FOUNDATION Engineering

Design of shallow foundation

Prepared by :

Prof. Alka M. ShahCivil Engineering Department

Institute of TechnologyNirma 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

Major purpose is to transmission of load in such a way soil is not over-stressed and does not undergo deformation

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

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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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SHALLOW FOUNDATION

Advantages:

a) Cost (affordable)b) Construction Procedure (simple)c) Material (mostly concrete)d) Labour (doesn’t need expertise)

Disadvantages:

e) Settlementb) Foundation gets subjected to pullout, torsion

etcc) Irregular ground surface(slope, retaining

wall)

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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) Reinforced concrete footings

2) STRAP FOOTING

3) COMBINED FOOTING i) Rectangular ii) Trapezoidal

4) RAFT 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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WALL FOOTING

A spread footing for a continuous wall is called strip footing. (Used to support load bearing walls)

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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.

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ISOLATED COLUMN FOOTING

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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

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COMBINED FOOTING

Rectangular footing Trapezoidal footing

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3) 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

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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

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RAFT / MAT FOUNDATION

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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

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FACTORS AFFECTING FOUNDATION SELECTIONFACTORS AFFECTING FOUNDATION CHOICE:

Sub-surface soil Ground water table conditions Type of structure Magnitude of load Tolerances of structure i.e. permissible values of settlement and tilt etc.

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PERMISSIBLE DIFPERENTUL SRTTLEMENTS AND TILT IS:1904

BASIC REQUIREMENT OF FOUNDATION

Foundation must be properly located considering any future influence which could adversely affect its performance

Soil supporting must be safe against shear failure Foundation must not settle or deflect Should safe against sliding and overturning

DESIGN OF FOUNDATION INCLUDE

1. Soil Design

2. Structural Design

Geotechnical Shallow Foundation Design

Soil Design of foundation Include:

Loads coming on foundation W (DL, LL, etc.)

Determination of Allowable Bearing pressure of soil, qa

Determination of area of foundation

Area = [W + Self weight of foundation(10% of W)] / qa

Layout of foundation: such a way that C.G. of foundation coincides

with C.G. of load

Upward soil pressure on footing p = W/A < A. B. P

THANK YOU

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