pile foundations v1.00 oct2010 (1)

8/11/2019 Pile Foundations v1.00 Oct2010 (1)

1/17

PILE FOUNDATIONS

CONTENTS:

1.0 Introduction

1.1 Choice of pile type

1.1.1 Driven (displacement) piles

1.1.2 Bored (replacement) piles

2.0 Analysis

2.0.1 Driving formulae

2.0.2 Soil mechanics

2.1 Piles in cohesive soil

2.1.1 Bored piles

2.1.2 Driven piles

2.1.3 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


2/17

-2-Pile Foundations v1.00 Oct2010

1.0 INTRODUCTION

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

A single pile can be defined asa 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 todifferential settlement.

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

(1970).

A pile carries the applied load via:

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

skin frictionin sands andadhesionin 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 pileswhich 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.


3/17


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.

Types of pile foundations

1.1 Choice of pile type

1.1.1 Driven or Displacement piles

a) Preformed piles:Advantages:- - may be inspected for quality and soundness

before driving- not liable to squeezing or necking- construction not affected by ground water- can be left protruding above G.L. (useful in

marine structures)

- can withstand high bending and tensilestresses

-can be driven in long lengths

Disadvantages: - unjointed types cannot easily be varied inlength

- may break during driving- uneconomic if the design is governed by

driving stresses rather than working stresses- noise and vibration during driving- displacement of soil may affect adjacent

structures- cannot be driven in situations of low head

room


4/17


5/17


2.0.1 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 theoretical

justification).

e.g. Hiley Formula;

Ru=W h n

s + c/2Where;

Ru= 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 thereforegenerally disapproved of by 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.

2.0.2 Analysis using soil mechanics

Load capacity of single piles

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

shaft resistancebase resistance

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

Qu= Qs+ Qbwhere :

Qu= ultimate pile capacity

Qs= ultimate shaft resistanceQb= ultimate base resistance

and

Qb= qbxAb= base bearing capacity xarea of base

Qs= surface area of shaft in contact with the soilxshear strength of the soil

Qs = ca d L (clays) ; where ca= adhesion

Qs= fs d L (sands) ; where fs= skin friction

where

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


6/17


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.

2.1 Piles in cohesive soil (clay/silt ; = 0o)

Ultimate pile capacity, Qu= Qb+ Qs

2.1.1 Bored piles

Base resistance, Qb(kN):

Qb = qbAb

= cuNcAbWhere

qb = base bearing capacity = cuNcAb =cross sectional area of pile base (m

2)

cu = undrained shear strength at base of pile

Nc = bearing capacity factor = 9.0 (intact clays) or= 6.75 (fissured) clays

Shaft resistance, Qs(kN):Qs = caAs

Where

ca = adhesion= cu= adhesion factor

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

cu = average undrained shear strength over length

of pile, L

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

Qu

Qs Qs

Qb


7/17


Class example 1

A 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/m2.

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

Determine the working load the pile could support assuming thefollowing:

i) = 0.7 for firm clay and 0.5 for stiff clay ; Nc= 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 shrinkage. [921 kN]

Class example 2

For the ground conditions and assumptions described in Example 1,determine the length of pile required to support a working load of1200 kN. [14.96m, say 15m]

2.1.2 Under-reamed piles

Often used in cohesive soils to increasethe base area of the pile, thereby

increasing 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).

Class example 3

A 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) = 0.3 ; Nc= 9b) A factor of safety of 3.0 should be applied to the base load

but full mobilisation of shaft adhesion can be assumed.[9.498MN]


8/17


9/17


Adhesion factors for short piles(L20 to 40d) driven into stiff clay(Tomlinson, 1987)


10/17


2.2 Piles in non-cohesive soil (sand/gravel ; c = 0)

Ultimate pile capacity, Qu= Qb+ Qs

2.2.1 Driven piles

Base resistance Qb:

Qb= qb. Ab

Where;

Ab= cross sectional area of pile base

qb= base bearing capacity = Nq v

andNq= bearing capacity factor, see chart

below

v = vertical effective stress at the base

of the pile

Qb= Nq vAb

(From Berezantsev et al 1961)

Qu

Qs Qs

Qb


11/17


The internal angle of friction , before the installation of the pile, is not easy to

determine since disturbance will occur during piling. The value used isobtained from correlations with the SPT Nvalues as shown below:

Critical depth, zc

As the depth of pile penetration increases, the vertical effective stress increasesand therefore the end bearing should increase. Field stress have shown,however, that end bearing does not increase continually with depth. A possible

explanation is that as increases the bearing capacity factor decreases.

This has lead to the concept of critical depth zc, below which shaft and base

resistance are considered to be constant (i.e. the values for zcand below).

The value of zcis determined from charts relating depth to - these are

somewhat tentative.

Shaft resistance Qs:Qs = fsAs

wherefs = skin friction on pile surface

= Kstanv

As =area of pile in contact with the soil= d L (cylindrical pile)

andKs = coefficient of horizontal effective stress

= angle of friction between pile surface and soil

v = average effective vertical stress

Qs = Kstan v d L

The method of installation affects the values of Ksand and they are usually

presented as one factor as shown below;


12/17


Class example 5

A 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 bulk

unit 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 surfacesbut this effect is diminished by the loosening of the sand.

Poulus(1980) suggests analysing as if for a driven pile but using reduced values

of v.Meyerhof (1976) suggests designing as if for a driven pile, but using one third ofthe base resistance and one half of the shaft resistance.


13/17


3.0 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 orFill 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. Consequentlythe values of friction or adhesion for the consolidating soil must be addedto theapplied load. Treat skin friction values as load on the pile and are not factored.

FILL(recentlyplaced)Compressesunder ownweight.

FILL

(Recentlyplaced)Compressesunder ownweight

SoftCLAY

Consolidatesdue to weight offill.

DenseGRAVEL

Does notcompress

DenseGRAVEL

Does notcompress

Class example 6

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

Fill (recent) 2.5m thick ( = 26.0 kN/m3; = 37o)

Medium SAND 3.0m thick ( = 17.0 kN/m3; N = 18)

Soft CLAY 2.0m thick ( sat= 22.0 kN/m3)

Compact SAND 9.0m thick ( sat= 22.0 kN/m3; N = 33)

The strength of the soft clay increases linearly from 18.0 kN/m2at 5.5m

below ground level to 36.0 kN/m

2

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.

[1256.3 kN]

4.0 WORKING LOAD OF PILES

In order to determine the working or safe loadthat a pile can carry, it isnecessary to apply factors of safety in order to limit the settlement to a

permissible value.


14/17


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.

working load =ultimate load

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, Qais the smallerof :

Qa=Qs+ Qb

OR Qa=Qs

+Qb

2 1 3

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 Qais smallerof :

Qa=Qs+ Qb

OR Qa=Qs

+Qb

2.5 1.5 3.5

Class example 7

Determine 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/m2at ground leveland at 5.0kN/m2per metre depth. Assume;

a. the top 1.0m of the pile does not support load due toclay/concrete shrinkageb. an adhesion factor, = 0.5; Nc= 9.0c. factors of safety of 1.5 and 3.0 on the shaft load and

base load respectively.[29.5m, say 30m]


15/17


5.0 SUMMARY

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

Piles in cohesive soil (clay/silt; = 0o)BORED PILES

Base resistance;Qb = cuNcAbwhere,

Ab= 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 = cuAs

where, = adhesion factor, usually taken as 0.45, but

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

cu = average undrained shear strength over length ofpile

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

Base resistance;

Qb = cuNcAb as above

Shaft resistance;

Qs = cuAswhere,

= adhesion factor dependent on depth ofpenetration and type of overburden, valuefound from graph

cu = average undrained shear strength over pilelength

As = 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 = 0)DRIVEN PILES

Base resistance;

Qb = Nqv' Abwhere,

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

Shaft resistance;

Qs = Kstanv' Aswhere,

Kstan= installation factor from graphv' = average effective vertical stress


16/17


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 bearing capacity.

Analyse as if for a driven pile but using reduced values of v', 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 =


17/17

-17-Pile Foundations v1 00 Oct2010

REFERENCES

Berezantsev et al (1961) Load bearing capacity and deformation of piledfoundationsProc. 5th Int Conf Soil Mechanics and Foundation Engineering,Paris, vol.2 pp.11 - 12

Burland, J B et al (1966) The behaviour and design of large-diameter bored pilesin stiff clayProceedings, Symposium on large bored piles ICE, London

Fleming, W G K et al (1985) Piling engineeringSurrey 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 J (1987) Pile design and construction practice3rd Ed, ViewpointPublications, Palladian Publications Ltd.

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

pile foundations v1.00 oct2010 (1)

Documents