shallow foundation, types and factor

introduction of shallow foundation,
types shallow foundation
depth and factor affecting it

vedio link
https://youtu.be/X-EwQTkcwjQ

1. SUBJECT
FOUNDATION ENGINEERING
TOPIC
DEFINITION OF SHALLOW FOUNDATION.
TYPES OF SHALLOW FOUNDATIONS.
DEPTH AND FACTOR AFFECTING IT.
PREPARED
BY
ER. SANJEEV SINGH
[email protected]

2. CONTENT
Definition of shallow foundation.
Types of shallow foundations.
Depth and factor affecting it.

3. SHALLOW FOUNDATION
Ashallowfoundation,accordingtoTerzaghi,isonewhosewidthis
greaterthanitsdepth(i.e,??????
??????/B≤1).
Shallowfoundationarelocatedjustbelowthelowerpartofthewallora
columnwhichtheysupport.
Footingarestructuralmember,madeofbrickwork,masonryor
concrete,thatareusedtotransmittheloadofthewallorcolumnsuch
theloaddistributedoveralargearea.

4. TYPES OF SHALLOW
FOUNDATIONS
Strip Foundation
Spread or Isolated Footing
Combined Foundation
Strap or cantilever foundation
Mat or Raft Foundation

5. STRIP FOUNDATION
Thefoundationwhoselengthisconsiderablygreaterthanits
widthiscalledstriporcontinuousfoundation.

6. SPREAD OR ISOLATED
FOOTING
Aspreadfootingisprovidedtosupportanindividualcolumn.
Aspreadfootingiscircular,squareorrectangularslabof
uniformthickness.
Sometimeitissteppedorhaunchedtospreadtheloadovera
largearea.

7. COMBINED FOUNDATION
Acombinedfootingsupportstwocolumns.
Itisusedwhenthetwocolumnsaresoclosetoeachother
thattheirindividualwouldoverlap.
Combinedfootingmayberectangularortrapezoidalinplan.

8. STRAP OR CANTILEVER
FOUNDATION
Astrapfootingconsistsoftwoisolatedfootingsconnectedwitha
structuralstrap.
Thestrapconnectsthetwofootingsuchthattheybehaveasoneunit.
Thestrapactasaconnectingbeamanddoesnottakeanysoilreaction.
Thestrapisdesignedasarigidbeam.
Astrapfootingismoreeconomicalthanacombinedfootingwhenthe
allowablesoilpressureisrelativelyhighandthedistancebetweenthe
columnsislarge.

9. MAT OR RAFT
FOUNDATION
Amatorraftfoundationisalargeslabsupportinganumber
ofcolumnsandwallundertheentirestructureoralargepart
ofthestructural.
Amatisrequiredwhentheallowablesoilpressureislowor
wherethecolumnsandwallsaresoclosethatindividual
footingwouldoverlapornearlytoucheachother.
Matfoundationareusefulinreducingthedifferential
settlementonnon-homogeneoussoil.
Wherethereisalargevariationintheloadsonindividual
columns.

10. DEPTH AND FACTOR
AFFECTING IT
Depth of top soil.
Frost depth.
Zone of soil volume change.
Water level.
Scour depth.
Underground defects.
Root holes.
Minimum depth.

11. DEPTH OF TOP SOIL
Thefootingshouldbelocatedbelowthetopsoilconsistingof
organicmatterswhichiseventuallydecompose.
Thetopsoilshouldberemovedoveranareaslightlylarger
thanthefooting.

12. FROST DEPTH
Thefootingshouldbecarriedbelowthedepthoffrostpenetration.
Ifthefootingislocatedatinsufficientdepth,itwouldbesubjectedtothefrost
damageduetoformationoficelensesandconsequentfrostheave.
Duringsummer,thawingoccursfromthetopdownwardsandthemeltedwateris
entrapped.
Asthesoilwaterfreezesandmelts,thefootingisliftedduringcoldweatherandit
settlesduringwarmweather.
Theshearstrengthofthesoilisalsodecreasedduringwarmweatherduetoan
increaseinwatercontent.
Topreventfrostdamage,thefootingsshouldbeplacedbelowthefrostdepth,
whichmaybe1mormoreincoldclimate.

13. ZONE OF SOIL VOLUME
CHANGE
Somesoils,especiallyclayshavinghighplasticity,suchas
blackcottonsoil,undergoexcessivevolumechange.Suchas
soilshrinksupondryingandswelluponwetting.
Thevolumechangesaregenerallygreatestneartheground
surfaceanddecreasewithincreasingdepth.
Largevolumechangesbeneathafootingmaycausealternate
liftinganddroping.

14. WATER LEVEL
Thefootingshouldbeplacedabovegroundwatertableasfar
aspossible.
Thepresenceofgroundwaterwithinthesoilimmediately
aroundafootinginundesirableasitreducethebearing
capacityofthesoilandtherearedifficultiesduring
construction.

15. SCOUR DEPTH
Thefootingslocatedinstream,onwaterfrontsorother
locationswherethereisapossibilityofscouring,shouldbe
placedbelowthepotentialscourdepth.

16. UNDERGROUND
DEFECTS
Thedepthoffootingsisalsoaffectedbythepresenceof
undergrounddefects,suchasfaults,cavesandmines.
Iftherearemanmadediscontinuities,suchassewerlines,
watermains,undergroundcables,theseshouldbeshiftedor
thefootingrelocated.

17. ROOT HOLES
Iftherearerootholesorcavitiescausedbyburrowing
animalsorworms,thefootingshouldbeplacedbelowsucha
zoneofweakenedsoil.

18. MINIMUM DEPTH
Is:1904-1978specifiesthatallfoundationshouldextendtoa
depthofatleast50cmbelowthenaturalgroundsurface.
However,incaseofrocks,onlytopsoilshouldberemoved
andthesurfaceshouldbecleaned.
Where q is the intensity of loading
(??????
??????)min=
??????
γ
(
1−??????????????????’
2
1+??????????????????’
)