SEEPAGE ANALYSIS & FLOWNETS.pdf

Seepage Analysis,Quick Sand condition and Flownets are explained in a very lucid manner

1. SEEPAGE ANALYSIS
AND FLOWNETS
GOURHARI BISWAS

2. SEEPAGE ANALYSIS
•Seepage-Itistheflowofwaterunder
gravitationalforcesthroughapermeable
medium.
•Flowofwatertakesplacefromapointof
highheadtoapointoflowhead.Theflow
isgenerallylaminarinnature.
•SeepageVelocity-Theapparentvelocity
withwhichgroundwatermovesthrough
thebulkoftheporousmedium.

3. Effective, Neutral and Total Stress
Effective, Neutral and Total Stress
Two types of stresses act within a saturated soil mass and
both of them make total stress. One is named effective
stress or intergranular pressure and the another is named
neutral stress or pore water pressure.
Neutral stress-stress carried by the pore water and it is
the same in all directions, also called ‘pore water pressure’
and is designated by u.
At a depth z below the water table :u = Ꝩw . z
Neutral stress is transmitted through the pore present in the
soil.Neutral stress does not bring any change either in void
ratio or in any engineering properties of soil. It acts equally
on all sides.

4. contd...
EffectiveStress-differencebetweenthetotalstressand
theneutralstress;thisisalsoreferredtoasthe
intergranularpressureandisdenotedby:
σ͞=σ–u
Effectivestressistransmittedfromonegraintoanother
grainthroughtheirpointsofcontactsinasoilmass.
Suchstressiseffectiveinchangingthevoidratioofsoil
massandinchangingotherengineeringpropertiessuch
asshearstrengthetc.
HydraulicGradient:-Lossofheadordissipationofhead
perunitdistanceofflow
Itisdenotedbyiandisgivenbyi=h/L
SeepagePressure:-Seepagewaterflowingthroughasoil
massexertsaforceorpressuretothesoilmassinthe
directionofflow,knownasSeepagepressure.

5. QUICK SAND CONDITION
Whenflowtakesplaceinupwarddirectionthe
seepagepressurealsoactsintheupward
directionandtheeffectivepressureisreduced.If
theseepagepressurebecomesequaltothe
pressureduetosubmergedweightofthesoil
theeffectivepressurereducedtozero.Insucha
casecohesionlesssoilloosesitsshearstrength
andthesoilparticleshaveatendencytomove
upinthedirectionofflow.Thisphenomenonof
liftingofsoilparticlesisknownasQuick
condition,boilingconditionorQuicksand
condition.
Thesoilthatexperiencesquicksandcondition
wouldloseshearstrengthandbearingcapacity.

6. QUICK SAND CONDITION
Whenthesoilparticlesare
inastateofcritical
equilibriumthetotal
upwardforceatthebottom
ofthesoilbecomesequal
tothetotalweightofthe
soilparticlesabovethe
bottomsection.
Atcriticalcondition,
(h+L).Aγw=γsat.L.A
h/L=ic=(G-1)/(1+e)

7. How to Avoid Quicksand Condition?
Itcanbepreventedbyloweringthe
watertableatthesitebefore
excavationorbyincreasingthe
lengthofupwardflow.

8. PHERATIC LINES
•Phreaticlinealsoknownas
theseepagelineorsaturation
lineisanimaginarylinewhich
separatessaturatedzonefrom
unsaturatedzoneinanearthen
dam.Itisalsoknownas
Hydraulicgradientline.
•Alongthephreaticline,the
atmosphericpressureexists
(whichisequaltozero)atthe
faceofphreaticline.
•Abovethephreaticlinethereis
anegativehydrostaticpressure
inthedamsectionandbelow
whichthereispositive
hydrostaticpressure.

9. FLOWNETS
•Whenwaterflowsthrough
theinterconnectingvoidsof
asoilmassitfollowsa
certainpath.
•Flownetisthegraphical
representationofflowof
water thgrough the
interconnectingvoidsofthe
soilmass.
•Aflownetsconsistsoftwo
groupsofcurves.Theseare
flowlinesandequipotential
lines.Thesecurvesbeara
fixedrelationshiptoeach
other.

10. Some Definitions
•FLOWLINES--arethepathswhichwaterparticlefollow
inthecourseofseepagethroughasoilmassunder
laminerflowcomdition.Waterflowsfromthepointof
higherheadtolowhead.Flowlinesrepresentthepattern
offlow.
•EQUIPOTENTIALLINES--arethelinesformedbyjoining
thepointsofsameheadorpotentialontheflowlines.
•FLOWNETS-Ifwedrawtheflowlinesandthe
equipotentiallinesforagivenflowofwaterthrougha
givensoilwegetanetlikesketchwhichisknownasflow
net.
•FlowChannels-Thespacebetweentwoadjacentflow
linesiscalledflowpathorchannel.
•Field-Thefigureformedontheflownetbetweenanytwo
adjacentflowlinesandadjacentequipotentiallinesis
refferdtoasfield.

11. Properties of Flow net:
1.Flowlinesandequalpotentiallinesintersecteachother
at90degrees.
2.Theareasboundedbytheflowlinesandequalpotential
linesformapproximatesquares.
3.Flownetsmustsatisfytheboundaryconditionsofflow
field.
4.Quantityofwaterflowingthrougheachflowchannelis
thesame.
5.Thepotentialdropinanytwoconsecutiveequalpotential
linesissame/constant.
6.Flowlinesandequalpotentiallinesaresmoothcurves.
7.Flowlinesdoshowrefractionattheinterfacebetween
twosoilshavingdifferentcoefficientofpermeability.
8.Darcy’slawisvalid.

12. APPLICATION OF FLOWNETS
1.Estimationofseepagelossesfromreservoirs:Itis
possibletousetheflownetinthetransformedspaceto
calculatetheflowunderneaththedam.
2.Determinationofupliftpressuresbelowdams:Fromthe
flownet,thepressureheadatanypointatthebaseofthe
damcanbedetermined.Theupliftpressuredistribution
alongthebasecanbedrawnandthensummedup.
3.Checkingthepossibilityofpipingbeneathdams:Atthe
toeofadamwhentheupwardexithydraulicgradient
approachesunity,boilingconditioncanoccurleadingto
erosioninsoilandconsequentpiping.Manydamsonsoil
foundationshavefailedbecauseofasuddenformationofa
pipedshapeddischargechannel.

13. Determination of seepage:
For the seepage through a flow net for
isotropic condition
Total Discharge, q = k.h.Nf/Nd
where, k = co-efficient of permeability
h= head causing flow
Nf= Total numbers of flow channels
Nd= Total numbers of potential drops.

14. Determination of seepage pressure