good

1. 112
Deep Foundations

2. 113
6.0 Construction and selection of deep foundations
Types of Foundation
The foundations can be divided into two main types, namely: Shallow foundations; and Deep
foundations. Terzaghi categorized foundations into the above categories based on the depth of the
foundation below the existing ground surface and classified the deep foundations as the foundations,
whose depth is more than the width of the foundation. However, classification of the foundations based
on the Terzaghi’s concept serves very little purpose in design and construction of the foundations. For
example, an individual footing foundation, having a depth of embedment more than the width, is
designed and constructed in the same way as a footing, whose depth of embedment is less than the
width. Therefore, for engineering purposes foundations should be classified so that there is a clear
difference between the design and construction of the two types. For this purpose, classification of the
foundations based on the load transfer mechanism to the soil or rock is more appropriate. According to
this classification, as shown in Figure 1.1, foundations with horizontal spreading of the superstructure
load are considered as shallow foundations whereas foundations with vertical load distribution are
classified as deep foundations. Therefore, spread footings, combined footings and raft foundations,
where concentrated forces are distributed laterally, are considered as shallow foundations. Similarly,
piles are the most commonly used type of foundations where vertical distribution of the load takes
place.
Figure 1.1 - Classification of the foundation based on the load transfer mechanism.
Classification of piles
The British Standard Code of Practice for Foundations (BS 8004) places in three categories. These are
as follows.
Large displacement piles - comprise of solid-section piles or hollow-section piles with a closed end,
which are driven or jacked into the ground and thus displace the soil. All types of driven, and driven
and cast-in-place piles come into this category.
Small-displacement piles are also driven or jacked into the ground but have a relatively small cross-
sectional area. They include rolled steel H- or I-sections, and pipe or box sections driven with an open
end such that the soil enters the hollow section. Where these pile types plug with soil during driving
they become large displacement types.
F
F
(b)
Horizontal distribution
of the force
Vertical
distribution
of the force
(a)

3. 114
Replacement piles are formed by first removing the soil by boring using a wide range of drilling
techniques. Concrete may be placed into an unlined or lined hole, or the lining may be withdrawn as
the concrete is placed. Performed elements of timber, concrete, or steel may be placed in drilled holes.
Types of piles in each of these categories can listed as follows.
Large displacement piles (driven types)
1. Timber (round or square section, jointed or continuous).
2. Precast concrete (solid or tubular section in continuous or jointed units).
3. Prestressed concrete (solid or tubular section).
4. Steel tube (driven with closed end).
5. Steel box (driven with closed end).
6. Fluted and tapered steel tube.
7. Jacked-down steel tube with closed end.
8. Jacked-down solid concrete cylinder.
Large displacement piles (driven and cast-in-place types)
1. Steel tube driven and withdrawn after placing concrete.
2. Precast concrete shell filled with concrete.
3. Thin-walled steel shell driven by withdrawable mandrel and then filled with concrete.
Small-displacement piles
1. Precast concrete (tubular section driven with open end).
2. Prestressed concrete (tubular section driven with open end).
3. Steel H-section.
4. Steel tube section (driven with open end and soil removed as required).
5. Steel box section (driven with open end and soil removed as required).
Replacement piles
1. Concrete placed in hole drilled by rotary auger, baling, grabbing, airlift of reverse-circulation
methods (bored and cast-in-place).
2. Tubes placed in hole drilled as above and filled with concrete ass necessary,,
3. Precast concrete units placed in drilled hole.
4. Cement mortar or concrete injected into drilled~hole.:
5. Steel sections placed in drilled hole.
6. Steel tube drilled down.
Composite piles
Numerous types of piles of composite construction may be formed by combining units in each of
the above categories, or by adopting combinations of piles in more than one category. Thus
composite piles of a displacement type can be formed by jointing a timber section to a precast
concrete section, or a precast concrete pile can have an H-section jointed to its lower extremity.
Composite piles consisting of more than one type can be formed by driving a steel or precast
concrete unit at the base of a drilled hole, or by driving a tube and then drilling out the soil and
extending the drill hole to form a bored and cast-in-place pile.

4. 115
In Sri Lanka mostly cast in-situ concrete piles or driven piles are used for pile foundations.
Selection of pile type
Bored piles are constructed by drilling a hole into the ground and filling the hole with concrete
after inserting a reinforcement cage. In comparison, driven piles are constructed by driving a
preformed pile into the ground through application of hammer blows or vibration to the top of
the pile. General factors that should be considered in selecting the type of the pile between
driven piles and bored & cast in-situ piles are discussed in this section while detailed
construction procedures of the two piling methods are discussed in subsequent sections of this
chapter.
Bored piles can be very effectively used when a hard layer is present at shallow depths. The
structural loads can be easily transferred to the hard layer and generally the cast in-situ bored
piles are designed as end bearing piles. Other main advantage of bored piles is its ability to
penetrate minor obstructions, such as boulders, which cannot be penetrated using driven piles.
Such obstructions are commonly found in residual formations in the form of unewathered core
stones commonly referred to as boulders. A driven pile, terminated on a boulder, has a lower
carrying capacity due to the possibility of pile slipping along the face of the boulder and hence,
undergoing large displacement. Therefore, in such situations, use of driven piles is not advisable.
It is very common to find thick weathered or fractured rock above the solid bedrock. There are
certain places, where weathering profile is variable and the rock head is steeply sloping. Under
such situations, it is very important to socket the pile into the bedrock so that the full end bearing
could be mobilized without slipping of the pile toe. Thick weathered zones can be penetrated
using cast in-situ boring techniques and the structural loads can be transferred to the underlying
strong solid rock formations. Compared to driven piles, the ability of the bored piles to penetrate
fractured rock is a tremendous advantage in such situations.
The length of a bored pile can be adjusted easily and in a variable soil or bedrock profile, it is a
definite advantage. Diameter can also be varied and if large diameter piles are used, additional
cost associated with the construction of the pile cap, connecting a group of driven piles, can be
eliminated. The noise and ground vibrations associated with the other driven piling
methodologies are greatly reduced and installation process can be carried out even in a highly
built up area without environmental concerns. Moreover ground heaving associated with
installation of large volume displacement piles is not present with replacement type piles such as
bored and cast in-situ piles. The structural design of the pile should be carried out only
considering the working stresses and reinforcement required for driving and handling stresses
are not needed as in driven piles.
Drilling and concreting is carried out at a certain depth below the ground surface, and in most
cases under a drilling mud. The contamination of the concrete with the drilling slurry, formation
of voids within the pile, necking due to flowing of the sides into the unlined bore, collapsing of
the sides are some of the difficulties associated with bored and cast piles. Moreover, improper
cleaning of the pile bottom can cause considerable reduction in the end bearing capacity of bored
and cast in-situ piles. Formation of a thick hardened layer of bentonite along the sides of the
drilled hole is possible if the bentonite slurry is kept in the borehole for a long period of time.
During concreting the inability of the rising concrete surface to remove the hardened bentonite
may reduce the skin friction along the shaft of bored piles. Therefore, a proper quality
controlling program during the installation process is essential to minimize the defects in the
bored and cast in-situ piles.
As a preformed pile is used, certain concerns associated with bored piles such as necking,
improper cleaning of the borehole, defects due to mixing of concrete and drilling mud etc. are

5. 116
not present with driven piles. When the amount of soil replaced during the installation of the pile
is considered, driven piles fall into small or large volume displacement type. If a pile with a
large volume such as a precast concrete pile is used, the amount of soil displaced during
installation of the pile is large and it is considered as a large volume displacement pile. Whereas
if a pile with a relatively small volume such as a steel pile having a H or I section, is used as a
driven pile the amount of soil displaced during driving is small and such piles are classified as
small volume displacement piles. As a certain amount of soil is displaced during the installation
of a driven pile, the surrounding soil is compacted and thus the soil surrounding the pile is
improved giving rise to increase in the carrying capacity. In comparison, bored piles are
classified as replacement piles and the stress surrounding a borehole for a bored pile is relaxed
resulting reduction of the carrying capacity. Furthermore, the reduction of the skin frictional
capacity due to usage of drilling agents such as bentonite slurry is not present with driven piles.
The structural integrity and the capacity of driven piles are enhanced as the pile is formed under
controlled conditions above the ground surface. But the cast in-situ piles are formed below the
ground surface and in most situations under water making such high level of quality controlling
impossible. Another advantage of the driven piles is the ability to use the piling material
depending on the availability. As an example, when timber is available in abundant, timber piles
can be used for the foundation. Eventhough, noise and vibration generated during driving is very
critical in built up areas, such concerns are not significant in remote areas. Therefore, driven
piles can be used in such situations. If the noise is a concern special techniques such as silent
pile drivers can be used to reduce such environmentally unfriendly effects.

6. 117
Driven displacement piles
Timber piles
In many ways, timber is an ideal material for piling It has a high strength to weight ratio,
it is easy to handle., it is readily cut to length and trimmed after driving, and in favourable
conditions of exposure durable species have an almost indefinite, life. Timber piles used
in their most economical form consist of round untrimmed logs which are driven butt
uppermost. The practice of squaring the timber can be detrimental to its durability since it
removes the outer sapwood which is absorptive to creosote or some other liquid
preservative. The less absorptive heart-wood is thus exposed and instead of a pile being
encased by a thick layer of well-impregnated sapwood, there is only thin layer of treated
timber which can be penetrated by the hooks or slings used in handling the piles, or
stripped off by obstructions in the ground
Timber piles, when situated wholly below ground-water level, are resistant to fungal
decay and have an almost indefinite life. However, the portion above ground-water level
in a structure on land is liable to decay. Although creosote or other preservatives extend
the life of timber in damp or dry conditions they will not prolong its useful life
indefinitely. Therefore it is the usual practice to cut off timber piles just below the lowest
predicted ground-water level and to extend them above this level in concrete. If the
ground-water level is shallow the pile cap can be taken down below the water level.
Bark should be removed from round timbers where these arc to be treated with
preservative. If this is not done the bark reduces the depth of impregnation. Also the bark
should he removed from piles carrying uplift loads by skin friction in case it should
become detached from the trunk, thus causing the latter to slip. Bark need not be removed
from piles carrying compression loads or from fender piles of uncreosoted timber
(hardwoods are not treated because they will not absorb creosote or other liquid
preservatives).
The timber should be straight-grained and free from defects which could impair its
strength and durability. BS 8004 states that a deviation in straightness from the centre-
line of up to 25 mm on a 6 m chord is permitted for round logs but the centre-line of a
sawn timber pile must not deviate by more than 25 mm from a straight line throughout its
length. The Swedish Code SBS-S 23:6 (1968) permits a maximum deviation of 1% of
length between two arbitrarily selected measuring points which must be at least 3 m
apart.
The requirements of BS 8004 of the working stresses in timber piles merely state that
these should not exceed the green permissible stresses given in CP 112 for compression
parallel to the grain for the species and grade of timber being used. The Code suggests
that suitable material will be obtained from stress grades ss and better. Timber piles are
usually in a wet environment when the multiplying factors should be used to convert the
dry properties to the wet conditions. When circulating the working stress on a pile
allowance must he made for bending stresses due to eccentric and lateral loading and to
eccentricity caused by deviations in the straightness and inclination of a pile Allowance
must also be made for reduction in the cross-sectional area due to drilling or notching and
to the taper on a round log.

7. 118
This limitation is applied in order to avoid the risk of damage to a pile by driving it to
some arbitrary ‘set’ as required by a dynamic pile-driving formula and also to avoid a
high concentration of stress at the toe of a pile end bearing on a hard stratum. Damage to
pile during driving is most likely to occur at its head and toe.
The problems of splitting of the heads and unseen ‘brooming’ and splitting of the toes of
timber piles occur when it is necessary to penetrate layers of compact or cemented soils
to reach the desired founding level. This damage can also occur when attempts are made
to drive deeply into dense sands and gravels or into soils containing boulders, in order to
mobilize the required skin-frictional resistance for a given uplift or compressive load.
Judgment is required to assess the soil conditions at a site so as to decide whether or not it
is feasible to drive a timber pile to the depth required for a given load without damage, or
whether it is preferable to reduce the working load to a value which permits a shorter pile
to be used. As an alternative, jetting or pre-boring may be adopted to reduce the amount
of driving required. The temptation to continue hard driving in an attempts to achieve an
arbitrary set for compliance with some dynamic formula must be resisted. Cases have
occurred where the measured set achieved per blow has been due to the crushing and
brooming of the pile toe and not to the deeper penetration required to reach the bearing
stratum.
Damage to a pile can be minimized by reducing: as far as possible the number of hammer
blows necessary to achieve the desired penetration, and also by limiting the height of
drop of the hammer. This necessitates the use of a heavy hammer which should at least
be equal in weight to the weight of the pile for hard driving conditions, and to one-half of
the pile weight for easy driving. The German Code (DIN 183.04) limits the hammer drop
to 2.0 m normally and to 2.5 m exceptionally.
The lightness of timber pile can be an embarrassment when driving groups of piles
through soft clays or silts to a point bearing on rock. Frictional, resistance in the soft
materials can be very low for a few days after driving, and’ the effect of pore pressures
caused by driving adjacent piles in the group may cause the pile already driven to rise out
of the ground due to their own buoyancy relative to that of the soil. The only remedy is to
apply loads to the pile heads until all the piles in the area have been driven.
Heads of timber piles should be protected against splitting during driving by means of a
mild steel hoop slipped over the pile head or screwed to it. A squared pile toe can he
provided where piles are terminated in soft to moderately stiff clays. Where it is
necessary to drive them into dense or hard materials a cast steel point should be provided
(Figure 1.2). As an alternative to a hoop, a cast steel helmet can be fitted to the pile head
during driving. The helmet must be deeply recessed and tapered to permit it to fit well
down over the pile head, allowing space for the insertion of hardwood packing.

8. 119
Figure 1.2 - Typical shoes to be used with timber piles. (Technical specification EI
02G001, US Army Corp of Engineers)
Precast concrete piles
Precast concrete piles have their principal use in marine and river structures, i.e. in
situations where the use of driven-and-cast-in-situ piles is impracticable or uneconomical.
For land structures unjointed precast concrete piles are frequently more costly than
driven-and-cast-in-situ types for two main reasons.
1. Reinforcement must be provided in the precast concrete pile to withstand the
bending and tensile stresses which occur during handling and driving. Once
the pile is in the ground, and if mainly compressive loads are carried, the
majority of this steel is redundant.
2. The precast concrete pile is not readily cut down or extended to suit variations
in the level of the bearing stratum to which the piles are driven.
However, there are many situations for land structures where the precast concrete pile can
be more economical. Where large numbers of piles are to be installed on easy driving
conditions the savings in cost due to the rapidity of driving achieved may outweigh the
cost of the heavier reinforcing steel necessary. Reinforcement may be need in any case to
resist bending stresses due to lateral loads or tensile stresses from uplift loads. Where
high-capacity piles are to be driven to a hard stratum savings in the overall quantity of
concrete compared with cast-in-situ piles can be achieved since higher working stresses
can be used. Where piles are to be driven in sulphate-hearing ground or into aggressive
industrial waste materials, the provisions of sound high-qua1ity dense concrete is
ensured. The problem of varying the length of the pile can be overcome by adopting a
jointed type.
From the above remarks it can be seen that there is still quite a wide range of
employment for the precast concrete pile, particularly for projects where the costs of
establishing a precasting yard can be spread over a large number of piles. The piles can
be designed and manufactured in ordinary reinforced concrete, or in the form of pre-
tensioned or post-tensioned prestressed concrete members. The ordinary reinforced
concrete pile is likely to be preferred for a project requiring a fairly small number of

9. 120
piles, where the cost of establishing a production line for prestressing work on site is not
justifiable and where the site is too far from an established factory to allow the
economical transportation of prestressed units form the factory to the site. Precast
concrete piles in ordinary reinforced concrete are usually square or hexagonal and of
solid cross-section for units of short or moderate length, but for saving weight long piles
are usually manufactured with a hollow interior hexagonal, octagonal or circular sections.
The interiors of the piles can be filled with concrete after driving. This is necessary to
avoid bursting where piles are exposed to severe frost action. Alternatively drainage
holes can be provided to prevent water accumulating in the hollow interior. To avoid
excessive flexibility while handling and driving the usual maximum lengths of square
section piles and the range of working loads applicable to each size are shown in Table
1.1.
Where piles are designed to carry the applied loads mainly in end bearing, e.g., piles
driven through soft clays into medium-dense or dense sands, economies in concrete and
reductions in weight for handling can be achieved by providing the piles with an enlarge
toe.
Table 1.1 - Working loads and maximum lengths for ordinary precast concrete piles of
square section.
Pile size
(mm square)
Range of working Loads
(kN)
Maximum length
(m)
250 200 – 300 12
300 300 – 450 15
350 350 – 600 18
400 450 – 750 21
450 500 – 900 25
BS 8004 requires that piles should be designed to withstand the loads or stresses and to
meet other serviceability requirements during handling, pitching, driving and in service in
accordance with the current standard Code of Practice for the structural use of concrete.
If normal mixes are adopted a 40-grade concrete with a minimum 28-day cube strength of
40 N/mm2
is suitable for hard to very hard driving and for all marine construction. For
normal or easy driving, a 25-grade concrete is suitable. This concrete has a minimum 28-
day cube strength of 25 N/mm2
.
To comply with the requirements of BS 8110 precast piles of either ordinary or
prestressed concrete should have nominal cover to the reinforcement as follows.
Exposure conditions Normal cover for concrete grade of
25 30 40 50 and
over
Buried concrete and concrete continuously
under water
40 mm 30 mm 25 mm 20 mm
Alternative wetting and drying and freezing 50 mm 40 mm 30 mm 25 mm
Exposed to sea water and moorland water
with abrasion
__ __ 60 mm 50 mm

10. 121
Concrete cast in shell piles are constructed by driving a steel shell to a required depth by
using a mandrel and filling the shell with concrete after withdrawing the mandrel. Other
three types of concrete piles should be designed considering:
i. Bending stresses developed during handling;
ii. Dynamic stresses developed during driving; and
iii. Stresses due to working loads.
Longitudinal reinforcements are used to carry bending stresses developed during
handling of the precast concrete piles and lateral loads acting on the pile under working
condition. The bending moment diagram of single point handling and the corresponding
handling arrangement for minimum bending moment are given in Figure 1.3(b).
Similarly, the bending moment diagram and the arrangement for minimum bending
moment for double handling point mechanism is given in Figure 1.3(a). Steel stirrups are
used to carry driving stress acting on the pile. However, if the pile is subjected to static
vertical working loads, the reinforcement provided for handling and driving is mostly
redundant under working loads.
Figure 1.3 - Double and single lifting of precast piles: (a) Double lifting, bending moment
diagram and minimum bending moment; and (b) Single lifting, bending moment diagram
and the minimum bending moment.
Prestressed concrete piles have certain advantages over those of ordinary reinforced
concrete. Their principal advantage is in their higher strength to weight ratio, enabling
long slender units to be lifted and driven. However, slenderness is not always
advantageous since a large cross-sectional area may be needed to mobilize sufficient

11. 122
resistance in skin friction and end bearing. The second main advantage is the effect of the
prestressing in closing up cracks caused during handling and driving. This effect,
prestressed pile increased durability which is advantageous in marine structures and
corrosive soils.
The nominal mixes for precast reinforced concrete piles are related to the severity of
driving, and the working stresses appropriate to these mixes are shown in Table 2.6.
For economy in materials, prestressed concrete piles should be made with designed
concrete mixes with a minimum 28-day works cube strength of 40 N/mm2
. Metal shoes
are not required at the toes of precast concrete piles where they are driven though soft of
lose soils into dense sands and gravels or firm to stiff clays. A blunt pointed end appears
to be just as effective in achieving the desired penetration in these soils as a more sharply
pointed end and the blunt points is better for maintaining alignment during driving. A
cast-iron or cast-steel shoe fitted to a pointed toe may be used for penetrating rocks or for
splitting cemented soil layers.
During driving of the piles using an impact hammer, a compression stress wave travels
through the pile in the downward directions and reflected at the pile toe to travel upward
direction towards the pile top. If the end resistance at the pile toe is high (fixed end
condition) the reflected wave is compression and on the other hand, low resistance near
the pile toe results in tensile reflection at the pile toe. As a result, driving stresses are
maximum near the pile top and pile toe with reduced driving stresses in the middle
portion of the pile shaft. Therefore, more steel stirrups are provided near the pile top and
pile toe to take up the high driving stresses generated during driving. The requirements of
steel stirrups as specified in BS 8004 are given in Table 1.2 below.
Table 1.2 - The requirements of steel stirrups as specified in BS 8004 for driven precast
piles.
Volume of steel at head
and toe of pile
Volume of steel in
body of pile
Other requirements
0.6% gross volume over
distance of 3  pile width
from each end
0.2% of gross
volume spaced at
not more than ½ 
pile width
Lapping of shot bars with
main reinforcement to be
arranged to avoid sudden
discontinuity
Steel piles
Steel piles have the advantages of being robust, light to handle, capable of carrying high
compressive loads when driven on to a hard stratum, and capable of being driven hard to
a deep penetration to reach a bearing stratum or to develop a high skin-frictional
resistance, although their cost per metre run is high compared with precast concrete piles.
They can be designed as small displacement piles, which is advantageous in situations
where ground heave and lateral displacement must be avoided. They can be readily cut

12. 123
down and extended where the level of the bearing stratum varies; also the head of a pile
which buck1es during driving can be cut down an re-trimmed for further driving. They
have a good resilience and high resistance to buckling and bending forces. Types of steel
piles include plain tubes, box-sections, H-sections, and tapered and fluted tubes (Mono-
tubes). Hollow section-piles can be driven with open ends. If the base resistance must be
eliminated when driving hollow-section piles to a deep penetration, the soil within the
pile can be cleaned out by grabbing, by augers, by reverse water circulation drilling, or by
airlift. It is not always necessary to fill hollow-section piles with concrete. In normal
undisturbed soil conditions they should have an adequate resistance to corrosion during
the working life of a structure.
Where hollow-section piles are required to carry high compressive loads they may be
driven with a closed end to develop the necessary end-bearing resistance over the pile
base area. Where deep penetrations are required they may be driven with open ends and
with the interior of the pile closed by a stiffened steel plate bulkhead located at a
predetermined height above the toe. An aperture should be provided in the bulkhead for
the release of water, silt or soft clay trapped in the interior during driving. In some
circumstances the soil plug within the pile may itself develop the required base
resistance.
Concrete filling of light-gauge steel tubes is required after driving is completed because
the steel may be torn buckled or may suffer corrosion losses. Piles formed from thin steel
shells driven by means of an internal mandrel, which is withdrawn before filling the
shells with concrete.
The facility of extending steel piles for driving to depths greater than predicated from soil
investigation data has already been mentioned. The practice of welding-on additional
length of pile in the leaders of the piling frame is satisfactory for land structures where
the quality of welding may not be critical. A steel pile supported by the soil can continue
to carry high compressive loads even though the weld is partly fractured by driving
stresses. However, this practice is not desirable for marine structures where the weld
joining the extended pile may be above sea-bed level in a zone subjected to high lateral
forces and corrosive influences.
Bored and Cast In-situ concrete piles
Due to the presence of hard rock layers at relatively shallow depths, bored and cast in-situ
piles are very often used in Sri Lanka. Therefore, the construction procedure of bored and
cast in-situ piles are discussed here.
Replacement piles are installed by first removing the soil by a drilling process and then
construction the pile by placing concrete or some other structural element in the drilled
hole. As mentioned previously, bored piles are constructed by drilling a hole in the
ground and filling it with concrete with or without inserting a reinforcement cage. Since
the borehole in most cases is unlined, there is a possibility of flowing soft soils into the
borehole and forming a ‘necking’ in the pile shaft. Moreover, there could be collapsing of

13. 124
the sides in cohesionless soils and fallen out debris may mix with fresh concrete resulting
in weak spots in the pile shaft. Furthermore, concreting is mostly done underwater
making it impossible to compact fresh concrete. Therefore, special construction
methodologies and precautions had to be followed to ensure a defect free sound bored
and cast in-situ pile. Compared with the construction of shallow foundations,
construction of deep foundation is a challenging task as the construction is carried out at
deeper levels without directly observing it. As a result, indirect quality control measures
should be adopted during the construction process.
In most sites, the ground water table is located at shallow depths and the top soil layers
contain cohesionless soils. Due to the loose soil conditions at the top levels of the ground,
the probability of collapsing of the ground is more. Therefore, it is very common to
install a casing of about 5 to 6m length at the top level of the borehole. If the ground
condition at the lower levels of the subsurface doesn’t contain very loose sandy material,
very often casing of the entire hole is not done and drilling is continued with filling the
hole with bentonite slurry. The cutting through the overburden is usually done by
auguring or chiseling and the cutting debris are removed from the hole using a bucket or
wash boring techniques. Drilling above the water table is usually done using an auger as
shown in Figure 1.4(a) and Figure 1.4(b) shows some auguring tools used for drilling.
(a)

14. 125
(b)
Figure 1.4 – Auguring above the water table: (a) Auguring during drilling; and (b) some
auguring tools used.
Figure 1.5 shows the installation of temporary casing during the drilling process for bored
and cast in-situ piles. After installation of the casing, the center of the casing is checked
as shown in Figure 1.6. During the drilling process, it is very common to use a drilling
fluid, such as bentonite slurry, to keep the sides of the borehole stable. The borehole is
filled with drilling fluid, when the borehole reaches the ground water table. For this
purpose, a bentonite reservoir is formed either surrounding the pile bore or away from the
pile bore location. Figure 1.7 shows a bentonite reservoir surrounding the pilebore and
Figure 1.8 shows a bentonite reservoir away from the pilebore location.
Figure 1.5 – Installation of a temporary casing.

15. 126
Figure 1.6 – Checking the location of the center of the pile.
Figure 1.7 – Bentonite reservoir surrounding the pile bore.

16. 127
Figure 1.8 – Bentonite reservoir away from the location of the pilebore.
Use of Bentonite as a drilling mud
Two types of natural bentonite exist: swelling bentonite which is also called sodium
bentonite and non-swelling bentonite or calcium bentonite. Sodium bentonite expands as
it can absorb several times its dry weight of water. It is mostly used in drilling mud in the
oil and gas well drilling industries as it exhibits low filter loss. However, non-swelling (or
low-swelling) bentonite has much higher filter or fluid loss than swelling sodium
bentonite and hence, it is not effective as a drilling fluid. As it is commonly accepted, the
drilling mud should perform or facilitate following tasks:
i. Remove cuttings produced by the bit at the bottom of the hole and carry them
to the surface;
ii. Lubricate and cool the drill bit during operation, as friction causes high
temperatures down-hole that can limit tool life and performance;
iii. Maintain hydrostatic equilibrium so that water from the surrounding soil do
not enter the borehole causing the wall to flow, kink and blow out. This is
achieved by adjusting the mud weight (density);
iv. Build a filter cake (or skin) on the wall of the drilled hole, preventing fluid
loss by mud invasion of penetrated formations; and
Pump used to circulate
bentonite

17. 128
v. Support and prevent caving of the wall of the hole.
Typically if 3% or more of bentonite powder is dispersed in water, a viscous slurry is
formed which is thick when allowed to stand but becomes thin when agitated. This
phenomenon is referred to as thixotropy.
Bentonite slurry provides the stability to the borehole walls by two main actions: (i)
Formation of a filter skin termed “cake” at the interface of the slurry and the walls of the
excavated hole; and (ii) higher lateral pressure of the dense slurry pushing against the
filter skin and the walls of the excavated hole. The concreting of the hole should be done
in such a way to displace the slurry in the hole with the fresh concrete. However, if the
slurry full of hole is kept for a long period of time, a thicker and harder “cake” will be
formed on the internal walls of the borehole. If the soil surrounding the pile shaft is
permeable, the water in the bentonite slurry may seep into the surrounding area forming a
thicker filter cake. Some researchers have shown that it is possible to form a thin cake of
few millimeters even in clayey soils, which is quite impermeable. The formation of filter
cake in clayey soils to electrical forces or chemical reaction of bentonite suspension on
the wall of the borehole. It is argued that if the shear strength of the filter cake formed is
more than that of the fluid concrete, it cannot be scoured by the rising concrete surface
during concreting and may be left in place resulting in degradation of the development of
skin friction.
It is believed that the formation of the major portion of the filter cake, and hence the
reduction of the major portion of the skin friction capacity, takes place within first few
hours of the construction time and further increase in construction time have minor effect
on the reduction of the skin friction capacity. The formation of the filter cake, which
reduces the development of skin friction, takes place at a higher rate within first few
hours between the end of drilling and concreting. The delay time between the end of
drilling and concreting should be minimized to reduce the effects of the filter cake on the
development of skin friction in bored and cast in-situ piles. .
Formation of the filter cake takes place as the water in the bentonite slurry seeps to the
surrounding area in sandy soils or chemical action between bentonite slurry and the
surrounding clayey soils. Within the rock socketed length of the pile, seeping of the water
takes place through the cracks in the rock mass. If the cracks are open and filled with
high permeable debris, large quantity of water may seep into the surrounding area and
formation of the filter cake may be enhanced. However, if the rock mass surrounding the
socketed length is impervious, the filter cake formed may be limited to a very thin layer.
If the bottom of the borehole is cleaned immediately before concreting, there is a high
probability that the thin layer of the filter cake formed within the rock socket may be
scraped off.
Drilling below the water table
The drilling below the water table can be carried out using rotary drilling or percussion
drilling. If a rotary drilling method is used, a drilling bucket, as shown in Figure 1.9, is
used to remove material from the pile bore and it is emptied, as shown in Figure 1.10. On
the other hand, a chisel is lifted and dropped to loosen the material in the percussion
drilling and then wash boring is used to remove the debris from the pile bore. Figure 1.11

18. 129
shows rigs used in the percussion drilling.
Figure 1.9 – Removing debris from the pile bore.
Figure 1.10 – emptying the drilling bucket.

19. 130
Figure 1.11 – Percussion drilling rigs.
Figure 1.12 – Rotary drilling tool used to drill through rock.

20. 131
Termination of pile bore
During the drilling process for the piles, rotary drilling or percussion drilling techniques
are used to drill through the rock. Since the coring through the rock is very rarely done,
the residue coming out consists of very small rock particles, which hardly gives any
indication of the quality of the bedrock. Therefore, the quality of the bedrock should be
established by some other means.
It is not uncommon in Sri Lanka to find sites with fairly thick weathered rock layers
overlying the sound bedrock. In this type of sites, very often large variation in pile
lengths are reported within very short distances. Due to the steep dip angle of the bedrock
and the highly fractured nature of the bedrock, a pile termination criterion plays a special
significance in this type of sites. A detail site investigation including the investigation of
the bedrock is a must for these sites to design a suitable pile foundation and to plan the
construction phase of the foundation. Another weakness in the site investigation
procedure adopted in Sri Lanka is the lack of coordination between the site investigation
firm and the client and/or the consultant. If the site investigation firm informs the site
conditions, for example the variation of the bedrock profile and the quality of the bedrock
at the site, to the client and/or the consultant during the field investigation phase then, the
site investigation program can be modified to suit the site conditions.
If the establishment of bedrock was not properly done during the site investigation
process, identification of the bedrock in a variable bedrock profile is highly questionable.
In a site, where the bedrock elevation highly varies across the site, during the site
investigation stage rock drilling should be carried out at reasonable number of points
across the site to establish the bedrock level with a relatively high RQD. Such
investigation will not only give more data needed for the design of the pile foundation but
also will provide very important information needed for planning the construction
process as well.
In a typical site with varying bedrock profile, it is very difficult to identify the bedrock
and estimate the pile socketing length during drilling for the piles. Some piling
contractors use highly subjective criteria such as penetration rate of the drilling tool as a
guide to establish the bedrock level. However, socketing length and the termination
criterion of the piles based on the rate of penetration of drilling tools could be highly
misleading as the drilling through the bedrock could give high and low penetration rates
depending on the weathered nature of the fractures in the upper part of the bedrock and
the quality of the cutting tool.
The pile termination criterion, for a site with varying bedrock profile preferably should be
done after installation of a test pile near a location of a borehole used for field
investigation. The information obtained during drilling for the test pile and the load test
result obtained from the test pile should be used for determination of the carrying
capacity of piles in the site and in deciding the termination criterion to be used for
installation of the production piles. It should noted here that a test pile should be loaded
upto twice the working load as specified in section 6.2 of ICTAD/DEV/15, not upto 1.5
times the working load as testing of a working pile.
Once the termination criteria for the site are established, the drilling process should start
from one side of the site and proceed forward. Level of the bedrock should be marked on

21. 132
a site map and preferably contours of the pile tip elevation should be plotted. The contour
map should be updated as the drilling progress and the pile tip elevation contours should
always be compared with the elevation of the bedrock established during the initial site
investigation process. Large variation of the pile tip elevation of a new pile from the
elevation shown by the contours should be carefully studied. For example, if the bedrock
is encountered at a higher elevation than the expected elevation of the bedrock from the
already established bedrock contours, drilling should be done to make sure that the pile
has not hit a core stone in the weathered rock layer.
Cleaning of the Borehole before Concreting
This is another very important aspect, which is not given due attention, during
construction of the piles. If the pile bottom is not properly cleaned before concreting is
done, there could be a layer of waste material present between the bottom of the pile and
the bedrock. As this material consists of unconsolidated loose debris, when the pile
bottom is loaded, it will undergo large settlement. The debris that is present may consist
of:
i. Granular material from the drilling operation through rock and soil, which is
in suspension with the drilling mud, may settle to the bottom of the borehole;
ii. Small block-like portions of soil and rock from the unlined wall of the
borehole may dislodge and fall down to the bottom of the borehole; and
iii. Ground water percolated through the pervious silty and sandy layers may
transport and deposit significant amount of sandy and silty material at the
bottom of the unlined borehole.
Even if the concreting procedure is methodical to give a defect free pile shaft, the
presence of the loose material below the pile bottom severely hamper the load carrying
capacity of the pile due to large settlement it undergoes.
Through the surveys carried out in Sri Lanka it is found that about 5% of the piles are not
according to the specifications and are categorized as ‘defective’. The analysis of the
load-settlement curves and the site conditions of the ‘defective’ piles indicated that the
piles have undergone large plunging type settlement under very small end bearing
resistance due to the presence of a relatively soft layer below the bottom of the pile.
Therefore, the most probable reason for presence of a soft layer beneath the pile toe is the
improper cleaning of the bottom of the borehole before concreting.
Concreting of Cast in-situ Bored Piles
The borehole may be dry, partially or completely filled with fluid before concreting.
Concreting under dry conditions should be done from dropping concrete from the ground
surface so that concrete ‘free falls’ onto the base of the borehole. A hoper or a guide
trunk should be used at the ground surface level to avoid contamination of the concrete
with soil near the ground surface level. The mix design of the concrete should be done to
produce a workable mix, which is self compacting without segregation. However,
segregation of fresh concrete may take place when the falling concrete hits the
reinforcement cage. To reduce the segregation of concrete this way, some contractors

22. 133
sprinkle cement powder on to the reinforcement cage before concreting.
Concreting a borehole partially or completely filled with fluid is a difficult task and
requires careful planning and supervision to construct a defect free pile. Since it is not
possible to observe the actual concreting process taking place down the borehole, some
indirect quality controlling measures should be adopted to ensure defect free pile. The
contractor, in consultation with the consultant to the project, should device a suitable
quality control program prior to the beginning of the piling process. In devising such
quality control program, due consideration should be given to the subsurface conditions
of the site. It is very often observed that the piling contractors don’t change their
construction process to suite the subsurface condition.
The concreting of the pile under water should be carried out using a tremie pipe. The
tremie pipe should be watertight and the interior surface should be free from any
projections for unhindered passage of concrete through it. Typically 125mm to 200 mm
diameter tremie pipes are used to concrete bored piles in Sri Lanka. Usually larger
diameter pipes are used to concrete large diameter piles and/or concrete with large
aggregates. It should be reiterated here that before the commencement of concreting,
drilling mud at the bottom of the borehole should be checked for contamination.
The tremie pipe is assembled inside the borehole, which is full of bentonite slurry. The
funnel (or hoper) is attached at the top of the assembled tremie pipe, which is long
enough to reach the pile bottom as shown in Figure 1.13(a) A plug is placed at the bottom
of the hoper and a small volume of suitable buoyant material is placed between the
bottom of the fresh concrete in the funnel and slurry in the tremie pipe as shown in Figure
1.13(b). The purpose of the buoyant material is to keep the first batch of concrete mixing
with the slurry in the tremie pipe. Otherwise, during the falling of the first batch of
concrete through the tremie tube, washing of concrete and mixing it with bentonite slurry
may take place significantly weakening the concrete. The hopper is filled with concrete
with the removable plug placed at the bottom of the hopper. Then, the plug is jerked out
allowing fresh concrete to shoot down under its own weight to the bottom of the
borehole. Concrete rapidly moving down the tremie pipe may push the drilling mud in
the pipe through the bottom as shown in Figure 1.13(c). Thus the first charge of concrete
is placed and the bottom of the tremie is immersed in fresh concrete to create a sealed
environment inside the tremie from the drilling mud outside. There are two potential
problems associated with initial charging of tremie with concrete:
(i) Segregation of concrete during placement and;
(ii) Entrapment of air inside the tremie pipe.
To avoid these problems, the tremie should be filled slowly after placing the initial
charge. During the time period, from initial charging of the pile to end of concreting, the
bottom of the tremie pipe should be always kept below the top surface of the concrete
inside the borehole. The depth of embedment of the tremie pipe in the borehole should be
about 1.5m to 3.0m and higher depth of embedment should be used for concreting large
diameter piles.

23. 134
Figure 1.13 - Concreting a borehole using tremie pipe: (a) Tremie is assembled in the
borehole; (b) A plug is placed at the bottom of the hopper and filled with concrete; (c)
Plug is removed and concrete moving through the tremie; and (d) Concreting continued
with bottom of tremie pipe immersed in fresh concrete.
The specific gravity of the drilling mud may go up with the degree of contamination of
the drilling mud with silt and other debris and the specific gravity of the drilling mud
should be less than 1.25 before the beginning of the concreting process. If the drilling
mud is contaminated with drilling debris and other substances, additional recycling or
substitution of the suspension is necessary so that the flow of fresh concrete can readily
replace the drilling mud at the bottom of the borehole. Concreting should be done in a
continuous operation without any interruptions. Therefore, the site engineer should make
necessary arrangements for continuous supply of concrete without delay. A contingency
plan should also in place to supply concrete if delay in the expected supply of concrete
happened due to some unforeseen reasons.
It is observed at most of the sites, that the tremie pipe is lifted up and lowered rapidly to
facilitate rapid flow of concrete. Since rapid lifting and lowering of the tremie causes the
mixing of drilling mud and the concrete within a certain zone surrounding the tremie,
such practice should be minimized or such movement should be limited to a small height.
Due to some reason if the tremie bottom is taken out of the fresh concrete, placement of
concrete should be stopped and the following procedure should be adopted in
recommencing the concreting process.

24. 135
 The tremie should be gently lowered on to the surface of the
previously placed concrete with very little penetration. The tremie
should be filled with high slump concrete with higher cement content
and a new initial charging of the tremie should be done to displace the
laitance/scum at the top of the old concrete surface with fresh concrete.
 The tremie should be pushed further slowly making fresh concrete
sweep away laitance/scum in its way.
However, if there is any delay in recommencing the concreting of the borehole, the above
procedure may not be applied as replacement of laitance/scum of set or partially set
concrete cannot be effectively carried out. In such situations, a new pile fully or partially
replacing the problematic pile should be introduced.
Withdrawing the casing is another important process that has to be done during the
concreting process. The rate of withdrawing the casing is the governing factor. If casing
is withdrawn too fast, the
Minipiles and micropiles
Minipiles are defined in CIRIA report PGI(2.10)
as piles having a diameter of less than 300
mm, with working loads in the range of 50 to 500kN. The term “micro-pile is given to
those in the lower range of diameter. They can be installed by a variety of methods.
Some of these are:
i. Driving small-diameter steel tubes followed by injection of grout with or
without withdrawal of the tubes;
ii. Driving thin wall shells in steel or reinforced concrete which are Oiled with
concrete and left in place;
iii. Drilling holes by rotary auger, continuous flight auger, or percussion
equipment followed by placing a reinforcing cage and in-situ concrete in a
manner similar to conventional bore pile construction;
iv. Jacking-down steel tubes, steel box-sections. or precast concrete sections. The
sections may be jointed by sleeving or dowelling.
The principal use of minipiles is for installation in conditions of low headroom such as
underpinning work or for replacement of floors of buildings damaged by subsidence.
Factors governing choice of type of pile
The advantages and disadvantages of the various forms of pile described in 2.2 to 2.5
affect the choice of pile for any particular foundation project and these are summarized as
follows:
Driven displacement piles
Advantages
1. Material forming pile can be inspected for quality and soundness before driving.
2. Not liable to ‘squeezing’ or ‘necking’.
3. Construction operations not affected by ground water.

25. 136
4. Projection above ground level advantageous to marine structures.
5. Can be driven in very long lengths.
6. Can he designed to withstand high bending and tensile stresses.
Disadvantages
1. Unjointed types cannot readily be varied in length to suit varying level of bearing
stratum.
2. May break during driving, necessitating replacement piles.
3. May suffer unseen damage which reduces carrying capacity.
4. Uneconomical if cross-section is governed by stresses due to handling and driving
rather than by compressive, tensile, or bending stresses caused by working conditions.
5. Noise and vibration due to driving may be unacceptable.
6. Displacement of soil during driving may lift adjacent piles or damage adjacent
structures.
7. End enlargements, if provided, destroy or reduce skin friction over shaft length.
Driven-and-cast-in-place displacement piles
Advantages
1. Length can easily be adjusted to suit varying level of beating stratum
2. Driving tube driven with closed end to exclude ground water
3. Enlarged base possible
4. Formation of enlarged base does not destroy or reduce shaft skin friction
5. Material in pile not governed by handling or driving stresses
6. Noise and vibration can be reduced in some types by driving with internal drop-
hammer
Disadvantages
1. Concrete in shaft liable to be defective in soft squeezing soils or in conditions of
artesian water flow where withdrawable-tube types are used.
2. Concrete cannot be inspected after installation.
3. Length of some types limited by capacity of piling rig to pull out driving tube
4. Displacement may damage fresh concrete in adjacent piles or lift these piles or
damage adjacent structures. .
5. Noise and vibration due to driving may be unacceptable
6. Cannot be used in river or marine structures without special adaptation.
7. Cannot be driven with very large diameters.
8. End enlargements arc of limited size in dense or very stiff soils.
9. When light steel sleeves arc used in conjunction with withdrawable driving tube, skin
friction on shaft will be dest toyed or reduced.

26. 137
Bored-and-Cast-in-Place replacement piles
Advantages
1. Length can readily he varied to suit variation in level of bearing stratum.
2. Soil or rock removed during boring can be inspected comparison with site
investigation data
3. In-situ loading tests Can be made in large-diameter pile boreholes, or penetration tests
made in small boreholes.
4. Very large (up to 7.3m diameter) bases can be formed in favourable ground.
5. Drilling tools can break up boulders or other obstructions which cannot be penetrated
by any form of displacement pile.
6. Material forming pile is not governed by handling or driving stresses.
7. Can be installed in very long lengths.
8. Can be installed without appreciable noise or vibration.
9. No ground heave.
10. Can be installed in conditions of low headroom.
Disadvantages
1. Concrete in shaft liable to squeezing or necking in soft soils where conventional types
are used.
2. Special techniques needed for concreting in water-bearing soils.
3. Concrete cannot be inspected after installation.
4. Enlarged bases cannot be formed in cohesionless soils.
5. Cannot be extended above ground level without special adaptation.
6. Low end-bearing resistance in cohesionless soils due to loosening by conventional
drilling operations
7. Drilling a number of piles in group can cause loss of ground and settlement of
adjacent structures.

27. 138
Design of Piles

28. 139
7.0 Design of Piles
Design criteria
Similar to a shallow foundation, failure of a structurally intact pile can be caused due to
two reasons: (i) shear failure of the soil surrounding the pile and; (ii) excessive settlement
of the foundation. Therefore, the task of the foundation designer is to find out an
economical pile to carry the working load with a low probability of shear failure while
keeping the resulting settlement to within allowable limits. In designing a single pile
against shear failure, it is customary to estimate the maximum load that can be applied to
a pile without causing shear failure, generally referred to as the ultimate carrying
capacity.
As in the case of shallow footings, two design approaches, (1) Allowable Stress Design
(ASD) method and (2) Load Resistance Factor Design (LRFD) method are available for
piles. The following sections will mostly elaborate the ASD method. The allowable stress
design (ASD) requires the following conditions:
Allowable loads
alln QFSP / (6.1 )
where
nP = ultimate resistance of pile
allQ = allowable design load
FS = factor of safety
The ultimate working load that can be applied to a given pile depends on the resistance
that the pile can produce in terms of side friction and point bearing (Figure 6.2). As the
pile is loaded at the pile top, the pile tends to move in the downward direction relative to
the surrounding soil. Therefore, the surrounding soil offers resistive force against that
relative movement. Hence the expression for the allowable load Pa on a pile would take
the following form:
FS
PP
Q
supu
all

 (6.3)
where
Ppu = ultimate point capacity
Psu = ultimate side friction
Determination of the ultimate carrying capacity of piles
There are mainly two different methods available to estimate the ultimate carrying
capacity of piles: i. static methods, and ii. dynamic methods.
Static methods can be further divided into following methods:
a. Using strength parameters of soil and/or rock;
b. Using empirical correlations and in-situ test results; and
c. Using static pile load test results.

29. 140
Dynamic methods can also be sub-divided into following methods:
a. Using pile driving equations;
b. Using the wave equation method; and
c. Dynamic testing of the piles.
Figure 2.1 Load carrying mechanism of piles.
Static methods of estimation of the ultimate carrying capacity of piles
The ultimate carrying capacity of piles is the maximum load that can be applied on the
pile without causing shear failure of the surrounding soil both along the pile shaft and at
the pile bottom. As the skin friction may not be uniform along the pile shaft, the skin
friction is estimated by adding the skin friction along the pile shaft.
 siupu PPP ,
However, it is observed that the deformation required to develop the ultimate point
bearing capacity is much higher compared to the deformation required to develop the
ultimate skin frictional capacity. Therefore, some define the ultimate carrying capacity of
the pile as summation of the ultimate skin friction and the developed end bearing capacity
when the skin friction reaches the ultimate value.
 usipu PPP ,
The total pullout resistance of the pile may be estimated using the following Equation:
Friction along the pile
shaft (skin friction)
Resistance at the pile point

30. 141
pusiu WPT   ,
uP = Ultimate (maximum) pile capacity in compression-usually defined as that load
producing a large penetration rate in a load test
Tu = Ultimate pullout capacity
Pp, u = ultimate pile tip capacity – seldom occurs simultaneously with ultimate skin
resistance capacity  usiP , : neglect for floating piles (which depends only on skin
resistance)
pP = tip capacity developed simultaneously with  usiP , : neglect for “floating piles”
 siP = skin resistance developed simultaneously with ultimate tip resistance Pp, u
: neglect for point bearing piles
 usiP , = ultimate skin resistance developing simultaneously with some tip
resistance Pp
Wp = weight of the pile being pulled
 = summation process over I soil layers making up the soil profile over length
of pile shaft embedment
The ultimate capacity of a pile can be generally written as:
bbusuu WPPP 
suP = ultimate shaft resistance
buP = ultimate base resistance
Wb = weight of the pile
Skin Friction
Development of skin friction in piles
A pile, which is in contact with the soil along its shaft, is loaded as shown in Figure
2.2(a). Due to the higher stiffness of the pile material relative to that of the soil, as the
load on the pile is applied, the pile tends to move in the downward direction relative to
the surrounding soil. This is similar to the situation, where two solid objects in contact
with each other, one of the objects tries to move relative to the other object. Naturally, a
resistive force is developed between the two objects to resist that attempted movement.
If two soil and pile elements in contact with each other are considered, the pile element
tends to move in the downward direction relative to the surrounding soil element as
shown in Figure 2.2(b). An imaginary space is created between the pile and the soil
elements, in reality they are in contact with each other. As the pile element tends to move

31. 142
in the downward direction relative to the surrounding soil element, the soil element also
moves with it and the downward moving soil element applies an upward resistive force
(fs) on the pile element, as shown in Figure 2.2(b). The pile element apply an equal an
opposite downward force on the soil element.
As the downward displacement of the pile element increases, the resistive force
developed between the pile and the soil elements is increased as shown in Figure 2.2(c).
However, that resistive force cannot increase indefinitely. The resistive force developed
reaches a maximum, commonly referred to as the ultimate skin friction (fs,u). The
relationship between the skin frictional force and the downward deflection of the pile
element can be approximated as shown in Figure 2.2(c). The downward displacement
required to mobilize the ultimate skin friction resistance is relatively low and is in the
rage of 5 – 10mm. After the ultimate skin friction is mobilized, the pile and the soil
elements start to slip.
Figure 2.2 – Development of skin friction on pile.
Load Transfer Curves.
The axial force variation in the pile with the depth is referred to as the load transfer curve.
If the pile is not subjected to negative skin friction, the axial force in the pile is maximum
at the pile top and is equal to the applied force. Considering the static equilibrium of the
section of the pile upto a depth z, the following Equation could be written:

fs
fs
Pile element Soil element
Imaginary separation between
pile and soil elements
(a)
(b)
(c)
Relative displacement of
the pile element ()
Skin friction developed (fs)
fs,u
o

32. 143
1sazat fPP 
From the above Equation it is clear that if the skin friction is acting in the upward
direction, the axial force decreases with the depth.
Considering the equilibrium of the small element of length dz shown in Figure, following
equilibrium equation could be written:
sdzzaaz dfPP   )(
Re-arranging the terms in the above Equation,
)( dzzaazs PPdf 
The skin friction at a given section is equal to the difference in the axial force at that
section.
Figure 2.2 - Axial force along the pile axis.
fs1
dfs
(a)
(b)
z
dz
Pa Pat
Paz
Pa(z+dz)

33. 144
Figure 2.4 load transfer curves obtained by increasing the load acting on the top of a pile
in clayey soil.
Figure 2.5 load transfer curves obtained by increasing the load acting on the top of a pile
in sandy soil.

34. 145
If the load transfer curve is vertical at a given section, it indicates that the skin friction in
that section is zero. Figures 2.4 and 2.5 are load transfer curves obtained by varying the
force acting at the top of piles and clay and sand respectively. Careful observation of
Figure 2.4 clearly indicates that when the force on the pile is increased from 300 kips, the
shape of the curves do not change significantly but the curves are shifted to the right (i.e.
the difference in the axial force between any two sections of the pile shaft does not
change significantly). This is due to reaching of the skin friction to the ultimate value.
Once the skin friction reaches the ultimate value along the entire pile shaft, the additional
load increased at the pile top directly increases the end bearing at the pile bottom. The
other point to note is the mobilization of the end bearing capacity. Initially only a very
small portion of the end bearing capacity is mobilized. But as the skin friction reaches the
ultimate capacity, the end bearing resistance increases significantly, finally reaching a
situation where load increment at the pile top is causing equal increment in the end
bearing resistance. From this it could be concluded that the initial load increments on
piles are taken up by mobilizing skin friction and very minimal end bearing is mobilized.
However, closer to failure load increments are entirely taken by increase in end bearing.
The load transfer curves shown in the Figure 2.5 also confirms the above facts and clearly
indicates that for HP 14 x 89 pile the shape of the load transfer curve do not change
significantly after 100 kips load at the pile top. This indicates that the skin friction has
reached the ultimate value by then. However, the end bearing capacity increases beyond
that. It was earlier mentioned that the skin friction reaches the ultimate capacity at low
deformation levels whereas the end bearing reaches the ultimate capacity at a very high
deformation levels.
Estimation of the ultimate skin frictional capacity of piles
Using soil strength parameters
As explained earlier, the development of the skin friction is due to the movement of the
pile shaft relative to the surrounding soil. At any level of the pile shaft a normal force (fn)
is applied on the pile shaft from the surrounding soil as shown in Figure 2.6.

35. 146
Figure 2.6 – Development of frictional resistance.
Assuming that the soil element in contact with the pile is also solid, theory of friction
between two solid objects can be used to find out the frictional resistance developed.
From fundamental concepts in frictional resistance between two solid objects, the
frictional resistance (ffr) developed can be expressed as:
 tannnfr fff 
Where,
 - Coefficient of friction between the two objects; and
 - Friction angle between the two objects.
At a given location of the pile shaft, the normal force fn may be assumed to be a constant.
The friction angle, , is not a constant and it increases with the relative displacement
between the two objects. As the displacement of the pile element, as shown in Figure 2.2,
increases the frictional resistance, ffr, increases upto the ultimate value, fufr. The maximum
friction angle between the soil and the pile element is taken as a, which is generally a
function of the angle of internal friction of the soil. Therefore, the ultimate frictional
resistance (fufr) may be expressed as:
annufr fff  tan
Dividing both sides of the above Equation by the surface area of the element, As:
anufr  tan
Where
fs
fs
Pile element Soil element
Imaginary separation between
pile and soil elements
(a)
(b)
fn

36. 147
ufr - Ultimate unit frictional resistance
n - Stress normal to the pile – soil interface
In addition to the frictional resistance developed between the pile and the soil, a unit
adhesive resistance, ca, may also be developed if the soil is cohesive. Therefore, the total
ultimate skin frictional resistance, us, may be expressed as:
aufrus c
By substitution,
anaus c  tan
It is generally observed that the:
'
vsn K  
Where Ks – coefficient of lateral earth pressure
v
/
- effective vertical stress at the level considered.
The lateral earth pressure coefficient is a function of the soil type, stress history and the
amount of disturbance caused to the surrounding by the pile installation process.
Thus,
asvaus Kc  tan
and
dzCP us
L
us 
0
 dzKcCP asva
L
us  tan
0
 
where
C = pile perimeter
L = length of pile shaft
Estimation of the skin friction in clayey soils
In clayey soils, the undrained condition is critical. Therefore, the ultimate skin frictional
resistance should be estimated using undrained strength parameters.
For clayey soils under undrained condition  = 0 and hence, a = 0. Therefore,
aus c
It was found that:
uus c 
Where
cu - Undrained cohesion of clay.
Different researches have suggested different relationships for  and one of the very
widely used simple relationship is given below:

37. 148
Figure2.7 – Relationship between  and undrained strength (Su)
Example
Estimate the ultimate skin frictional resistance of the 400 mm x 400 mm, 9m long precast
concrete pile driven into clyey soil having undrained cohesion of 50 kPa.
Solution
From Figure 2.7,  = 0.9 (Using the relationship proposed by Bowles (1996))
509.0 xcuus   = 45 kPa
dzCP us
L
us 
0
cu = 50 kPa
400 mm x 400 mm driven
pile
9 m
Bowles (1996)

38. 149
Since the us is constant with the depth, the total ultimate skin frictional capacity may be
estimated by multiplying us by the total surface area of the pile shaft.
Therefore,
  94.04.0245 xxxPus  = 648 kN.
Ultimate skin frictional capacity is 648 kN.
Exercise
Estimate the ultimate skin frictional capacity of the 9m long, 400 mm x 400 mm square
pile driven into a subsurface consisting of two clay layers having undrained cohesion of
25 kPa and 50 kPa as shown below. Estimate the ultimate skin frictional capacity of the
pile.
Estimation of the skin friction in clayey soils
The general skin friction capacity can be expressed by the Equation derived earlier:
asvaus Kc  tan
For sandy soils, ca = 0 and hence the above Equation is simplified to:
asvus K  tan
It is a very difficult task to accurately estimate the value of the lateral earth pressure
coefficient as it may depend on the type of soil, method of installation etc. As a result,
there are different methodologies proposed by various researches to estimate the
coefficient of lateral earth pressure closer to the pile. In this course, the -method
proposed by Burland (1972) is discussed.
-method
Burland (1972) made the following assumptions:
cu = 25 kPa
400 mm x 400 mm driven
pile
9 m
cu = 50 kPa
4 m

39. 150
1. Due to remolding adjacent to the pile, the effective stress cohesion intercept
reduced to zero;
2. The horizontal stress acting on the pile after dissipation of pore pressure is at least
equal to the horizontal stress prior to the installation of the pile (K0 condition); and
3. The major shear distortion during pile loading is confined to a relatively thin zone
around the pile shaft, and drainage of this thin zone either occurs rapidly during loading
or has already
occurred in the delay between driving and loading.
Considering the above assumptions, the skin friction can be expressed as:
aovus K  tan
Where
Ko - Coefficient of lateral earth pressure at rest and for normally consolidated
soils it is equal to (1 - sin).
Substituting Kotan a = , the ultimate skin frictional capacity is given by:
vus  
A particular attractive feature of this method is that if we use '
0 and1   SinK
then the range of  is from 0.27 to 0.30 in the pracital (range of 25°
to 45°
). This method
is more of then used with piles driven in cohesionless soil (when 0,0'
  ).
Variation of the effective overburden pressure (v) closer to the pile
In undisturbed ground, the effective overburden pressure increases with the depth, as the
weight of the soil above a certain level increases with the depth. However, this situation
changes closer to a pile when it is installed in the ground. Closer investigation of the skin
friction Equation given above reveals that at higher depths, if the effective overburden
pressure increases with the depth, the skin friction capacity should be very high. But
researches have found that the skin friction of piles do not increase without bounds with
the depth. The results of skin friction variation in sandy soils measured using model piles
are presented in Figure 2.8. It is clear from the variation of the skin friction with the
depth that the skin friction varies upto a certain depth but beyond that it remains constant.

40. 151
Figure 2.8 – Variation of the skin friction with the depth (Vesic, 1967)
Based on the results of the research it is concluded that the effective vertical overburden
pressure closer to the pile is not similar to the vertical effective overburden pressure
under in-situ undisturbed conditions. The presence of the pile tends to change the stress
conditions closer to the pile. The pile provides some arching action and tends to reduce
the overburden pressure beyond a certain critical depth as shown in Figure 2.9.
Zc
W. T.
L
(a) (b)
wetdw+(Zc-dw)(sat- w)
wetdw+(L-Zc)(sat- w)

41. 152
Figure 2.9 – (a) Vertical effective vertical stress distribution closer to the pile; and (b)
Vertical effective vertical stress distribution away from the pile.
It should be noted here that the skin friction developed on piles in sand varies with the
depth and the total resistance should be estimated considering such variations. The
relationship shown in Figure 2.10 can be used to determine the critical depth of a pile.
Figure 2.10 – Critical depth (zc) / pile diameter vs friction angle of the soil.
The angle of internal friction 1, prior to the installation of the pile, should be modified as
follows before using with Figure 2.10.
For driven piles;
10
4
3
1  o

For bored piles;
31  oo

Example I
A driven 400mm square, 9.0m long pile is installed in sandy soil layer having angle of
internal friction  = 32o
and cohesion c = 0. The water table is present at 1m below the
ground surface and the unit weight of soil above and below the water table are 16 kN/m3
and 17 kN/m3
respectively.
i. Determine the skin friction distribution along the pile; and
ii. Estimate the total ultimate skin frictional resistance on the pile.

42. 153
Solution
i.
Equivalent diameter (d) of the pile can be determined by finding the diameter of the pile
having the same cross sectional area as the square pile.
mm
xB
d 451
42


Modification of the friction angle;
ooo
x 341032
4
3
10
4
3
1  
From Figure 2.10, Zc/d = 6, therefore, the critical depth Zc = 2.70m.
Effective overburden pressure at the water table level = 116x = 16 kPa
Effective overburden pressure at the critical depth level =   0.170.216  wsat 
= 28.2 kPa.
The angle adhesion, a = 0.75 = 24o
  21.0tansin1  a
Ultimate skin friction:
At the water table level = 1621.0 xv  = 3.36 kPa
At the critical depth level = 2.2821.0 xv  = 5.92 kPa.
Since the vertical effective overburden pressure closer to the pile remains constant
beyond the critical depth and  is also constant, the skin friction is constant below the
critical depth level.
wet= 16 kN/m3
Sat= 17 kN/m3
1 m
W. T
8 m
400 mm x 400 mm driven pile
 = 32o

43. 154
The skin friction distribution can be graphically shown as below:
ii.
The total ultimate skin friction can be estimated by determining the area of the skin
friction variation with the depth.
Total skin friction upto the WT =  0.136.3
2
1
x = 1.68 kN
Total SF from 1.0 to 2.70m =   0.170.292.536.3
2
1
 = 7.79 kN
Total SF from 2.70 to 9.0m =  7.20.992.5 x = 37.3 kN
Total SF = 46.8 kN
Example II
A 800 mm diameter, 22 m long bored pile is installed through the subsurface shown in
the following Figure. The water table is present at 2 m below the ground surface and the
unit weight and the strength properties of the different layers are also given in the same
Figure.
i. Determine the skin friction distribution along the pile; and
ii. Estimate the total ultimate skin frictional resistance on the pile.
Zc=2.7m
16 kPa
28.2 kPa
28.2 kPa
3.36 kPa
5.92 kPa
5.92 kPa
Overburden pressure
variation closer to the pile
Ultimate skin friction
variation along the pile shaft

44. 155
Solution
i.
The three layers present in the subsurface may be labeled from the top to bottom as L1,
L2 and L3 respectively.
Modification of the friction angle of the LI;
ooo
2933231  
From Figure 2.10, Zc/d = 5.5, therefore, the critical depth Zc = 4.4m. the critical depth is
within L1.
Effective overburden pressure at the water table level = 215x = 30 kPa
Effective overburden pressure at the critical depth level =   0.24.430  wsat 
= 47.3 kPa.
Below the critical depth level, the effective overburden pressure closer to the pile is
constant with the depth.
Ultimate skin friction: For the bored and cast in-situ concrete piles, the angle of adhesion
a is assumed to be equal to the angle of friction of the soil.
The angle adhesion of L1, a =  = 32o
  29.0tansin1  a
The angle adhesion of L2, a =  = 37o
  30.0tansin1  a
wet= 15 kN/m3
Sat= 17 kN/m3
2 m
W. T
7 m
800mm
diameter
bored pile
Medium dense
sand  = 32o
Dense sand  = 37o
and  = 18 kN/m3
Very dense weathered
rock layer  = 40o
and
 = 20 kN/m3
5 m
8 m

45. 156
The angle adhesion of L3, a =  = 40o
  30.0tansin1  a
Within L1:
At the water table level SF = 3029.0 xv  = 8.7 kPa
At the critical depth level SF = 3.4729.0 xv  = 13.7 kPa.
SF within L2 = 3.473.0 xv  = 14.2 kPa
SF within L3 = 3.473.0 xv  = 14.2 kPa
The skin friction distribution can be graphically shown as below:
ii.
The total ultimate skin friction can be estimated by determining the area of the skin
friction variation with the depth.
Total skin friction upto the WT =  0.27.8
2
1
x = 8.7 kN
Total SF from 2.0 to 4.4m =   0.24.47.137.8
2
1
 = 26.9 kN
Total SF from 4.4 to 9.0m =  4.40.97.13 x = 63.0 kN
Total SF from L2 =  0.82.14 x = 113.6 kN
Total SF from L3 =  0.52.14 x = 71.0 kN
Total SF = 283.2 kN
Zc=2.7m
30 kPa
47.3 kPa
47.3 kPa
8.7 kPa
13.7 kPa
Overburden pressure
variation closer to the pile
Ultimate skin friction
variation along the pile shaft
14.2 kPa
14.2 kPa
14.2 kPa

46. 157
Determination of the end bearing capacity
The pile bottom is pressed against the soil beneath the pile toe and the bearing capacity
failure of the soil can occur. However, as the pile toe is at a greater depth below the
ground surface, the failure mode is normally the local shear failure and the failure pattern
is similar to the one shown in Figure 2.11.
Figure 2.11– Failure pattern below the pile toe.
As the failure pattern is different from the ones observed for shallow foundations, the
same bearing capacity equation, used for the estimation of the ultimate carrying capacity
of shallow foundations, may be used with modified bearing capacity factors.
BNNqcNq qcend
2
1


Where,
Nc, Nq, N = Bearing capacity factors
B = Width of the pile
q = Effective overburden pressure at the toe of the foundation
 = Unit weight of the material below the pile toe.
It is generally observed that the third term of the above bearing capacity Equation is
small compared to other two terms. Therefore, the third term of the above Equation is
neglected, if the width of the foundation is not large.
6 – 10 B
2 – 4 B

47. 158
Skempton (1951) suggested the chart given in Figure 2.12 to obtain the bearing capacity
factor Nc.
Figure 2.12 – Bearing capacity factor Nc (Skempton, 1951)
It is evident from the chart given above that for a circular or square footing the maximum
value of the bearing capacity factor is 9.0 for L/Br ratio greater than about 4.0. The chart
given in Figure 2.13 is proposed by Berezantzev et al. (1961) for the estimation of the
bearing capacity factor N.
Figure 2.13 – Bearing capacity factor for N (Berezantzev et al., 1961)
The  value of the soil should be modified as below before using with the above chart.

48. 159
For driven piles,
2
40'
1 



For bored piles, 3'
1  
Where '
1 = angle of internal friction prior to installation of pile
Example I
Estimate the ultimate end bearing capacity of the driven 400mm square pile driven 9m
into a clay layer having undrained cohesion 25 kPa.
For normally or slightly over consolidated soils, the undrained capacity is critical,
Therefore,  = 0 and Nq = 0.
cend cNq 
Since L/B > 4, Nc = 9.0
0.950xqend  = 450 kPa
Ultimate end bearing load, Pend = 450x0.4x0.4 = 72 kN.
Example II
Estimate the ultimate end bearing capacity of the 400 mm x 400mm driven pile shown in
the following diagram.
cu = 50 kPa
400 mm x 400 mm driven
pile
9 m

49. 160
Solution.
Equivalent diameter (d) of the pile can be determined by finding the diameter of the pile
having the same cross sectional area as the square pile.
mm
xB
d 451
42


Modification of the friction angle;
ooo
x 341032
4
3
10
4
3
1  
From Figure 2.10, Zc/d = 6, therefore, the critical depth Zc = 2.70m.
Effective overburden pressure at the water table level = 116x = 16 kPa
Effective overburden pressure at the critical depth level =   0.170.216  wsat 
= 28.2 kPa.
The modified  angle to be used in Figure 2.13 =
2
40'
1 



= 36o
N = 90
902.28 xqNq qend  = 2538 kPa
Pend = 460 kN.
Exercise
Five boreholes are driven in a proposed building site to investigate the subsurface
condition for a 20-storey building. The subsurface at the site consists of loose silty sand,
stiff clay, completely weathered rock, and fractured rock. A typical subsurface condition
in a borehole and the estimated shear strength parameters are given in the following
wet= 16 kN/m3
Sat= 17 kN/m3
1 m
W. T
8 m
400 mm x 400 mm driven pile
 = 32o

50. 161
Figure. As a trial design, 1000 mm diameter bored piles socketed 1m into the fractured
rock is considered. As a design engineer attached to the firm involved in the design,
Estimate the ultimate skin friction of a single pile upto the top surface of the weathered
rock layer.
Empirical correlations
There are large number of empirical correlations that can be used to estimate the skin
friction and end bearing of piles. However, these correlations should be used very
carefully as they are valid under the subsurface condition used to develop them.
Skin Friction
Correlations with the SPT blow counts:

51. 162
Meyerhof (1956, 1976) proposed the following correlation for the estimation of skin
friction (fus):
55Nf mus  (kPa)
Where
m = 2.0 for piles with large volumes displacement
= 1.0 for small volume displacement piles
N55 = Statistical average of the blow count in the stratum.
Shioi and Fukui (1982) suggested the following empirical correlations for the estimation
of the skin frictional resistance.
For driven piles: 55,0.2 sus Nf  for sand; = 55,10 sN for clay (kPa)
For bored piles: 55,sus Nf  for sand; = 55,5 sN for clay (kPa)
Where
Ni,55 = Average blow count in the material indicated for the pile or pile segment length.
Correlations with the Cone Penetration Test (CPT)
Meyerhof (1956) and Thorburn and Mac Vicar (1971) suggested the following
relationship based on the CPT results:
cus qf 005.0 (kPa)
Where
qc = cone penetration resistance in kPa.
When the side friction (qs) of the cone is measured:
sus qf  (for small volume displacement piles) and;
sus qtof )0.25.1( (for large volume displacement piles)
End bearing
Correlations with the SPT blow counts:
Meyerhof (1956, 1976) proposed the following relationship for the estimation of the end
bearing capacity.
  N
B
L
Nq b
end 38040  (kPa)
Where
N = Statistical average of the SPT N55 numbers in a zone of about 8B above to

52. 163
3B below the pile point.
B = Width or diameter of the pile.
Lb = Pile penetration depth into point-bearing stratum.
Correlations with the Cone Penetration Test (CPT)
Japanese use the following relationship to estimate the end bearing capacity:
cend qq  (in units of qc)
Where
qc = Statistical average of the SPT N55 numbers in a zone of about 8B above to
3B below the pile point.
Estimation of the ultimate carrying capacity from the pile driving formulae
This method is commonly used for the estimation of the ultimate carrying capacity of
driven piles. This method is based on two fundamental assumptions:
i. The pile is a rigid body with no elastic deformations; and
ii. The dynamic resistance of the pile is equal to the static resistance of the soil.
Most of the pile driving equations are based on the energy conservation during the
driving process and the equations of motion. Consider the hammer and the pile
immediately before the impact and after the impact shown in Figure 2.14.
Figure 2.14 – Hammer and pile velocities immediately before and after the impact.
u
up
W
Wp vp
v
Immediately before the
impact
Immediately after the
impact

53. 164
The energy transfer from the hammer to the pile and the resulting deformation of the pile
can be diagrammatically shown as given in Figure 2.15.
Figure 2.15 – Energy transfer and the deformation of the pile during a single hammer
blow.
Considering the velocities of the pile and the hammer before and after the impact, and the
deformation of the pile during and after impact following relationships can be obtained.
g
Wv
WHeE f
2
2
1 
The efficiency of impact is
   
    1
2
22
22
22
22
E
E
vgwvgw
ugwugw
e
pp
pp
iv 



The law of impulse gives :
   pp
p
uv
g
W
uv
g
W

The coefficient of elastic restitution, n, is
p
p
vv
uu
n



Assuming 0pv , and eliminating u, up, and v,
p
p
iv
WW
WnW
e



2
The energy left after impact is











p
p
fivf
WW
WnW
WHeWHeeE
2
2
Various pile driving equations are developed by simplification of the above derived
Equation. Some of the commonly used pile driving Equations are given in the following
Table 2.1.

54. 165
Table 2.1 Commonly used pile driving equations
Formula Equation for Ru Remarks
Senders
S
WH
Engineering
News CS
WH

C = 1.0 in. for drop hammer
0.1 in. for steam
hammer
0.1 WWp in. for
steam hammer on very
heavy piles
Eytelwein
(Dutch)
PWW
W
S
WH

.
Weisbach 2
2













l
SAE
L
WHAE
L
SAE PPP
Hiley
  P
Pf
WW
WnW
CCCS
WHe



2
321
.
2/1
See Tables 4.2, 4.3 and 4.4
for values of 321 ,,, CCCef ,
and n.
Janbu












S
WH
ku
1  dedu CCk  11
WWC Pd 15.075.0 
2
/ AESWHLe 
Danish
  2
1
/2 Pf
f
AEWHLeS
WHe

See Table 4.2 for ef values
Gates  SWHef 10log6.5 10
Units are inches and tons
(short)
 SWHef 25log0.4 10
Units are metric tons (1000
kg) and centimeters

55. 166
Exercise I
Precast concrete piles with 350 mm x 350 mm cross sectional area and a length of 12m
are to be driven for the abutment of a bridge using a 2 ton hammer with a height of drop
of 1m. Estimate the termination set to be achieved if the working load on a pile is 400 kN
and a factor of safety 3 is required against ultimate failure, using Gates method.
Gates equation







S
WHeR fult
25
log0.4 10
Units are in metric tons (1000 kg) and centimeters. The symbols carry the usual
meanings.
Exercise II
The subsurface at a bridge site consists of a 2m thick recently placed fill followed by
5m thick normally consolidated clay layer, which is underlain by a thick hard
weathered rock layer. 12 m long precast concrete piles with 350 mm x 350 mm cross
sectional area are to be driven at this site for the bridge abutment using a 2 ton
hammer with a height of drop of 1m. Estimate the termination set to be achieved if
the working load on a pile is 400 kN and a factor of safety 3 of is required against
ultimate failure, using
a. Hiley method; and
b. Janbu’s method.
Hiley pile driving formula:


















p
pf
ult
WW
WnW
CCCS
WHe
P
2
321 2/)(
where
AE
LP
C ult
2
C1 = 3mm C3 = 2.5 mm
n = 0.4 ef = 0.75
W = Drop weight H = Drop height
L = Length of the pile A = Cross sectional area of the pile
Wp=Weight of the pile S = Set of the pile during driving
E = Young’s modulus of the pile material
Janbu’s formula:













S
WH
K
P
u
ult
1

56. 167
Where









d
e
du
C
CK

1







W
W
C
p
d 15.075.0






 2
AES
WHL
e
(Assume unit weight of concrete and Young’s modulus of concrete as 24 kN/m3
and 27 x
106
kPa respectively)

57. 168
3.0 Estimation of the settlement of a vertically loaded single pile
As any other foundation, the design of the pile foundations should be safe against
excessive settlements. Therefore, settlement of the pile should be estimated and checked
against the allowable settlement of the foundation. The settlement estimation methods
could be divided mainly into three types:
i. Methods involving empirical correlations;
ii. Semi – elastic approaches involving Load –transfer methods considering the
axial force at various points along the pile shaft;
iii. Methods based on theory of elasticity that involves the use of Midlin (1936)
equations for subsurface loading within the semi-infinite mass; and
iv. Use of the numerical methods such as Finite Element Method
Empirical correlations:
Meyerhof (1959) Method
Based on the field load test results on piles in sandy soils, Meyerhof suggested that the
settlement could be obtained from the Equation [3.1] if the applied load has a factor of
safety more than three against the applied load.
F
db
30
 [3.1]
Where
db - diameter of the pile base
F - Factor of safety on ultimate load (Must be > 3.0)
It should be noted here that there is no soil properties nor applied load is in the settlement
estimation equation and hence the validity of this method is highly questionable.
Focht (1976) method
Focht proposed an empirical equation to estimate the settlement of a pile using the
movement ratio, the ratio between the settlement of the pile () and settlement of the pile
acting as a column under the working load. Based on observation of piles in clayey soils
Focht suggested that the use of Equations [3.2] or [3.3] to estimate the settlement of a
single pile.
5.0
Col

if col > 8mm [3.2]
and
0.1
Col

if col < 8mm [3.3]

58. 169
Methods based on theory of elasticity that involves the use of Mindlin (1936)
equations
Various researchers have used this approach to estimate the settlement of a single pile. In
most of these approaches, the pile is divided into a number of uniformly loaded elements,
and a solution is obtained by imposing compatibility between the displacements of the
pile and the adjacent soil for each element of the pile.
The displacement of the pile are obtained by considering the compressibility of the pile
under the axial loading. The soil displacements are obtained in most cases by using
Mindlin’s equations for the displacements within a soil mass caused by loading within
the mass. The difference between the various methods lies in the assumptions made
regarding the distribution of shear stress along the pile.
The method derived by Poulos and Davis (1968) is described below. The method
assumes a floating or frictional pile in a semi-infinite mass as shown in Figure 3.1.
Figure 3.1 – pile soil model used by Poulos and Davis (1968) for settlement estimation.
It is assumed in almost all the settlement analysis of piles that the pile and soil are stress-
free and that no residual stresses exists in the pile resulting from its installation. This
could be a false assumption for most of the practical situations. However, this error could
be somewhat minimized by selection of appropriate material properties.
If conditions at the pile-soil interface remain elastic and no slip occurs, the movement of
the pile and the soil should be equal. In the solution process only the vertical
displacement compatibility is considered and no lateral displacement of the pile is
considered.
P
D
L
h
Soil Young’s
modulus, Es,
and Poisson
ration, vs
Young’s
modulus of
the pile
material is Ep

59. 170
The results of the analysis carried out by Poulos and Davis (1968) are presented in terms
of a parameter referred to as the relative stiffness (k) of pile. The relative stiffness factor
k is defined by Equation [3.4].
s
p
A
E
E
Rk  [3.4]
Where RA is the area ratio defined by Equation [3.5]
c
p
A
A
A
R  [3.5]
Where Ap – area of the pile cross section
Ac - Area bounded by the outer circumference of the pile.
Consider a pipe pile of outer diameter of Do and inner diameter of Di as shown in Figure
3.2. The relative are RA is given by Equation [3.6].
 
2
22
4
4
o
io
A
D
DD
R



 [3.6]
Figure 3.2 – Cross section of a pipe pile.
If the pile has a solid cross section without any cavities within it, the area ratio RA is
equal to unity.
Separation of the skin friction and end bearing capacities
The theory presented in Poulos and Davis (1996) can be used to determine the skin
friction distribution along the pile shaft and the hence to separate the skin friction and end
bearing.
A uniform floating pile in a semi-infinite elastic medium, the ratio between the skin
friction and the average skin friction for piles with K=5000 and K=50 are shown in
Figure 3.3. the variation shown in Figure 3 is obtained assuming no-slip condition
between the pile and the soil. It is clear from the Figure 3.3 that the stress distribution
becomes highly non-uniform, when the pile stiffness factor is smaller due larger
settlement of the pile near the top of the pile as a result of high compressibility of the
pile. However, as the pile stiffness becomes higher, the skin friction distribution becomes
more or less uniform. The Poisson ratio of the soil has a negligible effect on the skin
friction distribution.
Do
Di

60. 171
Figure 3.3 – Stress distribution along the pile shaft of a floating pile.
If the elastic modulus of the bearing layer is Eb and the elastic modulus of the material
along the pile shaft is Es, the load transfer curves of end bearing piles, with different
Eb/Es, are shown in Figure 3.4.
Figure 3.4 – Variation of the axial force with the depth of the pile.
Based on the Mindlin (1936), Poulos and Davis (1996) suggested the following
methodology in estimation of the settlement of a single pile.

61. 172
Settlement of a floating pile
According to Poulos and Davis (1996), the settlement of a single pile (ρ) may be
expressed as given in Equation [3.7].
DE
PI
s
 [3.7]
Where
P - Applied axial force
I - Settlement influence factor
Es - Elastic modulus of the surrounding material along the pile shaft
D - Diameter of the pile
Settlement influence factor (I)
Settlement influence factor I can be expressed as:
hvko RRRII  [3.8]
Where Io – Settlement influence factor for an incompressible pile (k=) in a semi infinite
elastic medium with a Poisson ratio =0.5. Io could be obtained from Figure 3.5.
Figure 3.5 – Settlement influence factor Io

62. 173
Rk, Rv, and Rh are the modification factors, which could be obtained from Figures 3.6, 3.7
and 3.8 respectively.
Figure 3.6 – Modification factor Rk
Figure 3.7 – Modification factor Rv

63. 174
Figure 3.8 – Modification factor Rh
Settlement of an end bearing pile
According to Poulos and Davis (1996), the settlement of a single pile (ρ) may be
expressed as given in Equation [3.9].
DE
PI
s
 [3.9]
Where
P - Applied axial force
I - Settlement influence factor
Es - Elastic modulus of the surrounding material along the pile shaft
D - Diameter of the pile
Settlement influence factor (I)
Settlement influence factor I can be expressed as:
bvko RRRII  [3.10]
Where Io – Settlement influence factor for an incompressible pile (k=) in a semi infinite
elastic medium with a Poisson ratio =0.5. Io could be obtained from Figure 3.5.
Rk, Rv, and Rb are the modification factors, which could be obtained from Figures 3.6, 3.7
and 3.9 respectively.

64. 175
Figure 3.9 – Modification factor Rb
Estimation of the settlement of piles through layered medium
It is very rarely that the piles are installed through homogeneous medium. In reality, piles
are generally installed through layered soil mediums. Therefore, estimation of the piles
through layered medium should be performed. Figure 3.10 shows the settlement
influence factor (Io) estimated by various methods for a two layer medium with different
moduli ratio. The settlement influence factor (Io) estimated from more sophisticated
methods agree well with that estimated using the weighted average of the elastic moduli
of the two layers. Therefore, weighted average of the elastic moduli of the layered
medium is used in the estimation of the settlement of piles through layered medium as
shown in Figure 3.11 and Equation [3.11]. Similarly, the Poison ratio of the layered
medium is estimated using Equation [3.12].

65. 176
Figure 3.10 – Settlement influence factor (Io) of a two layer medium.
Figure 3.11 – Layered medium
The elastic modulus to be used in the settlement estimation is given by Equation [3.11]
and that for the Poison ratio is given by the Equation [3.12]
hn
h3
h2
h1
Ei
E1 vi
E2 v2
E3 v3
Ei vi
En vn

66. 177




 n
i
i
n
i
ii
av
h
hE
E
1
1
[3.11]




 n
i
i
n
i
ii
av
h
h
E
1
1

[3.12]
Example I
The thickness and elastic compressibility properties of the soil and rock layers near one
borehole are as follows:
Layer Thickness
(m)
Elastic modulus
(kPa)
Poisson ratio
Organic clay layer 7 2000 0.3
Medium dense sand
layer
8 15000 0.2
Completely
weathered rock
layer
13 50000 0.2
Highly fractured
rock
2 100000 0.2
Bedrock 1 200000 0.1
If a 800 mm diameter bored pile installed near this borehole is socketed 1m into the
bedrock layer, estimate the settlement of the pile under a working load of 2500 kN.
(Elastic modulus of concrete is 31.7 x 106
kPa)
Solution
31741
31
1000002135000081500072000



xxxx
Eavg
22.0
31
2.022.0132.0873.0



xxxx
Avg
1000
31741
107.31 6

x
E
E
k
Avg
p

67. 178
hvKo RRRII 
Io = 0.05
Rk = 1.2
Rv = 0.89
Rb = 0.4
Settlement of a pile is
mm
x
xxxx
DE
PI
s
2
8.031741
)4.089.01.105.0(2500
1 
Exercise
If the drained compressibility parameters, given in Table, are assumed for the subsurface
layers and the bedrock shown in Figure, estimate the expected final settlement of a 600
mm bored pile installed upto the bedrock.
Table
Layer Drained Young’s
Modulus (kPa)
Poisson ratio (/) Thickness (m)
Fine sand
layer
10000 0.2 3
NC clay layer 15000 0.3 5
Weathered
rock layer
30000 0.2 6
Bed rock 150,000 0.1
Figure

68. 179
Pile Groups

69. 180
7.0 Design of Pile Groups Subjected to Vertical Compressive Loads
Introduction
Depending on the carrying capacity of individual piles and the working load acting
through the structural elements, such as columns, there are situations that a single pile is
not capable of supporting the structural load. In such situations, it is customary to use a
group of piles to support such structural loads. Like any other type of foundations, the
pile group should also be designed considering:
i. Shear failure of the pile group – should have a reasonable factor of safety
against ultimate shear failure of the soil supporting the group; and
ii. The settlement of the group under the working loads – The settlement of the
pile group under the working load should be less than the allowable settlement
limit of the structure.
General configurations of pile groups are shown in Figure 4.1. When several piles are
clustered as shown in Figure 4.1, it is reasonable to expect that the soil pressures
produced from either side friction or point bearing will overlap as shown in Figure 4.2.
Figure 4.1 – Typical pile group patterns: (a) for isolated pile groups; and (b) for