2-basics-of-foundation.ppt

Prepared by :
Prof. Alka M. Shah
Civil Engineering Department
Nirma University

WHAT IS FOUNDATION ?
 Lowest part of a structure directly in contact with the ground and transmits all
the loads to the sub-soil below it.
 Majority of structure fails due to failure of their foundation
PURPOSE
 To support structure
 Distributes the loads over a larger area
 Minimizes the differential settlements
 Increases stability & prevents overturning
 Distribute non-uniform load uniformly to the soil
2

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•Foundation: Part of the structure which is in direct contact with the
ground to which the loads are transmitted.
•Footing: Lower most portion of foundation which is in direct
contact with the sub soil is called footing.
Foundation and Footing are same?

FOUNDATIONS
SHALLOW
FOUNDATION
D ≤ B
DEEP
FOUNDATIONS
D > B
TYPES OF FOUNDATION
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SHALLOW FOUNDATION
 Bearing capacity of soil is more.
 ground water table(W.T) is low.
 dewatering of foundation is not required.
 top layers of soil are uniform and stable.
 Load on the structure is less.
DEEP FOUNDATION
 Bearing capacity of soil is low.
 ground water table(W.T) is high.
 dewatering of foundation is costly and difficult.
 top layers of soil are non uniform and unstable.
 Load on the structure is more.
SUITABILITY OF FOUNDATION
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TYPES OF SHALLOW FOUNDATION
1) SPREAD FOOTING
i) Continuous footing (strip or wall)
ii) Isolated column footing a) Square
b) Circular
c) Rectangular
iii) Inverted arch footing
iv) Reinforced concrete footings
2) STRAP FOOTING
3) COMBINED FOOTING
i) Rectangular
ii) Trapezoidal
4) RAFT FOUNDATION
5) GRILLAGE FOUNDATION 6

1) SPREAD FOOTING
Carries a single column
Used to spread out loads from walls of columns over a wider
area
Consists of concrete slabs located under each structural
column and a continuous slab under load-bearing walls.
Commonly used due to their low cost & ease of
construction.
Used in small to medium size structures with moderate to
good soil condition.
For high rise buildings it is not efficient.
7

Wall Footing
Wall footing with
masonry offsets (Stepped
wall footing)
(SBC is low)
Simple wall footing
without
masonry offsets
(SBC is high)
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STRIP/WALL FOOTING
 A spread footing for a continuous wall is called strip
footing. (Used to support load bearing walls)
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INVERTED ARCH FOOTING
 Used on soft soils to reduce depth of foundation.
 End columns must be strong to resist outward
pressure caused by arch action.
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REINFORCED CONCRETE FOOTING
o Used where the walls are subjected to heavy loads &
soil bearing capacity is low

ISOLATED COLUMN FOOTING
 Used to support individual columns.
13

2) COMBINED FOOTING
 Supports two columns
 the two columns are so close to each other that their
individual footings would overlap
 one column is placed right at the property line
 C.G. of column load and centroid of the footing should
coincide
15

COMBINED FOOTING
 Rectangular footing  Trapezoidal footing
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A COMBINED FOOTING WILL DEFORM AS SHOWN IN FIG BELOW
 The eccentric loading condition upon the left end,
due to the restrictions of a property line, will
generate tensile stresses on the top of the footing.
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3) CANTILEVER/STRAP FOOTING
 Two isolated footings connected with a strap or beam
 Connects in such a way that they behave as one unit
 The strap acts as a connecting beam and does not take any soil
reaction. The strap is designed as rigid beam.
 It does not provide bearing
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 to connect an eccentrically loaded column footing to an interior
column.
 Used to transmit the moment caused from an eccentricity to the
interior
 Can be used instead of combined footing if the distance
between two columns is large
 More economical than combined footings
19

4) RAFT FOUNDATION
 Large reinforced concrete slab supporting no. of columns
& walls
 area of spread footings or combined footings exceeds
about 50 percent of the gross area of the building
 Greater weight & continuity of a mat provides sufficient
resistance
20

Raft/Mat Foundation is suitable under following conditions
 Structural loads are heavy or the soil condition is poor
 Soft or loose soils having less bearing capacity
 Foundation soil is non-homogeneous and prone to excessive
differential settlements
 Lateral loads are non-uniform
 Columns are so close that their individual footings would
overlap
 To resist water pressures when groundwater extends above
the floor level of the lowest basement of a building
 There is a large variation in the loads on individual columns
23

5) GRILLAGE FOUNDATION
 Used to transmit heavy loads from steel columns to a soil having low bearing
capacity
 Avoids deep excavation
 Reduces pressure intensity
 Lighter & economical
 Used in masonry bridge seat, bus stands, railways etc
 Made up of a rolled steel joints (RSJ) known as grillage beams
Types of Grillage foundation
 Steel grillage foundation
 Timber grillage foundation
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 A type of foundation often used at the base of a column.
It consists of one, two or more tiers of steel beams
superimposed on a layer of concrete, adjacent tiers
being placed at right angles to each other, while all tiers
are encased in concrete.
 This is dependable foundation and is used in those
place where the load of the structure is pretty and
bearing capacity of soil comparatively poor
 The grillage foundation helps in distributing the load
over a wider area of subsoil.
 The grillage foundation helps in avoiding deep
excavations as the necessary base area is provided for
the load of transmission
 This type of foundation generally used for heavy
structure columns piers and steel stanchions etc.

STEEL GRILLAGE FOUNDATION
 Consists of rolled steel
joists in single or
double tiers
27

TIMBER GRILLAGE FOUNDATION
 Uses timber planks &
timber beams
 No concrete is
embedded between the
timber joists
 Used in waterlogged
areas
29

GRILLAGE FOUNDATION FOR WOODEN/TIMBER POST
30

SHALLOW FOUNDATION
Advantages:
a) Cost (affordable)
b) Construction Procedure (simple)
c) Material (mostly concrete)
d) Labour (doesn’t need expertise)
Disadvantages:
a) Settlement
b) Foundation gets subjected to pullout, torsion etc
c) Irregular ground surface(slope, retaining wall)
31

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TYPES OF DEEP FOUNDATION
1) PILE FOUNDATION
2) CAISSONS OR WELL FOUNDATION
3) COFFER DAMS

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•Pile: Column-like foundations going deep into the subsoil.
•Piles are structural members that are made of steel, concrete or
timber.
•Preferred:
i)When soil bearing capacity is poor.
ii) Non availability of proper bearing strata at shallow depth.
iii) pumping of sub soil water would be too costly.
iv)Heavy loads from super structure for which shallow foundation
will be uneconomical.
1) Pile foundation

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1) Pile foundation
• Based on function:
-End bearing: Transfer load to through soil or water…
-Friction: Transfer load by means of skin friction…
-Compaction: Compacting loose granular material and
thus increasing SBC…
-Tension(uplift): Anchor down the structures subjected to
uplift due to hydraulic pressure or due to overturning…
Classification of pile foundation

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1) Pile foundation
Classification of pile foundation
-Anchor: Provide anchorage against horizontal pull…
-Fender and dolphins: Protect against impact from ships.
-Sheet: To reduce seepage and uplift under hydraulic structures
like dams…

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Under reamed piles
 Under reamed piles are bored cast-in-situ concrete piles
having one or more number of bulbs formed by enlarging the
pile stem.
 These piles are best suited in soils where considerable
ground movements occur due to seasonal variations, filled
up grounds or in soft soil strata.
 Provision of under reamed bulbs has the advantage of
increasing the bearing and uplift capacities. It also provides
better anchorage at greater depths.
 These piles are efficiently used in machine foundations, over
bridges, electrical transmission tower foundation sand water
tanks

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1) Caissons
o The word caissons derived from the French word caisse meaning a box
o Box like structure, round or rectangular which sunk from the surface of
either land or water to some desired depth
o Used for placing foundation under water

44
Types of Caissons
Box caisson
(open at top
and closed at
bottom)
Pneumatic caisson
(closed at top and
open at bottom)
Open caisson or wells
(open at top and bottom)
Single, Multiple and Cylinder

45
Box caissons
o Strong water tight vessel
o Built on land and floated up to site and then sink in to the water
oUsed when loads are not heavy
o Bearing stratum is available at shallow depth
oWater depth is 6 to 8 m
o Foundation bed not required
oMaterial is loose

47
Open or Well caissons
( Well Foundation)
o Used for buildings and bridge where soil liable to scour
o No firm bed is available

48
Pneumatic caissons
o It is useful when well foundation is not possible
o Depth of water 12 to 35 m

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1) Cofferdams
o Cofferdam is a temporary structure which is built in a river, lake or dam
to remove water from the area and make possible to construct
o Require in dams, docks and bridge pier

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Types of Cofferdams
1. Earth fill cofferdam

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3. Rock fill crib cofferdam

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6. Cellular cofferdam
o Mostly used for de-watering areas in places where depth of water is from
18 to 21 m
o Two types 1. Diaphragm cellular cofferdam
2. Circular cellular cofferdam
Circular cellular cofferdam

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Diaphragm cellular cofferdam

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Difference between caisson and cofferdam
CAISSON
o Permanent structure used
for placing foundation in
correct position
o It is build on dry bed,
floated to the desired site
and sunk to desired depth
o Preferred when work area
is small and water depth
is more
o Used where driving of
sheet pile is impossible
COFFERDAM
o Temporary structure used
for construction work under
reasonably dry condition and
demolished after completion of
work
o It is build in river or lake
o Preferred when work area is
large and water depth is small
o Used where driving of sheet
pile is easily possible

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FOUNDATION IN BLACK COTTON SOIL
o Black cotton soil is very dangerous to place foundation and bearing capacity
is about 50 to 100 kn/m2
o Variation in the volume change of soil extent from 20 to 30 % of original
volume
o Alternate swelling and shrinkage results in formation of cracks which are of
15 to 20 cm wide and 2.5 to 4 m deep

60
Types of Foundation in black cotton soil
1. Strip (wall) or pad (column) foundation
o Suitable when load is medium and swelling pressure is little
Foundation in black cotton soil

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o Suitable when swelling pressure is high and having high shrinkage
properties
o Used at place where high rainfall
Foundation in black cotton soil

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2. Pier Foundation
o Suitable foe wall carrying higher load
o Gap between arch and ground would permit free vertical movement of soil
during swelling and shrinkage

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Precaution For The Safety of Foundation In Black Cotton Soil
o For heavy structure raft or pile foundation is preferable
o If possible remove black cotton soil
o Construction work should be carried out in dry season
o Black cotton soil not allowed to come in direct contact with the foundation
o R.C.C. bands 10 to 15 cm deep should placed at plinth, lintel level
o Gap on either side of foundation should filled with sand
o Below foundation, 30 cm thick layer of moorum must provided

FACTORS AFFECTING FOUNDATION SELECTION
PRIMARY FACTORS AFFECTING FOUNDATION CHOICE:
 Sub-surface soil
 Ground water table conditions
 Type of structure
 Magnitude of load
SECONDARY FACTORS AFFECTING FOUNDATION CHOICE:
 Construction access, methods and site conditions
 Environmental factors
 Building Codes and Regulations
 Impact on surrounding structures
 Construction schedule
 Construction risks

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GROUND WATER TABLE CONDITIONS
Presence of groundwater near a footing is undesirable
because:
 Footing construction below GWT is difficult and expensive,
 Groundwater around a footing can reduce the strength of soils,
 It may also cause hydrostatic uplift,
 Frost action may increase,
 Waterproofing problems.
 If Ground water table is high, raft or grillage foundation is
desirable.
 If ground water table is low or at a high depth from foundation
than column foundation is desirable.

66
SUBSURFACE CONDITION AND BUILDING STRUCTURAL
REQUIREMENTS

2-basics-of-foundation.ppt

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