pile foundation

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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

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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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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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

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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

Hollow tube, orH-section

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

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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;

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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Dimension

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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

carrying

capacity of

an under-

reamed

pile may be

worked out

from the

following

expression:

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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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pile foundation

ground water

uncased driven

minimum stem

large diameter

reinforced

soft clay

end bearing

uncased pile

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