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Bearing CapacityBearin

g Capacity

foundations are designed tofoundations are designed totransmit load from the structure theytransmit load from the structure they

support to the soilsupport to the soil foundations are generally groupedfoundations are generally grouped

into two categories:into two categories:

A. Shallow FoundationsA. Shallow Foundations

B. Deep FoundationsB. Deep FoundationsRationhome.com


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Strip Footings to support wall loadsStrip Footings to support wall loads

ectangular and #rape,oidal Footings forectangular and #rape,oidal Footings fortwo columns (com-ined footing) ortwo columns (com-ined footing) ormachine -asemachine -ase

cCarthy* +cCarthy* + thth !d.!d.

cCarthy* +cCarthy* + thth !d.!d.Rationhome.com


5/21

AF# or A# FoundationsAF# or A# Foundations

#o lower the -earing pressure and reduce #o lower the -earing pressure and reducedi erential settlement on soils with lowdi erential settlement on soils with low-earing capacity or erratic or aria-le-earing capacity or erratic or aria-leconditionsconditions


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6/21

F/"A#$%& FoundationsF/"A#$%& Foundations

where deep deposits of compressi-le*where deep deposits of compressi-le*cohesi e soil are present and piles arecohesi e soil are present and piles areimpracticalimpractical-uilding0s su-structure is a com-ination-uilding0s su-structure is a com-inationmat and caisson to create a rigid -o1mat and caisson to create a rigid -o1weight of earth displaced -y foundation isweight of earth displaced -y foundation ise'ual to total weight of structure* there-ye'ual to total weight of structure* there-y


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7/21

Deep FoundationsDeep Foundations

used when soil near surface hasused when soil near surface haspoor load2-earing capacitypoor load2-earing capacity

they transmit load through wea3they transmit load through wea3soil strata (o er-urden) to stronger*soil strata (o er-urden) to stronger*load2-earing stratum (eg.* -edroc3*load2-earing stratum (eg.* -edroc3*

dense sand and gra el* etc.)dense sand and gra el* etc.)

loose soil

bedrock

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8/21

ypes o eepypes o eepFoundationsFoundations

where load2-earing stratumwhere load2-earing stratumno more than 4 m deepno more than 4 m deepnot used much any morenot used much any more

P$! SP$! S


CA$SS"%CA$SS"%SS


where o er2where o er2-urden no-urden nomore than 5 2more than 5 26 m thic36 m thic3replacingreplacingpierspiersRationhome.com


9/21

P$/!SP$/!S

deep o er2deep o er2

-urden more-urden morethan 5 2 6 mthan 5 2 6 mthic3thic3

7arious types7arious typesand placementand placementmethodsmethods

Craig* +Craig* + thth !d.!d.Rationhome.com


10/21

Structural e'uirementsStructural e 'uirements

8.8. Factor of Safety against &eneral ShearFactor of Safety against &eneral ShearFailure of supporting soil is normallyFailure of supporting soil is normallyre'uired to -e in the range 9.4 ;.


11/21

@ltimate Bearing Capacity*@ltimate Bearin g Capacity* '' f f

#he least pressure that would cause shear #he least pressure that would cause shearfailure of supporting soil immediatelyfailure of supporting soil immediately-elow and ad?acent to a foundation-elow and ad?acent to a foundation

Craig* +Craig* + thth Rationhome.com


12/21

modes ofmodes offailure:failure:

on low compressi-ility (dense or sti ) soilson low compressi-ility (dense or sti ) soilsplastic e'uili-rium throughout supportplastic e'uili-rium throughout supportand ad?acent soil massesand ad?acent soil masseshea ing on -oth sides of foundationhea ing on -oth sides of foundation>nal slip (mo ement of soil) on one side>nal slip (mo ement of soil) on one sideonly causing structure to tiltonly causing structure to tilt

&eneral Shear&eneral ShearFailureFailure

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13/21

on highly compressi-le soilson highly compressi-le soilsonly partial de elopment of plasticonly partial de elopment of plastice'uili-riume'uili-riumonly slight hea ing on sidesonly slight hea ing on sidessigni>cant compression of soil undersigni>cant compression of soil underfooting -ut no tiltingfooting -ut no tilting

/ocal Shear Failure/ocal Shear Failure

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14/21

on loose* uncompacted soilson loose* uncompacted soilsertical shearing around edges of footingertical shearing around edges of footing

high compression of soil under footing*high compression of soil under footing*hence large settlementshence large settlements

no hea ing* no tiltingno hea ing* no tilting

Punching ShearPunchi ng Shear

FailureFailure

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#er,aghi0s #heory #er ,aghi0s #heory

strip footing of in>nite length and width Bstrip footing of in>nite length and width B

uniform surcharge* 'uniform surcharge* '


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transition -etweentransition -etween && :: ACD BC&ACD BC&(,ones or radial shear or slip fans)(,ones or radial shear or slip fans)a-o e !DC&F: plastic e'uili-riuma-o e !DC&F: plastic e'uili-rium-elow !DC&F: elastic e'uili-rium-elow !DC&F: elastic e'uili-rium

Craig* +Craig* +thth

!d.!d.

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%eglecting the shear strength of the soil%eglecting the shear strength of the soila-o e depth D implies that this soil is aa-o e depth D implies that this soil is asurcharge: 'surcharge: '


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Bearing Capacity FactorsBearin g Capacity Factors

%% * %* %cc and %and % '' are -earing capacity factorsare -earing capacity factorsand are deri ed from arious sourcesand are deri ed from arious sources

Craig* +Craig* + thth Rationhome.com


19/21

&eneral Shear Failure of Footings (@ltimate&eneral Shear Failure of Foot ings (@ltimateBearing Capacity)Bear ing Capacity)

qcc f DN S N cS N Bq ++= )()(5.0

)45(tan 22)tan( += e N q

)cot()1( = qc N N

)4.1tan()1( = q N N FOOTING

TYPE S

S c

Strip 1.0 1.0

Square 0.8 1.2

ircular 1.! 1.2

"ecta#$ular )(2.01 L B )(2.01 L B+

theory wastheory wasde eloped for stripde eloped for stripfootingsfootingsto adapt to s'uare*to adapt to s'uare*circular andcircular andrectangularrectangularshapes* #er,aghi shapes* #er,aghi

Pec3 de elopedPec3 de elopedshape factors hereshape factors herewhich are stillwhich are stillwidely used today:widely used today:

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All l B iA B


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Allowa-le BearingA owa e Bear n gCapacityCapacity

the allowa-le -earing capacity* 'the allowa-le -earing capacity* ' aa is theis thealue used in the design of footing si,ealue used in the design of footing si,ein %orth America* a factor of safetyin %orth America* a factor of safety

against general shear failure* F is appliedagainst general shear failure* F is appliedto the ultimate -earing capacity* 'to the ultimate -earing capacity* ' f f ::

F

qq f a =

q

cc

a DN F

N S c N S B

q

+

+=

)()(5.0in Britain* F is not applied to thein Britain* F is not applied to thesurcharge:surcharge:

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S3empton0s %S3 empton0s % cc 7alues7aluesif undrained shearif undrained shear

strengthstrengthparameters areparameters areused for the designused for the designthen a special casethen a special casearises:arises:sincesince uu

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