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Bearing Capacity

foundations are designed to transmitload from the structure they support to

the soil foundations are generally grouped into

two categories:

A. Shallow Foundations

B. Deep Foundations

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Shallow Foundations

the most common (and cheapest) typeof shallow foundations areSPREAD FOOTINGS

square spreadfootings to supportindividual columns(also circular)

McCarthy, 6th Ed.

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Strip Footings to support wall loads

Rectangular and Trapezoidal Footings for twocolumns (combined footing) or machine base

McCarthy, 6th Ed.

McCarthy, 6th Ed.

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RAFT or MAT Foundations

To lower the bearing pressure and reduce

differential settlement on soils with low bearingcapacity or erratic or variable conditions

McCarthy, 6th Ed.

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FLOATING Foundations

where deep deposits of compressible, cohesivesoil are present and piles are impractical

buildings substructure is a combination matand caisson to create a rigid box

weight of earth displaced by foundation isequal to total weight of structure, thereby

minimizing settlement from consolidation

McCarthy, 6th Ed.

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Deep Foundations

used when soil near surface has poorload-bearing capacity

they transmit load through weak soilstrata (overburden) to stronger, load-bearing stratum (eg., bedrock, dense

sand and gravel, etc.)

loose soil

bedrock

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Types of Deep Foundations

where load-bearing stratumno more than 5 m deep

not used much any more

PIERS

McCarthy, 6th Ed.

CAISSONS

McCarthy, 6th Ed.

where over-burden no more

than 8 - 9 mthick

replacing piers

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PILES

deep over-burden

more than 8 - 9 mthick

Various types and

placementmethods

Craig, 6th Ed.

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Structural Requirements

1. Factor of Safety against General Shear Failureof supporting soil is normally required to be inthe range 2.5 3.0

2. Tolerable amount of settlement; in particular,differential settlement should not causesignificant damage to structure nor interferewith function

3. Secondary to these, during construction, thereshould be no adverse affect on adjacentstructures or services

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Ultimate Bearing Capacity, qf

The least pressure that would cause shearfailure of supporting soil immediately belowand adjacent to a foundation

Craig, 6th Ed.

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modes of failure:

on low compressibility (dense or stiff) soils

plastic equilibrium throughout support and

adjacent soil masses heaving on both sides of foundation

final slip (movement of soil) on one side only

causing structure to tilt

General Shear Failure

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on highly compressible soils

only partial development of plastic equilibrium

only slight heaving on sides

significant compression of soil under footingbut no tilting

Local Shear Failure

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on loose, uncompacted soils

vertical shearing around edges of footing

high compression of soil under footing, hencelarge settlements

no heaving, no tilting

Punching Shear Failure

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Terzaghis Theory

strip footing of infinite length and width B

uniform surcharge, q0 on surface of isotropic,homogeneous soil

Rankine active wedge, ABC: forces

Passive zones, ADE () & BGF ()

Craig, 6th Ed.

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transition between &:ACD & BCG (zonesor radial shear or slip fans)

above EDCGF: plastic equilibrium

below EDCGF: elastic equilibrium

Craig, 6th

Ed.

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Neglecting the shear strength of the soil abovedepth D implies that this soil is a surcharge:q0 =gD

Terzaghis general equation:

the more general case is a footing at depth D

qf= 0.5gBNg + cNc + gDNq

Contribution of: Soil Self

Weight

Shear

Strength

Surcharge

Craig, 6th Ed.

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Bearing Capacity Factors N

g, N

cand N

qare bearing capacity factors and

are derived from various sources

Craig, 6th Ed.

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General Shear Failure of Footings (UltimateBearing Capacity)

qccf DNSNcSNBq ggg )()(5.0

)45(tan2

2)tan( eNq

)cot()1( qc NN

)4.1tan()1( g qNN

FOOTINGTYPE

S Sc

Strip 1.0 1.0

Square 0.8 1.2

Circular 1.6 1.2

Rectangular )(2.01 LB

)(2.01 LB

theory was developedfor strip footings

to adapt to square,circular andrectangular shapes,Terzaghi & Peckdeveloped shape

factors here whichare still widely usedtoday:

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Allowable Bearing Capacity

the allowable bearing capacity, qa is the valueused in the design of footing size

in North America, a factor of safety against

general shear failure, F is applied to theultimate bearing capacity, qf:

F

qq

f

a

q

cc

a DNF

NScNSB

q g

g gg

)()(5.0

in Britain, F is not applied to the surcharge:

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Skemptons Nc Values if undrained shear

strength parametersare used for thedesign then a specialcase arises:

since u = 0, Nq = 1and:

DNcq cuf g

values of Nc areacquired from

Skemptons ChartC i 6th Ed