Deep foundation

DEFINITION
1.

Deep Foundations are those
 in which the depth of the foundation is very large
in comparison to its width.
 Which are not constructed by ordinary methods
of open pit excavations.

When Used?


In cases where



The strata of good bearing capacity is not available near the
ground
The space is restricted to allow for spread footings



In these cases the foundation of the structure has to be
taken deep with the purpose of attaining a bearing stratum
which is suitable and which ensures stability and durability
of a structure.



The bearing stratum is not the only case. There may be
many other cases. For example, the foundation for a bridge
pier must be placed below the scour depth, although
suitable bearing stratum may exist at a higher level.

Forms of Construction


Most common forms of construction pertaining to deep
foundations are:


Pile Foundation
construction)

(more



Cofferdams



Caisson or Well Foundation

commonly

used

in

building

Pile Foundations


The term ‘Pile Foundation’ denotes a construction for the
foundation of a wall or pier which is supported on piles.



Where Used :






stratum of required bearing capacity is at greater depth
steep slopes are encountered
Compressible soil or water-logged soil or soil of made-up type

Examples: Piles are used for foundation for buildings, trestlebridges and water front installations (piers, docks etc ).



Advantages:




Provides a common solution to all difficult foundation site
problems
Can be used for any type of structure and in any type of soil

Pile Foundations(contd.)
Situations Which Demand Pile Foundation :
















Sub-soil water table is so high that it can easily affect the other foundations.
Load coming form the structure is heavy and non uniform.
Where grillage or raft foundations are either very costly or their adoption
impossible due to local difficulties.
When it is not possible to maintain foundation trenches in dry condition by
pumping, due to very heavy inflow of seepage or capillary water.
When it is not possible to timber the excavation trenches in the case of deep
strip foundation. (strip foundation- spread footing under wall ).
When overlay soil is compressible, and water-logged and firm hard bearing
strata is located at quite a large depth.
When structures are located on river-bed or sea-shore and foundations are
likely to be scoured due to action of water.
Large fluctuations in sub-soil water level.
Canal or deep drainage lines exist near the foundations.
In the construction of docks, piers and other marine structures they are used as
fender piles.

Types of Piles Based on Function
a) Classification based on Function or Use
1.
Bearing Piles or End Bearing Piles
2.
Friction Piles or Skin Friction Piles
3.
Sheet Piles
4.
Tension Piles or Uplift Piles
5.
Anchor Piles
6.
Batter Piles
7.
Fender Piles
8.
Compaction Piles

(contd)
Bearing Piles


Driven into the ground until a hard stratum is reached.



Acts

as

pillars

supporting

the

super-structure

and

transmitting the load to the ground.


Piles, by themselves do not support the load, rather acts
as a medium to transmit the load from the foundation to
the resisting sub-stratum.

(contd)
Friction Piles (Floating Piles)










Piles are driven at a site where soil is weak or soft to a considerable
depth and it is not economical or rather possible to rest the bottom
end of the pile on the hard stratum,
Load is carried by the friction developed between the sides of the pile
and the surrounding ground ( skin friction).
The piles are driven up to such a depth that skin friction developed at
the sides of the piles equals the load coming on the piles.
Skin friction should be carefully evaluated and suitable factor of safety
applied, as it is this which is supporting the whole of structure over its
head.
The load carrying capacity of friction pile can be increased by
increasing diameter of the pile

driving the pile for larger depth

grouping of piles

making surface of the pile rough

(contd)

(contd)
Sheet Piles

Sheet piles are never used to provide vertical support but
mostly used to act as retaining walls. They are used for the
following purposes:









To construct retaining walls in docks, and other marine
works.
To protect erosion of river banks.
To retain the sides of foundation trenches.
To confine the soil to increase its bearing capacity.
To protect the foundation of structures from erosion by river
or sea.
To isolate foundations from adjacent soils.

(contd)

Figure: Sheet Piles

(contd)
Anchor Piles

Piles are used to provide anchorage against horizontal pull from sheet
piling wall or other pulling forces.

Batter piles:

Piles are driven at an inclination to resist large horizontal and inclined
forces.
Fender piles:

Piles are used to protect concrete deck or other water front structures
from the abrasion or impact caused from the ships or barges.

Ordinarily made up of timber.
Compaction piles:

When piles are driven in granular soil with the aim of increasing the
bearing capacity of the soil, the piles are termed as compaction piles.

(contd)

Figure: Under-reamed Piles

Types of Piles Based on Materials
a) Classification based on Materials
1.

Timber Piles

2.

Concrete Piles

3.

Composite Piles

4.

Steel Piles

5.

Sand Piles

(contd)
1. Timber Piles:












Transmission of load takes place by the frictional resistance
of ground and the pile surface.
Economical to support light structure.
Piles
made
from
timber
of
tree
like
Sal, Teak, Deodar, Babul, Khair etc.
Khair piles can stand action of sea water and thus used for
marine works.
May be circular, square in x-section.
Piles are driven with the help of pile driving machine in which
drop hammers delivers blows on the pile head.
Brooming of pile head is prevented by providing an iron ring
of less than 25mm in diameter than the pile head at the pile
top.

(contd)
1. Timber Piles:













To facilitate driving, the lower end is pointed and provided with a cast iron
conical shoe.
Piles should not be spaced less than 60 cm center to center, the best
spacing is 90 cm c/c. closer spacing destroys frictional resistance.
Max load should not exceed 20 tonnes.
Piles are subjected to decay for alternate dry and wet condition (on
account of variation of ground water level)
As such , timber piles are cut a little below the lowest water-mark and
capped with concrete, steel grillage, stone or timber.
If timber capping is used, the cap should be permanently under water.
Diameter varies from 30 to 50cm.
Length should not be more than 20 times the least sectional dimension.

(contd)
Advantages of Timber Piles:








Economical where timber is easily available.
Can be driven rapidly & as such saves time.
Because of elasticity, timber piles are recommended for sites
subjected to unusual lateral forces e.g. ship, ferry terminals.
Do not need heavy machinery and elaborate technical supervision.
Being light, they can be easily handled.
They can be easily withdrawn if needed.

(contd)
Disadvantages of Timber Piles:









Timber piles must be cut off below the permanent ground water
level to prevent decay.
Liable to decay or deteriorate by salt water/insects.
Restricted length. It is rather difficult to procure piles in required
size and length.
Low bearing capacity.
They are not very durable unless suitably treated.
It is difficult or rather impossible to drive these piles into hard
stratum

(contd)

(contd)

Figure: Timber Pile

Types of Concrete Piles
Concrete Piles are of 3 types:

Pre-cast Piles

Cast in situ Piles

Prestressed Concrete Piles

Concrete Piles ( contd)
Pre-cast Piles:

Reinforced
concrete
piles,
molded
in
circular, square, rectangular or octagonal form.

Cast and cured in the casting yard, then transported to the
site of driving.

If space available it can be cast and cured near the work
site.

Driven in similar manner as timber piles with the help of
piles drivers.

Diameter normally varies from 35cm to 65cm, length varies
from 4.5m to 30m.

Pre-cast Piles:

Function of reinforcement in a pre-cast pile is to resist the
stresses during handling, driving and final loading on the
pile rather than strengthen the pile to act as a column.

Longitudinal reinforcements usually 20mm to 50mm in
diameter, stirrups 6mm to 10mm in dia.

For 90 cm length at head and toe, stirrups spacing is 8cm
c/c and for remaining intermediate length it is about 30cm
c/c.

Circular piles are seldom tapered. When tapered piles
length is restricted to 12m.

A concrete cover of 5cm is maintained throughout, over the
main steel bars.

Advantages of Pre-cast Piles:

Very effective

Simple quality control

Improves the entire area
Disadvantages of Pre-cast Piles:

Limited in length

Difficult to transport

Not suitable for densely built up area

Requires costly equipment

Cast-in-Situ Piles:

Cast in position inside the ground.

First of all a bore is dug by driving a casing pipe into the
ground.

Then the soil from the casing is jetted out and filled with
cement concrete after placing necessary reinforcement in
it.
Cast-in-situ piles are of two types:
I.
Cased Cast-in-Situ Piles: metallic shell is left inside
the ground along with the core
II.
Uncased Cast-in-Situ Piles: metallic shell is withdrawn

Advantages of Cast-in-Situ Concrete Piles:

Not limited in length

Can be cast at any place

Requires less equipment

Cost is less and is depended on the size
Disadvantages of Cast-in-Situ Concrete Piles:

Quality control is difficult

Load carrying is mostly done through end bearing only

Skin frictional resistance is very low.


Figure: Cast-in-Situ Pile

Advantages of Concrete piles:

Durability is independent of ground water level.

For large size and greater bearing power number of piles
required is much less.

Can be cast to any length, size or shape.

Can be used to marine work without any treatment.

Material required for manufacture is easily obtainable.

Concrete piles can be monolithically bonded into pile cap
which is not possible in wooden piles.

Disadvantages of Concrete piles:

Costlier than timber piles.

Can not be driven rapidly.

Required elaborate tech supervision and heavy driving
machines.

Must be reinforced to withstand handling stresses.


The greatest disadvantage of large weightt and difficulty in
handling of pre-cast pile is eliminated by prestressed
concrete piles.

The weight is reduced by casting 200mm to 300mm
diameter fiber tubes inside the piles at the time of
concreting.

The pre tensioning cables are subjected to required pull
(tension) in the casting bed.

The fiber tube is held in position inside the form work and
the piles reinforced with pre stressed cables are concreted
in a row.


Prestressed concrete piles are provided with lifting hooks
at 1/5th ( 0.2L, L= length of pile ) of pile length from each
end.

Piles length 50 times the thickness →single point pick up

More than 50 times the thickness →two point pick up at
0.2L from either end.

Piles 500 sq. mm and smaller→ cast solid.

Piles over 500 sq. mm may be cast with 200mm to 300mm
cored hole (void).

Pre stressed piles are always pre- cast.

Advantages of Prestressed Concrete Piles

It has greater ability to withstand extremely hard driving.

It is more durable in sea water because of absence of
crack.

It has greater column capacity.

It has lesser handling costs because of light weight.

It requires lesser pick-up points.

It has larger moment of inertia than the conventional piles
of same dimension since the concrete is all in
compression.

Composite Piles (contd)




Piles of two different materials are driven one over the
other, so as to enable them to act together to perform the
function of a single pile.
This type of composite pile is used with the object of
achieving economy in the cost of piling work.

Selection of Type of Pile
The nature of the ground, where piling operation is to be
carried out, determines to a large extent the choice of type
of pile to be used.
In addition, the other important factors which must be
considered in this regard are:















The nature of the structure.
Loading conditions.
Elevation of the ground water level with respect to the pile
cap.
Probable length of pile required.
Availability of materials and equipment.
Factors which may cause deterioration of pile.
Probable cost of pile.

Causes of Failure of Piles















Load on the pile is more than the designed load.
Defective workmanship during casting of the pile.
Displacement of reinforcement during casting.
Bearing pile resting on a soft strata.
Improper classification of soil.
Improper choice of the type of pile.
Insufficient reinforcement in the pile.
Decay of timber piles due to attack by insects.
Buckling of piles due to inadequate lateral support.
Defective method adopted for driving the pile.
Incorrect assessment of the bearing capacity of the pile.
Lateral forces not considered in the design of piles.

Pile Driving
I.
II.
III.
IV.

By Drop Hammer.
By Steam Hammer.
By Water Jets (Wash Boring ).
By Boring (Auger Boring).

Cof ferdams



Cofferdams may be defined as a temporary structure
constructed in a river or a lake or any other water bearing
surface for excluding water form a given site to enable the
building operation to be performed on dry surface.
Cofferdams may be divided into the following category based
on the materials used during construction:
 Earthier cofferdam.
 Rock fill cofferdam.
 Single-walled cofferdam.
 Double-walled cofferdam.
 Crib V
 Cellular cofferdam.(Circular or diaphragm type)

Earthen Cofferdams






It essentially consists of an earthen embankment built
around the area to be enclosed.
It is constructed where the depth of the water is not much
and the velocity of the current is very low.
The top width of the embankment is not less than 1 m and
the slopes vary from 1:1.5 to 1:2. The earth used to built
these cofferdams is a mixture od clay and sand or clay and
gravel

Rockfill Cofferdam




If the depth of water to be retained by the embankment of
cofferdam is of order of 1.8 to 3 m, stone or rubble is used
for the embankment.
The stones are assembled in a required shape and the
voids ae partially fillled with earth and stone chips.

Single walled Cofferdams






This type of cofferdam is used in places where the area to
be enclosed is very small and he depth of water is more,
say 4.5 to 6m.
Timber piles known as guide piles are first driven in to the
river bed. Longitudinal runners called wales are then bolted
to these at suitable distance apart. Steel or wooden sheet
piles are then driven into the river bed along the wales and
secured to them by bolts.
To increase the stability the sheets on two faces are braced
by trussed arrangement of struts.

Double walled Cofferdam







For cofferdams required to enclose large areas in deep
waters, single-walled type becomes uneconomical as large
sections of struts would be necessary to resist water, so
double walled cofferdams are used.
Their construction is similar except instead of one wall two
walls with a gap in between is used.
This can be used for waters with a depth of upto 12 m.
The thickness of the water is equal to the depth of water
when it is 3m, and 3m plus half the depth of water if it is
more.

Crib Cofferdam








In deep waters where it is difficult to insert the sheet piles or
guide piles into the hard bed below, Crib cofferdam is used.
A crib is a framework of horizontal timbers installed in
alternate courses to form pockets which can be filled with
earth and stones.
The size of the crib depends on the depth of water and the
velocity of water flow.
The crib is constructed on ground and then floated to the
water surface. Sand and other loose material overlying the
impervious hard bed is dredged out and the crib sunk to the
bed. The space inside the crib is filled with stone or other
materials to make it stable against sliding and overturning.
Timber and steel sheet piles are then driven around the crib.

Cellular Cofferdam









When he height of the water is from 18 to 21 m, this type is
used to dewater the large areas.
They are commonly used during the construction of marine
structures like dams, whares etc.
They are constructed by driving straight web sheet piles
arranged in form of a series of inter connected cells. Finally
the cells are filled with clay to make them stable. The two
common shapes of cellular cofferdams are:
a) Circular type
b) Diaphragm type

Caissons






Caissons are water light structures made up of wood, steel or
reinforced concrete, constructed in connection with excavation
for foundations of bridges, piers, abutments in river and lake
dock structure fore shore protection etc.
The caisson remains in its pose and ultimately becomes as
integral parts of the permanent structure.
Caisson can be broadly classified into the following three types:
 Open Caisson
 Box Caisson (Floating Caisson)
 Pneumatic Caisson

Open Caissons







Depending on shape they are classified in to:
-Single wall open caisson: This is a box type structure with
no top or bottom mainly consisting of vertical walls.
-Cylindrical open caisson: This may be defined as a
cylindrical shell made up of timber, masonry, steel or
reinforced concrete shod with a cutting edge and which is
sunk by excavating the soil within the shell. It is also known
as well caisson.
-Open caisson with dredging walls: This type of caisson has
the distinction of being emoyed for the deepest foundation
for bridge piers, abutments etc. The caisson is square or
rectangular in pplan which is sub divided into smaller
sections from inside forming open walls

Box Caisson


It is similar to open caisson except it is closed at the bottom.
Caisson is cast and cured on land and then launched in
water and towed to the site for sinking. They are used where
the strata of sufficient bearing capacity is available near the
ground.

Pneumatic Caisson



This type of caisson is closed at top and open at bottom.
The water is excluded from the caisson chamber by means
of compressed air. The working chamber and shafts are
made air tight. In order that the workmen can work
underneath the caisson and water may not find its way
inside from below, the pressure of the compressd air in the
shaft is just kept a little higher than the water at that depth.

Bibliography
Text: Building Construction by Sushil Kumar
 Images: Internet


Deep foundation

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