Foundation Types and Their

Selections

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 Definition: The lowest artificially built part of a structure
which transmits the load of the structure to the ground.
 The foundation of a structure is always constructed below
ground level to increase the lateral stability of the structure.

firm weak soil

ground
bed rock bed rock
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 Purposes of Foundations
 Foundations are used for the following purposes:
i. To distribute the load of the structure over a large
bearing area so as to bring intensity of loading within
the safe bearing capacity of the soil lying underneath.
ii. To load the bearing surface at a uniform rate so as to
prevent unequal settlement.
iii. To prevent the lateral movement of the supporting
material.
iv. To secure a level and firm bed for building operations.
v. To increase both the overturning and sliding stability
of the structure as a whole.

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Types of Foundations
Foundations can be broadly classified into the following two
categories based on depth:
 Shallow foundations
 Deep foundations
1. Shallow Foundations :- are foundations provided immediately
beneath the lowest part of the structure, near to the ground
level.
Shallow foundations are further classified into the following
types:
Spread or Isolated footings
Combined footing
Cantilever footing
Continuous (strip) or wall footing
Raft (Mat) foundation
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A) Spread or Isolated Footings (simple, stepped or slope type)

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Isolated footing are used when:
 the soil has sufficient strength within a short depth below
the ground level.
 the soil has adequate bearing capacity.
 Super structural load is very small
The major advantages of selecting the Isolated footings are:
 Economical when columns are placed at longer distances.
 Ease of Constructability- Excavation, Form-work,
Reinforcement placement and placing of Concrete is at
ease.
 Workmen with little or no knowledge can easily construct
an Isolated Footing.

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B) Combined Footing

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Combined footing is required :
 Whenever two columns are nearby together, inducing
overlap of adjacent isolated footings
 Where soil bearing capacity is low, inducing overlap of
adjacent isolated footings
 When column end is situated near the property line and
the footing can not be extended
 When the super structural load to be transferred to the
foundation is very high
Types of Combined Footing:
 Rectangular
 Trapezoidal: Trapezoidal footing is provided when the load
on one of the column is larger than the other column.

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C) Cantilever or Strap Footing

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• Strap footing consists of two isolated footings connected with
a structural strap. The strap connects the footing such that
they behave as one unit.
• The strap does not remain in contact with soil and does not
transfer any pressure to the soil.
A strap footing is more economical than a combined footing
when:
• The allowable soil pressure is relatively high and distance
between the columns is large.
• A normal combined footing is impractical due to the required
large excavation.

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D) Continuous or wall footing or strip footing

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A strip footing is another type of spread footing which is
provided for a load bearing wall. A strip footing can also be
provided for a row of columns which are so closely spaced that
their spread footings overlap or nearly touch each other. In such
cases, it is more economical to provide a strip footing than to
provide a number of spread footings in one line. A strip footing is
also known as continuous footing.

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E) Mat (Raft) Foundation

Flat plate mat foundation Two-way beam and slab (Ribbed mat)

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A raft foundation is a solid reinforced concrete slab covering
entire area beneath the structure and supporting all the
columns. Such foundation due to its own rigidity minimizes
differential settlements.
Raft Foundation is preferably used :
• When the column loads are heavy or when the safe bearing
capacity of soil is very low
• In a places like seashore area, coastal area where the water
table is very high
• In swampy areas dominated by soft soil
• When the columns and walls are so close that individual
footings would overlap or nearly touch each other (when the
area covered by the footing is greater than 50% of the total
plan area)

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The drawbacks of raft foundation :
 Sometimes, mat foundations need heavy reinforcement in
certain areas, which can add up to the price of the
manufacture.
 The edges of the mat foundation, if not properly take care
of, may erode with time. However, when maintained well,
these edges can last as long as the building it supports.
 Sometimes, the design can become very complex and
thus, requires really skillful and experienced engineers as
well as workers.
 Frost can have an adverse effect on the mat formation.

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 2. Deep Foundations
• Deep foundation are those in which the depth of foundation is
very large in comparison to its width.(D>B).
• When the soil at or near the ground surface is not capable of
supporting a structure, deep foundations are required to
transfer the loads to deeper strata.
Deep foundations are therefore, used:
“When surface soil is unsuitable for shallow foundation, and a
firm stratum is so deep that it cannot be reached economically
by shallow foundations.”
The most common types of deep foundations are piles, piers and
caissons.
A) Pile: is a long (slender) vertical load transferring member
made of timber, steel or concrete.

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Pile foundations are divided into two types they are:-
 Pre cast Piles: Precast piles are casted at factory and
transported to the site. This kind of piles are readymade
and used where there is less place to cast pile. Precast
piles are not economical and requires more money to
transport piles to the site.

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 Cast-in-situ piles: The piles which are casted on site. And don’t
require any transportation is called cast-in-situ piles. The cast-
In-situ concrete piles are casted in position inside the ground
and need not to be reinforced in ordinary cases.
B) Pier: A pier is a vertical column of relatively large cross-
section than a pile.

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C) Caisson (Drilled shafts) : A caisson is a type of foundation of
the shape of hollow prismatic box, which is built above the
ground and then sunk to the required depth as a single unit.
• A pier and caisson differ basically only in the method of
construction. The caissons, has action similar to pile
foundations, but are high capacity cast-in-situ foundations. It
resists loads from structure through shaft resistance, toe
resistance and / or combination of both of these. The
construction of drilled shafts or caissons are done using an
auger.

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Advantages and Disadvantages of Caissons
 Advantages of Caissons:
• Economics
• Minimizes pile cap needs
• Slightly less noise and reduced vibrations
• Easily adaptable to varying site conditions
• High axial and lateral loading capacity
 Disadvantages of Caissons:
• Extremely sensitive to construction procedures
• Not good for contaminated sites
• Lack of construction expertise
• Lack of Qualified Inspectors

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 Pier Foundation Caisson Pile Foundation

Pier foundation is a type of deep foundation, Caissons are watertight structures made up of
Pile foundation is a type of deep foundation, in
which consists of a cylindrical column of large wood, steel or reinforced concrete built above
which the loads are taken to a low level by
diameter to support and transfer large the ground level and then sunken into the
means of vertical timber, concrete or steel.
superimposed loads to firm strata below. ground.

The types of pile foundation are end-bearing

The types of pier foundations are masonry or The types of caissons are box, open, piles, friction piles, compaction piles, anchor
concrete piers and drilled caissons. pneumatic, monolithic, floating, excavated etc. piles, tension or uplift piles, sheet and batter
piles etc.

Caisson is putting a box into underwater and Pile is a column of material driven by a
Pier is inserted down to the bedrock.
pouring it with concrete. piledriver.

Pier has a footing. Caisson doesn’t have a footing. Pile doesn’t have a footing.

Pier is typically dug out and cast in place

Caissons are driven into surface condition. Piles are driven into surface condition.
using forms.

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 Foundations on Black Cotton Soil
 Black cotton soils and other expansive soils have typical characteristics of shrinkage and
swelling due to moisture movement through them.

 When moisture enter between the soil particles under some hydrostatic pressure, the
particles separate out, resulting in increase in the volume.

 This increase in volume is commonly known as swelling. If this swelling is checked or

restricted high swelling pressure, acting in the upward direction, will be induced.

 This would result in several cracks in the walls and may some times damage the structural
such as lintels, beams, slabs etc.

 During summer season, moisture moves out of the soil and consequently, the soil shrinks.

 Shrinkage cracks are formed on the ground surface. These shrinkage cracks some times
also known as tension cracks, may be 10 to 15 cm wide on the ground surface.

 Black cotton soils and other expansive soils are dangerous due to their shrinkage and
swelling characteristics.

 In addition, these soils have very poor bearing capacity, ranging from 5 t/m2 to 10 t/m2.
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 General Principles of Foundation Design
• The usual approach to a normal foundation-engineering problem
is:
• To prepare a plan of the base of the structure showing the various
columns, load-bearing walls with estimated loads, including dead
load, live load, moments and torques coming into the foundation
units.
• To study the tentative allowable bearing pressures allocated for
the various strata below the ground level, as given by the soil
investigation report.

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• To determine the required foundation depth. This may be the
minimum depth based on soil strength or structural requirement
considerations.

• To compute the dimensions of the foundation based on the given

loading and allowable bearing pressure.

• To estimate the total and differential settlements of the

structure.

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 Loads on Foundation
A foundation may be subjected to two or more of the following
loads.
a) Dead load:
• Weight of structure
All material permanently attached to structure
Static earth pressure acting permanently against
the structure below ground surface.
Water pressure acting laterally against basement
walls and vertically against slab.
b) Live load: Temporary loads expected to superimpose on the
structure during its useful life.

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c) Wind load:- lateral load coming from the action of wind.
d) Earth-quake load:- lateral load coming from earth- quake
motion.
e) Dynamic load:- load coming from a vibrating object
(machinery).

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 Contact Pressure Distribution Under Foundations
 Concentrically loaded footing  Eccentrically loaded footing

(Eccentric Loads or Moments)

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 Settlement of Foundations
l1 l2 l3

Ground Level

Original foundation level

1
 max

2 3
1,2,3 = Differential settlement.,  = Greatest differential settlement.
max = maximum total sett., l1,l2,l3= Bay width, /l = angular distortion

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 Selection of Foundation Type
In selecting the foundation type the following points must be
considered;
• Function of the structure
• Loads it must carry
• Subsurface conditions (Bearing Capacity)
• Cost of foundation in comparison with the cost of the
superstructure.
• The depth of foundations of adjacent buildings

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END

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