Geotechnical-Engineering details study paper

Survey

1. Department of Civil
Engineering
CEB 3103
GEOTECHNICAL ENGINEERING I
SOIL WATER AND WATER FLOW
Prepared by
R.Elakya, Assistant Professor

2. Department of Civil
Engineering
SOIL WATER
Waterpresentinthevoidspacesofasoilmassiscalled‘Soil
Water’
Thesub-surfacewaterwhichoccupiesthevoidsinthesoil
abovethegroundwatertable.
Movementofwaterintosoil-Infiltration
Downwardmovementofwaterwithinthesoil-Percolation,
PermeabilityorHydraulicconductivity
SOIL WATER
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3. FORMS OF SOIL WATER
Therearemainlytwoformsofsoil
water.
Gravitationalwater
Freewater
Groundwater
Capillarywater
Heldwater
Adsorbedwater
Capillarywater
Structuralwater
Department of Civil
Engineering
SOIL WATER
Fig. 1 Soil water
3
Source: Fig. 1 -https://www.tutorvista.com/biology/types-of-soil-conservation

4. Gravitational water
Thewaterinthesoilduetothemovementofwaterunder
gravitationalforces.
Free water:
Similarpropertiesasthatofliquidwater
Movesundertheinfluenceofgravity,orduetodifferencein
hydrostaticpressurehead.
Sources-precipitation,run-off,floodwater,meltingsnow,
waterfromcertainhydraulicoperations.
Department of Civil
Engineering
SOIL WATER
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5. Ground water:
Fillsupthevoidsinthesoiluptothegroundwatertableand
translocatesthroughthem.
Fillscoherentlyandcompletelyallvoidswhichmakesthesoil
completelysaturated.
Groundwatersubjectedtoatmosphericpressure-Ground
watertable
Elevationofthegroundwatertableatagivenpoint-Ground
waterlevel
Department of Civil
Engineering
SOIL WATER
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6. Capillary water :
Waterinasuspendedcondition,heldbytheforcesofsurface
tensionwithintheintersticesandporesofcapillarysizeinthe
soil.
Retainedasminutebodiesofwaterfillingpartofthepore
spacebetweenparticles.
Department of Civil
Engineering
SOIL WATER
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7. Held water
Waterheldinsoilporesorvoidspacesbecauseofcertain
forcesofattraction.
Adsorbedwater:
Stronglyattractedtosoilmineralsurfacesbyelectrostatic
forcesespeciallyclays.
Drysoilmassadsorbwaterfromatmosphereevenatlow
relativehumidityknownashygroscopicwatercontent.
Waterlostfromanair-drysoilwhenheatedto105ºC.
Neitheraffectedbygravitynorbycapillaryforcesandwould
notmoveintheliquidform.
Department of Civil
Engineering
SOIL WATER
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8. Structuralwater:
Chemicallycombinedasapartofthecrystalstructureofthe
mineralofthesoilgrains
Cannotbeseparated/removedwhensubjectedtoloading
conditionsorovendryingto105ºC-110ºC
Department of Civil
Engineering
SOIL WATER
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9. STRESSES IN SOIL
Stresses(TotalStress)withinasoilmasscausedbyexternalloads
appliedtothesoilandalsoself-weightofthesoil.
Totalstressincreaseswithdepth(Z)andwithunitweightofsoil
(ɣ).
Atanypointinsideasoilmass,resistedbythesoilgrainsand
waterpresentintheporesorvoids(saturatedsoil).
Vertical total stress at depth Z,σ
v=ɣ.Z
Fig. 2 Stress in soil mass
STRESSES IN SOIL
Department of Civil
Engineering
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Source: Fig. 2
http://environment.uwe.ac.uk/geocal/SoilMech/stresses/stresses.htm

10. Belowawaterbody,thetotalstressisthesumoftheweightof
thesoiluptothesurfaceandtheweightofwaterabovethis.
σ
v=ɣ.Z+ɣ
w.Z
w
Fig. 3 Stress in submerged soil mass
Department of Civil
Engineering
STRESSES IN SOIL
10
Source: Fig. 3
http://environment.uwe.ac.uk/geocal/SoilMech/stresses/stresses.htm

11. Pore Pressure/Neutral stress
Porewaterpressure(u)-Pressureofgroundwaterheldwithina
soilorrock,ingapsbetweenparticles(pores).
Porewaterpressuresbelowthephreaticlevelofthegroundwater
aremeasuredwithpiezometers.
Magnitudeoftheporewaterpressureatwatertable-zero.
Belowthewatertable,porewaterpressure-positive.
u= Ɣ
w . h
Ɣ
w –Unit weight of water
Fig 4. Pore water pressure in soil mass
Department of Civil
Engineering
STRESSES IN SOIL
11
Source: Fig.4
http://environment.uwe.ac.uk/geocal/SoilMech/stresses/stresses.htm

12. Effective Stress/ Inter-granular Pressure
Effectivestress-Pressuretransmittedthroughgraintograinatthe
contactpointsthroughasoilmasscausingdisplacements.
CompressionandShearstrengthofthesoildependsoneffective
stress.
Effectivestress(σ')actingonasoiliscalculatedfromtwo
parameters,totalstress(σ)andporewaterpressure(u)according
to:
σ‘ = σ –u
Fig. 5 Total stress, Effective stress and Pore water pressure
Department of Civil
Engineering
STRESSES IN SOIL
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Source: Fig. 5 –Schofield and Wroth, “Critical State Soil Mechanics”

13. STRESSES IN SOIL
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Engineering
STRESSES IN SOIL
Fig. 6 Schematic representation of Total stress, Effective stress and Pore
water pressure
13
Source: Fig. 6
http://environment.uwe.ac.uk/geocal/SoilMech/stresses/stresses.htm

14. Example 1
Forthesoildepositshownbelow,drawthetotalstress,porewater
pressureandeffectivestressdiagrams.Thewatertableisatground
level.
Department of Civil
Engineering
STRESSES IN SOIL
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15. Solution:
Total stress
At -4m, σ = 1.92 x 4 = 7.68 T/m
2
At -11m, σ = 7.68 + 2.1 x 7 = 22.38 T/m
2
Pore water pressure
At -4 m, u = 1 x 4 = 4 T/m
2
At -11 m, u = 1 x 11 = 11 T/m
2
Effective stress
At -4 m , σ‘ = 7.68 -4 = 3.68 T/m
2
At -11m , σ‘ = 22.38 -11 = 11.38 T/m
2
Department of Civil
Engineering
STRESSES IN SOIL
15

16. Example 2
Determinetheneutralandeffectivestressatadepthof16mbelowthe
groundlevelforthefollowingconditions:Watertableis3mbelow
groundlevel;G=2.68;e=0.72;averagewatercontentofthesoilabove
watertableis8%.
Solution:
Department of Civil
Engineering
STRESSES IN SOIL
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17. Department of Civil
Engineering
STRESSES IN SOIL
17

18. Department of Civil
Engineering
STRESSES IN SOIL
18

19. PERMEA BILITY OF SOIL
Darcy'slawstatesthatthereisalinearrelationshipbetweenflow
velocity(v)andhydraulicgradient(i)foranygivensaturatedsoil
understeadylaminarflowconditions.
Iftherateofflowisq(volume/time)throughcross-sectionalarea
(A)ofthesoilmass,Darcy'sLawcanbeexpressedas
v=q/A=k.i
where
k–permeabilityofsoil(cm/sec)
i–hydraulicgradient(Δh/L)
Δh-differenceintotalheads
L–Lengthofsoilmass
SOIL PERMEABILITY
Department of Civil
Engineering
Fig. 7 Flow of water in soil
19Source: Fig. 7 -NPTEL

20. What is permeability of soil?
Permeabilityisdefinedasthepropertyofaporousmaterial
whichpermitsthepassageorseepageofwaterthroughits
interconnectingvoids.
Rateofpermeabilityvariesbasedonvoidspacesbetween
thegrains(irregularshapeoftheindividualparticles)
Department of Civil
Engineering
SOIL PERMEABILITY
Fig. 8 Comparison of Permeabilityof different soil
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Source: Fig.8 -https://www.pinterest.com/jvonstorch/muro-contenci/

21. PERMEABILITY FOR DIFFERENT SOILS
Fordifferentsoiltypesaspergrainsize,theordersofmagnitudefor
permeabilityareasfollows:
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Engineering
SOIL PERMEABILITY
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22. FACTORS AFFECTING SOIL PERMEABILITY
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Engineering
SOIL PERMEABILITY
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23. DETERMINATION OF CO -EFFICIENT OF PERMEABILITY
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Engineering
SOIL PERMEABILITY
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24. CONSTANT HEAD PERMEABILITY TEST
Quantityofwater(Q)thatflowsunderagivenhydraulicgradient
throughasoilsampleofknownlength&crosssectionalareaina
giventime(t).
Waterisallowedtoflowthroughthecylindricalsampleofsoil
underaconstanthead.
For testing of pervious, coarse grained soils
k = Coefficient of permeability
Q = total quantity of water
t = time
L = Length of the coarse soil
Department of Civil
Engineering
SOIL PERMEABILITY
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25. CONSTANT HEAD PERMEABILITY TEST SETUP
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Engineering
SOIL PERMEABILITY
Fig. 9 Constant Head Permeability test setup
25Source: Fig. 9 -Venkatramaiah, C., “Geotechnical Engineering”

26. FALLING HEAD PERMEABILITY TEST
Relativelyforlesspermeablesoils
Waterflowsthroughthesamplefromastandpipeattachedtothe
topofthecylinder.
Theheadofwater(h)changeswithtimeasflowoccursthrough
thesoil.Atdifferenttimestheheadofwaterisrecorded.
t=time
L=Lengthofthefinesoil
A=crosssectionareaofsoil
a=crosssectionareaoftube
k=Coefficientofpermeability
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Engineering
SOIL PERMEABILITY
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27. FALLING HEAD PERMEABILITY TEST SETUP
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Engineering
SOIL PERMEABILITY
Fig. 10 Falling Head Permeability test setup
27
Source: Fig. 10 -Venkatramaiah, C., “Geotechnical Engineering”

28. Example 3
Asampleinavariableheadpermeameteris8cmindiameterand10cm
high.Thepermeabilityofthesampleisestimatedtobe10×10
–4
cm/s.If
itisdesiredthattheheadinthestandpipeshouldfallfrom24cmto12
cmin3min.,determinethesizeofthestandpipewhichshouldbeused?
Solution:
Department of Civil
Engineering
SOIL PERMEABILITY
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29. Department of Civil
Engineering
SOIL PERMEABILITY
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30. Example 4
Thedischargeofwatercollectedfromaconstanthead
permeameterinaperiodof15minutesis500ml.Theinternal
diameterofthepermeameteris5cmandthemeasureddifference
inheadbetweentwogaugingpoints15cmverticallyapartis40
cm.Calculatethecoefficientofpermeability.
Solution:
Department of Civil
Engineering
SOIL PERMEABILITY
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31. PERMEABILITY –STRATIFIED SOIL DEPOSITS
Soildepositconsistsofanumberofhorizontallayershaving
differentpermeabilities,theaveragevalueofpermeability
canbeobtainedseparatelyforbothverticalflowand
horizontalflow,ask
Vandk
Hrespectively.
Considerastratifiedsoilhavinghorizontallayersof
thicknessH
1,H
2,H
3,etc.withcoefficientsof
permeabilityk
1,k
2,k
3,etc.
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Engineering
SOIL PERMEABILITY
Fig. 11 Permeability of stratified soil deposits
31Source: Fig. 11 -NPTEL

32. For vertical flow
For horizontal flow
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Engineering
SOIL PERMEABILITY
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33. Example 5
Ahorizontalstratifiedsoildepositconsistsofthreelayerseach
uniforminitself.Thepermeabilitiesoftheselayersare8×10
–4
cm/s,52×10
–4
cm/s,and6×10
–4
cm/s,andtheirthicknessesare
7,3and10mrespectively.Findtheeffectiveaveragepermeability
ofthedepositinthehorizontalandverticaldirections.
Solution:
Department of Civil
Engineering
SOIL PERMEABILITY
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34. Department of Civil
Engineering
SOIL PERMEABILITY
34

35. QUICK SAND CONDITION
Quicksandformsinsaturatedloosesandwhensuddenlyagitated.
Whenwaterinthesandcannotescape,itcreatesaliquefiedsoil
thatlosesstrengthandcannotsupportweight.
Inthecaseofupwardsflowingwater,seepageforcesopposethe
forceofgravityandsuspendthesoilparticlescausingloseof
strength.
Thecushioningofwatergivesquicksand,andotherliquefied
sediments,aspongy,fluid-liketexture.
Objectsinliquefiedsandsinktothelevelatwhichtheweightof
theobjectisequaltotheweightofthedisplacedsoil/watermix
andthesubmergedobjectfloatsduetoitsbuoyancy.
SOIL LIQUEFACTION
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Engineering
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36. MECHANISM
Anupwardflowopposestheforceofgravityandcauseto
counteractcompletelythecontactforces.
Effectivestressisreducedtozeroandthesoilbehaveslikea
veryviscousliquid-Quicksandcondition.
Thisconditionoccursincoarsesiltorfinesandsubjectto
artesianconditions.
Department of Civil
Engineering
SOIL LIQUEFACTION
Fig. 12 Quick sand condition -Mechanism
Video link : https://www.youtube.com/watch?v=eImtYyuQCZ8
36Source: Fig.12 -NPTEL

37. Contd….
At the bottom of the soil column,
During quick sand condition, the effective stress is reduced to zero.
wherei
cr=criticalhydraulicgradientThisshowsthatwhenwaterflows
upwardunderahydraulicgradientofabout1,itcompletelyneutralizesthe
forceonaccountoftheweightofparticles,andthusleavestheparticles
suspendedinwater.
Department of Civil
Engineering
SOIL LIQUEFACTION
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38. SOIL LIQUEFACTION
Liquefactionisaspecialcaseofquicksand.
Inthiscase,suddenearthquakeforcesimmediatelyincrease
theporepressureofshallowgroundwater.
Thesaturatedliquefiedsoillosesstrength,causingbuildingsor
otherobjectsonthatsurfacetosink.
Video link : https://www.youtube.com/watch?v=ZMWKTuRgJjY
Department of Civil
Engineering
SOIL LIQUEFACTION
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39. REFERENCES
AroraKR.,“SoilMechanicsandFoundationEngineering”,
StandardPublishers,2011.
Venkatramaiah,C.,“GeotechnicalEngineering”,NewAge
InternationalPublishers,NewDelhi,6
th
edition,2018.
https://nptel.ac.in/courses.php
https://en.wikipedia.org/
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Engineering
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