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1.0 INTRODUCTION
Piles are used where a structure cannot be supported satisfactorily on a shallow
foundation.
A single pile can be defined as “a long slender, structural member used to
transmit 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 the
economic use of conventional footings.
Where the soil layer(s) immediately underlying a structure are soft
or poorly compacted.
Where the soil layer(s) immediately underlying a structure are
moderately or highly variable in nature.
On sites where the soil strata, and in some cases the ground
surface are steeply inclined.
On river or shoreline sites where tidal or wave action or scouring
may 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 to
differential settlement.
For more detailed treatment of piling methods. pile types and design, refer to
the 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 of
the pile. This is called
skin friction in sands and
adhesion 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 into
two types:
a) Driven or displacement piles – which are usually
preformed before being driven, jacked, screwed or
hammered into the ground.
b) Bored or replacement piles – which first require a hole
to be bored into which the pile is then formed, usually of
reinforced concrete.
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Pile Foundations v1.00 Oct2010
Piles may also be classified according to how they achieve their load carrying
capacity;
end bearing piles or
friction piles.
In the majority of cases however, the load carrying capacity is dependent on
both the end bearing and shaft friction.
NOTE:Pile design must be accompanied by in situ load testing. Eurocode 7
emphasises that pile design must be based on static load tests or on
calculations 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 tensile
stresses
-can be driven in long lengths
Disadvantages: - unjointed types cannot easily be varied in
length
- 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
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Pile Foundations v1.00 Oct2010
b) Cast in place piles
Advantages: - length can easily be adjusted
- ground water can be excluded by driving with
a 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 are
used
- concrete cannot be inspected after installation
- length may be limited if tubes are to be
extracted
- displacement may damage adjacent
structures
- noise and vibration may be unacceptable
1.1.2 Bored or replacement piles
a) Cast in place piles:
Advantages: - length can be varied
- removed soil can be compared with design
data
- penetration tests can be carried out in
boreholes
- very large bases can be formed in favourable
ground
- drilling tools can break up boulders and other
obstructions
- 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 soft
soils
- special techniques required for concreting in
water 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
2.0 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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Pile Foundations v1.00 Oct2010
2.0.1 Driving Formulae
There are many different expressions – all try to relate the energy needed to
drive 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/2
Where;
Ru = ultimate driving resistance
W = weight of hammer
h = fall of hammer
n = efficiency of blow, found from graph
s = set or penetration/blow
c = total temporary compression of pile
Driving formulae take no account of soil type or conditions and are therefore
generally 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 vertical
loads:
shaft resistance
base resistance
At failure the ultimate values of both these resistances are mobilised to give:
Qu = Qs + Qb
where :
Qu = ultimate pile capacity
Qs = ultimate shaft resistance
Qb = ultimate base resistance
and
Qb = qb x Ab = base bearing capacity x area of base
Qs = surface area of shaft in contact with the soil
x shear 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 pile
L = length of pile in contact with the soil
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Piles usually penetrate several different soil types, each providing different shaft
resistances and the total shaft resistance is the summation of the individual
values.
The weight of the pile is usually ignored in the above equations, since it is
approximately 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 = qb Ab
= cu Nc Ab
Where
qb = base bearing capacity = cu Nc
Ab = cross sectional area of pile base (m2
)
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 = ca As
Where
ca = adhesion
= cu
= adhesion factor
[usually taken as 0.45, but may vary from
1.0 for soft clays to
0.3 for overconsolidated clays]
cu = average undrained shear strength over length
of pile, L
d = diameter of pile
L = length of pile in contact with soil stratum
Qu
Qs Qs
Qb
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Pile Foundations v1.00 Oct2010
Class example 1
A bored pile, 750mm diameter and 12.0m long, is to be installed on a
site 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 the
following:
i) = 0.7 for firm clay and 0.5 for stiff clay ; Nc = 9
ii) Factors of safety of 1.5 and 3.0 are applied to the shaft
load and base load respectively
iii) 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 of
1200 kN. [14.96m, say 15m]
2.1.2 Under-reamed piles
Often used in cohesive soils to increase
the base area of the pile, thereby
increasing the base resistance.
For under-reamed piles the adhesion
should 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 the
under-ream and
c) top 1m of the pile (zone of seasonal
shrinkage).
Class example 3
A large under-reamed bored pile is to be installed in stiff clay with
undrained shear strength of 125kN/m2
. The main shaft of the pile is
1.5m diameter and the base of the under ream is 4.5m diameter with a
height of 3.0m and the total length of the pile from the ground level to
the base of the under ream is 27m.
Determine the working load of the pile in MN, assuming the following:
a) = 0.3 ; Nc = 9
b) 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]
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2.1.3 Driven piles
Base resistance Qb:
Qb = cu Nc Ab (as above)
Shaft resistance Qs:
Qs = cu As
= cu d L
where;
= adhesion factor dependent on depth of
penetration and type of overburden, value
found from graph (see next page)
cu = average undrained shear strength over pile
length L
d = diameter of pile
L = length of pile in contact with soil stratum
Class example 4
A closed end pipe pile, 600mm diameter is driven to a depth of 15.0m
into a stiff clay. The undrained shear strength of the clay is 140.0kN/m2
.
Assume = 0.43
Determine the working load (kN) the pile could support with an overall
factor of safety of 2.5.
[778.0 kN]
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Adhesion factors for short piles(L<10d) driven into stiff clay
Adhesion factors for long piles(L>20 to 40d) driven into stiff clay
(Tomlinson, 1987)
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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‛
and
Nq = bearing capacity factor, see chart
below
v‛ = vertical effective stress at the base
of the pile
Qb = Nq v‛ Ab
(From Berezantsev et al 1961)
Qu
Qs Qs
Qb
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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 is
obtained from correlations with the SPT „N‟ values as shown below:
Critical depth, zc
As the depth of pile penetration increases, the vertical effective stress increases
and 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 zc and below).
The value of zc is determined from charts relating depth to ‛ - these are
somewhat tentative.
Shaft resistance Qs:
Qs = fs As
where
fs = skin friction on pile surface
= Ks tan v‛
As = area of pile in contact with the soil
= d L (cylindrical pile)
and
Ks = coefficient of horizontal effective stress
= angle of friction between pile surface and soil
v‛ = average effective vertical stress
Qs = Ks tan v‛ d L
The method of installation affects the values of Ks and and they are usually
presented as one factor as shown below;
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Class example 5
A 10.5m long concrete pile, 400mm square, is to be driven into a thick
deposit 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 ground
level.
Estimate the working load this length of pile will support assuming an
overall factor of safety of 2.5 and the sand has a saturated unit weight
of 20.0kN/m3
[949.2kN]
2.2.2 Bored piles
Boring holes in sands loosens an annulus of soil around the hole and reduces
horizontal stresses. Consequently bored piles in dense sands can be expected to
have low bearing capacity. Casting concrete in situ will produce rough surfaces
but 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 of
the base resistance and one half of the shaft resistance.
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3.0 NEGATIVE SKIN FRICTION
This term refers to the action (friction or adhesion) of soil layer/s acting with the
applied 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. Consequently
the values of friction or adhesion for the consolidating soil must be added to the
applied load. Treat skin friction values as load on the pile and are not factored.
FILL
(recently
placed)
Compresses
under own
weight.
FILL (Recently
placed)
Compresses
under own
weight
Soft
CLAY
Consolidates
due to weight of
fill.
Dense
GRAVEL
Does not
compress
Dense
GRAVEL
Does not
compress
Class example 6
A 300m square concrete driven pile driven 12.0m into a layered soils as
follows;
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/m2
at 5.5m
below ground level to 36.0 kN/m2
at a depth of 7.5m. A water table is
present at a constant depth of 5.5m below ground level.
Determine the safe working load of this pile by adopting factors of safety
of 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 load that a pile can carry, it is
necessary to apply factors of safety in order to limit the settlement to a
permissible 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 which
is 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 shaft
resistance values
For London Clay, Burland (1966) suggests that providing an overall factor of
safety of 2 is obtained, partial factors on the shaft and base of 1 and 3
respectively should be applied, so that the working load, Qa is the smaller of :
Qa =
Qs + Qb
OR Qa =
Qs
+
Qb
2 1 3
The first expression governs the design of straight shafted piles and the second
governs the design of large under reamed piles.
For soils other than London Clay, e.g. Glacial Till (boulder clay), where there is
uncertainty about the effects of installation, ground conditions etc, higher factors
of safety should be used so that the working load Qa is smaller of :
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 working
load of 4500kN in a thick deposit of clay with an undrained shear
strength increasing linearly with depth from 55.0kN/m2
at ground level
and at 5.0kN/m2
per metre depth. Assume;
a. the top 1.0m of the pile does not support load due to
clay/concrete shrinkage
b. an adhesion factor, = 0.5; Nc = 9.0
c. factors of safety of 1.5 and 3.0 on the shaft load and
base load respectively.
[29.5m, say 30m]
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5.0 SUMMARY
Types of pile: Driven or displacement piles Bored or replacement piles
Piles in cohesive soil (clay/silt; Φ = 0o
)
BORED PILES
Base resistance;
Qb = cu Nc Ab
where,
Ab = cross sectional area of pile base
cu = undrained shear strength at the base of the pile
Nc = bearing capacity factor
= 9.0 for intact clays or
= 6.75 for fissured clays
Shaft resistance;
Qs = α cu As
where,
α = adhesion factor, usually taken as 0.45, but
may vary from; 1.0 for soft clays to
0.3 for overconsolidated clays
cu = average undrained shear strength over length of
pile
As = surface area of pile in contact with soil stratum
DRIVEN PILES
Base resistance;
Qb = cu Nc Ab as above
Shaft resistance;
Qs = α cu As
where,
α = adhesion factor dependent on depth of
penetration and type of overburden, value
found from graph
cu = average undrained shear strength over pile
length
As = surface area of pile in contact with soil
stratum
Under-reamed piles
Increase 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 the
under ream.
Piles in non-cohesive soil (sand/gravel; c = 0)
DRIVEN PILES
Base resistance;
Qb = Nq σv' Ab
where,
Ab = cross sectional area of pile base
Nq = bearing capacity factor, found from graph
σv' = vertical effective stress at the base of the pile
Shaft resistance;
Qs = Kstanδ σv' As
where,
Kstanδ = installation factor from graph
σv' = average effective vertical stress
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AS = surface area of pile in contact with the soil
BORED PILES
Boring 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 σv', or use 1/3 of the base
resistance and 1/2 of the shaft resistance.
Negative skin friction
The action of fiction or adhesion acts WITH the applied loading i.e. against the pile
resistance. Consequently the values of friction or adhesion for the consolidating soil
must be added to the applied load. Do NOT factor down skin friction values.
Working load of piles
Apply factors of safety in order to limit the settlement to a permissible value.
For piles =<600mm diameter
Use an overall F of S of to give a settlement of <10mm.
For piles >600mm diameter
Apply 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 shaft
and base of 1 and 3 respectively, so that the working load, Qa is the smaller of:
Qa =
Qs + Qb
OR Qa =
Qs
+
Qb
2 1 3
The first expression governs the design of straight shafted piles and the second governs
the design of large under reamed piles.
For soils other than London Clay, where there is uncertainty about the effects of
installation, ground conditions etc, higher factors of safety should be used Qa is the
smaller of:
Qa =
Qs + Qb
OR Qa =
Qs
+
Qb
2.5 1.5 3.5
Note:
For negative skin friction, the above factors of safety are NOT applied to the
element of load acting against the pile resistance.
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REFERENCES
Berezantsev et al (1961) Load bearing capacity and deformation of piled
foundations Proc. 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 piles
in 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 J (1987) Pile design and construction practice 3rd Ed, Viewpoint
Publications, Palladian Publications Ltd.
Whitaker, T (1970) The design of piled foundations Oxford : Pergamon
