Pile Foundation Design: A Student Guide: Ascalew Abebe & DR Ian GN Smith
Pile Foundation Design: A Student Guide: Ascalew Abebe & DR Ian GN Smith
Pile Foundation Design: A Student Guide: Ascalew Abebe & DR Ian GN Smith
com
(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 any questions about the guide!)
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There are many texts on pile foundations. Generally, experience shows us that
undergraduates find most of these texts complicated and difficult to
understand.
This guide has extracted the main points and puts together the whole process of
pile foundation design in a student friendly manner.
Load on piles
Test piles
Factors of safety
1.2 Historical
1.3 Function of piles
1.4 Classification 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
1.4.5 Friction piles
1.4.6 Combination of friction piles and cohesion piles
1.4.7 .Classification of pile with respect to type of material
1.4.8 Timber piles
1.4.9 Concrete pile
1.4.10 Driven and cast in place Concrete piles
1.4.11 Steel piles
1.4.12 Composite piles
1.4.13 Classification of pile with respect to effect on the soil
1.4.14 Driven piles
1.4.15 Bored piles
1.5 Aide to classification of piles
1.6 Advantages and disadvantages of different pile material
1.7 Classification of piles - Review
Chapter 2 Load on piles
2.1 Introduction
2.2 Pile arrangement
Chapter 3 Load Distribution
3.1 Pile foundations: vertical piles only
3.2 Pile foundations: vertical and raking piles
3.3 Symmetrically arranged vertical and raking piles
3.3.1 Example on installation error
Chapter 4 Load on Single Pile
4.1 Introduction
4.2 The behaviour of piles under load
4.3 Geotechnical design methods
4.3.1 The undrained load capacity (total stress approach)
4.3.2 Drained load capacity (effective stress approach)
4.3.3 Pile in sand
4.4 Dynamic approach
Chapter 5 Single Pile Design
5.1 End bearing piles
5.2 Friction piles
5.3 Cohesion piles
5.4 Steel piles
5.5 Concrete piles
5.5.1 Pre-cast concrete piles
5.6 Timber piles (wood piles)
5.6.1 Simplified method of predicting the bearing capacity of timber piles
Chapter 6 Design of Pile Group
6.1 Bearing capacity of pile groups
6.1.1 Pile group in cohesive soil
6.1.2 Pile groups in non-cohesive soil
6.1.3 Pile groups in sand
Chapter 7 Pile Spacing and Pile Arrangement
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Chapter 8 Pile Installation Methods
8.1 Introduction
8.2 Pile driving methods (displacement piles)
8.2.1 Drop hammers
8.2.2 Diesel hammers
8.2.3 Pile driving by vibrating
8.3 Boring methods (non-displacement piles)
8.3.1 Continuous Flight Auger (CFA)
8.3.2 Underreaming
8.3.3 C.H.P
Chapter 9 Load Tests on Piles
9.1 Introduction
9.1.1 CRP (constant rate of penetration)
9.1.2 MLT, the maintained increment load test
Chapter 10 Limit State Design
10.1 Geotechnical category GC 1
10.2 Geotechnical category GC 2
10.3 Geotechnical category GC 3
10.3.1 Conditions classified as in Eurocode 7
10.4 The partial factors m, n, Rd
Pile foundations are the part of a structure used to carry and transfer the load of
the structure to the bearing ground located at some depth below ground
surface. The main components of the foundation are the pile cap and the piles.
Piles are long and slender members which transfer the load to deeper soil or
rock of high bearing capacity avoiding shallow soil of low bearing capacity The
main types of materials used for piles are Wood, steel and concrete. Piles made
from these materials are driven, drilled or jacked into the ground and connected
to pile caps. Depending upon type of soil, pile material and load transmitting
characteristic piles are classified accordingly. In the following chapter we learn
about, 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.
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Timber piles were driven in to the ground by hand or holes were dug and filled
with sand and stones.
The industrial revolution brought about important changes to pile driving system
through the invention of steam and diesel driven machines.
More recently, the growing need for housing and construction has forced
authorities and development agencies to exploit lands with poor soil
characteristics. This has led to the development and improved piles and pile
driving systems. Today there are many advanced techniques of pile installation.
A structure can be founded on piles if the soil immediately beneath its base
does not have adequate bearing capacity. If the results of site investigation
show that the shallow soil is unstable and weak or if the magnitude of the
estimated settlement is not acceptable a pile foundation may become
considered. Further, a cost estimate may indicate that a pile foundation may be
cheaper than any other compared ground improvement costs.
In the cases of heavy constructions, it is likely that the bearing capacity of the
shallow soil will not be satisfactory, and the construction should be built on
pile foundations. Piles can also be used in normal ground conditions to resist
horizontal loads. Piles are a convenient method of foundation for works over
water, such as jetties or bridge piers.
Carrying capacity is derived mainly from the adhesion or friction of the soil in
contact with the shaft of the pile (see fig 1.2).
Figure 1-1 End bearing piles Figure 1-2 Friction or cohesion pile
These piles transmit most of their load to the soil through skin friction. This
process of driving such piles close to each other in groups greatly reduces the
porosity and compressibility of the soil within and around the groups. Therefore
piles of this category are some times called compaction piles. During the
process of driving the pile into the ground, the soil becomes moulded and, as a
result loses some of its strength. Therefore the pile is not able to transfer the
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These piles also transfer their load to the ground through skin friction. The
process of driving such piles does not compact the soil appreciably. These
types of pile foundations are commonly known as floating pile foundations.
An extension of the end bearing pile when the bearing stratum is not hard, such
as a firm clay. The pile is driven far enough into the lower material to develop
adequate frictional resistance. A farther variation of the end bearing pile is piles
with enlarged bearing areas. This is achieved by forcing a bulb of concrete into
the soft stratum immediately above the firm layer to give an enlarged base. A
similar effect is produced with bored piles by forming a large cone or bell at the
bottom with a special reaming tool. Bored piles which are provided with a bell
have a high tensile strength and can be used as tension piles (see fig.1-3)
Timber
Concrete
Steel
Composite piles
Used from earliest record time and still used for permanent works in regions
where timber is plentiful. Timber is most suitable for long cohesion piling and
piling beneath embankments. The timber should be in a good condition and
should not have been attacked by insects. For timber piles of length less than
14 meters, the diameter of the tip should be greater than 150 mm. If the length
is greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is
essential that the timber is driven in the right direction and should not be driven
into firm ground. As this can easily damage the pile. Keeping the timber below
the ground water level will protect the timber against decay and putrefaction. To
protect and strengthen the tip of the pile, timber piles can be provided with toe
cover. Pressure creosoting is the usual method of protecting timber piles.
Pre cast concrete Piles or Pre fabricated concrete piles : Usually of square (see
fig 1-4 b), triangle, circle or octagonal section, they are produced in short length
in one metre intervals between 3 and 13 meters. They are pre-caste so that
they can be easily connected together in order to reach to the required length
(fig 1-4 a) . This will not decrease the design load capacity. Reinforcement is
necessary within the pile to help withstand both handling and driving stresses.
Pre stressed concrete piles are also used and are becoming more popular than
the ordinary pre cast as less reinforcement is required .
The Hercules type of pile joint (Figure 1-5) is easily and accurately cast into the
pile and is quickly and safely joined on site. They are made to accurate
dimensional tolerances from high grade steels.
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Two of the main types used in the UK are: West’s shell pile : Pre cast,
reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel
and driven into the ground after a concrete shoe has been placed at the front of
the shells. Once the shells have been driven to specified depth the mandrel is
withdrawn and reinforced concrete inserted in the core. Diameters vary from
325 to 600 mm.
Franki Pile: A steel tube is erected vertically over the place where the pile is to
be driven, and about a metre depth of gravel is placed at the end of the tube. A
drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug
which then penetrates the soil and takes the steel tube down with it. When the
required depth has been achieved the tube is raised slightly and the aggregate
broken out. Dry concrete is now added and hammered until a bulb is formed.
Reinforcement is placed in position and more dry concrete is placed and
rammed until the pile top comes up to ground level.
Steel piles: (figure 1.4) steel/ Iron piles are suitable for handling and driving in
long lengths. Their relatively small cross-sectional area combined with their high
strength makes penetration easier in firm soil. They can be easily cut off or
joined by welding. If the pile is driven into a soil with low pH value, then there is
a risk of corrosion, but risk of corrosion is not as great as one might think.
Although tar coating or cathodic protection can be employed in permanent
works.
a) X- cross- b) H - cross-
c) steel pipe
section section
Figure 1-6 Steel piles cross-sections
Augered
Large-diameter under-reamed
Drilled-in tubes
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Mini piles
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Wood piles
-- The piles will rot above the ground water level. Have a limited bearing
capacity.
-- The piles are difficult to splice and are attacked by marine borers in salt
water.
+ Stable in squeezing ground, for example, soft clays, silts and peats pile
material can be inspected before piling.
+ Can be driven in long lengths. Can be carried above ground level, for
example, through water for marine structures.
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+ Can be inspected before casting can easily be cut or extended to the desired
length.
+ Relatively inexpensive.
+ An enlarged base can be formed which can increase the relative density of a
granular founding stratum leading to much higher end bearing capacity.
-- Damage piles consisting of uncased or thinly cased green concrete due to the
lateral forces set up in the soil, for example, necking or waisting. Concrete
cannot be inspected after completion. Concrete may be weakened if artesian
flow pipes up shaft of piles when tube is withdrawn.
-- Relatively expensive.
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-- Limited length.
+ Soil removed in boring can be inspected and if necessary sampled or in- situ
test made.
-- Water under artesian pressure may pipe up pile shaft washing out cement.
-- Cannot be readily extended above ground level especially in river and marine
structures.
-- Boring methods may loosen sandy or gravely soils requiring base grouting to
achieve economical base resistance.
+ The piles are easy to handle and can easily be cut to desired length.
+ Can be driven through dense layers. The lateral displacement of the soil
during driving is low (steel section H or I section piles) can be relatively easily
spliced or bolted.
- Task
LOAD ON PILES
2.1 Introduction
This section of the guide is divided into two parts. The first part gives brief
summary on basic pile arrangements while part two deals with load distribution
on individual piles.
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Piles can be arranged in a number of ways so that they can support load
imposed on them. Vertical piles can be designed to carry vertical loads as well
as lateral loads. If required, vertical piles can be combined with raking piles to
support horizontal and vertical forces.
often, if a pile group is subjected to vertical force, then the calculation of load
distribution on single pile that is member of the group is assumed to be the total
load divided by the number of piles in the group. However if a group of piles is
subjected to lateral load or eccentric vertical load or combination of vertical and
lateral load which can cause moment force on the group which should be taken
into account during calculation of load distribution.
In 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: In 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 on individual
piles.
The worked examples are intended to give easy follow through exercise that
can help quick understanding of pile design both single and group of piles. In
the second part, the comparison made between different methods used in pile
design will enable students to appreciate the theoretical background of the
methods while exercising pile designing.
Learning outcome
Normally, pile foundations consist of pile cap and a group of piles. The pile cap
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distributes the applied load to the individual piles which, in turn,. transfer the
load to the bearing ground. The individual piles are spaced and connected 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. Figure 2.1
bellow illustrates the three basic formation of pile groups.
Q = Vertically applied load
H = Horizontally applied load
Figure 2-1 Basic formation of pile groups
LOAD DISTRIBUTION
To a great extent the design and calculation (load analysis) of pile foundations
is carried out using computer software. For some special cases, calculations
can be carried out using the following methods…...For a simple understanding
of the method, let us assume that the following conditions are satisfied:
Each pile receives the load only vertically (i.e. axially applied );
……………………………………………
P = k.U
3.1
Since P = E.A
where:
k = material constant
U = displacement
The length L should not necessarily be equal to the actual length of the pile. In a group of piles,
If all piles are of the same material, have same cross-sectional area and equal length L , then
the value of k is the same for all piles in the group.
Let us assume that the vertical load on the pile group results in vertical, lateral and torsion
movements. Further, let us assume that for each pile in the group, these movements are small
and are caused by the component of the vertical load experienced by the pile. The formulae in
the forthcoming sections which are used in the calculation of pile loads, are based on these
assumptions.
Here the pile cap is causing the vertical compression U, whose magnitude is equal for all
members of the group. If Q (the vertical force acting on the pile group) is applied at the neutral
axis of the pile group, then the force on a single pile will be as follows :
……………………………………………
3.4
where:
Pv = vertical component of the load on any pile from the resultant load Q
If the same group of piles are subjected to an eccentric load Q which is causing rotation around
axis z (see fig 3.1); then for the pile i at distance rxi from axis z:
………………………………………
……3.5
rxi positive measured the same direction as e and negative when in the opposite direction.
e = distance between point of intersection of resultant of vertical and horizontal loading with underside
of pile (see figure 3.8)
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The sum of all the forces acting on the piles should be zero
………………………………………
……3.6
If we assume that the forces on the piles are causing a moment M about axis z-z then the sum
of moments about axis z-z should be zero (see figure 3.1 a& b)
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……………………3.7
MZ = MZ
……………………………………………………..3.8
applying the same principle, in the x direction we get equivalent equation.If we assume that the
moment MX and MZ generated by the force Q are acting on a group of pile, then the sum of
forces acting on a single pile will be as follows:
……………………………………3.9
if we dividing each term by the cross-sectional area of the pile, A, we can establish the working
stream :
Example 3.1
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As shown in figure 3.2, A group of Vertical piles are subjected to an eccentric force Q,
magnitude of 2600kN. Determine the maximum and the minimum forces on the piles. Q is
located 0.2 m from the x-axis and 0.15 m from the z-axis.
Solution
rxi
m
2
m m
2
kN kNm
m
6.48 12.153
Mxrzi/ r2zi
Q/n Mzrxi/ r2xi Pi
PILE
(520
kN (390* rxi)/ 6.48 kN
rzi)/12.153
a1 217 58 54 217-58-54 = 105** Minimum load 105 KN, carried by pile a1
a2 19 54 217-19-54 = 144
a3 19 54 217+19-54 = 182
a4 58 54 217+58-54 = 221
b1 58 0 217-58-00 = 157
b2 19 0 217-19-00 = 157
b3 19 0 217+19-00 = 236
b4 58 0 217+58-00 = 275
c1 58 54 217+58-54 = 221
c2 19 54 217-19+54 = 252
c3 19 54 217+19+54=290
Example 3.2
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A pile trestle shown on figure 3-3 consists of four vertical piles surmounted by a
1.2m thick pile cap. It carries a horizontal load applied to the surface of the cap
of 400kN. The only vertical load exerted on the pile group is the weight of the
pile cup. Determine the loads on the piles.
Solution:
1. Determine the magnitude of the vertical force: For a pile cape 4.000m square, weight of pile
cap is:
3 . resultant of vertical load and horizontal load cuts the underside of the pile cup at a point
1.06m from N.A. pile group. This can be achieved graphically. E.g. On a millimetre paper, in
scale, draw the pile cup. Taking the top of the pile cup draw the vertical component downward
as shown in figure 2-3 then taking the tip of the vertical component as reference point draw the
horizontal component perpendicular to the vertical component. By joining the two components
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establish the resultant force R. Measure the distance from the N.A to the cutting point of R at
the underside of the pile cup.
To resist lateral forces on the pile group, it is common practice to use vertical piles combined
with raking piles (see figure3-5) The example below illustrates how the total applied load is
distributed between the piles and how the forces acting on each pile are calculated.
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Figure 3-5 Load distribution for combined vertical and raking piles
To derive the formulae used in design, we first go through the following procedures:-
1. Decide the location of the N.A of the vertical and the raking piles in plan position.
(see example below).
2. Draw both N.A till they cross each other at point c, this is done in Elevation and move
the forces Q, H& M to point c (see fig.3.5 elevation).
3. Let us assume that the forces Q &M cause lateral and torsional movements at point
c.
4. Point c is where the moment M is zero. Y is the moment arm (see fig. 3.5)
Figure 3.6 shows that the resultant load R (in this case Q) is only affecting the vertical piles.
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Figure 3-6
Pv =
As shown in figure 3.6 the lateral force, H, is kept in equilibrium by the vertical and the raking
piles.
V = 0: m Pr cosine - n Pv = 0
where:
Pr = H/(m sin
Pv = H/(n tan )
Figure 3-7
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as a result of moment M:
ri measured perpendicular to the N.A of both the vertical and raking piles
Example 3.3
Figure 3.7 shows a pile group of vertical and raking piles subjected to vertical load Q = 3000 kN
and lateral load H = 250 kN. Determine the forces acting on each pile. The raking piles lie at an
angle of 4:1.
Solution:
First we determine the location of the neutral axis, N.A, of both the vertical piles and the raking
piles. From figure 3.7 we see that the number of vertical piles = 8 and the number of raking piles
=4
Here we assume ¢ through piles a1, a2, a3, a4 as a reference point and start measuring in the
positive direction of the X axis, where it is denoted on figure 3.10 as X-X
The neutral axis for the vertical piles is located at 0.75 m from the ¢ line of pile a1, a2, a3, a4.
(1.0 -0.75 )m = 0.25m X = 0.25 m, the distance to the vertical load Q.
where:
n·eo = 8·eO and the numbers 4, 2, 2 are number of piles in the same axis
Here we can assume that the ¢ for the raking piles b1and b4 as a reference line and calculate
the location of the neutral axis for the raking piles as follows:
3. Draw both neutral axis till they cross each other at point c. (see figure 3.9) and establish the
lever arm distance, Y, so that we can calculate the moment M, about C.
where 0.75 m is the location of N.A of raking piles from e o or from the N.A Of the vertical piles.
4. Establish the angle and calculate sin, cos, and tangent of the angle
tan = 0.25
sin = 0.24
cos. = 0.97
cos2 = 0.94
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Raking piles
ri measured perpendicular
to the neutral axis
Figure 3-10
Vertical Piles
a , a , a , a , ri = -0.75m
*As we can see the maximum load 279kN will be carried by pile c1 and the minimum load
233kN is carried by piles in row a1
Just as we did for the previous cases, we first decide the location of the neutral axis for both the
vertical and raking piles.
Extend the two lines till they intersect each other at point c and move the forces Q & H to point
C. (see fig.11)
In the case of symmetrically arranged piles, the vertical pile I is subjected to compression stress
by the vertical component Pv and the raking pile Pr is subjected to tension (see figure 3.11 - 12)
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Pv = k (U)
pr = k (U cos. ) = PV cos.
V = 0 Q - n Pv - m Pr cos. = 0
Pr = Pv cos. Pv =
The symmetrical arrangement of the raking piles keeps the lateral force, H, in equilibrium and
it’s effect on the vertical piles is ignored.
With reference to figure 3.13 Horizontal projection of forces yield the following formulae.
H=0
Figure 3-14
NB the lateral force H imposes torsional stress on half of the raking piles.
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Example3.4
Determine the force on the piles shown in figure 3.15. The inclination on the raking piles is 5:1,
the vertical load, Q =3600 kN the horizontal load, H =200 kN and is located 0.6 m from pile
cutting level.
Solution
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4. Establish the angle and the perpendicular distance r, of the piles from the neutral axis.
sin = 0.196
cos = 0.98
cos2 = 0.96
tan = 0.20
Raking piles
= (0.32 cos2 )
= (0.62 cos2 )
= (0.346+1.037) 2 = 2.07 m2
Vertical piles
ri = 0.5 m
Q PV =
H Pr =
M
PILE ar br bv cv cr dr
where:
ar, br, bv, cv, cr, dr represent raking and vertical piles on respective axis.
Until now we have been calculating theoretical force distribution on piles. However during
installation of piles slight changes in position do occur and piles may miss their designed
locations. The following example compares theoretical and the actual load distribution as a
result of misalignment after pile installation.
MZ = 500 0 = 0
After installation
Displacement of piles in the X-X direction measured, left edge of pile cap as reference point
(see figure 3.17)
(0.5+0.6+0.4+2.0+2.1+1.7) 1 = 6 e e = 1.22 m
Measured perpendicular to the new N.A, pile distance, ri, of each pile:
2 45.3 (0.79) 49
3 45.3 (-0.62) 55
5 45.3 (-0.82) 47
condition of the pile at the top and the end of the pile
soil characteristics
Nevertheless, calculation method that can satisfy all of these conditions will be
complicated and difficult to carry out manually, instead two widely used
simplified methods are presented. These two methods are refereed as
geotechnical and dynamic methods. This section too has worked examples
showing the application of the formulae used in predicting the bearing capacity
of piles made of different types of materials.
Learning outcome
Piles are designed that calculations and prediction of carrying capacity is based
on the application of ultimate axial load in the particular soil conditions at the
site at relatively short time after installation.
No end-bearing is mobilised up to this point. The whole of the load is carried by the skin friction on the
pile shaft see figure 4-1 I)
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The pile shaft is carrying its maximum skin friction and the pile toe will be carrying some load
At this point there is no further increase in the load transferred in skin friction but the base load will
have reached its maximum value.
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In order to separate their behavioural responses to applied pile load, soils are
classified as either granular/noncohesive or clays/cohesive. The generic
formulae used to predict soil resistance to pile load include empirical modifying
factors which can be adjusted according to previous engineering experience of
the influence on the accuracy of predictions of changes in soil type and other
factors such as the time delay before load testing.
Q = Qb + Qs - Wp or
Rc = Rb + Rs - Wp
Rt = Rs + Wp
Qb = Rb = base resistance
Qs = Rs = shaft resistance
In terms of soil mechanics theory, the ultimate skin friction on the pile shaft is
related to the horizontal effective stress acting on the shaft and the effective
remoulded angle of friction between the pile and the clay and the ultimate shaft
resistance Rs can be evaluated by integration of the pile-soil shear strength a
over the surface area of the shaft:
a = Ca + n tan a
a = Ca + KS v tan a
and
L = pile length
the ultimate bearing capacity, Rb, of the base is evaluated from the bearing capacity
theory:
……………………………………
………4.1
Nevertheless, in practise, for a given pile at a given site, the undrained shear
strength Ca varies considerably with many factors, including, pile type, soil type,
and methods of installations.
Ideally, Ca should be determined from a pile-load test, but since this is not
always possible, Ca is correlated with the undrained cohesion Cu by empirical
adhesion factor so that the general expression in e.q. (4-1) could be simplified
to the following expression:
…………………………………
…………4.2
For piles in clay, the undrained load capacity is generally taken to be the critical
value unless the clay is highly over consolidated. If the undrained or short-term
ultimate load capacity is to be computed, the soil parameters C, , , should
be appropriate to undrained conditions and v and vb should be the total
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stresses. If the clay is saturated , the undrained angle of friction u is zero, and
a (angle of friction between pile and soil) may also be taken as zero. In
addition, Nq = 1, N = 1, so that the eq in(4-1) reduces to:
……………………………………………
4.3
Where: Nc, Nq, N ,= bearing capacity factors and are functions of the internal
angle of friction of the soil, the relative compressibility of the soil and the pile
geometry.
For piles installed in stiff, over consolidated clays, the drained load capacity is
taken as design criterion. If the simplified assumption is made that the drained
pile-soil adhesion C¢ a is zero and that the term in eq (4-1)…involving N c, N
ignoring the drained ultimate bearing capacity of the pile may be expressed as :
………………………………………
……4.4
f ¢ a,= effective angle of friction between pile/soil and implied can be taken as f ¢
,
for piles in sand, and can be decided using table 10-5 & 10-6
If the pile soil adhesion Ca and term Nc are taken as zero in e.q (4-1)… and the
terms 0.5 d N is neglected as being small in relation to the term involving N ,
the ultimate load capacity of a single pile in sand may be expressed as follows:
………………………………………
……4.5
Most frequently used method of estimating the load capacity of driven piles is to
use a driving formula or dynamic formula. All such formulae relate ultimate load
capacity to pile set (the vertical movement per blow of the driving hammer) and
assume that the driving resistance is equal to the load capacity to the pile under
static loading they are based on an idealised representation of the action of the
hammer on the pile in the last stage of its embedment.
Usually, pile-driving formulae are used either to establish a safe working load
or to determine the driving requirements for a required working load.
specified load acting on the head of the pile
If a pile is installed in a soil with low bearing capacity but resting on soil beneath
with high bearing capacity, most of the load is carried by the end bearing.
In some cases where piles are driven in to the ground using hammer, pile
capacity can be estimated by calculating the transfer of potential energy into
dynamic energy . When the hammer is lifted and thrown down, with some
energy lose while driving the pile, potential energy is transferred into dynamic
energy. In the final stage of the pile’s embedment,On the bases of rate of
settlement, it is able to calculate the design capacity of the pile.
For standard pile driving hammers and some standard piles with load capacity
(FRsp,), the working load for the pile can be determined using the relationship
between bearing capacity of the pile, the design load capacity of the pile
described by: FRsp n FSd and table 5-1
The data is valid only if at the final stage, rate of settlement is 10 mm per ten
blow. And pile length not more than 20 m and geo-category 2 . for piles with
length 20 - 30 m respective 30 - 50 m the bearing capacity should be reduced
by 10 res. 25%.
Example 5.1
solution:
FRsp n. Fsd
***For piles 20m - 30m length, the bearing capacity should be reduced by
10%
Table value (table 5-1): Hammer weight = 4 ton fall height 0.45m
(interpolation)
Load on piles that are driven into friction material, for the most part the weight is
carried by friction between the soil and the pile shaft. However considerable
additional support is obtained form the bottom part.
In designing piles driven into friction material, the following formulas can be
used
…………………………
5.1
qcs = end resistance at the bottom of the pile within 4 pile diameter from the end
of the pile
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Example 5.2
Pile length 22 m, steel pile, friction pile, external diameter 100 mm, GC2,
solution:
qc
MPa
Z m( depth measured from ground
level to bottom of pile)
0m - 5 m 5.4
5 - 11 6.4
11 - 18 7.0
18 - 22 7.5
22 m 8.0
The values are slightly scattered then the usual while the rest of the condition is
favourable.
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n = 1.1
s = 0.5
m = 0.0025
Piles installed in clay: The load is carried by cohesion between the soil and the
pile shaft. Bearing capacity of the pile can be calculated using the following
formula for pile installed in clay.
………………………… 5.2
Where:
The adhesion factor is taken as 0 for the firs three meters where it is expected
hole room and fill material or week strata. For piles with constant cross-
sectional area the value of can be taken as 1.0 and for piles with uniform
cross-sectional growth the value of can be taken as 1.2 .
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Example 5.3
18 m wood pile is installed small end down in clay. Pile diameter is 125 mm at
the end and 10 mm/m increase in diameter. The undrained shear strength of
the soil, measured from the pile cut-off level is: 0-6 m = 12 kP 6-12 m = 16 kPa
12-18 m = 19 kPa. Determine the ultimate load capacity of the pile. Pile cut-off
level is 1.5m from the ground level. Rd = 1.7
solution
Because of the relative strength of steel, steel piles withstand driving pressure
well and are usually very reliable end bearing members, although they are
found in frequent use as friction piles as well. The comment type of steel piles
have rolled H, X or circular cross-section(pipe piles). Pipe piles are normally,
not necessarily filled with concrete after driving. Prior to driving the bottom end
of the pipe pile usually is capped with a flat or a cone-shaped point welded to
the pipe.
Strength, relative ease of splicing and sometimes economy are some of the
advantages cited in the selection of steel piles.
The highest draw back of steel piles is corrosion. Corrosive agents such as salt,
acid, moisture and oxygen are common enemies of steel. Because of the
corrosive effect salt water has on steel, steel piles have restricted use for
marine installations. If steel pile is supported by soil with shear strength greater
than 7kPa in its entire length then the design bearing capacity of the pile can be
calculated using the following formulas. Use both of them and select the lowest
value of the two:
………………………… 5.3
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………………………… 5.4
I = fibre moment
Example 5.4
Determine the design bearing capacity of a Steel pile of external diameter 100
mm, thickness of 10 mm. Treated against corrosion. pile. Consider failure in the
pile material. Cc of the soil is 18 kPa, favourable condition. S2
Steel BS 2172
solution :
n = 1.1
m = 0.9
==
The first formula gives us lower value, therefore, the design bearing capacity of
the pile is 0.3 MN
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==
Concrete piles may be pre-cast or cast-in place. They may be are reinforced,
pre-stressed or plain.
These are piles which are formed, cast to specified lengths and shapes and
cured at pre casting stations before driven in to the ground. Depending up on
project type and specification, their shape and length are regulated at the prefab
site. Usually they came in square, octagonal or circular cross-section. The
diameter and the length of the piles are mostly governed by handling stresses.
In most cases they are limited to less than 25 m in length and 0.5 m in diameter.
Some times it is required to cut off and splice to adjust for different length.
Where part of pile is above ground level, the pile may serve as column.
If a concrete pile is supported by soil with undrained shear strength greater than
7 MPa in its entire length, the following formula can be used in determining the
bearing capacity of the pile :
………………………… 5.5
………………………… 5.6
Cuc = characteristic undrained shear strength of the soil in the loose part of the
soil within a layer of 4.0 m
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Example 5.5
Solution:
ef = 1.3
lc /h = 20
FRd = m NU
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Ecc = 34 GPa
Timber piles are frequently used as cohesion piles and for pilling under
embankments. Essentially timber piles are made from tree trunks with the
branches and bark removed. Normally wood piles are installed by driving.
Typically the pile has a natural taper with top cross-section of twice or more
than that of the bottom.
To avoid splitting in the wood, wood piles are sometimes driven with steel
bands tied at the top or at the bottom end.
For wood piles installed in soil with undrained shear strength greater than 7kPa
the following formula can be used in predicting the bearing capacity of the pile:
…………………………
5.7
If the wood is of sound timber, (e.g. pinewood or spruce wood with a diameter >
0.13m), then (reduced strength) of the pile can be taken as 11MPa.
…………………………
5.8
where: Am, = area of pile at each 3.5 m section mid point of pile
Example 5.6
Determine the design bearing capacity of a pile 12m pile driven in to clay with
characteristic undrained shear strength 10KPa and 1.0kPa increase per metre
depth. Piling condition is assumed to be favourable and the safety class 2. The
pile is cut at 1.5m below the ground level. Top diameter of the pile is 180mm
and growth in diameter is 9mm/m.
*Often it is assumed that cohesive strength of the soil in the fires three meters is
half the values at the bottom.
solution:
First decide which part of the pile is heavily loaded. To do so, divide the pile
which is in contact with the soil in three parts or sections (see fig.4.1) in this
example the pile is divided into three 3.5m parts
Calculate and decide diameter of the pile at the mid point of each 3.5m section
(0.180+0.009(yi) ; yi growth per meter from the end point.
Calculate the shear strength of the soil at the mid point of each 3.5m section C mi
= (22 - 1(yi) ). Shear strength at the end of the pile = (10MPa + 1MPa (12m))=22
MPa
Decide the values of the partial coefficients from table (10-1 - 10-4)
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dmi=
ymi(see fig.
(0.180+0.009
5.4) Cmi = (22 - 1
Part yi
(yi)
m m
yti
Part m
m
T(top) 55.1 10.5 0.275 928 this part of the pile is highly loaded
Now using the equation in (6-7), we will check the pile for failure
n = 0.9
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n = 1.1
=
In consideration of failure in the pile material, the pile can be loaded up to 9.0
MPa
…………………………
5.9
Rd, = 1.7
= 1.2
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Pile groups driven into sand may provide reinforcement to the soil. In some
cases, the shaft capacity of the pile driven into sand could increase by factor of
2 or more.
But in the case of piles driven into sensitive clays, the effective stress increase
in the surrounding soil may be less for piles in a group than for individual piles.
this will result in lower shaft capacities.
................................(6.1)
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where:
Cb, Cs= average cohesion of clay around the group and beneath the group.
Nc = bearing capacity factor. For depths relevant for piles, the appropriate value
of Nc is 9
a free-standing group, in which the pile cap is not in contact with the underlying soil.
a "piled foundation," 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 ( the ratio of block capacity to the sum of individual piles capacity) the
higher the better.
In 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 Figure 6-2 b)
should be assessed.
For pile groups in cohesive soil, the group bearing capacity as a block may be calculated by
mans of e.q. 4-5 with appropriate Nc value.
For pile groups in non-cohesive soil, the group bearing capacity as a block may be calculated by
means of e.q. 4-7
In the case of most pile groups installed in sand, the estimated capacity of the block will be well
in excess of the sum of the individual pile capacities. As a conservative approach in design, the
axial capacity of a pile group in sand is usually taken as the sum of individual pile capacities
calculated using formulae in 4-8.
Calculate the bearing capacity and group efficiency of pile foundation installed in uniform clay of
bulk unit weight, of 20kN/m3 and undrained shear strength of Cu of 50kN/m2. The foundation
consists of 25 piles each 18m long ,0.4m in diameter and weight 60kN. The weight of the pile
cap is 600kN and founded 1m below the ground level. The adhesion factor for the soil/pile
interface has a value of 0.8
SOLUTION
(Wp +Wcap) - Ws = (60 25+(600-20 5.0 5.0 1.0)) - (20 18 (0.2)2 25 = 469kN
total load capacity of 25 piles = Ruc25 = (Rci = Rsi + Rbi) 25 - {(Wp +Wcap) - Ws} = 960 25 - 469
= 23531kN
surface area of pile group
weight of soil replaced by pile cap
It is vital importance that pile group in friction and cohesive soil arranged that
even distribution of load in greater area is achieved.
Large concentration of piles under the centre of the pile cap should be avoided.
This could lead to load concentration resulting in local settlement and failure in
the pile cap. Varying length of piles in the same pile group may have similar
effect.
For pile load up to 300kN, the minimum distance to the pile cap should be 100
mm
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for load higher than 300kN, this distance should be more than 150 mm.
where:
Example 7-1
Solution:
= 1.33m
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minimum distance to the edge of the pile = 0.1m B = 2 0.1 + 0.25 + 1.10 = 1.55m
here because of the descending nature of the pile diameter a lesser value can be taken , say 1.10m
In order to avoid damages to the piles, during design, installation Methods and
installation equipment should be carefully selected.
1. Dropping weight
2. Explosion
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3. Vibration
4. Jacking (restricted to micro-pilling)
5. Jetting
A hammer with approximately the weight of the pile is raised a suitable height in
a guide and released to strike the pile head. This is a simple form of hammer
used in conjunction with light frames and test piling, where it may be
uneconomical to bring a steam boiler or compressor on to a site to drive very
limited number of piles.
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Jetting: to aid the penetration of piles in to sand or sandy gravel, water jetting
may be employed. However, the method has very limited effect in firm to stiff
clays or any soil containing much coarse gravel, cobbles, or boulders.
The method is especially effective on soft ground and enables to install a variety
of bored piles of various diameters that are able to penetrate a multitude of soil
conditions. Still, for successful operation of rotary auger the soil must be
reasonably free of tree roots, cobbles, and boulders, and it must be self-
supporting.
During operation little soil is brought upwards by the auger that lateral stresses
is maintained in the soil and voiding or excessive loosening of the soil minimise.
However, if the rotation of the auger and the advance of the auger is not
matched, resulting in removal of soil during drilling-possibly leading to collapse
of the side of the hole.
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8.3.2 Underreaming
8.3.3 C.H.D.P
Figure 8-4, Continuous helical displacement piles: a short, hollow tapered steel
former complete with a larger diameter helical flange, the bullet head is fixed to
a hallow drill pipe which is connected to a high torque rotary head running up
and down the mast of a special rig. A hollow cylindrical steel shaft sealed at the
lower end by a one-way valve and fitted with triangular steel fins is pressed into
the ground by a hydraulic ram. There are no vibrations.
Displaced soil is compacted in front and around the shaft. Once it reaches the a
suitably resistant stratum the shaft is rotated. The triangular fins either side of its
leading edge carve out a conical base cavity. At the same time concrete is
pumped down the centre of the shat and through the one-way valve. Rotation of
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the fins is calculated so that as soil is pushed away from the pile base it is
simultaneously replaced by in-flowing concrete. Rates of push, rotation and
concrete injection are all controlled by an onboard computer. Torque on the
shaft is also measured by the computer. When torque levels reach a constant
low value the base in formed. The inventors claim that the system can install a\
typical pile in 12 minute. A typical 6m long pile with an 800mm diameter base
and 350mm shaft founded on moderately dense gravel beneath soft overlaying
soils can achieve an ultimate capacity of over 200t. The pile is suitable for
embankments, hard standing supports and floor slabs, where you have a soft
silty layer over a gravel strata.
Figure 8 -4 C.H.D.P.
Pile load test are usually carried out that one or some of the following
reasons are fulfilled:
compression test
uplift test
lateral-load test
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torsion-load test
the most common types of test loading procedures are Constant rate of
penetration (CRP) test and the maintained load test (MLT).
In the CRP (constant rate of penetration) method, test pile is jacked into the
soil, the load being adjusted to give constant rate of downward movement to
the pile. This is maintained until point of failure is reached.
Failure of the pile is defined in to two ways that as the load at which the pile
continues to move downward without further increase in load, or according
to the BS, the load which the penetration reaches a value equal to one-tenth of the
diameter of the pile at the base.
Fig.9-2, In the cases of where compression tests are being carried out, the
following methods are usually employed to apply the load or downward force
on the pile:
approach is particularly rigorous, and this guide adopts the partial factors
presented in the codes.
This category includes structures or parts of structures which do not fall within
the limits of Geotechnical Categories 1and 2.
conventional type of :
spread foundations;
raft foundations;
piled foundations;
walls and other structures retaining for supporting soil or water;
excavations;
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+ if large scale investigation was carried out and test results are reliable
+ failure is plastic
-- if failure is brittle
Actions can vary spatially, e.g. self-weights are fixed (fixed actions), but
imposed loads can vary in position (free actions). The duration of actions
affections affects the response of the ground. It may cause strengthening such
as the gain in strength of a clay by long-term loading, or weakening as in the
case of excavation slopes in clay over the medium or long term. To allow for
this Eurocode 7 introduces a classification related to the soil response and
refers to transient actions (e.g. wind loads), short-term actions (e.g. construction
loading) and long-term actions. In order to allow for uncertainties in the
calculation of he magnitude of actions or combinations of actions and their
duration and spatial distribution, Euorcode requires the design values of actions
Fd to be used for the geotechnical design either to be assessed directly or to be
derived from characteristic values Fk :
Fd = Fk
The partial factor m: this factor is applied as a safety factor that the
characteristic values of the material is divided by this factor. (m = material
index) and covers :
Table 10-1 partial factors on material properties for conventional design situations for ultimate limit states
Table 10-2 partial factors on material properties for conventional design situations for service limit state
Normally the design values, d , Ed, tan , can be decided using the following formulae:
fd = fk/( n m)
Ed = Ek /( n m)
Where:
f = reaction force
E = elastic module
Class n
A 1.0
B 1.1
C 1.2
pile b s
d N NC Nq