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Sep 13, 2014
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Deep Foundation
Shallow Foundations (Spread Footings) - Bearing Capacity
- Settlement
Deep Foundations - Load Capacity (bearing and friction)
- Settlement
- Negative Skin Friction
The loads are so high that there is not
enough plan area to accommodate the size
of the foundation required
Where Water Table is high (dewatering is
required)
The presence of adjacent buildings in
congested built-up areas imposes
restrictions on open excavation and calls
for construction of walls to restrain
displacement of existing buildings.
Necessity of Deep Foundations
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Necessity of Deep Foundations...
2. To resist uplift or overturning forces.
3. To control settlements when spread footings
are on marginal or highly compressible soil.
4. To control scour problems on bridge
abutments or piers.
5. In offshore construction to transmit loads
through the water and into the underlying soil.
6. To control earth movements, such as
landslides.
BATTER PILE (RAKER PILE) –
The pile which is installed
at an angle to the vertical.
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Piles are relatively long and slender members
used to transmit foundation loads through soil
strata of low bearing capacity to deeper soil or
rock having a higher bearing capacity.
Pile resistance is comprised of
- end bearing
- shaft friction
For many piles only one of these components is
important. This is the basis of a simple
classification
Pile Foundations
End Bearing Piles
ROCK
SOFT SOIL PILES
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Friction Piles
SOFT SOIL PILES
Strength
increases
with depth
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NEGATIVE SKIN FRICTION
For end bearing and skin friction to develop the pile
must move downwards in relation to the soil.
There are, however, occasions when after a pile has
been installed, the soil surrounding the pile begins
to move downwards in relation to the pile. When this
occurs, the soil exerts a downward drag on the pile.
This downward drag is called negative skin friction.
Consider a soil profile underlain by a hard stratum.
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NEGATIVE SKIN FRICTION
NEGATIVE SKIN FRICTION
Consider what happens on account of the following two
events:
1. A fill is placed at the ground surface above the soft clay –
the fill will induce the development of excess pore water
pressures in the soft clay and with time they will dissipate,
the effective stress will increase and the soft clay will
consolidate. As it consolidates it will move downwards in
relation to the pile since the pile is resting on firm stratum.
2. At this site for season the ground water table is lowered –
the lowering of the ground water table has the effect of
increasing the effective stress in the soft clay and it will
consolidate and move downwards in relation to the pile.
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NEGATIVE SKIN FRICTION
In both these situations, the effect of downward movement
of the soft clay in relation to the pile will be two folds:
1. The skin friction in soft clay helping to resist the load from
the superstructure will be wiped out
2. The downward movement of the soil will impose a drag
equal to the skin friction in the downward direction which
will have to borne by end bearing.
Qult = Qb – Qs
End bearing will have to support not just the load of the
superstructure but also the load on account of negative skin
friction acting on the pile surface.
Combinations of vertical, horizontal and
moment loading may be applied at the soil
surface from the overlying structure
For the majority of foundations the loads
applied to the piles are primarily vertical
For piles in jetties, foundations for bridge
piers, tall chimneys, and offshore piled
foundations the lateral resistance is an
important consideration
The analysis of piles subjected to lateral and
moment loading is more complex than
simple vertical loading because of the soil-
structure interaction.
Loads applied to Piles V
M H
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Individual Piles
Method of Estimating Load Capacity
Load Test
Dynamic Formula
Static Analysis
Fig. 20.22 Arrangements for conducting a Pile Load Test
Axial Pile Capacity – Pile Load Test Approach
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Fig. 20.23 Results from Pile Load Tests
a. Dense Sand or stiff clay; safe design load = Ultimate load/Factor
of safety
b. Loose sand or soft/medium stiff clay – No clear failure load;
Limiting settlement is used.
A typical criteria states that the safe design load shall be taken as the
lower of:
1. Half the load at which pile settlement is 10% of pile diameter
2. 2/3rd of load at which pile settlement is 12 mm
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3.14 Test Pile
A pile which is selected for load testing and which is subsequently used as a part of the foundation. The test pile may form a working pile itself, if subjected to routine load test up to 1.5 times the safe load.
3.15 Working Pile
A pile forming part of the foundation system of a given structure.
3.16 Trial Pile
One or more piles, which are not working piles, may be installed if required to assess the load-carrying capacity of a pile. These piles are tested either to their
a) ultimate load capacity or
b) to 2 times the estimated safe load.
IS2911: Definition-old code- check as per new code
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Steps in Rational Pile Design and Selection
Adequate Subsurface Investigation
Soil Profile Development
Appropriate Lab/Field Testing
Selection of Soil Design Parameters
Static Analysis
Applied Experience
Ultimate Bearing Capacity - Static Formula Method (Qu = Qp + Qs)
Embedded
Length = D
Qu = Ultimate Bearing Capacity
Qs = fAs
f = Unit Frictional
Resistance
AS = Shaft Area
qP = Unit Bearing
Capacity
AP = Area of Point
QP = qPAP
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There are many piling systems
The pile installation procedure varies
considerably, and has an important influence on
the subsequent response
Two main groups can be identified
- Displacement (Driven) piles
- Non-displacement (Bored) piles
Types of Pile
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SolidConcrete,or Timber
Hollow tubeClosed end
Steel or Concrete
Preformed
Tube formerwithdrawn
void filled withconcrete
Formed in-situ
Large
Hollow tube, orH-section
Steel
Screw
Small
Displacement
Types of Displacement Piles
(Precast)
Types of Bored Piles
Unsupported during
Construction
Permanently by Casing Temporarily
Void filled with Reinforced Concrete
Supported during
Construction
Bored Piles
(Non-displacement)
By Drilling Mud by casing
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Types of Pile Material
Concrete Steel Pipe
Timber Steel H Pre-cast Concrete
Composite
Is 2911: DESIGN AND CONSTRUCTION OF PILE FOUNDATIONS – CODE OF PRACTICE-Part 1 –Concrete Piles
Section 3 Driven precast concrete piles
Section 1 Driven cast-in-situ concrete piles
Section 2 Bored cast-in-situ concrete piles
Section 4 Precast piles in pre-bored holes
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Precast Driven Pile: (IS 2911: Part1-Section 3 Driven precast concrete piles
The pile constructed in concrete in a
casting yard and subsequently driven into
the ground when it has attained sufficient
strength.
Piles are inserted into the soil by the following methods: 1. Driving using a pile hammer.
2. Driving using a vibratory device.
3. Jacking the pile.
4. Drilling a hole (pre-drilling) and inserting
a pile into it.
5. Screw into the ground.
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Driven precast concrete piles
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Precast Concrete Plies
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SEGMENTAL PRECAST RCC PILES
Wherever final pile length is
so large that a single length
precast pile unit is either
uneconomical or
impracticable for installation,
the segmental precast RCC
piles with a number of
segments using efficient
mechanical jointing could be
adopted.
Excessive whipping during handling
pre-cast pile may generally be avoided
by limiting the length of pile to a
maximum of 50 times the least width. As
an alternatives segmental precast piling
technique could be used.
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3.1 Driven Cast-in-situ Pile (IS 2911: Part1-sec1)
The pile formed within the ground by driving a casing of uniform diameter (displacement piles), subsequently filling the hole with reinforced concrete.
For displacing the subsoil the casing is driven with a plug or a shoe at the bottom. When the casing is left permanently in the ground, it is termed as cased pile and when the casing is taken out, it is termed as uncased pile. The steel casing tube is tamped during its extraction to ensure proper compaction of concrete.
Fig. 27.11 Driven cast-in-situ pile encased in a mandrel driven thin steel shell
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Fig. 27.12 An uncased driven cast-in-situ pile
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An uncased driven cast-in-situ pile of compacted concrete
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SMALL DISPLACEMENT PILE
SMALL DISPLACEMENT PILE
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SMALL DISPLACEMENT PILE
BORED CAST IN SITU PILE (IS 2911: Part1-sec2) -A
pile formed within the ground by excavating or
boring a hole within the ground (non displacement ), with
or without the use of a temporary casing and
subsequently filling it with plain or reinforced
concrete.
When the casing is left permanently it is termed as
cased pile and when the casing is taken out it is
termed as uncased pile.
In installing a bored pile, the sides of the borehole
(when it does not stand by itself) is required to be
stabilized with the aid of, a temporary casing, or with
the aid of drilling mud of suitable consistency.
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Cast-in-situ pile: reinforcement insertion followed by concreting
Cast-in-situ pile: concreting followed by reinforcement insertion
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Drilling Methods
1)Dry method
2)Casing method
3)Slurry (or “wet”) method
possible only for competent soil profiles
needed for caving soils
Selection of the drilling method depends on the nature of the ground
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Difference between “Driven and cast in situ” piles and “Bored and cast in situ” piles?
Driven cast in-situ piles are
displacement piles in which a hole is
formed by driving a metallic shell or a
casing into the ground while bored and
cast-in situ piles are non displacement
piles in which hole is formed by boring
(i.e. excavating soil by auger etc.).
Advantages of bored and cast-in situ
Very little displacement & no risk of heave.
Soil can be checked & inspected.
Length of pile can be readily varied at site.
Piles of great length up to 50m can be made.
Large diameter piles with enlargement 2-3 time diameter of shaft is possible.
Piles can be installed without much noise, vibration
Piles can be installed with limited head room.
Feasible in strata with cobbles and boulders.
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Disadvantages of bored and cast-in situ
1. Installation of cast-in-situ piles requires careful supervision and quality control of all the materials used in the construction.
2. The method is quite cumbersome. It needs sufficient storage space for all the materials used in the construction.
3. The advantage of increased bearing capacity due to compaction in granular soil that could be obtained by a driven pile is not produced by a cast-in-situ pile.
4. Construction of piles in holes where there is heavy current of ground water flow or artesian pressure is very difficult.
Precast piles -Advantages 1. Piles can be precast to the required specifications.
2. Piles of any size, length and shape can be made in advance and used at the site. As a result, the progress of the work will be rapid.
3. A pile driven into granular soil compacts the adjacent soil mass and as a result the bearing capacity of the pile is increased.
4. The work is neat and clean. The supervision of work at the site can be reduced to a minimum. The storage space required is very much less.
5. Driven piles may conveniently be used in places where it is advisable not to drill holes for fear of meeting ground water under pressure.
6. Drivens pile are the most favored for works over water such as piles in wharf structures or jetties.
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Precast piles -Disadvantages 1.Precast or prestressed concrete piles must be properly
reinforced to withstand handling stresses during transportation and driving.
2. Advance planning is required for handling and driving.
3. Requires heavy equipment for handling and driving.
4. Since the exact length required at the site cannot be determined in advance, the method involves cutting off extra lengths or adding more lengths. This increases the cost of the project.
5. Driven piles are not suitable in soils of poor drainage qualities. If the driving of piles is not properly phased and arranged, there is every possibility of heaving of the soil or the lifting of the driven piles during the driving of a new pile.
6. Where the foundations of adjacent structures are likely to be affected due to the vibrations generated by the driving of piles, driven piles should not be used.
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Auger Cast-in-situ Piles
Non-displacement-Bored
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Fig. 27.17 Auger cast-in-situ pile
Advantage:
No casing
is required
for stablizing
the hole
CFA piles are typically installed
with diameters ranging from 0.3
to 0.9 m and lengths of up to 30 m
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Precast Piles in Pre-bored Holes (IS 2911: Part1-Section 4 Precast piles in prebored holes
A pile constructed in reinforced concrete in a
casting yard and subsequently lowered into
prebored holes and the annular space around the
pile ground is grouted through grouting duct.
Grouting Duct - A circular hole kept in the
center of a precast pile for the purpose of
grouting the annual space in the borehole around
the pile.
Small and large diameter piles (as per
IS2911)
Piles of 600mm or less in diameter are
commonly known as small diameter piles
while piles greater than 600mm dia are
called large diameter piles. The following
nominal diameters (in mm) are commonly
used in piling: 450, 500, 600, 750, 800,
900, 1000, 1100, 1200 and upto 2000 mm.
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6.6 Spacing of Piles{2911-(part1-sec1,2,3,4):2010} The center to center spacing of piles is
considered from two aspects, viz.,
a) practical aspects of installing the piles;
and
b) the nature of the load transfer to the soil
and possible reduction in the bearing
capacity of piles group.
C)Nature of load transfer to the soil and
possible reduction in the load capacity of
pile group.
6.6 Spacing of Piles: for End Bearing Pile{2911-(part1-sec1,2,3,4):2010} In case of piles founded on hard stratum
and deriving their capacity mainly from end
bearing the minimum spacing shall be 2.5
times the diameter of the circumscribing
circle corresponding to the cross-section of
the shaft.
In case of piles resting on rock, the
spacing of 2 times the said diameter may
be adopted. NOTE – In the case of piles of non-circular cross-section,
diameter of the circumscribing circle shall be adopted.
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6.6 Spacing of Piles: for Friction Pile {2911-(part1-sec1,2,3,4):2010} Piles deriving their bearing capacity mainly
from friction shall be spaced sufficiently
apart to ensure that the zones of soils from
which the piles derive their support do not
overlap to such an extent that their bearing
values are reduced.
Generally the spacing in such cases shall
not be less than 3 times the diameter of
the shaft.
6.6 Spacing of Piles:old version of code In case of loose sand or filling closer
spacing may be possible since displacement during the piling may be absorbed by vertical and horizontal compaction of the strata. Minimum spacing in such strata may be two times the diameter of the shaft.
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BORED COMPACTION PILE - A
bored cast in situ pile with or without
bulb(s) in which the compaction of
surrounding ground and freshly filled
concrete in pile bore is simultaneously
achieved by suitable method. If the
pile with bulb(s), it is known ‘under-
reamed bored compaction pile’.
UNDER-REAMED PILE
A bored cast in situ or bored
compaction concrete pile with an
enlarged bulb(s) made by either
cutting or scooping out the soil or
by any other suitable process.
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Fig. 27.16 Bored cast-in-situ under-reamed pile
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UNDER-REAMED PILES Under-reamed piles are bored cast in-situ and bored
compaction concrete types having one or more bulbs
formed by enlarging the borehole for the pile stem.
•These piles are suited for expansive soils which are
often subjected to considerable ground movements due
to seasonal moisture variations.
These also find wide application in other soil strata
where economics are favorable.
UNDER-REAMED PILES
When the ground consists of expansive
soil, for example, black cotton soils, the
bulb of under-reamed pile provide
anchorage against uplift due to swelling
pressure, apart from the increased
bearing, provided topmost bulb is
formed close to or just below the
bottom of active zone.
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All
Dimension
in mm
All
Dimension
in mm
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In deep deposits of expansive soils the minimum length of piles, irrespective of any other considerations, shall be 3.5 m below ground level.
If the expansive soil deposits are of shallow depth and overlying on non-expansive soil strata of good bearing or rock, piles of smaller length can also be provided.
In recently filled up grounds or other strata or poor bearing the piles should pass through them and rest in good bearing strata.
The minimum stem diameter of under-reamed pile can be 200 mm up to 5m depth in dry conditions, that is strata with low water table.
The minimum stem diameter for piles up to 5 m depth in strata with high water table within pile depth, shall be 300 mm for normal under-reamed pile and 250 mm for compaction under-reamed pile.
For piles of more than 5 m depth, the minimum diameter in two cases shall be 375 mm and 300 mm respectively.
The minimum diameter of stem for strata consisting of harmful constituents, such as sulphates, should also be 375 mm.
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The diameter of under-reamed bulbs
may vary from 2 to 3 times the stem
diameter, depending, upon the
feasibility of construction and design
requirements.
In bored cast in-situ under-reamed
piles and under-reamed compaction
piles, the bulb diameter shall be
normally 2.5 and 2 times the stem
diameter respectively.
For piles of up to 300 mm
diameter, the spacing of the bulbs
should not exceed 1.5 times the
diameter of the bulb. For piles of
diameter greater than 300 mm,
spacing can be reduced to 1.25
times the bulb diameter.
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From NBC 2005
The topmost bulb should be at a minimum depth of two times the bulb diameter.
In expansive soils it should also be not less than 2.75 m below ground level.
The minimum clearance below the underside of pile cap embedded in the ground and the bulb should be a minimum of 1.5 times the bulb diameter.
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Clayey Soils
— For
clayey soils,
the ultimate
load
carrying
capacity of
an under-
reamed
pile may be
worked out
from the
following
expression:
Clay
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f
-
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SAFE LOAD TABLE
The safe bearing, uplift and lateral loads
for under-reamed piles given in Table 1
apply to both medium compact (l0<N
<30) sandy soils
and clayey soils of medium (4<N < 8)
consistency including expansive soils.
The values are for piles with bulb
diameter equal to two-and-a-half times
the shaft diameter.
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For dense sandy (N>=30) and stiff clayey (N
>=8) soils, the safe loads in compression and
uplift obtained from Table 1 may be increased
by 25 percent.
For piles in loose (4< N <10) sandy and soft
(2< N <4) clayey soils, the safe loads should
be taken 0.75 times the values shown in the
Table. For very loose (N < 4) sandy and very
soft (N < 2) clayey soils the values obtained
from the Table should be reduced by 50
percent.
CONCRETE: Bored and Driven cast-in-situ piles including under-reamed piles
The minimum grade of concrete to be used for cast-in-situ piles shall be M-25 and the minimum cement content shall be 400 kg/m3 (Table5-IS456, M-25 mini. Is 300kg/m3)in all conditions.
For piles up to 6 m deep, concrete with minimum cement content 350 kg/m3 without provision for under-water concreting may be used under favourable non-ggressive subsoil condition and where concrete of higher strength is not needed structurally or due to aggressive site conditions.
The concrete in aggressive surroundings due to presence of sulphates, etc, shall conform to provision given in IS : 456-2000.
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MINIMUM CLEAR COVER
11.2.8.2 The minimum clear cover
over the longitudinal reinforcement
shall be 50 mm.
In aggressive environment of
sulphates etc, it may be increased to
75 mm.
Fig. 27.18 A micro pile
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