andreleased to strike the pile head. This is a simple form of hammer used in conjunction with lightframes and test piling, where it may be uneconomical to bring a steam boiler or compressor on to asite to drive very limited number of piles.

There are two main types of drop hammers/

*ingle@acting steam or compressed@air hammers

8/10/2019 Pile Foundation Design -Smith

26/38

ouble@acting pile hammers

%. *ingle@acting steam or compressed@air comprise a massive weight in the form of a cylinder3see fig.+@%4. *team or compressed air admitted to the cylinder raises it up the fixed pistonrod. t the top of the stroke, or at a lesser height which can be controlled by the operator,the steam is cut off and the cylinder falls freely on the pile helmet.

#. ouble@acting pile hammers can be driven by steam or compressed air. pilling frame is notrequired with this type of hammer which can be attached to the top of the pile by leg@guides,the pile being guided by a timber framework. When used with a pile frame, back guides arebolted to the hammer to engage with leaders, and only short leg@guides are used to preventthe hammer from moving relatively to the top of the pile. ouble@acting hammers are usedmainly for sheet pile driving.

8/10/2019 Pile Foundation Design -Smith

27/38

$igure 6-1 ile driving using ha**er

.2.2 "iesel hammers

lso classified as single and double@acting,in operation, the diesel hammer employs a ram which israised by explosion at the base of a cylinder. lternatively, in the case of double@acting dieselhammer, a vacuum is created in a separate annular chamber as the ram moves upward, and assistsin the return of the ram, almost doubling the output of the hammer over the single@acting type. !nfavourable ground conditions, the diesel hammer provide an efficient pile driving capacity, but theyare not effective for all types of ground.

.2.3 #ile driving by vibrating

5ibratory hammers are usually electrically powered or hydraulically powered and consists of contra@rotating eccentric masses within a housing attaching to the pile head. The amplitude of the vibrationis sufficient to break down the skin friction on the sides of the pile. 5ibratory methods are best suitedto sandy or gravelly soil.

8ettin%/ to aid the penetration of piles in to sand or sandy gravel, water jetting may be employed.6owever, the method has very limited effect in firm to stiff clays or any soil containing much coarsegravel, cobbles, or boulders.

5.3 )orin% &et,ods 6 nondisplace&ent piles7

8/10/2019 Pile Foundation Design -Smith

28/38

.3.1 Continuous /light 0uger C/0!

n equipment comprises of a mobile base carrier fitted with a hollow@stemmed flight auger which isrotated into the ground to required depth of pilling. To form the pile, concrete is placed through theflight auger as it is withdrawn from the ground. The auger is fitted with protective cap on the outlet atthe base of the central tube and is rotated into the ground by the top mounted rotary hydraulic motorwhich runs on a carrier attached to the mast. ;n reaching the required depth, highly workableconcrete is pumped through the hollow stem of the auger, and under the pressure of the concretethe protective cap is detached. While rotating the auger in the same direction as during the boringstage, the spoil is expelled vertically as the auger is withdrawn and the pile is formed by filling withconcrete. !n this process, it is important that rotation of the auger and flow of concrete is matchedthat collapse of sides of the hole above concrete on lower flight of auger is avoided. This may lead tovoids in filled with soil in concrete.

The method is especially effective on soft ground and enables to install a variety of bored piles ofvarious diameters that are able to penetrate a multitude of soil conditions. *till, for successfuloperation of rotary auger the soil must be reasonably free of tree roots, cobbles, and boulders, and it

must be self@supporting.

uring operation little soil is brought upwards by the auger that lateral stresses is maintained in thesoil and voiding or excessive loosening of the soil minimise. 6owever, if the rotation of the auger andthe advance of the auger is not matched, resulting in removal of soil during drilling@possibly leadingto collapse of the side of the hole.

0igure +@# )0 Process

.3.2 (nderreaming

special feature of auger bored piles which is sometimes used to enable to exploit the bearing

capacity of suitable strata by providing an enlarged base. The soil has to be capable of standingopen unsupported to employ this technique. *tiff and to hard clays, such as the 8ondon clay, areideal. !n its closed position, the underreaming tool is fitted inside the straight section of a pile shaft,

8/10/2019 Pile Foundation Design -Smith

29/38

and then expanded at the bottom of the pile to produce the underream shown in fig. [email protected],after installation and before concrete is casted, a man carrying cage is lowered and the shaft and theunderream of the pile is inspected.

0igure + @C a4hydraulic rotary drilling equipment b4 ).0., c4undrreaming tool openposition

.3.3 C..".#

0igure +@', )ontinuous helical displacement piles/ a short, hollow tapered steel former complete witha larger diameter helical flange, the bullet head is fixed to a hallow drill pipe which is connected to ahigh torque rotary head running up and down the mast of a special rig. hollow cylindrical steel shaft

sealed at the lower end by a one@way valve and fitted with triangular steel fins is pressed into theground by a hydraulic ram. There are no vibrations.

8/10/2019 Pile Foundation Design -Smith

30/38

isplaced soil is compacted in front and around the shaft. ;nce it reaches the a suitably resistantstratum the shaft is rotated. The triangular fins either side of its leading edge carve out a conicalbase cavity. t the same time concrete is pumped down the centre of the shat and through the one@way valve. :otation of the fins is calculated so that as soil is pushed away from the pile base it issimultaneously replaced by in@flowing concrete. :ates of push, rotation and concrete injection are all

controlled by an onboard computer. Torque on the shaft is also measured by the computer. Whentorque levels reach a constant low value the base in formed. The inventors claim that the system caninstall aQ typical pile in %# minute. typical Mm long pile with an +((mm diameter base and CJ(mmshaft founded on moderately dense gravel beneath soft overlaying soils can achieve an ultimatecapacity of over #((t. The pile is suitable for embankments, hard standing supports and floor slabs,where you have a soft silty layer over a gravel strata.

0igure + @' ).6..P.

Back to Top

Pile Foundation Design: A StudentGuide

LO! '#$' O" PIL#$

9.1 Introduction

Pile load test are usually carried out that one or some of the following reasons are fulfilled/

8/10/2019 Pile Foundation Design -Smith

31/38

To obtain back@figured soil data that will enable other piles to be designed.

To confirm pile lengths and hence contract costs before the client is committed to over all job

costs.

To counter@check results from geotechnical and pile driving formulae

To determine the load@settlement behaviour of a pile, especially in the region of the

anticipated working load that the data can be used in prediction of group settlement.

To verify structural soundness of the pile.

'est loadin%/ There are four types of test loading/

compression test uplift test

lateral@load test

torsion@load test

the most common types of test loading procedures are )onstant rate of penetration 3):P4 test andthe maintained load test 3-8T4.

.1.1 C# constant rate of penetration!

!n the ):P 3constant rate of penetration4 method, test pile is jacked into the soil, the load beingadjusted to give constant rate of downward movement to the pile. This is maintained until point offailure is reached.

0ailure of the pile is defined in to two ways that as the load at which the pile continues to movedownward without further increase in load, or according to the >*, the load which the penetrationreaches a value e=ual to onetenth of the diameter of the pile at the base.

0ig.@#, !n the cases of where compression tests are being carried out, the following methods areusually employed to apply the load or downward force on the pile/

platform is constructed on the head of the pile on which a mass of heavy material, termed

EkentledgeE is placed. ;r a bridge, carried on temporary supports, is constructed over the test pileand loaded with kentledge. The ram of a hydraulic jack, placed on the pile head, bears on a cross@head beneath the bridge beams, so that a total reaction equal to the weight of the bridge and its loadmay be obtained.

.1.2 'T5 the maintained increment load test

0ig.@%, the maintained increment load test, kentledge or adjacent tension piles or soil anchors areused to provide a reaction for the test load applied by jacking3s4 placed over the pile being tested.The load is increased in definite steps, and is sustained at each level of loading until all settlementshas either stop or does not exceed a specified amount of in a certain given period of time.

8/10/2019 Pile Foundation Design -Smith

32/38

0igure @% test load arrangement using kentledge

0igure @# test being carried out

Pile Foundation Design: A StudentGuide

8/10/2019 Pile Foundation Design -Smith

33/38

Li&it $tate !esi%n

Introduction

Traditionally, design resistance of foundations has been evaluated on an allowablestress basis that piles were designed with ultimate axial capacity between # and Ctimes than working load. 6owever structural design is now using a limit state design38*4 bases whereby partial factors are applied to various elements of the designaccording to the reliability with which the parameters are known or can becalculated. 8* approach is the base of all the 2urocodes, including that forfoundations design. !t is believed that 8imit state design has many benefits for theeconomic design of piling. The eurocode approach is particularly rigorous, and thisguide adopts the partial factors presented in the codes.

See Elements of Soil Mechanics(8th Edition)to learn how to designpiles (and other geotechnical structures) to Eurocode 7. Full details

are here....

#urocode divides investigation, design and implementation of geoconstructionsinto three categories. !t is a requirement of the code that project must be supervisedat all stages by personnel with geotechnical knowledge.

!n order to establish minimum requirements for the extent and quality ofgeotechnical investigation, deign and construction three geotechnical categoriesdefined. These are/ 9eotechnical )ategory %, #, C.

1;.1 +oetec,nical cate%or/ 1< +C 1

this category includes small and relative simple structures/

@for which is impossible to ensure that the fundamental requirements will besatisfied on the basis of experience and qualitative geotecnical investigationB

@with negligible risk for property and life.

9eotechnical )ategory % procedures will be only be sufficient in ground conditions

which are known from comparable experience to be sufficiently straight@forward thatroutine methods may be used for foundation design and construction. Fualitativegeotechnical investigations

1;.2 +eotec,nical Cate%or/< +C 2

This category includes conventional types of structures and foundations with noabnormal risks or unusual or exceptionally difficult ground or loading conditions.*tructures in 9eotechnical category # require quantitative geotechnical data andanalysis to ensure that the fundamental requirements will be satisfied, but routineprocedures for field and laboratory testing and for design and execution may be

used. Fualified engineer with relevant experience must be involved.

8/10/2019 Pile Foundation Design -Smith

34/38

1;.3 +eotec,nical Cate%or/< +C 3

This category includes structures or parts of structures which do not fall within thelimits of 9eotechnical )ategories %and #.

The following are examples of structures or parts of structures complying withgeotechnical category #/

conventional type of /

spread foundationsB raft foundationsB piled foundationsB walls and other structures retaining for supporting soil or waterB excavationsB bridge piers and abutmentsB embankment and earthworksB ground anchors and other tie@back systemsB tunnels in hard, non@fractured rock and not subjected to special water

tightness or other requirement.

+eotec,nical Cate%or/ 3includes very large or unusual structure. *tructuresinvolving abnormal risks or unusual or exceptionally difficult ground or loadingconditions and highly seismic areas. Fualified geotechnical engineer must beinvolved.

The following factors must be considered in arriving at a classification of a structureor part of a structure/

7ature and si1e of the structure 8ocal conditions, e.g. traffic, utilities, hydrology, subsidence, etc. 9round and groundwater conditions :egional seismicityI..

16.3.1 Conditions classified as in Eurocode ,

!n the code, conditions are classified as favourable or unfavourable.

Favourable conditions are as suc,:

=if experience shows that the material posses limited spreading characteristic

=if large scale investigation was carried out and test results are reliable

=the existence of well documented investigation carried out using reliable methodswhich can give reproducible results

8/10/2019 Pile Foundation Design -Smith

35/38

=if additional tests, investigations and supervisions are recommend

=high certainty in defining test results

=failure is plastic

*nfavourable conditions are as suc,:

if experience shows that the material posses spreading characteres

if test results shows large spreading than the normal conditions

if the extent of investigation is limited

limited experience and methods lucking reproducibility

where there is no recommendation for additional test, investigations andsupervision

uncertainty in analysing test results

if failure is brittle

2urocode & refers to foundation loadings as action. The se can be permanent as !nthe case of weights of structures and installations, or variable as imposed loading,or wind and snow loads. They can be accidental, e.g. vehicle impact or explosions.

ctions can vary spatially, e.g. self@weights are fixed 3fixed actions4, but imposedloads can vary in position 3free actions4. The duration of actions affections affectsthe response of the ground. !t may cause strengthening such as the gain in strengthof a clay by long@term loading, or weakening as in the case of excavation slopes inclay over the medium or long term. To allow for this 2urocode & introduces aclassification related to the soil response and refers to transient actions 3e.g. windloads4, short@term actions 3e.g. construction loading4 and long@term actions. !n order

to allow for uncertainties in the calculation of he magnitude of actions orcombinations of actions and their duration and spatial distribution, 2uorcoderequires the design values of actions+dto be used for the geotechnical designeither to be assessed directly or to be derived from characteristic values +k/

+d= +k

1;.4 ',e partial factors &, and ) have been introduced.

5alues of nis given in table %(@C

Partial coefficient(d/ this co@efficient is applied in consideration of deviation

between test results and future construction. 5alues of the nshould be between

%.' @ %.+

'able 1;1partial factors on &aterial properties for conventional desi%n situations for ulti&ate li&it states

-aterial property Partial factor m

tan %.%@ %.#J

modules %.# @ %.+

other properties %.M @ #.(

'able 1;2 partial factors on &aterial properties for conventional desi%n situations for service li&it state

-aterial property Partial factor m

modules %.# @ %.+

other properties %.M @ #.(

"or&all/ t,e desi%n values< d< #d< tan < can be decided usin% t,e follo>in% for&ulae:

fd= f#$!n ?/> )

Ed= E#$!n ?/> )

tand= tan#$!n ?/> )

%here:

f = reaction force

8/10/2019 Pile Foundation Design -Smith

37/38

= internal angle of friction

E = elastic module

'able 1;3 partial factor n

)lass n

%.(

> %.%

) %.#

'able 1;4 ad,esion factor

pile b s

)oncrete piles (.J (.((J

*teel piles (.J (.((#

timber piles 3wood piles4 (.J (.((

The table is used for qc ?%( -pa

'able 1;- )earin% factors "< "?< "C

d 7 7) 7q

#J M.'+ #(.& %(.&

#M &.M' ##.# %%.+

#& +. #C. %C.#

#+ %(.M #J.+ %'.&

# %#.J #&. %M.'

C( %'.& C(.% %+.'

C% %&.' C#.& #(.M

C# #(.M CJ.J #C.#

CC #'.' C+. #M.%

8/10/2019 Pile Foundation Design -Smith

38/38

C' #.( '#.# #.'

CJ C'.' 'M.% CC.C

CM '%. J(.M C&.&

C& '.% JJ.M '#.

C+ J+. M%.C '+.

C &(. M&. JM.(

'( +J.M &J.C M'.#

'% %(' +C. &C.

'# %#M C.& +J.'

'C %J' %(J .(

'' %( %%+ %%J

'J #C' %C' %CJ