4. Types-of Foundations.pdf

Foundation engineering lecture notes

1. 4. Foundation Types and Their
Selections
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2. •Definition: The lowest artificially built part
of a structure which transmits the load of
the structure to the ground is called
foundation.
•The foundation of a structure is always
constructed below ground level to increase
the lateral stability of the structure.
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3. bed rock
firm
ground
bed rock
weak soil
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4. Types of Foundations
Foundations can be broadly classified into the following two
categories:
–Shallow foundations
–Deep foundations
1.Shallow Foundations :-The foundations provided immediately
beneath the lowest part of the structure, near to the ground
level are known as shallow foundations.
Shallow foundations are further classified into the following types:
»Spread or Isolated footings
»Combined footing
»Cantilever footing
»Continuous or wall footing
»Raft foundation
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5. SpreadorIsolatedFootings:usedtosupportindividual
column.
•Thesearethemostcommontypeoffoundation,
primarilybecauseoftheircostandeaseofconstruction.
•Theyaremostoftenused:
–insmalltomediumsizestructures,
–onsiteswithmoderatetogoodsoilconditions,
–onsomelargestructureswhentheyarelocated
atsitesunderlainbyexceptionallygoodsoilor
shallowbedrock.
•Isolatedfootingsaresteppedtype,simpletypeorslope
type
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6. 6

7. 7

8. Combined Footing:supports two or sometimes
three column in a row.
•Combined footing is used when property lines,
equipment locations, column spacing or other
considerations limit the footing clearance at the
column locations.
•Combined footing can be:
•rectangular in shape if both the columns carry equal
loads, or
•trapezoidalif there is a space limitation and they
carry unequal loads.
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9. a
Plan
A
A
b
Plan
b
1
C
C
b
2
a
D
a a
Columns Columns
D
Footing
Footing
Section A.A Section C.C
Combined footing (rectangular) Combined footing (trapezoidal)
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10. 10
Combined Footing

11. Cantilever or Strap Footing: consists of two
individual footings connected by a beam called a
strap.
•Cantilever footing may be used:
•where the distance between the columns is so
great that a trapezoidal combined footing
becomes quite narrow, with resulting high
bending moments.
•The strap beam does not remain in contact with
soil so a strap doesn’t transfer any pressure to the
soil.
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12. 12

13. Property line
A
A
a
1
b
1 b
2
a
2
Strap beam
b
1
b
2
D
2
D
1
Section A-A
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14. Continuous or wall footing or strip
footing:
•Inthistypeoffooting,asinglecontinuous
reinforcedconcreteslabisprovidedas
foundationofloadbearingwall.
•Astripfootingisalsoprovidedforarowof
columnswhicharesocloselyspacedthattheir
spreadfootingsoverlapornearlytoucheach
other.
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15. Wall
Footing
Column
D
Footing
D
Section C-CSection A.A
Wall on footing
Columns on footing
b
b
b b
A
A
C
C
Columns
Wall
Plan Plan
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16. 16
Wall Footing

17. Mat (Raft) Foundation: is a combined footing that
covers the entire area beneath a structure and
supports all the columns.
•Foundation engineers often consider mats when
dealing with any one of the following conditions:
–The structural loads are so high or the soil
conditions so poor that spread footings would be
exceptionally large.
As a general rule of thumb, if spread footings would cover
more than 50 percent of the building footprint area, a mat
or some type of deep foundation will usually be more
economical.
–The soil is very erratic and prone to excessive
differential settlement.
–The structural loads are erratic, and thus increase
the likelihood of excessive differential settlement.
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18. –The lateral loads are not uniformly distributed through
the structure and thus may cause differential horizontal
movement in spread footings.
–The uplift loads are lager than spread footings can
accommodate.
–The bottom of the structure is located below ground
water table, so waterproofing is an important concern.
Because mats are monolithic, they are easier to
waterproof.
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19. Flat plate mat foundation Two-way beam and slab
(Ribbed mat)
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20. 20

21. 2.DeepFoundations:Whenthesoilatornearthe
groundsurfaceisnotcapableofsupportingastructure,
deepfoundationsarerequiredtotransfertheloadsto
deeperstrata.
•Deepfoundationsare,therefore,used
»whensurfacesoilisunsuitableforshallow
foundation,andafirmstratumissodeep
thatitcannotbereachedeconomicallyby
shallowfoundations.
•Themostcommontypesofdeepfoundationsarepiles,
piersandcaissons.
•Pile:isaslenderstructuralmembermadeofsteel,
concreteorwood.
•Apileiseitherdrivenintothesoilorformedin-situ
byexcavatingaholeandfillingitwithconcrete.
•
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22. •Pier: A pier is a vertical column of relatively large
cross-section than a pile.
»A pier is installed in a dry area excavating a
cylindrical hole of large diameter to the
desired depth and then backfilling it with
concrete.
•Caisson: 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.
•It is a watertight box or chamber used for laying
foundation under water.
•A pier and caisson differ basically only in the method of
construction
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23. 23

24. 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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25. •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.
If these are excessive the bearing pressure will have to
be reduced or the foundation taken to a deeper and
less compressible stratum or the structure will have
to be founded on piles or other special measures
taken
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26. 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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27. •c) Wind load:-lateral load coming from the action of wind.
-Local building codes provide magnitude of design
wind pressure.
•d) Earth-quake load:-lateral load coming from earth-quake
motion.
-The total lateral force (base shear) at the base of a
structure is evaluated in accordance with local
building code.
•e) Dynamic load:-load coming from a vibrating object
(machinery).
–In such case, separate foundation should be provided. The
impact effect of such loads should be considered in design.
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28. Pressure Distribution Beneath Foundations
•This is the distribution of the pressure between the base of the
foundation and the ground.
•The pattern of the distribution varies according to the stiffness
of the foundation.
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Pressure distribution under a rigid footing
a)On cohessionless soil
b)On cohesive soil
a)
b)

29. Pressure distribution under a flexible footing
a)On clay soil
b)On granular soil
a) b)
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30. Approximate contact pressure
distribution
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31. Eccentric Loads or Moments
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32. Eccentric Loads or Moments
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33. Two-way Eccentric Loads or
Moments
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34. .
e
b
e
a
a
b

max
 = P/ab (16e
b/b 6e
a/a)

min
For contact pressure to remain (+) ve
everywhere, 0.1
66
+
L
e
B
e
LB
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35. Settlement of Foundations
35
Ground Level
Original foundation level

max

1

2

3
l
1
l
2 l
3

1,
2,
3= Differential sett., = Greatest differential sett.

max = maximum total sett., l
1,l
2,l
3= Bay width, /l = angular distortion

36. NOSETTLEMENT*TOTALSETTLEMENT* DIFFERENTIAL
SETTLEMENT
Uniformsettlementisusuallyoflittleconsequenceinabuilding,but
differentialsettlementcancauseseverestructuraldamage
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37. 37

38. •From statistical analysis Skempton and MacDonald concluded
that as long as the angular distortion , /l of a building is less
than 1/300, there should be no settlement damage.
1.Recommendation of Skempton and MacDonald
i) Settlements on sand
a)isolated footings /l = 
max/600, 
max2inches
b) rafts /l = 
max/750, 
max2 1/2inches
ii) Settlements on clay
a) isolated footings /l = 
max/1000, 
max3.3inches
b) rafts /l = 
max/1250, 
max4 1/4inches
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39. 2.Recommendation of Bowles
Types of soil Type of foundations
Isolated Rafts
Sand 3.8cm 3.8-6.4cm
clay 6.4cm 6.4cm-10.2cm
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40. 3.Recommendation of EBCS7-1995
Types of
soils
Isolated rafts
Sand 5.0cm 5.0cm
clay 7.5cm 7.5cm
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41. 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
•Cost of foundation in comparison with the cost of the
superstructure.
•Having these points in mind one should apply the following
steps in order to arrive at a decision.
•Obtain at least approximate information concerning
the nature of the superstructure and the loads to be
transmitted to the foundation
•Determine the subsurface condition in a general way.
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42. •Consider each of the usual types of foundations in order to judge
whether or not
•They could be constructed under existing conditions.
•They are capable of carrying the required load.
•They experience serious differential settlements.
•The types that are found to be unsuitable should then be
eliminated.
•Undertake a detailed study of the most promising types. Such a
study may require additional information on loads and subsurface
conditions.
•Determine the approximate size of footing or the approximate
length and number of piles required
•Prepare an estimate for the cost of each promising type of
foundation.
•Select the type that represents the most acceptable compromise
between performance and cost.
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