pile foundation

PILE FOUNDATIONS

CONTENTS:

1.0 Introduction

1.1 Choice of pile type

1.1.1 Driven (displacement) piles

L.1.2 Bored (replacement) piles

2.O Analysis

2.0.1 Driving formulae

2.O.2 Soll mechanics

2.L Piles in cohesive soil

2.1.7 Bored piles

2.7.2 Driven piles

2.L.3 Under-reamed piles

2.2 Piles in non-cohesive soil

2.2.1 Driven piles

2.2.2 Bored piles

3.0 Negative Skin Friction

4.0 Working Load

5.0 Summary

REFERENCES

1,0 INTRODUCTION

Piles are used where a structure cannot be supported satisfactorily on a shallowfoundation.

A single pile can be defined as "a long slender, structural member used totransmit loads applied at its top to the ground at lower levels".

Examples of where piled foundations may provide a solution are:

. Where a soil layer of adequate bearing capacity lies too deep for theeconomic use of conventional footings.

. Where the soil layer(s) immediately underlying a structure are softor poorly compacted.

. Where the soil layer(s) immediately underlying a structure aremoderately or highly variable in nature.

. On sites where the soil strata, and in some cases the groundsurface are steeply inclined.

. On river or shoreline sites where tidal or wave action or scouringmay vary the amount of material near the surface.

. For structures transmitting very high concentrated loads.

. For structures transmitting significant horizontal or inclined loads.

. For structures which structurally or functionally may be sensitive tod iffe rentia I settlement.

For more detailed treatment of piling methods. pile types and design, refer tothe books by Tomlinson (1987), Poulos (1980), Fleming (1985) and Whitaker( 1e70).

A pile carries the applied load via:

1. A shear stress mobilised (developed) on the surface ofthe shaft ofthe pile. This is called

skin friction in sands andadhesion in clays.

2. Bearing capacity at the base of the pile, called end bearing.

From the point of view of both design and construction, piles are classified intotwo types:

a) Driven or displacement piles - which are usuallypreformed before being driven, jacked. screwed orhammered into the ground,

b) Bored or replacement piles - which first require a holeto be bored into which the pile is then formed, usually ofreinforced concrete.

Pile Foundations v1 .00 Oct2010

Piles may also be classified according to how they achieve their load carryingcapacity;

end bearing piles orfriction piles.

In the majority of cases however, the load carrying capacity is dependent onboth the end bearing and shaft friction.

NOTE:Pile design must be accompanied by in situ load testing. Eurocode 7emphasises that pile design must be based on static load tests or oncalculations that have been validated by these tests.

t=Trll

solt ll*ril IIIII--+J-----roLli I

End

bearing

tl+11--fl-lrfiH[{.,,,,H ,li IilI-Floating

or 1l'iction

Tvpes of pile foundations

1.1 Choice of pile type

1.1.1 Driven or Displacement oiles

a) Preformed piles:. Advantages: -

. Disadvantages: -

+

Undt'r-re amcd

Upiili Frec-strnding

may be inspected for quality and soundnessbefore drivingnot liable to squeezing or neckingconstruction not affected by ground watercan be left protruding above G.L. (useful inmarine structures)can withstand high bending and tensilestresses

-can be driven in long lengths

unjointed types cannot easily be varied inlengthmay break during drivinguneconomic if the design is governed bydriving stresses rather than working stressesnoise and vibration during drivingdisplacement of soil may affect adjacentstructu rescannot be driven in situations of low headroom

Pile Foundations vl .00 Oct2010

b) Cast in place piles. Advantages: - length can easily be adjusted

- ground water can be excluded by driving witha closed end

- enlarged base possible- design governed by working conditions- noise and vibration reduced by internal drop

hammer

. Disadvantages: - necking is possible where temporary tubes areused

- concrete cannot be inspected after installation- length may be limited if tubes are to be

extracted- displacement may damage adjacent

structu res- noise and vibration may be unacceptable

1.1.2 Bored or replacement oiles

a) Cast in place piles:. Advantages: - length can be varied

- removed soil can be compared with designdata

- penetration tests can be carried out inboreholes

- very large bases can be formed in favourableground

- drilling tools can break up boulders and otherobstructions

- pile is designed to working stresses- very long lengths possible- little noise and vibration during construction- no ground heave

. Disadvantages: - piles liable to squeezing and necking in softsoils

- special techniques required for concreting inwater bearing ground

- concrete cannot be inspected after installation- enlarged bases cannot be formed in

collapseable soil- cannot be easily extended above ground- boring may cause instability and settlement of

adjacent structures

2.O ANALYSIS OF PILES

Analysis of piles is quite complex and there are two main approaches:

1. Estimate the carrying capacity from driving formulae and load tests(only suitable for sands/gravels or stiff clay)

2, Calculate the carrying capacity from soil mechanics expressions.

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2.O.1 Driving Formulae

There are many different expressions - all try to relate the energy needed todrive the pile to the penetration of the pile (for which there is no theoreticaljustification).

e.g. Hiley Formula;

R,= Whns+c/2

Where;R, = ultimate driving resistanceW = weight of hammerh = fall of hammern = efficiency of blow, found from graphs = set or penetration/blowc = total temporary compression of pile

Driving formulae take no account of soil type or conditions and are thereforeoenerallv disaooroved of bv foundation engineers.

The only sure way is to drive some test piles and then carry out load tests -thereby finding the carrying capacity - time and cost are big disadvantages.

2.O.2 Analysis using soil mechanics

Load capacitv of single piles

There are two forms of resistance provide by the pile to the applied verticalloads:

. shaft resistance

. base resistance

At failure the ultimate values of both these resistances are mobilised to give:

and

Qu=Qs+Qu

ultimate pile capacityultimate shaft resistanceultimate base resistance

qb x Ab = base bearing capacity x area of basesurface area of shaft in contact with the soil

x shear strength of the soil

c6ndL (clays) ; where ca = adhesionfsndL (sands) ; where fs = skin friction

d = diameter of pileL = length of pile in contact with the soil

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where :

Qu=Qs=Qu=

Qo=Q.=

Q.=Qs=

where

Piles usually penetrate several different soil types, each providing different shaftresistances and the total shaft resistance is the summation of the individualvalues.

The weight of the pile is usually ignored in the above equations, since it isapproximately equal to the weight of soil removed or displaced.

2.t Piles in cohesive soil (claylsilt ; 0 = O")

Ultimate pile capacity, Q, = Qo + Qs

QU

2.1.1 Bored oiles

Base resistance, Qo (kN):

Where

Shaft resistance, Q.(kN) :

Where

QbI

I

= qb Ab

= cu N6 A6

= base bearing capacity = cu Nc

= cross sectional area of pile base (mz)

= undrained shear strength at base of pile

= bearins capac*v factor = ?:9J'fti3ji.il}?]"",1,

=CaAs

= adhesion= ad,= adhesion factor

[usually taken as 0.45, but may vary from1.0 for soft clays to0.3 for overconsolidated claysl

= average undrained shear strength over lengthof pile, L

= diameter of pile= length of pile in contact with soil stratum

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Qr

qb

AtCu

Nc

Qt

C6

Cu

dL

Class example 1A bored pile, 750mm diameter and 12.0m long, is to be installed on asite where two layers of clay exist;

Upper firm clay; 8.0m thick;undrained shear strength = 50.0 kN/m'z.

Lower stiff clay; 12.0m thick;undrained shear strength = 120.0kN/m2.

Determine the working load the pile could support assuming thefollowing:

i) o = O.7 for firm clay and 0.5 for stiff clay I N. = 9ii) Factors of safety of 1.5 and 3.0 are applied to the shaft

load and base load respectivelyiii) The top 1.0m of the firm clay is ignored due to

clay/concrete shrinkaqe. [921 kN]

Class example 2For the ground conditions and assumptions described in Example 1,determine the length of pile required to support a working load of1200 kN. 114.96m, say 15mI

2.1.2 Under-reamed oiles

Often used in cohesive soils to increasethe base area of the pile, therebyincreasing the base resistance.

For under-reamed piles the adhesionshould be ignored over the:

a) height of the under-ream.b) main shaft of the pile up to 2 shaft

diameters above the top of theunder-ream and

c) top 1m of the pile (zone of seasonalshrinkage).

Und et rea.tt

A

lq,

Class example 3A large under-reamed bored pile is to be installed in stiff clay withundrained shear strength of 125kN/m2. The main shaft of the pile is1.5m diameter and the base of the under ream is 4.5m diameter with aheight of 3.0m and the total length of the pile from the ground level tothe base of the under ream is 27m.

Determine the working load of the pile in MN, assuming the following:a)cr=0.3 i N.=9b) A factor of safety of 3.0 should be applied to the base load

but full mobilisation of shaft adhesion can be assumed.t9.498MNl

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2.1.3 Driven piles

Base resistance Qb:

Qn = cu N6 A6 (as above)

Shaft resistance Qs:a' = cr-cu A.

=c.6zrdLwhere;

cr = adhesion factor dependent on depth ofpenetration and type of overburden, valuefound from graph (see next page)

6 = average undrained shear strength over pilelength L

d = diameter of pileL = length of pile in contact with soil stratum

Class example 4A closed end pipe pile, 600mm diameter is driven to a depth of 15.0minto a stiff clay. The undrained shear strength of the clay is 140.OkN/m'z.

Assume c = 0,43

Determine the working load (kN) the pile could support with an overallfactor of safety of 2.5.

t778.O kNl


U

o

o

a

=c

Eit

1.00

0.75

0.50

sands ory gravels

stiff clay

50 100 150 200

Undrained shear strength ofclay (kNim'?)

Adhesion factors for short piles(L<1Od) driven into stiff clay

50 100 150 100 250

Urrtlrrinctl shcrr strenclh of clry {l,N/rrr:,

Adhesion factors for long piles(L>20 to 4Od) driven into stiff clay(Tomlinson, 1987)

250

.2s l

,J

0

t.00

0.75

0.50

0.25

00

sands or sandy gravels

I

ar overburden

- soft clav_--..-.-..-_......-\.-

\ srnds (1. = l0r/)--------....-.---\-.-.

*\sofr clay ,,t . 20d)

:--- srnds 1/. = J0r/)

no 0\'elil;:-; -:-

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2.2 Piles in non-cohesive soil (sand/gravel ; c = O)

Ultimate pile capacity, Qu = Qo + Qs

2.2.1 Driven piles

Base resistance Qb:Qo=Qo.Ao

Where;

1000

Au = cross sectional area of pile baseqb = base bearing capacity = Nq ov'

Nq = bearing capacity factor, see chartbelow

6v' = vertical effective stress at the baseof the pile

Qr = Nq ov' Ao

Nq

100

l0 L-t5 30 35 40

Angle of intemal friction @ '(From Berezantsev et al 1961)

QU

T

45


The internal angle of friction {', before the installation of the pile, is not easy todetermine since disturbance will occur during piling. The $' value used isobtained from correlations with the SPT'N'values as shown below:

StaDdard penetrarion resistance 'N' (blows/300 mm)

42

; "1{J.95518ai -16

.=. l-l:-z \1

.tt)

7

d'relared to Standard penetr.tion aesistance ^N'(Peck, Hanson and Thombum. 1974)

Critical depth, zcAs the depth of pile penetration increases, the veftical effective stress increasesand therefore the end bearing should increase. Field stress have shown,however, that end bearing does not increase continually with depth. A possibleexplanation is that as O' increases the bearing capacity factor decreases.

This has lead to the concept of critical depth zc , below which shaft and baseresistance are considered to be constant (i.e. the values for z. and below).

The value of zc is determined from charts relating depth to O' - these aresomewhat tentative,

Shaft resista nce Qs:Qr = f. A,

wherefs = skin friction on pile surface

= Ks tan 6 -ov'A, = area of pile in contact with the soil

= n d L (cylindrical pile)and

Ks = coefncient of horizontal effective stressE = angle of friction between pile sudace and soil-av' = average effective vertical stress

a. = K. tan 66v'n d L

The method of installation affects the values of K, and 6 and they are usuallypresented as one factor as shown below;

..0 r0 20 30 J0 50 60 70


1.6

Driven pilcs

/

Jacked piles

Bored piles

1.2

-o

! 0.8k

0..1

o:b :1-s "10

Initial angle of internal lriction @',

Class example 5A 10.5m long concrete pile, 400mm square, is to be driven into a thickdeposit of medium dense sand, with an SPT'N'value of 25 and a bulkunit weight of 20.0 kN/m2. The water table lies at 2.5m below groundlevel,

Estimate the working load this length of pile will support assuming anoverall factor of safety of 2.5 and the sand has a saturated unit weightof 20.0kN/m3

[949.2kN]

2.2.2 Bored piles

Boring holes in sands loosens an annulus of soil around the hole and reduceshorizontal stresses, Consequently bored piles in dense sands can be expected tohave low bearing capacity. Casting concrete in situ will produce rough sufacesbut this effect is diminished by the loosening of the sand.

Poulus(1980) suggests analysing as if for a driven pile but using reduced valuesof ou',Meyerhof (1976) suggests designing as if for a driven pile, but using one third ofthe base resistance and one half of the shaft resistance.


3.O NEGATIVE SKIN FRICTION

This term refers to the action (friction or adhesion) of soil layer/s acting with theapplied loading i.e. against the pile resistance. It is usually caused by either;

. Clay soil undergoing consolidation settlement or

. Fill material compacting over time

Negative skin friction is caused by a dragging down effect by the consolidating /compacting layer plus any overlying strata, see diagrams below. Conseguentlythe values of friction or adhesion for the consolidating soil must be added to theapplied load. Treat skin friction values as load on the pile and are !q! factored.

(recentlyplaced)Compressesunder ownweight.

Class example 6A 300m square concrete driven pile driven 12.0m into a layered soils asfollows;

Fill (recent) 2.5m thick (y = 26.0 kN/m3 4'= 371Medium SAND 3.0m thick 0 = l7.O kN/mi; N = 18)Soft CLAY 2.0m thick (y."t = 22.0 kN/m3)Compact SAND 9.0m thick (y.u1 = 22.0 kN/m3; N = 33)

The strength of the soft clay increases linearly from 18.0 kN/m2 at 5.5mbelow ground level to 36.0 kN/m2 at a depth of 7.5m. A water table ispresent at a constant depth of 5.5m below ground level.

Determine the safe working load of this pile by adopting factors of safetyof 1.5 and 2.5 for the shaft and end bearing resistance respectively.

[12s6.3 kNl

4.O WORKING LOAD OF PILES

In order to determine the working or safe load that a pile can carry, it isnecessary to apply factors of safety in order to limit the settlement to apermissible value.

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Different authors apply various factors of safety to different pile conditions.However the following values are generally accepted.

For piles up to 600mm diameter

An overall factor of safety of 2.5 should be adopted, to give a settlement whichis unlikely to exceed 10mm.

. ultimate loadworklnq loao = - *2.5

For piles larger than 600mm diameter

It is necessary to apply partial factors of safety to the ultimate base and shaftresistance values

For London Clay, Burland (1966) suggests that providing an overall factor ofsafety of 2 is obtained, partial factors on the shaft and base of 1 and 3respectively should be applied, so that the working load, Q" is the smal/er of :

^ a-t-a' oR q" = * * -Fva- 2

The first expression governs the design of straight shafted piles and the secondgoverns the design of large under reamed piles.

For soils other than London Clay, e.g. Glacial Till (boulder clay), where there isuncertainty about the effects of installation, ground conditions etc, higher factorsof safety should be used so that the working load Q" is smaller of :

.-, _ Q' + Qr ^o .,,, - -Qr _, Quva: 2.5 \-,^ v" - t.5 - 3.5

Class example 7Determine the length of a pile, 1200mm diameter, to support a workingload of 4500kN in a thick deposit of clay with an undrained shearstrength increasing linearly with depth from 55.0kN/m2 at ground leveland at 5,0kN/m2 per metre depth. Assume;

a. the top 1.0m of the pile does not support load due toclay/concrete shrinkage

b. an adhesion factor, c = 0.5; N. = 9.0c. factors of safety of 1.5 and 3.0 on the shaft load and

base load respectively.[29.5m, sav 3OmI


5.O SUMMARY

Types of pile: Driven or displacement piles Bored or replacement piles

Piles in cohesive soil (clay/sil$ O = Oo)BORED PILES

Base resistance;Qr = cuNgA6

where,Au = cross sectional area of pile basecu = undrained shear strength at the base of the pileNc = bearing capacity factor

= 9.0 for intact clays or= 6.75 for fissured clays

Shaft resistance;Qs = u.u As

wh e re,o = adhesion factor, usually taken as 0.45, but

may vary from; 1.0 for soft clays to0.3 for overconsolidated clays

c, = average undrained shear strength over length ofp ile

As = surface area of pile in contact with soil stratumDRIVEN PiLES

Base resistance;Qu = Cg Ns A6 as above

Shaft resistance;a. = oGA

where,o = adhesion factor dependent on depth of

penetration and type of overburden, valuefound from graph

c, = average undrained shear strength over pilelength

A" = surface area of pile in contact with soilstratum

Under-reamed pilesIncrease of the base area of the pile, thereby increasing the base resistance.The adhesion should be ignored for a distance of two diameters above the top of theunder ream.

Piles in non-cohesive soil (sand/gravel; c = O)DRIVEN PILES

Base resistance;q = Nq o"'fu

where,fu = cross sectional area of pile baseNq = bearing capacity factor, found from graphou' = vertical effective stress at the base of the pile

Shaft resistance;

a, = K,tan6 d"'Aswhere,

K,tan6 = installation Factor from graph6u' = average effective vertical stress

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As = surface area of pile in contact with the soil

BORED PILESBoring holes in sands loosens an annulus of soil around the borehole, hence low bearingcapacity.

Analyse as if for a driven pile but using reduced values of w', or use 1/3 of the baseresistance and 1/2 of the shaft resistance.

Negative skin frictionThe action of fiction or adhesion acts WITH the applied loading i.e. against the pileresistance. Consequently the values of friction or adhesion for the consolidating soilmust be added to the applied load. Do NOT factor down skin friction values.

Working load of pilesApply factors of safety in order to limit the settlement to a permissible value.

For piles =<600mm diameterUse an overall F of S of to give a settlement of <10mm.

For oiles >600mm diameterApply partial factors of safety to the base resistance and the shaft resistance.

For London Clay, an overall F of S of 2.0 is obtained, with partial factors on the shaftand base of 1 and 3 respectively, so that the working load, Qa is the smaller of:

.., - Q.+Qr t - Qoa"=--:z- OR Q,=fr ,The first expression governs the design of straight shafted piles and the second governsthe design of large under reamed piles.

For soils other than London Clay, where there is uncertainty about the effects ofinstallation, ground conditions etc, higher factors of safety should be used Qa is thesmaller of:

e"=i*k oR a"=+.+Note:For negative skin friction, the above factors of safety are NOT applied to theelement of load acting against the pile resistance,

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REFERENCES

Berezantsev et al (1961) Load bearing capacity and deformation of piledfoundations Proc. 5th Int Conf Soil Mechanics and Foundation Engineering,Paris, vol.2 pp.lI - t2

Burland, J B et al (L966) The behaviour and design of large-diameter bored pilesin stiff clay Proceedings, Symposium on large bored piles ICE, London

Fleming, W G K et al (1985) Piling engineering Surrey University Press /Halstead Press

Meyerhof, G G (1976) Bearing capacity and settlement of pile foundations,Proceedings, American Society of Civil Engineers 102(GT3), pp 195-228

Poulos H G and Davis. E H (1980) Pile foundation analysis and design John Wiley& Sons, New York.

Tomlinson, M I (1987) Pile design and construction practice 3rd Ed, ViewpointPublications, Palladian Publications Ltd.

Whitaker, T (1970) The design of piled foundations Oxford : Pergamon

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