SOIL PERMEABILITY.pdf

These slides describes the permeability of soil in a very lucid manner. This has been posted specially for the students of Diploma and Degree Engineering courses.

1. GEOTECHNICAL
ENGINEERING
CHAPTER-III
PERMEABILITY AND SHEAR
STRENGTH OF SOIL
PART-I
PERMEABILITY OF SOIL
PREPARED BY
GOURHARI BISWAS

2. PERMEABILITY OF SOIL
•Amaterialisporousifitcontains
interstices(voids).
•Theporousmaterialispermeableifthe
voidsareinterconnectedorcontinuous.
•Aliquidcanflowthroughapermeable
material.
•Electronphotomicrographsofevenvery
stiffclaysoilindicatethatthevoidsare
interconnected.

3. Permeability-Definition
•Permeability-Permeabilityisdefinedas
thepropertyofaporousmaterialwhich
permitsthepassageorseepageofwater(
oranyliquid)throughitsinterconnecting
voids.
•Permeabilityinfluidmechanicsandthe
earthsciences(commonlysymbolizedas
κ,ork)isameasureoftheabilityofa
porousmaterial(often,arockoran
unconsolidatedmaterial)toallowfluidsto
passthroughit.

4. Facts about Permeability
•ItisaveryimportantEngineeringproperty
ofsoil.
•Gravelsarehighlypermeableandstiffclay
isleastpermeable.
•Amaterialhavingcontentiousvoidsis
knownaspermeable.Asoiliscalledhighly
permeablewhenwatercanflowthroughit
veryeasily.
•Thepermeabilityofamediumisrelatedto
theporosity,butalsototheshapesofthe
poresinthemediumandtheirlevelof
connectedness.

5. LAMINAR AND TURBULANT FLOW
•Flowofwaterthroughsoilsmayeitherbe
alaminarfloworaturbulentflow.
•Inlaminarflowconditioneachfluidparticle
travelalongadefinitepathwhichnever
crossesthepathofanyotherparticle.
•Whileinturbulentflowtravelpathsof
particlesareirregularandtwisting,
crossingatrandom.

6. Fields of Civil Engineering where Knowledge
of Permeability is Required
•Calculationofupliftpressureunderhydraulic
structureandtheirsafetyagainstpiping
•Groundwaterflowtowardswellsanddrainage
ofsoil
•Calculationofseepagethroughthebodyof
earthdamsandstabilityofslopes
•Determinationofrateofsettlementofa
saturatedcompressiblesoillayerors.
•Constructionoffoundationofstructuresbelow
groundwatertable.

7. DARCY’S LAW
•Darcy'slawisanequationthatdescribesthe
flowofafluidthroughaporousmedium.Thelaw
wasformulatedbyHenryDarcybasedonthe
resultsofexperimentsontheflowofwater
throughbedsofsand,formingthebasisof
hydrogeology,abranchofearthsciences.
•Darcy'slawstatesthatthereisalinear
relationshipbetweenflowvelocity(v)and
hydraulicgradient(i)foranygivensaturated,
isotropicandhomogeneoussoilundersteady
laminarflowconditions,i.e,flowvelocitythrough
aporousmediaisproportionaltohydraulic
gradient.

8. Mathametical Expression of Darcy’s Law
Iftherateofflowisq
(volume/time)throughcross-
sectionalarea(A)ofthesoil
mass,Darcy'sLawcanbe
expressedas
v∞i
Or,v=q/A=k.i
wherek=permeabilityofthe
soil
i=∆h/L=HydraulicGradient
∆h=differenceintotalheads
L=lengthofthesoilmass

9. Coefficient of permeability(k)
Darcy’sLawv=q/A=k.i
Whenhydraulicgradientisunity,i.e.i=1,q
=kAOrv=k,sincev=q/A
FromthisrelationwecandefineCoefficient
ofpermeability(k)astheaveragevelocityof
flowthatwilloccurthroughthetotalcross
sectionalareaofsoilunderunithydraulic
gradient.
Unitofkiscm/sec.M/dayetc.

10. Coefficient of Permeability
Permeability(k)isan
engineeringpropertyof
soilsandisafunctionof
thesoiltype.Itsvalue
dependsontheaverage
sizeoftheporesandis
relatedtothedistributionof
particlesizes,particle
shapeandsoilstructure.
Fordifferentsoiltypesas
pergrainsize,theordersof
magnitudeforpermeability
areasfollows:
Soil k (cm/sec)
Gravel 10
0
Coarse sand 100to 10-1
Medium sand 10-1to 10-2
Fine sand 10-2to 10-3
Silty sand 10-3to 10-4
Silt 1 x 10-5
Clay 10
-7
to 10
-9

11. Factors affecting permeability of soils
•ParticleSize:ItwasstudiedbyAllenHazenthatthe
coefficientofpermeability(k)ofasoilisdirectly
proportionaltothesquareoftheparticlesize(D).Thus
|permeabilityofcoarsegrainedsoilisverylargeas
comparedtothatoffinegrainedsoil.Thepermeabilityof
coarsesandmaybemorethanonemilliontimesas
muchthatofclay.
•Impuritiesinwater:Anyforeignmatterinwaterhasa
tendencytoplugtheflowpassageandreducethe
effectivevoidsandhencethepermeabilityofsoil.
•Voidratio(e):Foragivensoil,thegreaterthevoidratio,
thehigherthevalueofthecoefficientofpermeability.
Here'e'isthevoidratio.

12. contd...
•DegreeofSaturation:Ifthesoilisnotfullysaturated,it
containsairpockets.Thepermeabilityisreducedduetothe
presenceofairwhichcausesablockagetothepassageof
water.Consequently,thepermeabilityofapartiallysaturated
soilisconsiderablysmallerthanthatoffullysaturatedsoil.
Infact,Darcy'sLawisnotstrictlyapplicabletosuchsoils.
•Adsorbedwater:Finegrainedsoilshavealayerof
adsorbedwaterstronglyattachedtotheirsurface.This
adsorbedlayerisnotfreetomoveundergravity.Itcauses
anobstructiontotheflowofwaterintheporesandhence
reducesthepermeabilityofsoils.
•Entrappedairandorganicmatter:Airentrappedinthesoil
andorganicmatterblockthepassageofwaterthroughsoil,
hencepermeabilityconsiderablydecreases.Inpermeability
tests,thesampleofsoilusedshouldbefullysaturatedto
avoiderrors.

13. CONSTANT HEAD PERMEA BILITY TEST
Intheconstanthead
permeameter,thehead
causingflowthroughthe
specimenremainsconstant
throughoutthetest.The
coefficientofpermeability(k)
isobtainedfromtherelation
Where q= discharge,
Q=totalvolumeofwater,
t=timeperiod,h=head
causingflow,L=lengthof
specimen, A= cross-
sectionalarea.

14. VARIABLE HEAD PERMEA BILITY TEST
The variable head
permeameterisusedto
measurethepermeabilityof
relativelylesspervioussoils.
The coefficient of
permeabilityisgivenby
Whereh1=initialhead,h2
=finalhead,t=timeinterval,
a=cross-sectionalareaofthe
liquidstandpipe,A=cross-
sectionalareaofthe
specimen,L=lengthof
specimen.

15. NEED AND SCOPE OF PERMEABILITY
TESTS OF SOIL
The test results of the permeability experiments
are used:
1.To estimate ground water flow.
2.To calculate seepage through dams.
3.To find out the rate of consolidation and settlement of
structures.
4.To plan the method of lowering the ground water table.
5.To calculate the uplift pressure and piping.
6.To design the grouting.
7.And also for soil freezing tests.
8.To design pits for recharging.